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We investigated a central antihypertensive effect of acupuncture in rostral ventrolateral medulla (RVLM) in spontaneously hypertensive rats (SHRs). In total, 56 rats were randomly divided into seven groups as follows: the SHR group, SHR+acupuncture (SHR+Acu) group, SHR+nonacupuncture (SHR+Non-acu) group, GW9662+acupuncture (GW9662+Acu) group, GW9662+GW1929 group, GW9662 group, and 2% DMSO group (n = 8 per group). The whole eight Wistar-Kyoto rats were assigned to the WKY group. The acupuncture treatment lasting for 14 days was performed at the Taichong acupoint (LR3) or at a nonacupoint (non-acu) once daily. The peroxisome proliferator-activated receptor (PPAR)-γ agonist GW1929 and the PPAR-γ inhibitor GW9662 were microinjected by the brain stereotactic technique. Blood pressure was measured by the tail-cuff method. Sympathetic vasomotor activity was determined by implanting in a telemetry electrocardiogram radio transmitter. The expression of PPARs in the RVLM of the rats was detected using Western blot. We demonstrated that acupuncture attenuated blood pressure, heart rate, and sympathetic vasomotor activity in SHRs. The protein expression of PPAR-γ was significantly increased in SHRs treated with acupuncture. The antihypertensive effects of acupuncture in SHRs were abrogated by microinjection bilaterally into RVLM of GW9662. Microinjection of GW1929 mimicked the antihypertensive effect of acupuncture. PPAR-γ expression was negatively correlated with blood pressure and sympathetic vasomotor activity in SHRs treated with acupuncture. These results suggested that acupuncture promoted a central antihypertensive effect by increasing the expression of PPAR-γ in RVLM of SHRs.

In thermal neutral condition, rats display cyclic variations of the vasomotion of the tail and paws, synchronized with fluctuations of blood pressure, heart rate, and core body temperature. "On-" and "off-" cells located in the rostral ventromedial medulla, a cerebral structure implicated in somatic sympathetic drive, 1) exhibit similar spontaneous cyclic activities in antiphase and 2) are activated and inhibited by thermal nociceptive stimuli, respectively. We aimed at evaluating the implication of such neurons in autonomic regulation by establishing correlations between their firing and blood pressure, heart rate, and skin and core body temperature variations. When, during a cycle, a relative high core body temperature was reached, the on-cells were activated and within half a minute, the off-cells and blood pressure were depressed, followed by heart rate depression within a further minute; vasodilatation of the tail followed invariably within ∼3 min, often completed with vasodilatation of hind paws. The outcome was an increased heat loss that lessened the core body temperature. When the decrease of core body temperature achieved a few tenths of degrees, sympathetic activation switches off and converse variations occurred, providing cycles of three to seven periods/h. On- and off-cell activities were correlated with inhibition and activation of the sympathetic system, respectively. The temporal sequence of events was as follows: core body temperature → on-cell → off-cell ∼ blood pressure → heart rate → skin temperature → core body temperature. The function of on- and off-cells in nociception should be reexamined, taking into account their correlation with autonomic regulations., (Copyright © 2014 the American Physiological Society.)

Metabolic syndrome (MetS), which is rapidly becoming prevalent worldwide, is long known to be associated with hypertension and recently with oxidative stress. Of note is that oxidative stress in the rostral ventrolateral medulla (RVLM), where sympathetic premotor neurons reside, contributes to sympathoexcitation and hypertension. This study sought to identify the source of tissue oxidative stress in RVLM and their roles in neural mechanism of hypertension associated with MetS. Adult normotensive rats subjected to a high-fructose diet for 8 weeks developed metabolic traits of MetS, alongside increases in sympathetic vasomotor activity and blood pressure. In RVLM of these MetS rats, the tissue level of reactive oxygen species was increased, nitric oxide (NO) was decreased, and mitochondrial electron transport capacity was reduced. Whereas the protein expression of neuronal NO synthase (nNOS) or protein inhibitor of nNOS was increased, the ratio of nNOS dimer/monomer was significantly decreased. Oral intake of pioglitazone or intracisternal infusion of tempol or coenzyme Q10 significantly abrogated all those molecular events in high-fructose diet-fed rats and ameliorated sympathoexcitation and hypertension. Gene silencing of protein inhibitor of nNOS mRNA in RVLM using lentivirus carrying small hairpin RNA inhibited protein inhibitor of nNOS expression, increased the ratio of nNOS dimer/monomer, restored NO content, and alleviated oxidative stress in RVLM of high-fructose diet-fed rats, alongside significantly reduced sympathoexcitation and hypertension. These results suggest that redox-sensitive and protein inhibitor of nNOS-mediated nNOS uncoupling is engaged in a vicious cycle that sustains the production of reactive oxygen species in RVLM, resulting in sympathoexcitation and hypertension associated with MetS., (© 2014 American Heart Association, Inc.)

Angiotensin (ANG)-(1-12) excites neurons via ANG II type 1 receptors (AT1Rs), which are present in the caudal ventrolateral medullary depressor area (CVLM). We hypothesized that microinjections of ANG-(1-12) into the CVLM may elicit decreases in mean arterial pressure (MAP), heart rate (HR), and sympathetic nerve activity. This hypothesis was tested in urethane-anesthetized adult male Wistar rats. Microinjections of ANG-(1-12) into the CVLM elicited decreases in MAP, HR, and greater splanchnic nerve activity (GSNA). ANG-(1-12)-induced responses consisted of initial (first 1-8 min) and delayed (8-24 min) phases. Prior microinjections of losartan, A-779, and captopril into the CVLM blocked initial, delayed, and both phases of ANG-(1-12) responses, respectively. Blockade of GABA receptors in the rostral ventrolateral medullary pressor area (RVLM) attenuated cardiovascular responses elicited by microinjections of ANG-(1-12) into the ipsilateral CVLM. Microinjections of ANG-(1-12) into the CVLM potentiated the reflex decreases and attenuated the reflex increases in GSNA elicited by intravenous injections of phenylephrine and sodium nitroprusside, respectively. These results indicate that microinjections of ANG-(1-12) into the CVLM elicit decreases in MAP, HR, and GSNA. Initial and delayed phases of these responses are mediated via ANG II and ANG-(1-7), respectively; the effects of ANG II and ANG-(1-7) are mediated via AT1Rs and Mas receptors, respectively. Captopril blocked both phases of ANG-(1-12) responses, indicating that angiotensin-converting enzyme is important in mediating these responses. GABA receptors in the RVLM partly mediate the cardiovascular responses to microinjections of ANG-(1-12) into the CVLM. Microinjections of ANG-(1-12) into the CVLM modulate baroreflex responses.

Hydrogen sulfide (H(2)S) acts as an endogenous gaseous transmitter in the central nervous system and plays important roles in regulating cardiovascular function. The rostral ventrolateral medulla (RVLM) is a putative critical central region in the control of sympathetic vasomotor tone and plays an important role in the baroreflex by integrating the inputs from a variety of visceral and somatic stimuli. In this study, we tested the hypothesis that H(2)S decreases sympathetic vasomotor tone through ATP-sensitive potassium channels (K(ATP)) in the RVLM. The arterial blood pressure (ABP), heart rate (HR), and renal sympathetic nerve activity (RSNA) of anesthetized rats were recorded. Bilateral microinjections of sodium hydrosulfide (NaHS; 4, 8, and 16 mM, 50 nl), an H(2)S donor, into the RVLM decreased ABP, HR, and RSNA in a dose-dependent manner. Preinjection of glibenclamide (40 μM, 50 nl), a K(ATP) channel blocker, abolished the sympathoinhibitory effects of NaHS (8 mM, 50 nl). Preinjection of a nitric-oxide synthase inhibitor, N(ω)-nitro-l-arginine methyl ester (200 μM, 50 nl) partially inhibited the sympathoinhibitory effects of NaHS. Prior microinjection of 1,4-dihydro-2,6-dimethyl-5-nitro-4-(2-[trifluoromethyl]phenyl)pyridine-3-carboxylic acid methyl ester (Bay K8644) (1 μM, 50 nl), an agonist of Ca(2+) channels, did not alter the effects of NaHS. Infusion of hydroxylamine (30 mM, 50 nl), a cystathionine β-synthase inhibitor, increased ABP, HR, and RSNA. Taken together, these findings suggest that exogenous H(2)S in the RVLM inhibits sympathetic vasomotor tone by opening K(ATP) channels. Nitric-oxide signaling may partially be involved in the sympathoinhibitory effect of H(2)S in the RVLM.

Immunohistochemistry in Wistar rats identified serotonergic neurons in 8 cores of the medulla oblongata belonging to the so-called "bulbar vasomotor center". Using morphometry revealed that the proportion of serotonergic neurons located in the projection of the cores studied ranged from 17 to 26 %, and value of the index rised to 34-40 % only in the cores of the back seam. Single immunopositive cells that can perform the integrative function in the regulation ofhemodynamics were detected between the cores as well as between the cores and pathways.

The ventral medulla oblongata is critical for cardiorespiratory regulation. Here we review previous literature relating to sites within the ventral medulla that have been identified as having a 'cardiovascular' or 'respiratory' function. Together with the maps generated here, of sites from which cardiovascular and respiratory responses were evoked by glutamate microinjection, specific 'cardiovascular' regions have been defined and delineated. Commonly investigated regions, including the vasopressor rostral ventrolateral medulla (RVLM) and vasodepressor caudal ventrolateral medulla (CVLM), or areas only described by others, such as the medullary cerebral vasodilator area, are included for completeness. Emphasis is given to the caudal medulla, where three pressor regions, the caudal pressor area (CPA), the intermediate pressor area (IPA) and the medullo-cervical pressor area (MCPA), caudal to the vasodepressor CVLM were defined in the original data provided. The IPA is most responsive under pentobarbitone rather than urethane anaesthesia clearly delineating it from both the rostrally located CPA and the caudally located MCPA. The description of these multiple pressor areas appears to clarify the confusion that surrounds the identification of the 'CPA'. Also noted is a vasopressor region adjacent to the vasodepressor CVLM. Apart from the well described ventral respiratory column, a region medial to the pre-Bötzinger is described, from which increases in both phrenic nerve frequency and amplitude were evoked. Limitations associated with the technique of glutamate microinjection to define functionally specific regions are discussed. Particular effort has been made to define and delineate the regions with respect to ventrally located anatomical landmarks rather than the commonly used ventral surface or dorsal landmarks such as the obex or calamus scriptorius that may vary with the brain orientation or histological processing. This should ensure that a region can easily be defined by all investigators. Study of defined regions will help expedite the identification of the role of the multiple cell groups with diverse neurotransmitter complements that exist even within each of the regions described, in coordinating the delivery of oxygenated blood to the tissues.

Several forms of experimental evidence gathered in the last 37 years have unequivocally established that the medulla oblongata harbors the main neural circuits responsible for generating the vasomotor tone and regulating arterial blood pressure. Our current understanding of this circuitry derives mainly from the studies of Pedro Guertzenstein, a former student who became Professor of Physiology at UNIFESP later, and his colleagues. In this review, we have summarized the main findings as well as our collaboration to a further understanding of the ventrolateral medulla and the control of arterial blood pressure under normal and pathological conditions.

It is well known that the ventrolateral medulla contains neurons involved in the tonic and reflex control of the cardiovascular system. Two regions within the ventrolateral medulla were initially identified: the rostral ventrolateral medulla (RVLM) and the caudal ventrolateral medulla (CVLM). Activation of the RVLM raises arterial blood pressure and sympathetic nerve activity, and activation of the CVLM causes opposite effects. The RVLM premotor neurons project directly to sympathetic preganglionic neurons and are involved in the maintenance of resting sympathetic vasomotor tone. A significant proportion of tonic activity in the RVLM sympathetic premotor neurons is driven by neurons located in a third region of the ventrolateral medulla denominated caudal pressor area (CPA). The CPA is a pressor region located at the extreme caudal part of the ventrolateral medulla that appears to have an important role controlling the activity of RVLM neurons. In this brief review, we will address the importance of the ventrolateral medulla neurons for the generation of resting sympathetic tone related to arterial blood pressure control focusing on two regions, the RVLM and the CPA.

The present study explores how elevations in brain P(CO(2)) increase the sympathetic nerve discharge (SND). SND, phrenic nerve discharge (PND) and putative sympathoexcitatory vasomotor neurons of the rostral ventrolateral medulla (RVLM) were recorded in anaesthetized sino-aortic denervated and vagotomized rats. Hypercapnia (end-expiratory CO(2) from 5% to 10%) increased SND (97 +/- 6%) and the activity of RVLM neurons (67 +/- 4%). Injection of kynurenic acid (Kyn, ionotropic glutamate receptor antagonist) into RVLM or the retrotrapezoid nucleus (RTN) eliminated or reduced PND, respectively, but did not change the effect of CO(2) on SND. Bilateral injection of Kyn or muscimol into the rostral ventral respiratory group (rVRG-pre-Bötzinger region, also called CVLM) eliminated PND while increasing the stimulatory effect of CO(2) on SND. Muscimol injection into commissural part of the solitary tract nucleus (commNTS) had no effect on PND or SND activation by CO(2). As expected, injection of Kyn into RVLM or muscimol into commNTS virtually blocked the effect of carotid body stimulation on SND in rats with intact carotid sinus nerves. In conclusion, CO(2) increases SND by activating RVLM sympathoexcitatory neurons. The relevant central chemoreceptors are probably located within or close to RVLM and not in the NTS or in the rVRG-pre-Bötzinger/CVLM region. RVLM sympathoexcitatory neurons may be intrinsically pH-sensitive and/or receive excitatory synaptic inputs from RTN chemoreceptors. Activation of the central respiratory network reduces the overall sympathetic response to CO(2), presumably by activating barosensitive CVLM neurons and inhibiting RTN chemoreceptors.

Chemical stimulation of a region extending from the most caudal ventrolateral medulla into the upper cervical spinal cord evoked large sympathetically mediated pressor responses. These responses were not dependent on the integrity of the rostral ventrolateral medulla (RVLM) and may be mediated by glutamatergic neurons embedded in the white matter that project to the thoracic spinal cord. We term this new region the medullo-cervical pressor area (MCPA). This region is distinct from the caudal pressor area, because blockade of the RVLM with muscimol inhibited this pressor response but not that evoked from the MCPA. This is the first study to provide functional evidence for a cardiovascular role for neurons in the cervical spinal cord white matter that innervate sympathetic preganglionic neurons (Jansen and Loewy, 1997). Using retrograde tracing, in combination with immunohistochemistry and in situ hybridization, we identified two groups of spinally projecting neurons in the region. Approximately 50% of neurons in one group were excitatory because they contained vesicular glutamate transporter 1 (VGluT1)/VGluT2 mRNA, whereas the other contained a mixed population of neurons, some of which contained either VGluT1/VGluT2 or GAD67 (glutamic acid decarboxylase 67) mRNA. Despite the fact that activation of the MCPA causes potent sympathoexcitation, it does not act to restore arterial pressure after chemical lesion of the RVLM so that a role for this novel descending sympathoexcitatory region remains to be elucidated.

The present study examined the role of inducible nitric oxide synthase (iNOS) in the rostral ventrolateral medulla (RVLM) of the brain stem, where the vasomotor center is located, in the control of blood pressure and sympathetic nerve activity. Adenovirus vectors encoding iNOS (AdiNOS) or beta-galactosidase (Adbetagal) were transfected into the RVLM in Wistar-Kyoto (WKY) rats. Blood pressure and heart rate were monitored using a radiotelemetry system. iNOS expression in the RVLM was confirmed by immunohistochemical staining or Western blot analysis. Mean arterial pressure significantly increased from day 6 to day 11 after AdiNOS transfection, but did not change after Adbetagal transfection. Urinary norepinephrine excretion was significantly higher in AdiNOS-transfected rats than in Adbetagal-transfected rats. Microinjection of aminoguanidine or S-methylisothiourea, iNOS inhibitors, or tempol, an antioxidant, significantly attenuated the pressor response evoked by iNOS gene transfer. The levels of thiobarbituric acid-reactive substances, a marker of oxidative stress, were significantly greater in AdiNOS-transfected rats than in Adbetagal-transfected rats. Dihydroethidium fluorescence in the RVLM was increased in AdiNOS-transfected rats. In addition, nitrotyrosine-positive cells were observed in the RVLM only in AdiNOS-transfected rats. Intracisternal infusion of tempol significantly attenuated the pressor response and the increase in the levels of thiobarbituric acid-reactive substances induced by AdiNOS transfection. These results suggest that overexpression of iNOS in the RVLM increases blood pressure via activation of the sympathetic nervous system, which is mediated by an increase in oxidative stress.

We studied the effects of electrical stimulation of the inferior alveolar nerve (IAN) on cardiovascular responses in cats. There was statistical correlation between cardiovascular response and prestimulus mean arterial blood pressure (MABP) and heart rate (HR). A trigeminal depressor response (TDR) was induced when the prestimulus MABP and HR were above 95 mm Hg and 140 beats/min, respectively. We investigated further to identify the vasomotor regulating center and neural transmitters involved in TDR. In the medulla, electrical stimulation of the dorsomedial medulla, the infratrigeminal nucleus (IFT), and the rostral ventrolateral medulla (RVLM) induced a vasopressor response. We confirmed that neurons in the RVLM were retrogradely labeled by wheat germ agglutinin-conjugated horseradish peroxidase injection into the nucleus intermediolateralis of the spinal cord. The vasopressor response induced by IFT stimulation was similar to that induced by IAN stimulation. Vasodepressor responses were induced when the caudal ventrolateral medulla, the nucleus tractus solitarius, the lateral tegmental field, the trigeminal nucleus interpolaris, the trigeminal spinal tract, and the paramedian reticular nucleus were stimulated. These responses, however, were not similar to the vasodepressor response induced by IAN stimulation but were similar to the cardiovascular response induced by vagal afferent stimulation. After spinalization or lesion of the RVLM, MABP and HR decreased and TDR completely disappeared. Inhibitory synaptic ligands and receptors were localized using immunohistochemical techniques. Neurons immunopositive for adrenaline, noradrenaline, and gamma-aminobutyric acid (GABA), and adrenaline alpha(2A), GABA(A), GABA(B), and glycine receptors were distributed along the sympatho-reflexive route including the RVLM and IFT. These results suggest that TDR could be induced as negative feedback to sympathetic hyperactivity whenever MABP and HR are high, because of the inhibitory control of the RVLM., (Copyright 2004 Wiley-Liss, Inc.)

Sympathetically-mediated vasoconstriction of cutaneous vessels is critical for thermoregulation in the cold. We determined whether cold-induced sympathetic discharge depends on activity of neurons in the rostral medullary raphé. In urethane-anesthetized rats and rabbits, cooling the trunk skin by a water jacket reproducibly increased cutaneous sympathetic discharge recorded in the tail (rats) and the ear pinna (rabbits). When neurons in the rostral medullary raphé were inhibited by microinjection of glycine (30-100 nmol in 60-200 nl in rats), or muscimol (1 nmol in 100 nl in rabbits), cutaneous sympathetic activity was silenced and no longer responded to cooling (7+/-3 and 5+/-2% of pre-injection increase in rats and rabbits, respectively, P < 0.01). Our data demonstrate that activity of rostral medullary raphé neurons is important for the CNS mediation of cold-induced increases in sympathetic cutaneous vasomotor nerves.

Neurones within the rostral ventromedial medulla project to the intermediolateral cell column of the spinal cord where they may influence sympathetic preganglionic neuronal activity controlling cutaneous vascular beds. Here we assess whether such neurones contribute to cutaneous sympathetic vasoconstrictor activity in a fever-like state induced by i.c.v. injection of E-series prostaglandin. In urethane-anaesthetised rats, we recorded population sympathetic activity to the tail (an index of vasoconstrictor discharge regulating cutaneous thermoregulatory circulations). A survey of the effects of GABA microinjections (200 mM; 60-80 nl; 111 sites in 57 rats) demonstrated that those into the rostral ventromedial medulla (in the region of raphe pallidus and magnus; approximately bregma -10 to -12 mm) markedly decreased (51%-100%) population sympathetic cutaneous vasoconstrictor activity during "normothermic control." In contrast, injections at sites dorsal and lateral to this region tended to produce either a smaller decrease or have no effect. In heat-clamp (nine animals: body temperature 40-41 degrees C) cutaneous vasoconstrictor activity was decreased by 83+/-5%. I.c.v. prostaglandin E(1) (100 ng and above) restored activity to, or above, control levels in these animals and where body temperature was maintained at control levels (12 animals: body temperature 35.5-36.5 degrees C). The depressant action of GABA was sustained in both conditions. GABA did not significantly influence concurrently recorded splanchnic nerve activity and heart rate in any condition although both were increased following i.c.v. prostaglandin E(1) (500 ng). This study is the first to demonstrate that inhibition of neuronal activity within the rostral ventromedial medulla decreases sympathetic cutaneous vasoconstrictor activity during normothermic control and following i.c.v. prostaglandin E(1) (both with and without heat-clamp). Therefore, sympathetic premotor neurones in this area contribute to vasoconstrictor drive in these conditions. In contrast, we were unable to demonstrate that the same area had a substantial involvement in the control of splanchnic nerve activity or heart rate, even when these were enhanced following i.c.v. prostaglandin E(1).

Systemic administration of the gastrointestinal hormone cholecystokinin (CCK) selectively inhibits splanchnic sympathetic vasomotor discharge and differentially affects presympathetic vasomotor neurons of the rostroventrolateral medulla (RVLM). Stimulation of the sympathoexcitatory region of the periaqueductal grey (PAG) produces profound mesenteric vasoconstriction. In this study, our aim was to identify phenotypically different populations of RVLM presympathetic vasomotor neurons using juxtacellular neuronal labelling and immunohistochemical detection of the adrenergic neuronal marker phenylethanolamine-N-methyl transferase (PNMT) and to determine whether the PAG provides functional excitatory input to these neurons. Fifty-eight percent (36/62) of RVLM presympathetic neurons were inhibited by systemic administration of CCK. These cells had conduction velocities (3.6 +/- 0.2 m/sec) in the non-C-fiber range consistent with neurons possessing lightly myelinated spinal axons. Of these, 79% (22/28) were excited by PAG stimulation, and 59% (10/17) were not immunoreactive for PNMT. Conversely, 42% (26/62) of RVLM presympathetic neurons were either unaffected or activated by CCK administration and had slower conduction velocities (1.4 +/- 0.3 m/sec) than cells inhibited by CCK. Fifty percent (11/22) of these cells were driven by PAG stimulation, and most (11/14 or 79%) were PNMT-positive. These results suggest that cardiovascular responses elicited by PAG stimulation occur via activation of non-C1 and C1 RVLM presympathetic neurons. RVLM neurons inhibited by CCK were more likely to be driven by PAG stimulation and may be a subset of neurons responsible for driving gastrointestinal sympathetic vasomotor tone. CCK-induced inhibition of a subpopulation of RVLM presympathetic neurons may be implicated in postprandial hyperemia and postprandial hypotension., (Copyright 2003 Wiley-Liss, Inc.)

1. We have previously shown that intracisternal administration of endothelin-1 (ET-1) elicited cardiorespiratory responses acting on the ventral surface of the medulla oblongata (VSM) subjacent to the rostral ventrolateral medulla (RVLM). In this study, we examined whether vasomotor and respiratory neurons in RVLM participate in above-mentioned responses and whether those neurons respond to direct iontophoretic application of ET-1 and/or an ET-A receptor antagonist, FR139317. 2. Unit activity of vasomotor, respiratory, or nociceptive neurons in RVLM was recorded together with arterial blood pressure (AP) and heart rate (HR) in urethaneanesthetized Sprague-Dawley rats. 3. Intracisternal administration or topical application of ET-1 (0.1-1 pmol) to VSM caused excitation of the majority of vasomotor neurons (15/18) and respiratory neurons (10/11) but not in nociceptive neurons (0/7). Changes in neuronal activity were in similar time course with corresponding changes in AP and HR. Iontophoretic application of ET-1 to the vicinity of recording neuron caused excitation in 19 of 21 vasomotor neurons without affecting AP nor HR. Remaining two neurons were insensitive to ET-1. FR139317 did not affect basal activity of the vasomotor neurons but inhibited ET-1-evoked excitation. Twenty-four of 40 respiratory neurons were excited and 13 were inhibited by iontophoretic application of ET-1. Five of ET-1-excited respiratory neurons were inhibited by FR139317 alone while six of ET-1-inhibited neurons were not affected by FR139317 alone. In both cases, FR139317 inhibited the effect of simultaneously applied ET-1. Iontophoretic application of ET-1 excited only one out of 10 nociceptive neurons so far tested. 4. These results support the view that intracisternally administered ET-1 alters activity of vasomotor and respiratory neurons in the RVLM, at least in part by acting directly on neurons themselves and hence causes systemic cardiorespiratory changes. Majority of vasomotor and respiratory neurons should express ET-A receptors and some respiratory neurons are under tonic excitatory control by ET-1.

1. Evidence gathered over the last 30 years has firmly established that the rostral ventrolateral medulla (RVLM) is a major vasomotor center in the brainstem, harboring sympathetic premotor neurons responsible for generating and maintaining basal vasomotor tone and resting levels of arterial blood pressure. Although the RVLM has been almost exclusively classified as a vasopressor area, in this report we review some evidence suggesting a prominent role of the RVLM in muscle vasodilation during defense-alerting responses. 2. Defense-alerting reactions are a broad class of behavior including flexion of a limb, fight/flight responses, apologies, etc. They comprise species-distinctive motor and neurovegetative adjustments. Cardiovascular responses include hypertension, tachycardia, visceral vasoconstriction, and muscle vasodilation. Since defense-alerting reactions generally involve intense motor activation, muscle vasodilation is regarded as a key feature of these responses 3. In anesthetized or unanesthetized-decerebrate animals, natural or electrical stimulation of cutaneous and muscle afferents produced hypertension, tachycardia, and vasodilation restricted to the stimulated limb. 4. Unilateral inactivation of the RVLM contralateral to the stimulated limb abolished cardiovascular adjustments to stimulation of cutaneous and muscle afferents. Within the RVLM glutamatergic synapses mediate pressor responses, whereas GABAergic synapses mediates muscle vasodilation. 5. In urethane-anesthetized rats, electrical stimulation of the hypothalamus elicited hypertension, tachycardia, visceral vasoconstriction, and hindlimb vasodilation. The hindlimb vasodilation induced by hypothalamic stimulation is a complex response, involving reduction of sympathetic vasoconstrictor tone, release of catecholamines by the adrenal medulla, and a still unknown system that may use nitric oxide as a mediator. 6. Blockade of glutamatergic transmission within the RVLM selectively blocks muscle vasodilation induced by hypothalamic stimulation. 7. The results obtained suggest that, besides its role in the generation and maintenance of the sympathetic vasoconstrictor drive, the RVLM is also critical for vasodilatory responses during defense reactions. The RVLM may contain several, distinctive mechanisms for muscle vasodilation. Anatomical and functional characterization of these pathways may represent a breakthrough in our understanding of cardiovascular control in normal and/or pathological conditions.

1 The role of peripheral 5-hydroxytryptamine (5-HT(3)) receptors and cholecystokinin type 1 (CCK(1)) receptors in the inhibitory effects of phenylbiguanide (PBG) and CCK on arterial blood pressure, heart rate and the discharge of presympathetic vasomotor neurones of the rostral ventrolateral medulla (RVLM) was studied in alpha-chloralose-anaesthetized rats. 2 CCK (1 and 4 micro g kg(-1), i.v.) and PBG (2 and 10 micro g kg(-1), i.v.) reduced arterial blood pressure and heart rate, and inhibited the discharge of single RVLM presympathetic vasomotor neurones in a dose-related manner. 3 Devazepide (0.5 mg kg(-1), i.v.), a selective CCK(1) receptor antagonist, blocked the effects of CCK on arterial blood pressure, heart rate and neuronal discharge but did not significantly alter these responses to PBG. MDL72222 (0.1 mg kg(-1), i.v.), a selective 5-HT(3) receptor antagonist, blocked the effects of PBG on arterial blood pressure, heart rate and presympathetic neuronal discharge. MDL72222 attenuated the effects of CCK on arterial blood pressure, heart rate and RVLM presympathetic neuronal discharge. Vehicle did not significantly alter any of the responses to CCK or PBG. 4 These experiments suggest that systemically administered CCK acts directly through CCK(1) receptors to modulate sympathetic vasomotor function. In addition, the actions of CCK also are partly dependent on activation of 5-HT(3) receptors. CCK may release 5-HT which then acts at 5-HT(3) receptors to produce sympathetic vasomotor inhibition. In contrast, the actions of PBG are entirely dependent on 5-HT(3) receptors and are independent of any actions at the CCK(1) receptor.

Aim: Neurons in the rostral ventrolateral medulla (RVLM) that project directly to sympathetic preganglionic neurons in the spinal cord play a critical role in maintaining tonic activity in sympathetic vasomotor nerves. Intracellular recordings in vivo from putative RVLM presympathetic neurons have demonstrated that under resting conditions these neurons display an irregular tonic firing rate, and also receive both excitatory and inhibitory synaptic inputs. This paper will briefly review some recent findings on the role of glutamate, GABA and angiotensin II (Ang II) receptors in maintaining the tonic activity of RVLM presympathetic neurons., Results: Based on these findings, the following hypotheses will be discussed: (1) RVLM neurons receive tonic glutamatergic excitatory inputs, which originate from both medullary and supramedullary sources; (2) at least some neurons that project to and tonically inhibit RVLM presympathetic neurons are themselves tonically inhibited by GABAergic inputs originating from neurons in the caudalmost part of the ventrolateral medulla (caudal pressor area); (3) under normal conditions, Ang II receptors in the RVLM do not contribute significantly to the tonic activity of RVLM presympathetic neurons, but may do so in abnormal conditions such as heart failure or neurogenic hypertension; (4) RVLM presympathetic neurons maintain a significant level of tonic resting activity even when glutamate, GABA and Ang II receptors on the neurons are completely blocked. Under these conditions, the tonic activity is a consequence either of the intrinsic membrane properties of the neurons (autoactivity) or of synaptic inputs mediated by receptors other than glutamate, GABA or Ang II receptors., Conclusion: The current evidence indicates that the resting activity of RVLM presympathetic neurons is determined by the balance of powerful tonic excitatory and inhibitory synaptic inputs. Ang II receptors also contribute to the raised resting activity of these neurons in some pathological conditions.

Previous studies in the rat have shown that a significant proportion of the tonic activity of presympathetic neurons in the rostral ventrolateral medulla (RVLM) is dependent on the tonic activity of neurons within the caudal pressor area (CPA), located in the most caudal part of the caudal ventrolateral medulla (CVLM). In this study, we determined the extent to which tonically active neurons in the CPA contribute to sympathetic vasomotor tone, and we also investigated the pharmacological mechanisms by which these neurons affect the tonic activity of RVLM presympathetic neurons. In anaesthetised rabbits, bilateral injections of the neuroinhibitory compound muscimol into the CVLM at the level of the most caudal part of the lateral reticular nucleus, which corresponds to the anatomical location of the CPA as mapped in the rat, resulted in an immediate profound hypotension and almost complete abolition of renal sympathetic nerve activity (rSNA). In contrast, microinjections into surrounding regions had little or no effect or else evoked a delayed hypotensive response. The hypotensive and sympathoinhibitory response evoked by inhibition of the CPA was greatly delayed by prior injections of the GABA receptor antagonist bicuculline into the RVLM. In contrast, injections of the glutamate receptor antagonist kynurenic acid into the RVLM did not alter the hypotensive and sympathoinhibitory response. The results indicate that neurons within the CPA tonically inhibit other neurons, which, in turn, inhibit RVLM sympathoexcitatory neurons, via a GABAergic synapse. This disinhibition of RVLM neurons by CPA neurons is essential for maintaining resting sympathetic vasomotor tone.

1. Barosensitive, bulbospinal neurons in the rostral ventrolateral medulla (RVLM), which provide the major tonic excitatory drive to sympathetic vasomotor neurons, are prominently inhibited by GABA. 2. A major source of the GABAergic inhibition to presympathetic RVLM neurons arises from an area immediately caudal to the RVLM, known as the caudal ventrolateral medulla (CVLM). 3. Arterial baroreceptor afferents projecting to the nucleus tractus solitarius (NTS) provide a major tonic excitatory input to GABAergic CVLM neurons. These CVLM cells are a critical component for baroreflex-mediated changes in presympathetic RVLM neuronal activity, sympathetic nerve activity (SNA) and arterial pressure (AP). 4. Some GABAergic CVLM neurons are tonically activated by inputs independent of arterial baroreceptors or the NTS, providing a GABAergic-mediated inhibition of the presympathetic RVLM neurons that is autonomous of baroreceptor inputs. 5. GABAergic CVLM neurons appear to play two distinct, yet important, roles in the regulation of sympathetic vasomotor tone and AP. They dampen immediate changes in AP via the baroreflex and tonically inhibit the activity of the presympathetic RVLM neurons by baroreceptor-independent mechanisms. This baroreceptor-independent, GABAergic inhibition of presympathetic RVLM neurons may play an important role in determining the long-term level of sympathetic vasomotor tone and AP.

The main source of excitatory drive to the sympathetic preganglionic neurons that control blood pressure is from neurons located in the rostral ventrolateral medulla (RVLM). This monosynaptic input includes adrenergic (C1), peptidergic, and noncatecholaminergic neurons. Some of the cells in this pathway are suspected to be glutamatergic, but conclusive evidence is lacking. In the present study we sought to determine whether these presympathetic neurons express the vesicular glutamate transporter BNPI/VGLUT1 or the closely related gene DNPI, the rat homolog of the mouse vesicular glutamate transporter VGLUT2. Both BNPI/VGLUT1 and DNPI/VGLUT2 mRNAs were detected in the medulla oblongata by in situ hybridization, but only DNPI/VGLUT2 mRNA was present in the RVLM. Moreover, BNPI immunoreactivity was absent from the thoracic spinal cord lateral horn. DNPI/VGLUT2 mRNA was present in many medullary cells retrogradely labeled with Fluoro-Gold from the spinal cord (T2; four rats). Within the RVLM, 79% of the bulbospinal C1 cells contained DNPI/VGLUT2 mRNA. Bulbospinal noradrenergic A5 neurons did not contain DNPI/VGLUT2 mRNA. The RVLM of six unanesthetized rats subjected to 2 hours of hydralazine-induced hypotension contained tenfold more c-Fos-ir DNPI/VGLUT2 neurons than that of six saline-treated controls. c-Fos-ir DNPI/VGLUT2 neurons included C1 and non-C1 neurons (3:2 ratio). In seven barbiturate-anesthetized rats, 16 vasomotor presympathetic neurons were filled with biotinamide and analyzed for the presence of tyrosine hydroxylase immunoreactivity and/or DNPI/VGLUT2 mRNA. Biotinamide-labeled neurons included C1 and non-C1 cells. Most non-C1 (9/10) and C1 presympathetic cells (5/6) contained DNPI/VGLUT2 mRNA. In conclusion, DNPI/VGLUT2 is expressed by most blood pressure-regulating presympathetic cells of the RVLM. The data suggest that these neurons may be glutamatergic and that the C1 adrenergic phenotype is one of several secondary phenotypes that are differentially expressed by subgroups of these cells., (Copyright 2002 Wiley-Liss, Inc.)

1. The present article reviews several aspects of opioid signalling in the rostral ventrolateral medulla (RVLM) and their implications for the neural control of blood pressure. 2. In the RVLM, preproenkephalin (PPE) mRNA is expressed by bulbospinal cells that are strongly barosensitive. These putative presympathetic neurons includes C1 and non-C1 neurons. 3. In the RVLM, PPE mRNA is also present in GABAergic neurons that do not project to the thoracic spinal cord. 4. Rostral ventrolateral medulla presympathetic cells receive enkephalinergic inputs and express mu-opioid receptors (MOR). Some of their synaptic inputs also contain MOR. 5. Pre- and post-synaptic modulation of RVLM presympathetic neurons by MOR agonists has been demonstrated in slices of neonate brain. The post-synaptic effect is inhibitory (increased gK). Presynaptic effects include disfacilitation (reduction of glutamate release) and possibly dishinhibition (reduction of GABA release). 6. In conclusion, opioid signalling plays a pervasive role in the medullospinal network that controls sympathetic tone and arterial pressure. Opioid peptides are made by the presympathetic, presumably excitatory, cells of the RVLM and by local GABAergic inhibitory neurons. In addition, RVLM presympathetic neurons are also controlled by opioid peptides at the pre- and post-synaptic level. mu-Opioid receptors are found post-synaptically, whereas presynaptic receptors probably include both mu and delta subtypes. Conditions that trigger the release of opioid peptides by presympathetic neurons or by inputs to these cells are not fully understood and may include decompensated haemorrhage and certain types of peripheral sensory stimulation related to acupuncture.

The central neural circuit mediating baroreceptor control of sympathetic vasomotor outflow involves an excitatory projection from arterial baroreceptors to nucleus tractus solitarius, an excitatory projection from nucleus tractus solitarius to the caudal ventrolateral medulla, an inhibitory projection from the caudal ventrolateral medulla to the rostral ventrolateral medulla (RVLM), and an excitatory projection from the RVLM to sympathetic preganglionic neurons in the spinal cord. For this circuit to be operational, the relevant neurons in the RVLM must be tonically active. Indeed, numerous studies have demonstrated that RVLM vasomotor neurons are tonically active; however, little is known regarding the nature of the tonic excitatory drive to these neurons. We present a model in which RVLM vasomotor neurons are tonically excited by inputs to the RVLM that can be blocked by the excitatory amino acid receptor antagonist, kynurenic acid, as well as an input from the caudal ventrolateral medulla that is not sensitive to kynurenic acid.

To study the role of the caudal pressor area (CPA) on the ventral surface of medulla oblongata in vasotonia of the skeletal muscles and kidneys, perfusion pressures of vessels in skeletal muscles and kidneys were recorded to observe effects of microinjection of L-glutamate (L-glu) into CPA. The results are as follows. L-glu induced a significant increase of artery pressure (AP), perfusion pressure of muscles (PPm) and perfusion pressure of kidney (PPk), which was markedly attenuated by prior injection of phentolamine or propranolol to the bilateral pressor area. These results indicate that the vascular roles of CPA were mainly mediated via alpha- and/or beta-receptors.

We examined the involvement of caudal ventrolateral medulla (CVLM) in respiratory control. Microinjection of glutamate (Glu) into CVLM decreased systemic arterial blood pressure (SAP) and altered phrenic nerve activities (PNA). Among 143 depressor sites, 55% (78/143) increased respiratory frequency (Rf), while 72% altered PNA amplitude (36% increased and 36% decreased). A small but significant positive correlation was observed between the magnitudes of depressor responses and inhibition of PNA amplitude (r = 0.1718, n = 143), indicating a substantial cross-talk between depressor and PNA inhibitory neurons. Furthermore, microinjections of acetylcholine (ACh) mimicked the Glu-induced depressor responses. However, ACh did not alter Rf, but still reduced PNA amplitude. Our findings suggest that Rf-regulating and depressor neurons are two separate neuronal populations, coexisting in CVLM. The PNA inhibitory and depressor neurons, in contrast, could have stronger correlation.

1. The question of whether neurones in the paraventricular nucleus (PVN) of the hypothalamus have an excitatory influence on reticulo-spinal vasomotor neurones of the rostral ventrolateral medulla (RVL) has been addressed in this study using anaesthetized rats. 2. Extracellular microelectrode recordings were made from sixty vasomotor neurones in the RVL, identified by their cardiac cycle-related probability of discharge, by the decrease in activity in response to an increase in arterial blood pressure produced by intravenous phenylephrine and by the increase in activity in response to a decrease in blood pressure produced by intravenous nitroprusside. 3. More than 70 % of these RVL vasomotor neurones were identified as spinally projecting by antidromically activating their axons via a stimulating electrode in the lateral funiculus of the T2 or T10 segment of spinal cord. 4. Activation of neurones at different sites in the PVN with a microinjection of d,l-homocysteic acid (DLH) elicited either pressor or depressor responses. 5. At PVN pressor sites fifteen RVL vasomotor neurones were shown to be activated prior to the blood pressure change. A further twenty RVL vasomotor neurones were observed to decrease activity following the blood pressure rise. At PVN depressor sites twelve RVL neurones were inhibited prior to the blood pressure change whereas another thirteen identified RVL neurones increased their discharge following the fall in blood pressure. 6. In three rats single shock electrical stimulation at a PVN pressor site, first identified with DLH, elicited a single or double action potential in thirteen RVL neurones with a latency of 27 +/- 1 ms. 7. It is concluded that PVN neurones may elicit increases in blood pressure via excitatory connections with RVL-spinal vasomotor neurones, and that other PVN neurones may elicit decreases in blood pressure via inhibitory connections with these RVL neurones.

Experiments have been carried out to investigate the chemical substrate in the rostral ventrolateral medulla (RVLM) underlying the depressor responses induced by activation of the greater splanchnic nerve (GSPL) afferent fibres of the rat. In anaesthetised rats with urethane and alpha-chloralose, microinjection of bicuculline, a GABA(A) receptor antagonist, into the RVLM, attenuated largely the depressor responses elicited by electrical stimulation of the GSPL afferent fibres, while strychnine or saline had no effect. In 18 RVLM neurons (including seven identified cardiovascular neurons), iontophoresis of bicuculline also significantly blocked the inhibition evoked by stimulation of the GSPL afferent inputs. We suggest that the depressor responses induced by stimulation of the GSPL afferent fibres involve a GABA(A)-receptor-mediated mechanism in the RVLM in rats.

A distinct subpopulation of neurons in the rostral and ventrolateral part of the medulla oblongata (RVL) plays a key role in controlling sympathetic vasomotor tone. To characterize these neurons under conditions in which all cell-to-cell interactions are eliminated, RVL neurons were acutely dissociated from 13- to 19-day old rats. Cells projecting to the upper thoracic segments were retrogradely labeled with fluorescent beads. Fifty-two percent (17/33) of examined spinally projecting neurons were catecholaminergic, as demonstrated by single-cell reverse transcription-polymerase chain reaction or immunocytochemistry. No spontaneous (capacitive) spikes were revealed in the tight seal cell-attached configuration. Whole cell recordings were made from 54 spinally projecting neurons using Cs(+)- or K(+)-containing pipettes. No spontaneous firing was observed in current-clamp mode with K(+)-based pipettes (membrane potential, -61.5 +/- 2.3 mV). Step depolarizations (300- or 400-ms pulses, up to 100 pA) evoked regular firing or one to four spikes. Several voltage-gated currents, resembling the transient and persistent Na+, delayed rectifier and low- and high-threshold Ca2+, were revealed in voltage-clamp mode. These results show that isolated spinally projecting RVL neurons display no pacemaker-like activity. Because data from the literature indicate that these neurons are capable of generating such activity under different experimental conditions, the factors responsible for different behavior need to be determined. Dissociated RVL neurons provide a useful new model for studying biophysical and other properties of neurons involved in blood pressure control.

To test in a catechol-specific and dynamic manner for the existence of a powerful long-lasting inhibition arising from barosensitive afferents that depresses the activity of adrenergic neurons in the rostral ventrolateral medulla (RVLM), in vivo voltammetry was used before and after acute sino-aortic deafferentation. Rats were anaesthetized with pentobarbital or halothane and ventilated with a mixture of air and oxygen. Snares were inserted around the vagus, the glossopharyngeal and the superior laryngeal nerves. After placing the animal prone in the stereotaxic frame and stabilization at a high mean arterial pressure (MAP approximately 120 mmHg), the snares were rapidly closed to produce complete barodeafferentation, assessed by loss of heart rate responses and changes in renal nerve sympathetic activity in response to vasoactive agents. Recording of a catechol signal was maintained in the RVLM during deafferentation. Under pentobarbital-induced anaesthesia (n = 5), deafferentation did not lead to a significant change in the catechol signal within the deafferented group. Under halothane-induced anaesthesia and phenylephrine-induced high baseline pressure (n = 5), no changes in the catechol signal were observed upon deafferentation (not significant vs. sham animals: n = 5). This failure to demonstrate a major increase in catechol activity upon deafferentation does not fit with the hypothesis that a powerful tonic baroreflex-mediated inhibition depresses the activity of adrenergic RVLM barosensitive bulbospinal neurons, even when the baseline MAP is high. Rather, these data are compatible with weak or no inhibition of catechol activity by the baroreceptors and a nonessential role of adrenergic neurons within the baroreceptor reflex arc itself: the adrenergic neurons may not be in series within this arc but in parallel with the arc. This interpretation is in keeping with newer schemas of autonomic core circuitry that are devoid of adrenergic neurons.

Sympathoexcitatory neurons in the rostral ventrolateral medulla (RVLM) play a key role in the tonic maintenance of resting arterial pressure. Removal of tonically active inhibitory inputs to the RVLM provided by the caudal ventrolateral medulla (CVLM) elicits a large increase in arterial pressure. The present study addresses the hypothesis that excitatory amino acids (EAA) provide the excitation of the RVLM responsible for the increase in arterial pressure that occurs after withdrawal of CVLM-mediated inhibition of the RVLM. In rats anesthetized with either alpha-chloralose or urethan, inhibition of the CVLM by local injection of muscimol markedly elevated arterial pressure. Subsequent injection of the EAA receptor antagonist kynurenic acid into the RVLM caused arterial pressure to fall to levels comparable to those that occur-with total autonomic blockade. In contrast, injection of kynurenic acid into the RVLM of control rats had little effect on arterial pressure. These results indicate that the large increase in arterial pressure produced by inhibition of the CVLM is mediated by EAA excitation of RVLM neurons. Furthermore, these data suggest that EAA play a prominent role in the tonic excitation of RVLM neurons, but, in intact rats, inhibition of EAA in the RVLM elicits no change in arterial pressure because of removal of inhibitory as well as excitatory drives of the RVLM.

The gigantocellular depressor area (GiDA) is a functionally defined subdivision of the medullary gigantocellular reticular formation where vasodepressor responses are evoked by glutamate microinjections (Aicher, S. A., D. J. Reis, D. A. Ruggiero, and T. A. Milner. Neuroscience 60: 761-779, 1994). The present experiments sought to determine whether the GiDA 1) tonically inhibits the sympathetic nervous system; 2) is necessary for baroreflex function; and 3) is functionally distinct from adjacent vasodepressor regions in the medullary reticular formation, including the midline raphe nuclei and the caudal ventrolateral medulla (CVL). Excitotoxic lesions of the GiDA abolished the baroreflex and significantly increased sympathetic nerve activity in anesthetized rats. Equivalent injections into the midline raphe nuclei elevated sympathetic activity but did not alter baroreflex responses. Therefore, the GiDA is functionally distinct from the raphe nuclei, although both contain tonically active sympathoinhibitory neurons. Because the effects of GiDA lesions were identical to those seen after lesions of the CVL, further studies were required to demonstrate that the GiDA and CVL are functionally and anatomically distinct. First, intramedullary injections of kynurenic acid produced hypertension and blocked the baroreflex when placed in the CVL, but not when placed in the GiDA. Second, muscimol inactivation of the RVL blocked the hypertension produced by excitotoxic lesions of the CVL, but failed to block the hypertension produced by similar lesions of the GiDA. Third, CVL neurons project to the RVL but not the spinal cord, whereas GiDA neurons project to the spinal cord but not the RVL. These studies show that the CVL and GiDA are both tonically sympathoinhibitory regions, but they are distinct with regard to their functional connectivity with other autonomic regions.

To locate premotor neurons controlling the cardiac sympathetic supply and to determine their relation to brain stem vasomotor pathways, the rostral ventrolateral medulla (RVLM) was mapped in seven chloralose-anesthetized cats, with the use of microinjections of sodium glutamate (5-10 nl, 0.1 M) to excite neuronal cell bodies. Cardiac sympathetic responses were recorded from the ipsilateral inferior cardiac nerve, while recordings were made simultaneously from postganglionic vasoconstrictor fibers to skeletal muscle (ipsilateral peroneal nerve). Baroreceptors were denervated to eliminate the reflex effects of blood pressure changes. Most of the 115 injected RVLM sites excited both sympathetic nerves. Inferior cardiac nerve activity increased by up to 395% (mean 105 +/- 86%, SD), and muscle vasoconstrictor activity increased by up to 487% (110 +/- 107%). Their relative response varied with injection site, however. For 16 of the most rostromedial injections, the inferior cardiac nerve-to-muscle vasoconstrictor response ratio exceeded that expected by two- to sevenfold; for 9 very caudolateral injections that ratio was strongly reversed, favoring muscle vasoconstrictors by two to fivefold. Intervening sites gave more equal responses. Overall, the response ratio varied systematically with injection site. These findings demonstrate that neurons with preferential or selective actions on the cardiac sympathetic outflow are present in the RVLM and are organized topographically. The simplest interpretation is that a population of selective cardiac sympathetic premotor neurons occupies a territory substantially overlapping, but centered rostromedially to, the population controlling vasoconstriction in muscle.

We evaluated the contribution of the rostral nucleus reticularis ventrolateralis (NRVL) to the vasomotor component in the spectrum of systemic arterial pressure (SAP) signals by quantifying the transfer function between electrical stimulation of this medullary nucleus and the SAP response. Sprague-Dawley rats anesthetized with pentobarbital sodium, paralyzed with pancuronium, and mechanically ventilated were used. Broad-band stimulation of the NRVL with computer-generated rectangular current pulses (10-50 microA, 1 ms), at a mean spike rate of 50 pulses/s and randomized modulation frequency of 0-3 Hz, elicited a site-specific and intensity-related pressor response. Intriguingly, the corresponding autospectrum of SAP signals exhibited prevailing power density only in the lower frequency range (0-0.8 Hz). This low-pass response characteristic was confirmed by the observation that 90% of the total magnitude of transfer function between NRVL stimulation and SAP response concentrated between 0 and 0.6 Hz. The magnitude of NRVL-SAP transfer function was significantly reduced by phentolamine or prazosin but appreciably enhanced by yohimbine. We conclude that the NRVL may contribute to the very-low (0-0.25 Hz)- and low (0.25-0.8 Hz)-frequency components of the SAP spectrum, which are belived to reflect sympathetic modulation on vasomotor activity via alpha-adrenergic neurotransmission.

In anesthetized and ventilated rats, activation of carotid chemoreceptors with intracarotid administration of 100 nmol sodium cyanide rapidly excited the spinal cord-projecting vasomotor neurons in the rostroventrolateral reticular nucleus (RVL) of the medulla oblongata and sympathetic nerves and increased arterial pressure. The chemoreflex sympathoexcitatory pressor responses were attenuated by an acute systemic administration of ethanol at 0.45 g/kg, but not at 45 mg/kg. The ethanol effects were observed at the level of RVL-spinal vasomotor neurons, in attenuating the neuronal responses to the chemoreflex excitation and direct iontophoresis of N-methyl-D-aspartic acid (NMDA) but without altering responses of the carotid sinus nerves to intracarotid cyanide. The effect of ethanol on the RVL neurons was further defined as blocking NMDA-evoked inward current in the corresponding spontaneously active RVL neurons in vitro. The results indicate that acute ethanol intoxication markedly influences NMDA receptor activation and arterial chemoreflexes. The relevance of the type of action to clinical hypertension in chronic and heavy drinkers is discussed.

The rostral ventrolateral medulla (RVLM) plays a primary role in the tonic and phasic control of arterial blood pressure. Stimulation of angiotensin receptors in this region appears to contribute to the tonic excitatory drive of RVLM neurons involved in the control of blood pressure, but the extent of this contribution has not been previously evaluated. The present study used bilateral microinjections of angiotensin receptor antagonists into the RVLM of chloralose-anesthetized rats to determine the degree to which tonic blood pressure was dependent upon this angiotensin-mediated input. Bilateral injection into the RVLM of 1 nmol of [Sar1, Thr8]angiotensin II or [Sar1, Ile8] angiotensin II decreased blood pressure approximately 40 mmHg. The decrease in blood pressure elicited by these angiotensin antagonists was nearly as great as that elicited by complete bilateral inhibition of the RVLM produced by local injections of muscimol or elicited by inhibition of the autonomic nervous system by intravenous injection of chlorisondamine. The decrease in blood pressure caused by injection of these angiotensin antagonists was localized to the RVLM and was dose related. Responses elicited by [Sar1, Thr8]angiotensin II were eliminated by coinjection of angiotensin. In addition to markedly decreasing resting blood pressure, 1 nmol of [Sar1, Thr8]angiotensin II injected into the RVLM, also completely antagonized the increase in blood pressure elicited by blocking the tonic inhibitory influence exerted on the RVLM by neurons in the caudal ventrolateral medulla. These results demonstrate that tonic stimulation of angiotensin receptors in the RVLM accounts for much of the excitatory sympathetic vasomotor drive emanating from the RVLM.

We examined whether vasomotor neurons in the rostroventrolateral reticular nucleus of the medulla oblongata might be responsible for an acute increase in arterial pressure, elicited by application of angiotensin II in the central nervous system, as suggested by others. In urethane-pentobarbital-anesthetized and ventilated rats, intracisternal administration of angiotensin II (1-30 nmol, infused over a period of 30 s) produced a dose-dependent pressor response, which was abolished by intracisternal application of [Sar1, Thr8]angiotensin II (100 nmol), an angiotensin II receptor antagonist. The pressor response, however, was neither preceded by nor associated with increased discharges of vasomotor neurons with slow- and fast-conduction axons in the rostroventrolateral reticular nucleus and of lumbar sympathetic chain and renal sympathetic nerves. Intravenous injections of [beta-mercapto-beta, beta-cyclopentamethylenepropinyl1,-O-Et-Tyr2, Val4, Arg8]vasopressin, a vasopressin receptor antagonist, largely abolished the central angiotensin II-induced pressor response, while a blockade of ganglionic transmission with hexamethonium and disruption of descending sympathoexcitatory output were ineffective. We conclude that central administration of angiotensin II, under the experimental conditions and at the doses, evokes an acute pressor response largely through the release of vasopressin, not by exciting vasomotor and sympathetic neurons.

Electrical stimulation of the medial prefrontal cortex (MPFC) reduces arterial blood pressure. To investigate the mechanism of this response, the effects of electrical and chemical stimulation of the MPFC on splanchnic and lumbar sympathetic nerve discharge and on the discharges of barosensitive neurons of the rostral ventrolateral medulla (RVLM) were studied in halothane-anesthetized rats. Electrical stimulation (20 Hz, 1 ms, 100 and sympathoinhibitory responses (reduced discharge of the splanchnic sympathetic nerve). Microinjection of glutamate (10 nmol/100 nl) into the MPFC also reduced arterial blood pressure and sympathetic discharge. Electrical stimulation (0.5 Hz, 1-ms pulse pairs, 3-ms interval, 150-300 microA) produced distinct patterns of splanchnic and lumbar sympathetic nerve discharge. A clear sympathoinhibitory phase with an onset latency of 146 +/- 14 ms was observed only in the case of the splanchnic sympathetic nerve activity. Electrical stimulation at depressor sites within the MPFC also inhibited the discharge of 10 of 21 RVLM barosensitive neurons tested. RVLM neurons were never excited by MPFC stimulation. These results indicate that the MPFC contains neurons that form part of a central sympathoinhibitory pathway.

In Carl Ludwig's laboratories in Leipzig, P. Owsjannikow (1871) and C. Dittmar (1873) performed precise, histologically controlled lesioning experiments into the question of the location of the vasomotor center in the medulla oblongata. A small area in the ventrolateral parts of the medulla with its caudal border 3 mm cranial to the obex and an extension of 3-4 mm in cranial direction was described as the vasomotor area which has to be left intact for normal vasomotor tone and reflexes. This finding, ignored for more than 100 years, is the most precise description of what is known today as the location of spinally projecting sympatho-excitatory neurones, named the RVML region. Already in these early publications the question of the origin of basic vasomotor tone within these areas was raised and the possibilities of automaticity and reflexogenic input were discussed. Both possibilities are still valid today and under current investigation. A specific chemosensitivity of the RVLM neurones has been demonstrated during the recent decade and its role with regard to the generation of the sympathetic tone and the nervous control of the entire cardiovascular system is being discussed.

A group of clinically useful antihypertensive agents, including clonidine, moxonidine, and rilmenidine, are all ligands at alpha(2)-adrenergic and imidazoline (I-) receptors, the latter principally of the I1-subclass. These agents all lower blood pressure by reducing the activity of tonically active sympathoexcitatory reticulospinal neurons of the C1 area of the rostroventrolateral medulla (RVL). They tonically excite preganglionic sympathetic neurons in the spinal cord by release of L-glutamate and mediate most reflexes influencing blood pressure. The RVL contains alpha(2)-adrenergic and I1-receptors, and there is evidence to suggest that both receptors may participate in the hypotensive actions of the drugs. However, because only activation of the alpha(2)-adrenergic receptors appears responsible for somnolence, the imidazoline-receptor agonists moxonidine and rilmenidine, both relatively selective for I-receptors, may have superior clinical utility in antihypertensive therapy, since they are sympatholytic and also suppress the generation of angiotensin II.

In anesthetized rats, stimulation of the arterial chemoreceptors excites reticulospinal vasomotor neurons of the nucleus rostroventrolateral reticularis of the medulla oblongata, via activating the neuronal NMDA receptors. Excitation of these neurons is responsible for reflex increases in sympathetic neuronal activity and arterial pressure. Additional doses (0.12-0.24 g/kg i.v.) of urethane dose-dependently reduced the elevations in the discharge rate of reticulospinal vasomotor and sympathetic neurons and of arterial pressure, elicited by stimulating the carotid chemoreceptors with intra-carotid injections of sodium cyanide (100 nmol/10 microl), without affecting discharges of the carotid chemoafferents. Microiontophoresis of urethane onto the reticulospinal vasomotor neurons reversibly inhibited the excitation elicited by stimulation of the chemoreceptors and by iontophoretically applied L-glutamate. The results indicate that the suppression of chemoreflexes by excessive amount of urethane is central and one of its blocking excitatory amino acid transmission onto these vasomotor neurons.

The membrane properties of visually identified, DiI retrogradely labeled bulbospinal neurons of the C1 adrenergic cell group were studied by whole-cell recordings in brainstem slices from 7- to 10-day-old rats. A post-hoc histochemical analysis allowed us to evaluate the electrophysiological properties of the C1 adrenergic neurons, a group of cells known to project to the sympathetic preganglionic neurons. Two types of cells were labeled: pacemaker and non-pacemaker neurons. In voltage-clamp mode, C1 pacemaker neurons exhibited a TTX-sensitive, persistent inward current that was activated between -55 and -50 mV and reached a peak between -40 and -30 mV. This current was significantly larger in the pacemaker neurons as compared to the non-pacemaker neurons and appeared to be a principal conductance driving the C1 pacemaker activity. Two other conductances modulated the frequency of pacemaker discharge: (1) an anomalous rectifier accelerated pacemaker frequency by three synergistic actions: (a) depolarizing it at rest, (b) increasing the slope of the pacemaker potentials, and (c) limiting hyperpolarizing membrane excursions; and (2) an A-type current which had two opposing actions: (a) slowing it by decreasing the slope of the pacemaker potential, and (b) accelerating it by repolarizing the fast action potential. Persistent sodium current functions as the driver potential responsible for the tonic firing pattern of the C1 bulbospinal neurons providing a cellular mechanism responsible for the descending excitatory drive imposed onto sympathetic preganglionic neurons. Thus, it may explain how C1 neurons may function to maintain vasomotor tone or modulate other autonomic functions. This study is the first attempt to analyze voltage-activated membrane conductances of RVLM neurons of known phenotype and axonal connections.

Extracellular single unit recordings were made from barosensitive neurons in the subretrofacial nucleus (SRF) of the rostral ventrolateral medulla in chloralose-anaesthetised cats. At the same time, single preganglionic neuron activity was recorded from filaments of the cervical sympathetic trunk (CST); barosensitive units were selected for study. Evidence for monosynaptic connections between the two neuron groups was sought by cross-correlation analysis of their ongoing activity. Cross-correlograms of 16/16 SRF/CST neuron pairs showed a broad peak (100-200ms wide), reflecting the synchronizing action of arterial baroreceptors on both neurons' activity. Two of the 16 cross-correlograms additionally showed a robust, statistically significant, narrow peak of a single 2 ms bin width, providing the first physiological demonstration that ventrolateral medullary neurons monosynaptically excite preganglionic sympathetic neurons. Deductions are made about the strength, convergence and divergence of the connection.

This study was done using Wistar rats to determine if the actions of propofol (22 +/- 1, 40 +/- 2, 64 +/- 3 and 103 +/- 3 mg.kg-1 x hr-1) decreased blood pressure and heart rate through depression of brain stem vasomotor centres. All rats were given atropine to block vagal influences on the heart. Propofol decreased renal nerve activity as well as blood pressure and heart rate in a dose-dependent manner. Infusion of the lowest dose of propofol (22 +/- 1 mg.kg-1 x hr-1) had no effect on blood pressure, heart rate and renal nerve activity. Infusion of propofol at 40 +/- 2 mg.kg-1 x hr-1 decreased renal activity by 22 +/- 4% (mean +/- SEM) and at 64 +/- 3 mg.kg-1 x hr-1 it decreased renal nerve activity by 36 +/- 6%. Finally, infusion of the largest dose of propofol (102 +/- 3 mg.kg-1 x hr-1) decreased nerve activity by 50 +/- 5%. The haemodynamic changes observed in our experiments during the infusion propofol paralleled the changes in sympathetic firing, suggesting that hypotension was caused by central actions of propofol to depress sympathetic firing. In experiments with bolus injections of propofol, the renal nerve activity returned to normal before arterial pressure and heart rate recovered. Because decreases in blood pressure and heart rate were longer-lasting than changes in renal nerve activity, a part of the vasodepression and bradycardia caused by propofol likely resulted from direct actions on blood vessels and the heart. Sympathetic and cardiovascular responses to blocking neurons in the ventrolateral medulla with microinjection of glycine were depressed by propofol.(ABSTRACT TRUNCATED AT 250 WORDS)

To examine the effect of hypoxia confined to the ventrolateral medulla we microinjected NaCN into the cat medulla (1.0 mm below the ventral surface) unilaterally and investigated cardio-respiratory changes. We studied anesthetized artificially ventilated animals and measured the electrical activity of phrenic and cervical sympathetic nerves and blood pressure. Histotoxic hypoxia depressed phrenic amplitude and elevated sympathetic tone and blood pressure. These responses were obtained predominantly from the region 5.0-8.0 mm caudal to the foramen caecum and 3.0-5.0 mm lateral to the midline (intermediate area). A study with 14C-cyanide showed that total and covalently bound cyanide was confined within a 1 mm diffusion sphere following microinjection. Isolated areas in both rostral and caudal medulla responded to cyanide with elevated sympathetic tone in the absence of phrenic nerve depression, suggesting dissociation of respiratory and vasomotor responses to hypoxia. Thus, the respiratory depression and vasomotor excitation produced by central hypoxia can be reproduced by hypoxia limited to discrete regions of the ventrolateral medulla.

In anesthetized rats, iontophoretic application of ATP excited the spinal cord-projection neurons in the rostral ventrolateral reticular nucleus of the medulla oblongata. The neuronal response to ATP was mimicked and then blocked by alpha,beta-methylene-ATP, a metabolically stable ATP analogue, and the response was abolished by suramin. Microinjections of ATP (3-100 pmol) into the rostral ventrolateral medulla produced a powerful pressor response. The results suggest that ATP may function as a neurotransmitter or neuromodulator involved in medullary regulation of cardiovascular functions.

Electrical stimulation of the prefrontal cortex in anaesthetised, paralysed rats evokes transient hypotension. In this study we have endeavoured to determine whether this evoked response is mediated by the spinal cord-projecting vasomotor neurones of the rostroventrolateral medulla (RVL). The responses of RVL-spinal vasomotor neurones to electrical stimulation of the prefrontal cortex caused a period of inhibition of the neuronal activity in the majority of cases (11 out of 13 neurones tested, 85%) and a short period of excitation in the remaining 2 neurones (15%). The prefrontal cortex-evoked inhibition of RVL-spinal vasomotor neurones was eliminated by iontophoretic application of bicuculline, a GABAa receptor antagonist, to the RVL-spinal vasomotor neurones. Microinjection of 50 nl of bicuculline methiodide into the same area of the RVL where the neurones have been identified converted the prefrontal-evoked hypotension into a vasopressor response. These findings indicate that the hypotension evoked by stimulating the prefrontal cortex is mediated by GABAergic inhibition of the RVL-spinal vasomotor neurones.

Recent studies have identified a region in the rostral ventromedial medulla (RVMM) of rats that appears to be involved in cardiovascular function. Since these studies used either microinjection of lidocaine or electrical stimulation, the exact contribution of intrinsic neurons as opposed to fibers of passage could not be determined. The present study was performed to map the location of neurons in RVMM from which changes in mean arterial pressure could be elicited by the microinjection of the excitatory amino acid analogue N-methyl-D-aspartic acid (NMDA) (20 ng/50 nl), which selectively activates cell bodies in barbiturate-anesthetized rats. Microinjection of NMDA into RVMM most often (53%) elicited pressor responses (31 +/- 7 mm Hg). On the basis of these responses, RVMM was determined to encompass a large portion of the nucleus gigantocellularis 0.5-1.5 mm lateral to the midline, 0.5-3.5 mm above the ventral surface, and extending from the rostral to the caudal pole of the facial nucleus. Depressor responses (-21 +/- 3 mm Hg) were found at all levels of RVMM but were most concentrated and of the largest magnitude in the rostral and caudal poles of RVMM. Microinjection of the inhibitory neurotransmitter glycine (500 mM) was used to determine whether neurons in RVMM were contributing to the maintenance of arterial pressure. Microinjection of glycine decreased arterial pressure (-15 +/- 2 mm Hg) throughout most of RVMM. Unexpectedly, increases in mean arterial pressure (24 +/- 3 mm Hg) were elicited by microinjection of glycine into the same region in RVMM in which NMDA most frequently elicited pressor responses.(ABSTRACT TRUNCATED AT 250 WORDS)