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In the present study we used atomic force microscopy (AFM) to examine the ligand binding properties of α7-containing nicotinic acetylcholine receptors (nAChRs) expressed on neurons from the ventral respiratory group. We also determined the effect of acute and prolonged exposure to nicotine on the binding probability of nAChRs. Neurons from neonatal (P5–P10) and juvenile (3–4 wk) rats were cultured. Internalization of Alexa Fluor 488–conjugated substance P was used to identify respiratory neurons that expressed neurokinin-1 receptors (NK1-R); a recognized marker of ventral respiratory group neurons. To assess functional changes in nAChRs, AFM probes conjugated with anti-α7 subunit nAChR antibody were used to cyclically interact the soma surface of NK1-R positive neurons. Measurements were made of the frequency of antibody adhesion to the α7-receptor subunit and of the detachment forces between the membrane attached receptor and the AFM probe tip. Addition of α-bungarotoxin (a specific antagonist of α-7 subunit-containing nAChRs) to the cell bath produced a 69% reduction in binding to the α-7 subunit (P

GABAergic neurones are interspersed throughout the nucleus tractus solitarii (NTS), and their tonic activity is crucial to the maintenance of cardiorespiratory homeostasis. However, the mechanisms that regulate the magnitude of GABAergic inhibition in the NTS remain unknown. We hypothesized that the level of GABAergic inhibition is proportionally regulated by the level of excitatory synaptic input to the NTS from baroreceptors. Using the in situ working heart-brainstem preparation in normotensive and spontaneously hypertensive rats, we blocked GABA(A) receptor-mediated neurotransmission in the NTS with gabazine (a specific GABA(A) receptor antagonist) at two levels of perfusion pressure (low PP, 60-70 mmHg; and high PP, 105-125 mmHg) while monitoring the immediate changes in cardiorespiratory variables. In normotensive rats, gabazine produced an immediate bradycardia consistent with disinhibition of NTS circuit neurones that regulate heart rate (HR) which was proportional to the level of arterial pressure (HR at low PP, 57 +/- 9 beats min(1); at high PP, 177 +/- 9 beats min(1); P < 0.001), suggesting that GABAergic circuitry in the NTS modulating heart rate was arterial pressure dependent. In contrast, there was no significant difference in the magnitude of gabazine-induced bradycardia in spontaneously hypertensive rats at low or high PP (HR at low PP, 45 +/- 10 beats min(1); at high PP, 58 +/- 7 beats min(1)). With regard to thoracic sympathetic nerve activity (tSNA), at high PP there was a significant reduction in tSNA during the inspiratory (I) phase of the respiratory cycle, but only in the normotensive rat (tSNA = 18.7 +/- 10%). At low PP, gabazine caused an elevation of the postinspiration phase of tSNA in both normotensive (tSNA = 23.7 +/- 2.9%) and hypertensive rats (tSNA = 44.2 +/- 14%). At low PP, gabazine produced no change in tSNA during the mid-expiration phase in either rat strain, but at high PP we observed a significant reduction in the mid-expiration phase tSNA, but only in the spontaneously hypertensive rat (tSNA = 25.2 +/- 8%). Gabazine at both low and high PP produced a reduction in the late expiration phase of tSNA in the hypertensive rat (low PP, tSNA = 29.4 +/- 4.4%; high PP, tSNA = 22.8 +/- 3%), whereas in the normotensive rat this was only significant at high PP (tSNA = 42.5 +/- 6.1%). Therefore, in the spontaneously hypertensive rat, contrary to the GABA(A) receptor-mediated control of HR, it appears that GABA(A) receptor-mediated control of tSNA in the NTS is arterial pressure dependent. This study provides new insight into the origin of GABAergic inhibition in NTS circuitry affecting heart rate and sympathetic activity.

Neurokinin-1 receptor (NK1-R) expressing neurons are densely distributed throughout the nucleus tractus solitarii (NTS). However, their fundamental role in arterial baroreflex function remains debated. Previously, our group has shown that activation of contraction-sensitive somatic afferents evoke substance P (SP) release in the NTS and resets the arterial baroreflex via activation of a GABAergic NTS circuit. Based on these findings, we hypothesized that modulation of arterial baroreflex function by somatic afferents is mediated by NK1-R dependent inhibition of barosensitive NTS circuits. In the present study, SP-conjugated saporin toxin (SP-SAP) was used to ablate NK1-R expressing NTS neurons. Contraction-sensitive somatic afferents were activated by electrically-evoked muscle contraction and the arterial baroreceptor–heart rate reflex was assessed by constructing reflex curves using a decerebrate, arterially-perfused preparation. Baseline baroreflex sensitivity was significantly attenuated in SP-SAP-treated rats compared with control rats receiving either unconjugated SAP or vehicle. Muscle contraction significantly attenuated baroslope in SAP and vehicle-treated animals and shifted the baroreflex curves to higher systemic pressure. In contrast, somatic afferent stimulation failed to alter baroslope or shift the baroreflex curves in SP-SAP-treated animals. Moreover, when reflex sensitivity was partially restored in SP-SAP animals, somatic stimulation failed to attenuate baroreflex bradycardia. In contrast, SP-SAP and somatic stimulation failed to blunt the reflex bradycardia evoked by the peripheral chemoreflex. Immunohistochemistry revealed that pretreatment with SP-SAP significantly reduced the number of NK1-R expressing neurons in the caudal NTS, while sparing NK1-R expressing neurons rostral to the injection site. This was accompanied by a significant reduction in the number of glutamic acid decarboxylase (GAD67) expressing neurons at equivalent levels of the NTS. These findings indicate that immunolesioning of NK1-R expressing NTS neurons selectively abolishes the depressive effect of somatosensory input on arterial baroreceptor–heart rate reflex function.

In the present study, we examined the role of the neurokinin-1 receptor (NK1R) in the modulation of respiratory rhythm in a functionally identified bradypnoeic region of the ventral respiratory group (VRG) in the in situ arterially perfused juvenile rat preparation. In electrophysiologically and functionally identified bradypnoeic sites corresponding to the Botzinger complex (BotC), microinjection of the selective NK1R agonist [Sar9-Met(O2)11]-substance P (SSP) produced a significant reduction in phrenic frequency mediated exclusively by an increase in expiratory duration (TE). The reduction was characterized by a significant increase in postinspiratory (post-I) duration with no effect on either late-expiratory duration (E2) or inspiratory duration (TI). In contrast, in a functionally identified tachypnoeic region, corresponding to the preBotzinger complex (Pre-BotC), control microinjection of SSP elicited tachypnoea. Pretreatment with the NK1R antagonist CP99994 in the BotC significantly attenuated the bradypnoeic response to SSP injection and blunted the increase in TE duration. This effect of SSP mimicked the extension of TE produced by activation of the Hering-Breuer reflex. Therefore, we hypothesized that activation of NK1Rs in the BotC is requisite for the expiratory-lengthening effect of the Hering-Breuer reflex. Unilateral electrical stimulation of the cervical vagus nerve produced bradypnoea by exclusively extending TE. Ipsilateral blockade of NK1Rs by CP99994 following blockade of the contralateral BotC by the GABAA receptor agonist muscimol significantly reduced the extension of TE produced by vagal stimulation. Results from the present study demonstrate that selective activation of NK1Rs in a functionally identified bradypnoeic region of the VRG can depress respiratory frequency by selectively lengthening post-I duration and provide evidence that endogenous activation of NK1Rs in the BotC appears to be involved in the expiratory-lengthening effect of the Hering-Breuer reflex. In conclusion, our findings demonstrate that selective activation of NK1Rs in discrete regions of the VRG can exert functionally diverse effects on breathing.

Since NTS neurons receive synaptic input from many sensory modalities, it is crucial to understand the neuronal mechanisms involved in synaptic processing. We have proposed that GABA-containing neurons in the NTS are the primary target for somatic afferent fibers activated by skeletal muscle contraction. In our model, local inhibition of baroreceptor signaling is necessary to counteract the increase in baroreceptor input such that NTS output is normalized and baroreflex sensitivity is maintained during exercise. This GABAergic mechanism, in conjunction with sympathoexcitation evoked by somatic afferents, preserves reflex sensitivity and resets the baroreflex, respectively. Unfortunately, there is insufficient data to date to support or refute the proposed role for GABA on baroreflex function during exercise. However, we feel that this model will be useful in formulating future experiments to explore these synaptic interactions.

Inhibitory neurotransmission plays a crucial role in the processing of sensory afferent signals in the nucleus of the solitary tract (NTS). The aim of this review is to provide a critical overview of inhibitory mechanisms that may be responsible for altering arterial baroreflex function during physical activity or exercise. Over a decade ago, the view of reflex control of cardiovascular function during exercise was revised because of the finding that the arterial baroreflex is reset in humans, enabling continuous beat-to-beat reflex regulation of blood pressure and heart rate. During the ensuing decade, many investigators proposed that resetting was mediated by central neural mechanisms that were intrinsic to the brain. Recent experimental data suggest that rapid and reversible changes in gamma-aminobutyric acid (GABA) inhibitory neurotransmission within the NTS play a fundamental role in this process. The hypothesis will be presented that baroreflex resetting by somatosensory input is mediated by: (1) selective inhibition of barosensitive NTS neurones; and (2) excitation of sympathoexcitatory neurones in the rostral ventrolateral medulla. Current research findings will be discussed that support an interaction between GABA and substance P (SP) signalling mechanisms in the NTS. An understanding of these mechanisms may prove to be essential for future detailed analysis of the cellular and molecular mechanisms underlying sensory integration in the NTS.

Respiratory and locomotor patterns are coupled during locomotion. The objectives of this study were to (1) demonstrate that respiratory rhythms are entrained by sensory input from somatic afferents, (2) establish whether the parabrachial nucleus mediates entrainment, (3) examine responses of single respiratory neurons in the ventral respiratory group (VRG) to somatic afferent stimulation, and (4) use a computational model of the pontomedullary respiratory network (Rybak et al., 2004a,b) to suggest neuronal mechanisms for entrainment.We used anin situpreparation in young rats that retained pontomedullary respiratory circuits and spinal pathways transmitting somatosensory input. We demonstrate that rhythmic stimulation of somatic afferents entrains respiratory rhythm on a 1:1 basis (1:1), increasing breathing frequency up to ∼1.4-2.2 times greater than spontaneous frequency. Stable entrainment occurred only when stimuli were delivered during expiration. Reversible blockade of the lateral parabrachial nucleus eliminated entrainment. Somatic afferent stimulation produced significant increases in the firing rate of augmenting expiratory (E2) neurons but shortened the firing duration of postinspiratory (post-I) neurons. A computational model reproduced 1:1 entrainment and other experimental findings based on the assumption that the somatic afferents initiate early onset of inspiration via activation of medullary E2 neurons. The model also predicted that afferent stimulation evoked transient hyperpolarization of ramp-inspiratory (ramp-I) neurons. This was confirmed experimentally by intracellular recording from ramp-I neurons. Our experimental and modeling results demonstrate that an entrainment pathway from somatic afferents to the VRG via the lateral parabrachial nucleus causes resetting of respiratory rhythm through excitation of E2 and consequent inhibition of post-I neurons.

The role of gamma-aminobutyric acid (GABA) in homeostatic control in the brainstem, in particular, in the nucleus tractus solitarius (NTS), is well established. However, to date, there is no detailed description of the distribution of GABAergic neurons within the NTS. The goal of the current study was to reexamine the efficacy of immunohistochemical localization of glutamic acid decarboxylase (GAD) protein, specifically the 67-kDa isoform (GAD67), as a marker for GABAergic neurons in the medulla and to provide a detailed map of GAD67-immunoreactive (-ir) cells within rat NTS by using a recently developed mouse monoclonal antibody. We describe a distribution of GAD67-ir cells in the medulla similar to that reported previously from in situ hybridization study. GAD67-ir cells were localized in regions known to contain high GABA content, including the ventrolateral medulla, raphe nuclei, and area postrema, but were absent from all motor nuclei, although dense terminal labeling was discerned in these regions. In the NTS, GAD67-ir was localized in all subregions. Semiquantitative analysis of the GAD67-ir distribution in the NTS revealed greater numbers of GAD67-ir cells medial to the solitary tract. Finally, dense GAD67 terminal labeling was found in the medial, central, intermediate, commissural, and subpostremal subregions, whereas sparse labeling was observed in the ventral subregion. Our findings support the use of immunohistochemistry for GAD67 as a marker for the localization of GABAergic cells and terminal processes in the rat brainstem. Furthermore, the reported heterogeneous distribution of GAD67-ir in the NTS suggests differential inhibitory modulation of sensory processing.

Arterial baroreceptor and skeletal muscle receptor afferents relay sensory information to the nucleus of the solitary tract (NTS) during exercise. Previous studies have suggested that skeletal muscle afferent input inhibits baroreflex function; however, detailed information on the role of muscle afferents and GABAergic mechanisms in the NTS is limited. Furthermore, identification of specific afferent modalities that activate GABAergic neurons in the NTS remains unknown. In the present study, we examined the neuroanatomical and physiological interactions between spinal dorsal horn cells that transmit contraction-sensitive input from skeletal muscle and GABAergic interneurons in the NTS. Biotinylated dextran amine (BDA, 10%, 25-100 nL) microinjection into dorsal horn of the cervical spinal cord was combined with glutamate decarboxylase (GAD) immunohistochemistry to visualize the nature of the relationship of BDA-labeled fibers in the NTS with GAD immunoreactivity (GAD-ir). BDA-labeled axons and terminal processes were localized in the medial, commissural, dorsomedial and dorsolateral subdivisions of the caudal NTS. Moreover, BDA-labeled fibers were observed in close proximity to GAD-ir structures throughout these regions of the NTS. The physiological interaction between skeletal muscle receptor and arterial baroreceptor afferents was investigated using an arterially perfused, decerebrate rat preparation. Activation of skeletal muscle afferents by electrically evoked twitch contraction of the forelimb attenuated baroreflex responsiveness (BR, calculated as the ratio of changes in heart rate to systemic pressure) from -1.5+/-0.3 bpm.mm Hg(-1) to -0.1+/-0.1 bpm.mm Hg(-1) (control versus contraction, P0.05, n=15). However, forelimb contraction failed to inhibit the reflex bradycardia evoked by activation of peripheral chemoreceptor afferents, indicating a reflex-specific action. Bilateral microinjection of bicuculline methiodide (BIC, 10 microM, 40-60 nL) into the caudal NTS restored baroreflex responsiveness during contraction (-1.6+/-0.2 versus -0.1+/-0.1 versus -1.5+/-0.2 bpm.mmHg(-1), control versus contraction versus contraction+BIC P0.05, n=8). We conclude that activation of ascending spinal neurons from the cervical dorsal horn by contraction-sensitive skeletal muscle afferents selectively inhibits arterial baroreceptor signaling in the NTS via activation of a GABAergic mechanism.

1. A vast array of peripheral receptors provide the central nervous system (CNS) with sensory signals that coordinate autonomic motor outflow to cardiovascular organs, such as the heart and peripheral vasculature, during locomotion. 2. Much of this sensory input is mediated by cardiovascular receptors located in blood vessels (arterial baroreceptors) and skeletal muscle (skeletal muscle ergoreceptors). 3. Several medullary nuclei are targets for cardiovascular receptors, including the nucleus tractus solitarius (NTS). 4. In the present review, the interaction between arterial baroreceptor and somatosensory receptor afferents in the NTS is examined while placing particular emphasis on the neurochemical and electrophysiological mechanisms involved in processing these signals. 5. Data from anaesthetized animals, as well as from an innovative working heart-brainstem preparation, will illustrate the potential role of GABAergic transmission on baroreceptor signalling in the caudal NTS during locomotion.

The nucleus tractus solitarii (NTS) is a medullary region involved in central transmission and processing of peripheral viscerosensory input (Spyer, 1994). Neural input from peripheral cardiorespiratory receptors (arterial baroreceptors, cardiac and pulmonary receptors, peripheral chemoreceptors) and supra-bulbar regions (hypothalamus, parabrachial nucleus, sensorimotor cortex) synapse in the NTS (Jordan et al. 1988; Felder & Mifflin, 1988; Ba-M'Hamed et al. 1996). In addition, neuroanatomical (Kalia et al. 1981; Nyberg & Blomqvist, 1984; Menetrey & Basbaum, 1987) and electrophysiological (Person, 1989; McMahon et al. 1992; Toney & Mifflin, 1994) studies have shown that somatic afferent fibres also project to and alter the discharge properties of NTS neurones. These findings suggest that the NTS may be a principal relay site for viscerosomatic sensory input. However, NTS neurones also contain an abundance of neuropeptides, including neuropeptide Y, angiotensin II, vasopressin, opioids, bradykinin, substance P and related tachykinins (Maley & Elde, 1982; Van Giersbergen & De Jong, 1992; Lawrence & Jarrott, 1996). Therefore, the NTS may perform integrative functions that are controlled in a highly complex and refined manner. The role of the tachykinin substance P in central cardiovascular regulation in the NTS has recently been an area of increasing interest. To date, there are several lines of evidence demonstrating that substance P may be involved in mediating central autonomic neurotransmission. First, biochemical and immunohistochemical evidence has demonstrated that the NTS contains substance P nerve fibres, terminals and receptors (Maley & Elde, 1982; Van Giersbergen et al. 1992; Sykes et al. 1994). Second, substance P immunoreactivity has been localized in the nodose and petrosal ganglia that contain cell bodies for primary baroreceptor, chemoreceptor and cardiopulmonary receptor afferent fibres (Helke et al. 1980, 1984). Third, in vivo microdialysis has shown that substance P is released from neurones in the NTS in response to electrical stimulation of aortic baroreceptor afferent fibres and activation of peripheral chemoreceptors by hypoxia (Lindefors et al. 1986; Morilak et al. 1988; Srinivasan et al. 1991). Fourth, localized microinjection of substance P in the NTS has been reported to evoke hypotension and bradycardia in rats (Hall et al. 1989; Chan et al. 1990). And finally, blockade of tachykinin receptors in the NTS attenuates the cardiovascular responses evoked by cardiac vagal receptors (Paton, 1998). Because some of these cardiovascular responses are similar to those evoked by the arterial baroreceptor reflex, it has been suggested that substance P may be the neurotransmitter for arterial baroreceptor afferent fibres (Gillis et al. 1981). However, it has also been suggested that substance P is not involved in modulating the arterial baroreceptor reflex (Feldman, 1995; Massari et al. 1998). Thus, although there is evidence that substance P is present and released by NTS neurones there remains debate as to whether substance P exerts a tonic modulatory action on central pathways involved in reflex autonomic control. Despite the lack of agreement over the action of substance P on central cardiovascular regulation, it has been reported that substance P is involved in the transmission of nociceptive and non-nociceptive stimuli in spinal pathways. For example, electrically induced contraction of the triceps surae releases substance P in the dorsal horn of the lower lumbar-upper sacral spinal cord (Wilson et al. 1993a,b). Selective blockade of substance P receptors in the spinal cord has also been reported to blunt the cardiovascular responses evoked by muscle contraction (Hill et al 1992; Wilson et al 1992). These findings suggest that substance P mediates the transmission of contraction-related afferent input in the dorsal horn of the spinal cord. Because these afferent fibres project to cardiovascular-related regions in the brainstem, including the NTS (Kalia et al. 1981; Nyberg & Blomqvist, 1984; Menetry & Basbaum, 1987), it is important to determine if neural input from contraction-sensitive skeletal muscle afferent fibres is a source of tachykininergic input. Furthermore, knowledge of the origin of substance P-releasing (SPergic) inputs to the NTS may provide insight into the peptidergic mechanisms in the lower brainstem that mediate the cardiovascular responses during muscular exercise. Two objectives are proposed in this study: (1) to determine if activation of skeletal muscle afferent fibres releases substance P in the NTS; and (2) to determine if the release of substance P is secondary to activation of cardiorespiratory receptors (arterial baroreceptors, cardiac receptors, pulmonary stretch receptors). We hypothesized that skeletal muscle contraction increases the extraneuronal concentration of substance P in the NTS independent of neural input transmitted by the carotid sinus and vagus nerves. A preliminary report of these findings has been published (Potts et al. 1997).

Because arterial baroreceptor and skeletal muscle receptor afferents project to cardiovascular regions in the lower brain stem such as the nucleus tractus solitarii (NTS), it is likely that the level of baroreceptor afferent input will modify the excitatory cardiovascular responses evoked by contraction-sensitive skeletal muscle afferents. The purpose of this study was to determine the effect of carotid sinus baroreceptor afferent input (CSA) on reflex heart rate (HR) and mean arterial pressure (MAP) responses evoked by activation of skeletal muscle receptor afferents (SMA). CSA input was servo controlled at three levels of carotid sinus pressure using the isolated carotid sinus preparation, and SMA input was varied by induced muscle contraction (L7-S1ventral root stimulation) or passive muscle stretch. Experiments were performed in α-chloralose-anesthetized and vagotomized dogs ( n = 9). When CSA input was low (106 ± 35 mmHg), electrically induced muscle contraction increased HR and MAP (30 ± 8 beats/min and 42 ± 12 mmHg, respectively, P < 0.05). However, when CSA input was high (221 ± 9 mmHg), the reflex changes in HR and MAP during muscle contraction were attenuated (6 ± 4 beats/min and 18 ± 4 mmHg, respectively, P< 0.05). Similarly, the sympathoexcitatory responses evoked by passive muscle stretch were attenuated in a baroreceptor-dependent manner. These results suggest that changing CSA input from low (106 mmHg) to high (221 mmHg) shifts the interaction from facilitation to inhibition. Therefore, it is concluded that the nature of the interaction (i.e., facilitation or inhibition) between the baroreflex and the exercise pressor reflex is dependent on the level of baroreceptor input. Moreover, our findings substantiate early studies showing that the level of afferent input from arterial baroreceptors is a powerful modulator of sympathoexcitation evoked by mechanically and metabolically sensitive skeletal muscle receptors.

A previous study has shown increased Fos-like immunoreactivity (FLI), a marker of neural activation, in the nucleus of the solitary tract (NTS) and the ventrolateral medulla (VLM) after static muscle contraction elicited by electrical stimulation of L7 and S1 ventral roots of the spinal cord in anesthetized, baroreceptor-intact cats. Because the electrically induced static muscle contraction reflexly increased arterial blood pressure, the concomitant activation of the arterial baroreceptor reflex during static muscle contraction may have resulted in some of the FLI labeling that was observed in the medulla. The purpose of this study was to determine regions in the medulla that are activated by muscle contraction in the absence of arterial baroreceptor input. Electrical stimulation of L7 and S1 ventral roots of the spinal cord was used to elicit static muscle contraction, and FLI in the medulla was determined in barointact and barodenervated cats. In barointact contraction cats, FLI was observed in the lateral reticular nucleus (LRN), NTS, lateral tegmental field (FTL), subretrofacial nucleus (SRF), and A1 region of the medulla. In barodenervated contraction cats, FLI increased in the same regions; however, the number of FLI-labeled cells in the NTS, FTL, and A1 region was significantly less than in barointact contraction animals. No significant difference in the number of FLI-labeled cells was found in the LRN and SRF between the two groups. These results clearly demonstrate that cardiovascular regions in the medulla are activated by input from afferent activity originating in skeletal muscle independently of concomitant arterial baroreceptor reflex activation.

Blood pressure and heart rate reflexly increase during static muscle contraction in anesthetized cats. Previous studies have demonstrated that vasopressin (AVP) and oxytocin (OT) may act as neuromodulators to regulate cardiovascular responses elicited by contraction of skeletal muscle. In this study, we tested the hypothesis that neurons containing AVP and OT in the paraventricular nucleus (PVN) and the supraoptic nucleus (SON) of the hypothalamus are activated during static muscle contraction. A laminectomy was performed to expose the spinal cord and the peripheral cut ends of L7 and S1 ventral roots were stimulated electrically to induce muscle contraction. Hypothalamic neurons activated during the muscle contraction were identified by Fos-like immunoreactivity (FLI). Static muscle contraction significantly increased FLI in the PVN and SON, compared with sham-operated cats. Double-staining of neurons in the PVN for AVP and OT showed that 22 +/- 4% of the AVP and 26 +/- 3% of the OT neurons in the PVN expressed FLI. In contrast, only 4 +/- 1% of the AVP and 3 +/- 1% of the OT neurons in the PVN were labeled with FLI in sham-operated animals. These results indicate that neurons in the PVN and SON of the hypothalamus were activated during static muscle contraction. Furthermore, as FLI was present in AVP and OT neurons, this suggests these neurons may constitute a part of the neural pathway involved in cardiovascular regulation during static muscle contraction.

In the present study, we used atomic force microscopy (AFM) to examine the ligand-binding properties of α7-containing nicotinic acetylcholine receptors (nAChRs) expressed on neurons from the ventral respiratory group. We also determined the effect of acute and prolonged exposure to nicotine on the binding probability of nAChRs. Neurons from neonatal (postnatal day 5-10) and juvenile rats (3-4 weeks old) were cultured. Internalization of Alexa Fluor 488-conjugated substance P was used to identify respiratory neurons that expressed the neurokinin-1 receptor (NK1-R), a recognized marker of ventral respiratory group neurons. To assess functional changes in nAChRs, AFM probes conjugated with anti-α7 subunit nAChR antibody were used to interact cyclically with the surface of the soma of NK1-R-positive neurons. Measurements were made of the frequency of antibody adhesion to the α7 receptor subunit and of the detachment forces between the membrane-attached receptor and the AFM probe tip. Addition of α-bungarotoxin (a specific antagonist of α7 subunit-containing nAChRs) to the cell bath produced a 69% reduction in binding to the α7 subunit (P0.05, n = 10), supporting specificity of binding. Acute exposure to nicotine (1 μM added to culture media) produced an 80% reduction in nAChR antibody binding to the α7 subunit (P0.05, n = 9). Prolonged incubation (72 h) of the cell culture in nicotine significantly reduced α7 binding in a concentration-dependent manner. Collectively, these findings demonstrate that AFM is a sensitive tool for assessment of functional changes in nAChRs expressed on the surface of living NK1-R-expressing medullary neurons. Moreover, these data demonstrate that nicotine exposure decreases the binding probability of α7 subunit-containing nAChRs.

Cardiovascular responses were obtained during cycling with graded levels of lower-body positive pressure (LBPP) applied to the exercising limbs. Seven men performed four incremental work rate (25 W.min-1) exercise (IWREx) tests to their limit of tolerance while exposed to 0, 15, 30, or 45 Torr LBPP. They also performed four, 6-min constant work rate exercise (CWREx) bouts at two work rates with LBPP's of 0 and 45 Torr. Cardiovascular data were obtained at rest and at 40%, 55%, 75%, and 90% of VO2peak, as well as at minute 5 of CWREx. LBPP did not alter VO2, HR, SV, or cardiac output (Qc) responses at rest or during exercise. However, both 30 and 45 Torr LBPP produced increases in MAP at rest and during exercise (P < 0.05). During CWREx, elevations in blood pressure were mediated via increases in TPR (P < 0.05). Only 45 Torr LBPP elicited a significantly greater blood pressure increase during exercise than rest, suggesting muscle blood flow restriction at this level of LBPP was sufficient to activate a muscle metabo-reflex. These findings suggest that the muscle metabo-reflex is not tonically active during dynamic exercise under normal conditions, but may instead require a critical reduction in muscle blood flow before it is activated.

GABAergic neurones are interspersed throughout the nucleus tractus solitarii (NTS), and their tonic activity is crucial to the maintenance of cardiorespiratory homeostasis. However, the mechanisms that regulate the magnitude of GABAergic inhibition in the NTS remain unknown. We hypothesized that the level of GABAergic inhibition is proportionally regulated by the level of excitatory synaptic input to the NTS from baroreceptors. Using the in situ working heart-brainstem preparation in normotensive and spontaneously hypertensive rats, we blocked GABA(A) receptor-mediated neurotransmission in the NTS with gabazine (a specific GABA(A) receptor antagonist) at two levels of perfusion pressure (low PP, 60-70 mmHg; and high PP, 105-125 mmHg) while monitoring the immediate changes in cardiorespiratory variables. In normotensive rats, gabazine produced an immediate bradycardia consistent with disinhibition of NTS circuit neurones that regulate heart rate (HR) which was proportional to the level of arterial pressure (HR at low PP, 57 +/- 9 beats min(1); at high PP, 177 +/- 9 beats min(1); P0.001), suggesting that GABAergic circuitry in the NTS modulating heart rate was arterial pressure dependent. In contrast, there was no significant difference in the magnitude of gabazine-induced bradycardia in spontaneously hypertensive rats at low or high PP (HR at low PP, 45 +/- 10 beats min(1); at high PP, 58 +/- 7 beats min(1)). With regard to thoracic sympathetic nerve activity (tSNA), at high PP there was a significant reduction in tSNA during the inspiratory (I) phase of the respiratory cycle, but only in the normotensive rat (tSNA = 18.7 +/- 10%). At low PP, gabazine caused an elevation of the postinspiration phase of tSNA in both normotensive (tSNA = 23.7 +/- 2.9%) and hypertensive rats (tSNA = 44.2 +/- 14%). At low PP, gabazine produced no change in tSNA during the mid-expiration phase in either rat strain, but at high PP we observed a significant reduction in the mid-expiration phase tSNA, but only in the spontaneously hypertensive rat (tSNA = 25.2 +/- 8%). Gabazine at both low and high PP produced a reduction in the late expiration phase of tSNA in the hypertensive rat (low PP, tSNA = 29.4 +/- 4.4%; high PP, tSNA = 22.8 +/- 3%), whereas in the normotensive rat this was only significant at high PP (tSNA = 42.5 +/- 6.1%). Therefore, in the spontaneously hypertensive rat, contrary to the GABA(A) receptor-mediated control of HR, it appears that GABA(A) receptor-mediated control of tSNA in the NTS is arterial pressure dependent. This study provides new insight into the origin of GABAergic inhibition in NTS circuitry affecting heart rate and sympathetic activity.

Rationale: Central angiotensin (Ang) II inhibits baroreflex and plays an important role in the pathogenesis of hypertension. However, the underlying molecular mechanisms are still not fully understood. Objective: Our objective in the present study was to characterize the signal transduction mechanism of phosphatidylinositol 3-kinase (PI3K) involvement in Ang II–induced stimulation of central neuronal activity in cultured neurons and Ang II–induced inhibition of baroreflex in spontaneously hypertensive rats (SHR) versus WKY rats. Methods and Results: Application of Ang II to neurons produced a 42% greater increase in neuronal firing in cells from the SHR than the WKY rat. Although the Ang II–mediated increase in firing rate was abolished entirely by the protein kinase (PK)C inhibitor GF109230 in the WKY, blockade of both PKC and PI3K activity was necessary in the SHR. This was associated with an increased ability of Ang II to stimulate NADPH oxidase–reactive oxygen species (ROS)–mediated signaling involving phosphorylation of the p47phox subunit of the NADPH oxidase and was dependent on the activation of PI3K in the SHR. Inhibition of PI3K resulted in the reduction of levels of p47phox phosphorylation, NADPH oxidase activity, ROS levels, and ultimately neuronal activity in cells from the SHR but not the WKY rat. In addition, in working heart–brainstem preparations, inhibition of PKC activity in the nucleus of the solitary tract in situ abolished the Ang II–mediated depression of cardiac and sympathetic baroreceptor reflex gain in the WKY. In contrast, PKC inhibition in the nucleus of the solitary tract of SHR only partially reduced the effect of Ang II on the baroreceptor reflex gain. Conclusions: These observations demonstrate that PI3K in the cardiovascular brainstem regions of the SHR may be selectively involved in Ang II–mediated signaling that includes a reduction in baroreceptor reflex function, presumably via a NADPH-ROS mediated pathway.

The role of substance P (SP) and its receptor, the neurokinin-1 (NK1R), in the generation of respiratory rhythm has received considerable attention, particularly at the Pre-Botzinger Complex of the ventral respiratory group (VRG). However, the functional role of SP and NK1R in other VRG regions has not been explored in detail. We review the current literature and describe recent data demonstrating that selective activation of NK1R in the Botzinger Complex (BotC) of the VRG evoked bradypnea by lengthening expiratory period. In addition, endogenous activation of NK1R in the BotC participates in the expiratory lengthening effect of the Hering-Breuer reflex. These data suggest that NK1R expressing neurons in different subregions of the VRG have functionally diverse roles and provide new insight on the modulatory role of SP on respiratory reflexes.

The role of substance P (SP) and its receptor, the neurokinin-1 (NK1R), in the generation of respiratory rhythm has received considerable attention, particularly at the Pre-Bötzinger Complex of the ventral respiratory group (VRG). However, the functional role of SP and NK1R in other VRG regions has not been explored in detail. We review the current literature and describe recent data demonstrating that selective activation of NK1R in the Bötzinger Complex (BötC) of the VRG evoked bradypnea by lengthening expiratory period. In addition, endogenous activation of NK1R in the BötC participates in the expiratory lengthening effect of the Hering-Breuer reflex. These data suggest that NK1R expressing neurons in different subregions of the VRG have functionally diverse roles and provide new insight on the modulatory role of SP on respiratory reflexes.

In severe hypoxia, respiratory rhythm is shifted from an eupneic, ramp-like motor pattern to gasping characterized by a decrementing pattern of phrenic motor activity. However, it is not known whether hypoxia reconfigures the spatiotemporal organization of the central respiratory rhythm generator. Using the in situ arterially perfused juvenile rat preparation, we investigated whether the shift from eupnea to gasping was associated with a reconfiguration of the spatiotemporal pattern of respiratory neuronal activity in the ventral medullary respiratory network. Optical images of medullary respiratory network activity were obtained from male rats (4-6 weeks of age). Part of the medullary network was stained with a voltage-sensitive dye (di-2 ANEPEQ) centred both within, and adjacent to, the pre-Botzinger complex (Pre-BotC). During eupnea, optical signals initially increased prior to the onset of phrenic activity and progressively intensified during the inspiratory phase peaking at the end of inspiration. During early expiration, fluorescence was also detected and slowly declined throughout this phase. In contrast, hypoxia shifted the respiratory motor pattern from eupnea to gasping and optical signals were restricted to inspiration only. Areas active during gasping showed fluorescence that was more intensive and covered a larger region of the rostral ventrolateral medulla compared to eupnea. Regions exhibiting peak inspiratory fluorescence did not coincide spatially during eupnea and gasping. Moreover, there was a recruitment of additional medullary regions during gasping that were not active during eupnea. These results provide novel evidence that the shift in respiratory motor pattern from eupnea to gasping appears to be associated with a reconfiguration of the central respiratory rhythm generator characterized by changes in its spatiotemporal organization.

Inhibitory neurotransmission plays a crucial role in the processing of sensory afferent signals in the nucleus of the solitary tract (NTS). The aim of this review is to provide a critical overview of inhibitory mechanisms that may be responsible for altering arterial baroreflex function during physical activity or exercise. Over a decade ago, the view of reflex control of cardiovascular function during exercise was revised because of the finding that the arterial baroreflex is reset in humans, enabling continuous beat-to-beat reflex regulation of blood pressure and heart rate. During the ensuing decade, many investigators proposed that resetting was mediated by central neural mechanisms that were intrinsic to the brain. Recent experimental data suggest that rapid and reversible changes in gamma-aminobutyric acid (GABA) inhibitory neurotransmission within the NTS play a fundamental role in this process. The hypothesis will be presented that baroreflex resetting by somatosensory input is mediated by: (1) selective inhibition of barosensitive NTS neurones; and (2) excitation of sympathoexcitatory neurones in the rostral ventrolateral medulla. Current research findings will be discussed that support an interaction between GABA and substance P (SP) signalling mechanisms in the NTS. An understanding of these mechanisms may prove to be essential for future detailed analysis of the cellular and molecular mechanisms underlying sensory integration in the NTS.

Encoding of sensory information by nucleus of the solitary tract (NTS) neurons is incompletely understood. Using extracellular single-unit recording in alpha-chloralose-urethane anesthetized cats, we have examined the discharge characteristics of NTS neurons to activation of somatic Adelta and C fiber afferents by skeletal muscle contraction evoked by electrical stimulation of lower lumbar/upper sacral ventral roots. Generally, somatic afferent stimulation evoked two distinct firing patterns. The first population (36/43 cells) increased their firing rate to brief somatic stimuli. A subset (21/27 cells) exhibited a rapid decay of their firing rate during sustained somatic stimulation. Peak instantaneous firing frequency (F(p)) increased proportionally with the intensity of somatic stimulation (105+/-4 vs. 119+/-4 vs. 139+/-4 Hz, 10, 20 and 40 Hz, respectively, P0.0001), whereas steady-state firing frequency (F(ss)) was not altered (25+/-2 vs. 27+/-2 vs. 27+/-2 Hz, 10, 20 and 40 Hz, respectively, P=0.72). Two indices were derived to quantify the decay properties. The decay rate constant (obtained from exponential curve fitting) was not altered by stimulation frequency (461+/-10 vs. 442+/-14 vs. 429+/-26 ms, 10, 20 and 40 Hz, respectively, P=0.415), nor was the decay index (derived to express the percent reduction in firing rate with respect to the initial peak firing rate; 76+/-2 vs. 77+/-2 vs. 81+/-2%, 10, 20 and 40 Hz, respectively, P=0.187). In contrast, the second population (seven of 43 cells) decreased their firing rate to stimulation. Of the NTS neurons tested for barosensitivity (29/36), none responded to pressure stimulation. These results have identified a population of somatosensitive NTS neurons that exhibit rapid firing rate decay properties during sustained stimulation. However, this population could faithfully encode phasic excitation during rhythmic somatosensory input. These results are discussed in relation to the role of somatosensory input on baroreflex function.

In addition to the well-defined role of dorsal horn neurons in pain transmission, neurons in the superficial laminae also provide a rich source of synaptic input to cardiovascular and respiratory centers in the medullary reticular formation. In this study, ascending projection neurons from the superficial laminae of the cervical enlargement were studied in the rat using the anterograde tracer biotinylated dextran amine (BDA). Ipsilateral microinjection of BDA into the cervical spinal cord (C6–C8) resulted in extensive labeling of dorsal horn neurons in laminae I–V. Axons and terminal processes of cervical dorsal horn cells projecting to the medulla were present in the cuneate nucleus (Cu), the nucleus of the solitary tract (NTS), the lateral reticular nucleus, (LRt) as well as the caudal and rostral ventrolateral medulla (VLM). The highest density of BDA labeling was found ipsilaterally in the Cu, LRt, caudal and rostral VLM, while a moderate density of labeling was present in the NTS caudal to the area postrema (AP). Moderate-to-weak labeling was also found in the LRt, the caudal and rostral VLM contralateral to the BDA injection. These results support the existence of a spinomedullary pathway that transmits noxious and innocuous Aδ and C fiber-mediated sensory signals to the medulla. Neurons in this ascending spinal pathway likely participate in the patterning of autonomic responses evoked by pain or during exercise.

We have previously shown that static muscle contraction induces the expression of c-Fos protein in neurons of the nucleus tractus solitarii (NTS) and that some of these cells were codistributed with neuronal NADPH-diaphorase [nitric oxide (NO) synthase]-positive fibers. In the present study, we sought to determine the role of NO in the NTS in mediating the cardiovascular responses elicited by skeletal muscle afferent fibers. Static contraction of the triceps surae muscle was induced by electrical stimulation of the L7 and S1 ventral roots in anesthetized cats. Muscle contraction during microdialysis of artificial extracellular fluid increased mean arterial pressure (MAP) and heart rate (HR) 51 ± 9 mmHg and 18 ± 3 beats/min, respectively. Microdialysis ofl-arginine (10 mM) into the NTS to locally increase NO formation attenuated the increases in MAP (30 ± 7 mmHg, P < 0.05) and HR (14 ± 2 beats/min, P > 0.05) during contraction. Microdialysis ofd-arginine (10 mM) did not alter the cardiovascular responses evoked by muscle contraction. Microdialysis of N G-nitro-l-arginine methyl ester (2 mM) during contraction attenuated the effects ofl-arginine on the reflex cardiovascular responses. These findings demonstrate that an increase in NO formation in the NTS attenuates the pressor response to static muscle contraction, indicating that the NO system plays a role in mediating the cardiovascular responses to static muscle contraction in the NTS.

The subretrofacial nucleus (SRF) is a region of the rostral ventrolateral medulla known to play a crucial role in sympathoexcitation. SRF neurons send direct projections to the intermediolateral cell columns of the spinal cord where they form synaptic contact with preganglionic sympathetic motor neurons. Activation of this neural pathway increases sympathetic outflow to the heart and blood vessels affecting cardiac function and vasomotor tone. Previous studies utilizing electrophysiological recording techniques and c-Fos expression have established that the activity of SRF neurons is increased during skeletal muscle contraction. However, the excitatory neurotransmitter mediating this increased activity remains in question. In the present study, static contraction of the triceps surae was induced by electrical stimulation of L7 and S1 ventral roots in anesthetized cats (n=12). Endogenous release of glutamate (Glu) from the SRF was recovered by microdialysis and measured by HPLC. Static muscle contraction for 4 min increased mean arterial pressure (MAP) 38+/-4 mmHg from a control level of 102+/-12 mmHg (P0.05). During muscle contraction the extracellular concentration of Glu recovered from the SRF increased from 623+/-117 to 1078+/-187 nM (P0.05). To determine the effect of muscle contraction on Glu release in the absence of synaptic input from other reflexogenic areas, contraction was repeated following acute sinoaortic denervation and vagotomy. Following this denervation, muscle contraction increased MAP 41+/- 4 mmHg (P0.05) and Glu concentration from 635+/-246 to 1106+/-389 nM (P0.05). Muscle paralysis prevented the increases in MAP and Glu concentration during ventral root stimulation. These results suggest that: (i) Glu is released in the SRF during activation of contraction-sensitive skeletal muscle afferent fibers in the cat; and (ii) synaptic input from other reflexogenic areas appears to be ineffective in modulating the release of Glu in the SRF during static muscle contraction.

Resetting of the arterial baroreflex is mediated by central (central command) or peripheral (exercise pressor reflex) mechanisms. The purpose of this study was to determine the effect of somatosensory input from skeletal muscle receptors on resetting of the carotid baroreceptor reflex. Resetting of the baroreflex was determined by measuring carotid sinus threshold pressure (Pth) during a ramp protocol that consisted of a linear increase in sinus pressure from 50 to 250 mmHg at ∼3 mmHg/s. Experiments were performed in seven α-chloralose-anesthetized and vagotomized dogs. To determine the effect of skeletal muscle afferent input on resetting, electrically induced muscle contraction was used to activate mechanically and metabolically senstive afferent fibers, whereas passive stretch of the hindlimb was used to activate predominantly mechanically sensitive afferent fibers. Pth for heart rate (HR) and arterial blood pressure (BP) during the control ramp protocol was 110 ± 4 and 118 ± 7 mmHg, respectively. Electrically induced muscle contraction increased hindlimb tension (5.7 ± 0.4 kg) and significantly increased Pth-HR and Pth-BP above control (135 ± 6 and 141 ± 5 mmHg, respectively; P< 0.05). Muscle paralysis prevented the increase in Pth-HR and Pth-BP during ventral root stimulation (104 ± 7 and 116 ± 5 mmHg, respectively; P = not significant). Passive muscle stretch ( n = 3) increased hindlimb tension (5.9 ± 0.9 kg) and significantly increased Pth-BP (125 ± 21 vs. 159 ± 16 mmHg, control vs. contraction; P< 0.05). There was no difference in the magnitude of Pth resetting between muscle contraction or stretch. The present study demonstrates that activation of skeletal muscle afferent fibers, by either muscle contraction or stretch, increases Pth of the carotid baroreflex. Therefore, neural input from skeletal muscle receptors resets the carotid baroreflex in a manner similar to that ascribed by central command.

Baroreflex regulation of cardiac output is determined by the performance of the heart as well as the available blood flow returning to the heart (i.e., venous return). We hypothesized that a decrease in arterial compliance (Ca) would affect carotid baroreflex control of cardiac output by altering the slope of the venous return curve (VR curve). Baroreflex control of systemic arterial pressure (Pa), central venous pressure (Pv), heart rate, cardiac output (CO), and peripheral vascular resistance ( R) were determined during bilateral carotid occlusion (BCO) in spontaneously hypertensive (hypertensive, HT) and Sprague-Dawley (normotensive, NT) rats. Ca was determined from the rate of arterial pressure decay when CO was transiently stopped, and the VR curve was obtained during graded inflation of a vascular balloon positioned in the right atrium. The inverse slope of the VR curve was used as an index of the resistance to venous return (RVR). The baseline slope of the VR curve was −50.5 ± 3.3 vs. −35.5 ± 2.6 ml ⋅ kg−1 ⋅ min−1 ⋅ mmHg−1in NT vs. HT, respectively ( P < 0.05). Control values of Pa (96 ± 5 vs. 124 ± 8 mmHg) and R[0.43 ± 0.04 vs. 0.80 ± 0.07 peripheral resistance units (PRU)] were reduced in NT, whereas Ca (0.062 ± 0010 vs. 0.036 ± 0.003 ml ⋅ kg−1 ⋅ mmHg−1) was elevated in NT vs. HT, respectively ( P < 0.05). Analysis of the pressure dependence of Ca demonstrated that Ca was a nonlinear function of Pa, and the exponential decay constant for the Ca-Parelationship was reduced in HT (0.0055 ± 0.0012 vs. 0.0012 ± 0.0002 min, NT vs. HT, P < 0.05). Baroreflex activation by BCO significantly increased Pa(ΔPa, 20 ± 4 vs. 28 ± 3 mmHg) and R(Δ R, 0.16 ± 0.04 vs. 0.24 ± 0.06 PRU) in NT vs. HT, respectively. However, BCO significantly decreased CO in NT but not HT (ΔCO, −24 ± 5 vs. −4 ± 6 ml ⋅ kg−1 ⋅ min−1, P < 0.05). In NT, RVR was increased 39 ± 9% during BCO ( P < 0.05), whereas RVR increased 8 ± 3% in HT ( P = NS). From these findings, we conclude that the difference in baroreflex control of CO is mediated, in part, by the reduction in Ca, which minimized the baroreflex-evoked increase in RVR.

To determine the potential of an inhibitory interaction between the carotid sinus baroreflex (CSB) and the exercise pressor reflex (EPR), both pathways were activated to produce sympathoexcitation. It was hypothesized that, under conditions when the baroreflex increased sympathetic outflow, the interaction between CSB and EPR would be inhibitory. Bilateral carotid occlusion (BCO), electrically induced muscle contraction (EMC), and passive muscle stretch (PMS) were used to evoke sympathoexcitation. BCO decreased sinus pressure 50 ± 5 mmHg, and the levels of muscle tension generated by EMC and PMS were 7 ± 2 and 8 ± 1 kg, respectively. This resulted in significant increases in mean arterial pressure (MAP) of 55 ± 9, 50 ± 7, and 50 ± 6 mmHg ( P = not significant, BCO vs. EMC vs. PMS) and in heart rate (HR) of 7 ± 2, 19 ± 4, and 17 ± 2 beats/min ( P < 0.05, BCO vs. EMC and PMS). When BCO was combined with EMC or PMS, the reflex increase in MAP was augmented (80 ± 8 and 79 ± 10 mmHg; BCO+EMC and BCO+PMS, respectively; P< 0.05). However, summation of the individual MAP responses was greater than the response evoked during coactivation (106 ± 11 and 103 ± 12 mmHg, respectively, P < 0.05). Because summing the individual blood pressure responses exceeded the response during coactivation, the net effect was that the CSB and EPR interacted in an occlusive manner. In contrast, summation of the individual chronotropic responses was the same as the response evoked during coactivation. Moreover, there was no difference in summation of the individual MAP or HR responses when muscle afferents were activated by either EMC or PMS. In conclusion, the interaction between the CSB and the EPR in control of MAP was occlusive when both reflexes were stimulated to evoke sympathoexcitation. However, summation of the reflex changes in HR was simply additive.

In anesthetized cats, static contraction of the hindlimb reflexly increases mean arterial pressure (MAP). This cardiovascular adjustment is reduced by the arterial baroreflex. Both of these reflex responses are mediated through activation of ventrolateral medullary (VLM) regions. We tested the hypothesis that the concentration of glutamate (Glu) increases in the caudal ventrolateral medulla (cVLM) during static hindlimb contractions in anesthetized cats, and that barodenervation reduces this elevation in Glu levels. Static contractions of the triceps surae muscle of one hindlimb were evoked by electrical stimulation of the peripheral ends of cut L7 and S1 ventral roots. After the insertion of the microdialysis probes and a 3-h recovery period, a 2-min static contraction increased MAP by 47 +/- 7 mmHg. The concentration of Glu increased from 606 +/- 189 to 1042 +/- 228 nM. These results were repeatable in that Glu, as well as MAP, rose by a similar amount in two subsequent contractions. By contrast, in a subset of cats paralyzed prior to the third contraction, neither MAP nor Glu were significantly increased over baseline levels during the third stimulation period. In a third group of cats, hindlimb contraction increased MAP and Glu levels. However, the Glu release was attenuated in subsequent contractions after these cats were barodenervated. During the same periods of stimulation, the denervation accentuated the rise in MAP. These data demonstrate that static contraction of the hindlimb increases the extracellular concentration of Glu in the cVLM. Further, our study implicates this neurotransmitter in the baroreflex mediated reduction of the pressor reflex response to static muscle contraction.

Despite the well-established fact that the carotid sinus baroreflex system has profound control over the physical properties of the systemic circulation, the resistance to venous return (RVR) seems to be invariant of such control. We hypothesized that this apparent paradox may be explained from the baroreflex changes in systemic arterial compliance. In 12 pentobarbital-anesthetized mongrel dogs, RVR was measured at controlled carotid sinus pressures (CSP) of 50 and 200 mmHg with normal and artificially increased arterial compliance. Arterial compliance was determined from the arterial pressure decay when systemic blood flow was stopped with total vena caval occlusion. Changing CSP between 50 and 200 mmHg changed RVR significantly only under the condition of artificially increased arterial compliance. A four-parameter lumped model of the systemic circulation revealed that the baroreflex changes in arterial compliance and arterial resistance, which occurred in opposite directions, prevented a change in RVR when CSP was changed. The data also suggested that approximately 75% of RVR was attributed to large and conduit veins, the resistances along which were insensitive to baroreflex control. We concluded that the invariance of RVR results from a combination of 1) baroreflex change in the arterial compliance, 2) baroreflex insensitivity of the resistance along large and conduit veins, and 3) spatially distinct location between the major site of reflex change in capacitance and the major site of compliance.

We examined the aortic baroreflex control of heart rate (HR) in seven healthy young men of average fitness (AF) and seven of high fitness (HF). The fitness level was determined by maximal oxygen uptake (AF = 42.9 +/- 1.1, HF = 62.3 +/- 1.8 ml.kg-1.min-1). Aortic baroreflex control of HR was determined during a steady-state increase of mean arterial pressure (MAP; AF, +15.0 +/- 2.1 and HF, +18.3 +/- 0.8 mmHg) with phenylephrine (PE) infusion combined with positive neck pressure (NP; AF, 18 +/- 2.0 and HF, 20 +/- 0.8 mmHg) to counteract the increased carotid sinus pressure and with low levels of lower body negative pressure to counteract the increased central venous pressure. There was no group difference in the increased MAP or NP, nor was there stage difference in MAP within either group during PE infusion. However, the isolated cardiac-aortic baroreflex gains (i.e., delta HR/delta MAP) were significantly less in the HF (0.16 +/- 0.02 and 0.14 +/- 0.03 beats.min-1.mmHg-1) than in the AF (0.52 +/- 0.08 and 0.59 +/- 0.07 beats.min-1.mmHg-1) subjects at PE + NP and PE + NP + lower body negative pressure. We concluded that during steady-state increases in MAP, the sensitivity of aortic baroreflex control of HR was significantly less in the HF than in the AF subjects.

Phase-coupling of the respiratory rhythm to locomotor activity has been reported in quadrupeds and humans [1]. Ventral respiratory group (VRG) neurons are involved in respiratory rhythm generation due, in part, to both intrinsic and synaptic interactions between pre-Botzinger and Botzinger complex neurons [2], as well as the influence of extrinsic synaptic drive to the VRG network. The aim of this study was to determine the role of neuro-genic input from somatosensory receptors on the bursting patterns of respiratory neurons in the VRG network. We hypothesized that neural input from contraction-sensitive somatosensory receptors entrained respiration by modulating the phasic activity of VRG neurons. Experiments were performed using the working heart-brainstem preparation [3]. Rats (70–100 g) were anesthetized, decerebrated pre-collicularly, paralyzed and perfused with a Ringers solution plus an oncotic agent. Somatosensory afferents were stimulated by intermittent contraction of the forelimb (3 ms pulses, 90–150 V, 10–15 g of developed tension). Phrenic (PNA) and central vagus (CVA) nerve activities were recorded using suction electrodes and the extra-cellular single-unit activity of VRG neurons was recorded using glass microelectrodes (tip resistance: 5–15 mΩ). Neurons were classified based on their bursting patterns relative to the phase of the respiratory cycle as inspiratory (I; n = 18), post-inspiratory (PI; n = 21) or augmenting expiratory (E-AUG; n = 12). Intermittent activation of somatosensory afferents entrained respiratory motor outflow to the frequency of forelimb contraction and generated a 1:1 phase-coupled rhythm. In order to determine the effect of somatosensory receptor activation on the bursting patterns of respiratory neurons, forelimb contraction was evoked during specific phases of the respiratory cycle. Once a respiratory neuron was identified and its ongoing activity was characterized, somatosensory afferents were activated by a single twitch contraction of the forelimb. When somatic afferents were activated during early PI phase 67% of PI neurons were inhibited (n = 14); whereas, 58% of E-AUG neurons were excited (n = 7). In contrast, only 33% of Ineurons were inhibited when the forelimb was contracted during early inspiration (n = 6). From these findings, we conclude that the phase-locking of respiratory rhythm to repetitive muscle contraction is mediated by somatosensory-evoked excitation of E-AUG neurons and inhibition of PIneurons. Our findings suggest that peripheral feedback from somatosensory afferents modulates respiratory network activity in discrete phases of the respiratory cycle during locomotion.