Search

The rostral ventrolateral medulla (RVLM) is an area of the brain stem that contains diverse neural substrates that are involved in systems critical for physiological function. There is evidence that aging affects some neural substrates within the RVLM, although age-related changes in RVLM molecular mechanisms are not well established. The goal of the present study was to characterize the transcriptomic profile of the aging RVLM and to test the hypothesis that aging is associated with altered gene expression in the RVLM, with an emphasis on immune system associated gene transcripts. RVLM tissue punches from young, middle-aged, and aged F344 rats were analyzed with Agilent's whole rat genome microarray. The RVLM gene expression profile varied with age, and an association between chronological age and specific RVLM gene expression patterns was observed [ P < 0.05, false discovery rate (FDR) < 0.3]. Functional analysis of RVLM microarray data via gene ontology profiling and pathway analysis identified upregulation of genes associated with immune- and stress-related responses and downregulation of genes associated with lipid biosynthesis and neurotransmission in aged compared with middle-aged and young rats. Differentially expressed genes associated with the complement system and microglial cells were further validated by quantitative PCR with separate RVLM samples ( P < 0.05, FDR < 0.1). The present results have identified age-related changes in the transcriptomic profile of the RVLM, modifications that may provide the molecular backdrop for understanding age-dependent changes in physiological regulation.

Hyperthermia is a potent activator of visceral sympathetic nerve discharge (SND), and the functional integrity of the rostral ventral lateral medulla (RVLM) is critically important for sustaining sympathoexcitation at peak hyperthermia. However, RVLM mechanisms mediating SND activation to acute heat stress are not well understood. Because RVLM GABA is tonically inhibitory to sympathetic nerve outflow, it is plausible to hypothesize that disinhibition of RVLM sympathetic neural circuits, via withdrawal of GABAergic tone, may affect SND regulation at peak hyperthermia. The effect of RVLM bicuculline (BIC; GABAA receptor antagonist, 100-200 pmol) microinjections on the level of renal SND in anesthetized rats was determined after internal body temperature (Tc) had been increased to 41.5°C. Temperature-control experiments involved RVLM BIC (100-200 pmol) microinjections, with Tc maintained at 38°C. As expected, acute heating significantly increased renal SND from control levels. Bilateral RVLM BIC microinjections at 41.5°C produced immediate and significant increases in renal SND above heating-induced levels of activation. Bilateral RVLM BIC microinjections at 38°C increased renal SND to similar levels as produced by RVLM BIC microinjections after Tc had been increased to 41.5°C (heating + RVLM BIC). These results demonstrate that a considerable level of RVLM GABAergic inhibition is sustained at peak hyperthermia, an interesting physiological response profile based on the significance of SND activation to cardiovascular regulation during heat stress.

Bacillus anthracis infection is a pathophysiological condition that is complicated by progressive decreases in mean arterial pressure (MAP). Lethal toxin (LeTx) is central to the pathogenesis of B. anthracis infection, and the sympathetic nervous system plays a critical role in physiological regulation of acute stressors. However, the effect of LeTx on sympathetic nerve discharge (SND), a critical link between central sympathetic neural circuits and MAP regulation, remains unknown. We determined visceral (renal, splenic, and adrenal) SND responses to continuous infusion of LeTx [lethal factor (100 μg/kg) + protective antigen (200 μg/kg) infused at 0.5 ml/h for ≤6 h] and vehicle (infused at 0.5 ml/h) in anesthetized, baroreceptor-intact and baroreceptor (sinoaortic)-denervated (SAD) Sprague-Dawley rats. LeTx infusions produced an initial state of cardiovascular and sympathetic nervous system activation in intact and SAD rats. Subsequent to peak LeTx-induced increases in arterial blood pressure, intact rats demonstrated a marked hypotension that was accompanied by significant reductions in SND (renal and splenic) and heart rate (HR) from peak levels. After peak LeTx-induced pressor and sympathoexcitatory responses in SAD rats, MAP, SND (renal, splenic, and adrenal), and HR were progressively and significantly reduced, supporting the hypothesis that LeTx alters the central regulation of sympathetic nerve outflow. These findings demonstrate that the regulation of visceral SND is altered in a complex manner during continuous anthrax LeTx infusions and suggest that sympathetic nervous system dysregulation may contribute to the marked hypotension accompanying B. anthracis infection.

Acute heat stress activates visceral sympathetic nerve discharge (SND) in young rats, and the functional integrity of the rostral ventrolateral medulla (RVLM) is required for sustaining visceral sympathoexcitation during peak increases in internal body temperature (T(c)). However, RVLM mechanisms mediating SND activation to hyperthermia remain unknown. In the present study, we investigated the role of RVLM ionotropic excitatory amino acid receptors in mediating visceral SND activation to heat stress in anesthetized, young rats. The effects of bilateral RVLM kynurenic acid (Kyn; 2.7 and 5.4 nmol), saline, or muscimol (400-800 pmol) microinjections on renal SND and splenic SND responses to heat stress were determined at peak hyperthermia (T(c) 41.5°C), during progressive hyperthermia (T(c) 40°C), and at the initiation of heating (T(c) increased from 38 to 38.5°C). RVLM Kyn microinjections did not reduce renal and splenic SND recorded during progressive or peak hyperthermia and did not attenuate SND activation at the initiation of heating. In fact, renal and splenic SND tended to be or were significantly increased following RVLM Kyn microinjections at the initiation of heating and during hyperthermia (40 and 41.5°C). RVLM muscimol microinjections at 39, 40, and 41.5°C resulted in immediate reductions in SND. These data indicate that RVLM ionotropic glutamate receptors are required for mediating visceral sympathoexcitation to acute heating and suggest that acute heating activates an RVLM ionotropic excitatory amino acid receptor dependent inhibitory input, which reduces the level of visceral SND to heating.

Sympathetic nerve discharge (SND) responses to hyperthermia are attenuated in aged rats without heart failure (HF) and in young HF (Y(HF)) rats, demonstrating that individually aging and HF alter SND regulation. However, the combined effects of aging and HF on SND regulation to heat stress are unknown, despite the high prevalence of HF in aged individuals. We hypothesized that SND responses to heating would be additive when aging and HF are combined, demonstrated by marked reductions in SND and mean arterial pressure (MAP) responses to heating in aged HF (A(HF)) compared with aged sham HF (A(SHAM)) rats, and in A(HF) compared with Y(HF) rats. Renal and splenic SND responses to hyperthermia (colonic temperature increased to 41.5°C) were determined in anesthetized Y(HF), young sham (Y(SHAM)), A(HF), and A(SHAM) Fischer rats. HF was induced by myocardial infarction and documented using echocardiographic, invasive, and postmortem measures. The severity of HF was similar in Y(HF) and A(HF) rats. SND responses to heating were attenuated in Y(HF) compared with Y(SHAM) rats, demonstrating an effect of HF on SND regulation in young rats. In contrast, A(HF) and A(SHAM) rats demonstrated similar SND responses to heating, suggesting a prominent influence of age on SND regulation in A(HF) rats. Splenic SND and MAP responses to heating were similar in Y(HF), A(HF), and A(SHAM) rats, indicating that the imposition of HF in young rats changes the regulatory status of these variables to one consistent with aged rats. These data suggest that the effect of HF on SND regulation to hyperthermia is age dependent.

Studies completed in human subjects have made seminal contributions to understanding the effects of age on sympathetic nervous system (SNS) regulation. Numerous experimental constraints limit the design of studies involving human subjects; therefore, completion of studies in animal models of aging would be expected to provide additional insight regarding mechanisms mediating age-related changes in sympathetic nerve discharge (SND) regulation. The present review assesses the current state of the literature regarding contributions from animal studies on the effects of advancing age on SND regulation, focusing primarily on studies that have used direct recordings of sympathetic nerve outflow. Few studies using direct SND recordings have been completed in animal models of aging, regardless of the fundamental component of SND regulation reviewed (basal levels, acute responsiveness, relationships between the discharges in sympathetic nerves, central neural regulation). SNS responsiveness to various acute stressors is altered in aged compared with young animals; however, mechanisms remain virtually unexplored. There is a marked dearth of studies that have used central neural microinjection techniques in conjunction with SND recordings in aged animals, making it difficult to develop an evidence-based framework regarding potential age-associated effects on central regulation of SND. Determination of age-related changes in mechanisms regulating SND is important for understanding relationships between chronic disease development and changes in SNS function; however, this can only be achieved by substantially extending the current knowledge base regarding the effects of age on SND regulation in animal studies.

Activation of the hypothalamic-pituitary-adrenal (HPA) axis and augmented plasma and tissue levels of IL-6 are hallmarks of heart failure (HF). Within the forebrain, cardiovascular homeostasis is mediated in part by the paraventricular nucleus (PVN) of the hypothalamus. IL-6, via binding to the IL-6 receptor (IL-6R)/glycoprotein 130 (gp130) complex influences cellular and physiological responses. Thus, in the current study, we hypothesized that PVN IL-6R protein and gene expression are upregulated in HF vs. sham-operated rats, whereas gp130 levels in the same tissues remain stable. Six weeks after coronary ligation surgery, hemodynamic measurements were obtained, and HF rats were divided into moderate noncongestive and severe chronic congestive groups based on cardiac indices. Plasma IL-6 levels were determined and changes in gene and protein expression of IL-6R and gp130 between sham-operated and HF rats were determined via real-time PCR and Western blot analyses, respectively. Plasma levels of IL-6 were elevated in rats with severe, but not moderate, HF compared with sham-operated controls. In both moderate and severe HF rats, protein but not gene expression of IL-6R was significantly increased in PVN tissue but not in non-PVN tissue, compared with sham-operated controls. Gene and protein levels of the gp130 subunit were not altered by HF in either tissue analyzed. Collectively, these data suggest that within the brain of HF rats, IL-6R expression is not a global change. Rather the increased IL-6 levels characteristic of HF may alter PVN-mediated physiological responses via enhanced expression of the IL-6R.

Splenic nerve denervation abrogates enhanced splenic cytokine gene expression responses to acute heating, demonstrating that hyperthermia-induced activation of splenic sympathetic nerve discharge (SND) increases splenic cytokine gene expression. Hypothermia alters SND responses; however, the role of the sympathetic nervous system in mediating splenic cytokine gene expression responses to hypothermia is not known. The purpose of the present study was to determine the effect of hypothermia on the relationship between the sympathetic nervous system and splenic cytokine gene expression in anesthetized F344 rats. Gene expression analysis was performed using a microarray containing 112 genes, representing inflammatory cytokines, chemokines, cytokine/chemokine receptors and housekeeping genes. A subset of differentially expressed genes was verified by real-time RT-PCR analysis. Splenic SND was decreased significantly during cooling (core temperature decreased from 38 to 30 degrees C) in splenic-intact rats but remained unchanged in sham-cooled splenic-intact rats (core temperature maintained at 38 degrees C). Hypothermia upregulated the transcripts of several genes, including, chemokine ligands CCL2, CXCL2, CXCL10, and CCL20, and interleukins IL-1alpha, IL-1beta, and IL-6. Gene expression responses to hypothermia were similar for the majority of cytokine genes in splenic-intact and splenic-denervated rats. These results suggest that hypothermia-enhanced splenic cytokine gene expression is independent of splenic SND.

Hypothermia produced by acute cooling prominently alters sympathetic nerve outflow. Skin sympathoexcitatory responses to skin cooling are attenuated in aged compared with young subjects, suggesting that advancing age influences sympathetic nerve responsiveness to hypothermia. However, regulation of skin sympathetic nerve discharge (SND) is only one component of the complex sympathetic nerve response profile to hypothermia. Whether aging alters the responsiveness of sympathetic nerves innervating other targets during acute cooling is not known. In the present study, using multifiber recordings of splenic, renal, and adrenal sympathetic nerve activity, we tested the hypothesis that hypothermia-induced changes in visceral SND would be attenuated in middle-aged and aged compared with young Fischer 344 (F344) rats. Colonic temperature (Tc) was progressively reduced from 38 degrees C to 31 degrees C in young (3 to 6 mo), middle-aged (12 mo), and aged (24 mo) baroreceptor-innervated and sinoaortic-denervated (SAD), urethane-chloralose anesthetized, F344 rats. The following observations were made. 1) Progressive hypothermia significantly (P < 0.05) reduced splenic, renal, and adrenal SND in young baroreceptor-innervated F344 rats. 2) Reductions in splenic, renal, and adrenal SND to progressive hypothermia were less consistently observed and, when observed, were generally attenuated in baroreceptor-innervated middle-aged and aged compared with young F344 rats. 3) Differences in splenic, renal, and adrenal SND responses to reduced Tc were observed in SAD young, middle-aged, and aged F344 rats, suggesting that age-associated attenuations in SND responses to acute cooling are not the result of age-dependent modifications in arterial baroreflex regulation of SND. These findings demonstrate that advancing chronological age alters the regulation of visceral SND responses to progressive hypothermia, modifications that may contribute to the inability of aged individuals to adequately respond to acute bouts of hypothermia.

We tested the hypothesis that central angiotensin II (ANG II) administration would activate splenic sympathetic nerve discharge (SND), which in turn would alter splenic cytokine gene expression. Experiments were completed in sinoaortic nerve-lesioned, urethane-chloralose-anesthetized, splenic nerve-intact (splenic-intact) and splenic nerve-lesioned (splenic-denervated) Sprague-Dawley rats. Splenic cytokine gene expression was determined using gene-array and real-time RT-PCR analyses. Splenic SND was significantly increased after intracerebroventricular administration of ANG II (150 ng/kg, 10 microl), but not artificial cerebrospinal fluid (aCSF). Splenic mRNA expression of IL-1beta, IL-6, IL-2, and IL-16 genes was increased in ANG II-treated splenic-intact rats compared with aCSF-treated splenic-intact rats. Splenic IL-1beta, IL-2, and IL-6 gene expression responses to ANG II were significantly reduced in splenic-denervated compared with splenic-intact rats. Splenic gene expression responses did not differ significantly in ANG II-treated splenic-denervated and aCSF-treated splenic-intact rats. Splenic blood flow responses to intracerebroventricular ANG II administration did not differ between splenic-intact and splenic-denervated rats. These results provide experimental support for the hypothesis that ANG II modulates the immune system through activation of splenic SND, suggesting a novel relation between ANG II, efferent sympathetic nerve outflow, and splenic cytokine gene expression.

In the present study, we established dose-response relationships between central administration of 4-hydroxy-2,2,6,6-tetramethylpiperidine 1-oxyl (Tempol, a superoxide dismutase mimetic) and the level of renal sympathetic nerve discharge (SND) and tested the hypothesis that intracerebroventricular (icv) Tempol pretreatment would attenuate centrally mediated changes in SND produced by icv ANG II administration. Urethane-chloralose-anesthetized, baroreceptor-denervated, normotensive rats were used. We found that icv Tempol administration produced dose-dependent sympathoinhibitory, hypotensive, and bradycardic responses. Mean arterial pressure and SND values were significantly increased after icv ANG II (150 ng/kg) administration, and these responses were abrogated after icv pretreatment with Tempol (75 micromol/kg) or losartan. Brain superoxide levels tended to be higher in ANG II-treated rats compared with rats treated with Tempol and ANG II. Tempol pretreatment did not prevent increases in SND level that were produced by acute heat stress, which indicates specificity in the effect of Tempol in reducing sympathoexcitation. These results demonstrate that icv Tempol administration influences central sympathetic neural circuits in a dose-dependent manner and attenuates SND responses to central ANG II infusion.

Whole body hyperthermia (WBH) has been used in experimental settings as an adjunct to radiochemotherapy for the treatment of various malignant diseases. The therapeutic effect of WBH has been hypothesized to involve activation of the immune system, although the effect of hyperthermia-induced activation of sympathetic nerve discharge (SND) on splenic immune function is not known. We tested the hypothesis that heating-induced splenic sympathoexcitation would alter splenic cytokine gene expression as determined using gene array and real-time RT-PCR analyses. Experiments were performed in splenic-intact and splenic-denervated anesthetized Sprague-Dawley rats (n=32). Splenic SND was increased during heating (internal temperature increased from 38 degrees to 41 degrees C) in splenic-intact rats but remained unchanged in nonheated splenic-intact rats. Splenic interleukin-1beta (IL-1beta), interleukin-6 (IL-6), and growth-regulated oncogene 1 (GRO 1) mRNA expression was higher in heated than in nonheated splenic-intact rats. Splenic IL-1beta, IL-6, and GRO 1 mRNA expression was reduced in heated splenic-denervated compared with heated splenic-intact rats, but did not differ between heated splenic-denervated and nonheated splenic-intact rats. These results support the hypothesis that hyperthermia-induced activation of splenic SND enhances splenic cytokine gene expression.

Renal and splanchnic sympathetic nerve discharge (SND) responses to heating are significantly reduced in senescent compared with young Fischer-344 (F344) rats (Kenney MJ and Fels RJ. Am J Physiol Regul Integr Comp Physiol 283: R513-R520, 2002). However, the functional significance of this finding is not known. We tested the hypothesis that blood flow distribution profiles to heating are altered in senescent (24 mo old) compared with mature (12 mo old) and young (3 mo old) F344 rats. Visceral organ, skeletal muscle, and tail blood flows were determined with the radionuclide-tagged microsphere technique before (control, 38 degrees C) and during heating that increased body temperature to 41 degrees C in anesthetized F344 rats. Vascular conductance in the kidney, stomach, large intestine, pancreas, spleen, and tail was significantly reduced during control before heating in senescent compared with young F344 rats. Heating significantly decreased kidney, stomach, small and large intestine, and pancreas vascular conductance in young and mature but not senescent F344 rats. Vascular conductance at 41 degrees C in the kidney and small intestine was significantly lower and in the stomach tended to be lower in young compared with senescent rats. Splenic conductance increased during heating in young and senescent rats but was highest in young rats. Tail conductance during heating was significantly increased in young rats but remained unchanged in mature and senescent rats. These results demonstrate a marked attenuation in heating-induced vascular conductance changes in senescent rats, suggesting an important functional consequence for the attenuated SND responses to heating in aged rats.

Acute heating in young rats increases visceral sympathetic nerve discharge (SND); however, renal and splanchnic SND responses to hyperthermia are attenuated in senescent compared with young Fischer 344 (F344) rats (Kenney MJ and Fels RJ. Am J Physiol Regul Integr Comp Physiol 283: R513-R520, 2002). Central mechanisms by which aging alters visceral SND responses to heating are unknown. We tested the hypothesis that forebrain neural circuits are involved in suppressing sympathoexcitatory responses to heating in chloralose-anesthetized, senescent F344 rats. Renal and splanchnic SND responses to increased (38 degrees C-41 degrees C) internal temperature were determined in midbrain-transected (MT) and sham-MT young (3-mo-old), mature (12-mo-old), and senescent (24-mo-old) F344 rats and in cervical-transected (CT) and sham-CT senescent rats. Renal SND remained unchanged during heating in MT and sham-MT senescent rats but was increased in CT senescent rats. Splanchnic SND responses to heating were higher in MT vs. sham-MT senescent rats and in CT vs. MT senescent rats. SND responses to heating were similar in MT and sham-MT young and mature rats. Mean arterial pressure (MAP) was increased during heating in MT but not in sham-MT senescent rats, whereas heating-induced increases in MAP were higher in sham-MT vs. MT young rats. These data suggest that in senescent rats suppression of splanchnic SND to heating involves forebrain and brain stem neural circuits, whereas renal suppression is mediated solely by brain stem neural circuits. These results support the concept that aging alters the functional organization of pathways regulating SND and arterial blood pressure responses to acute heating.

In the appendicular skeleton, substantial evidence demonstrates that somatosensory input from deep tissues including limb muscles and joints elicits somatosympathetic reflexes. Much less is known about the presence and organization of these reflexes from the axial skeleton. We determined if mechanical loading of the lumbar spine and lumbar paraspinal muscle irritation reflexively affects postganglionic sympathetic nerve discharge (SND) to the spleen and kidney. In 27 alpha-chloralose-anesthetized cats, the L2-4 multifidus muscles were injected with the inflammatory irritant mustard oil (20%, 60 microl total) and a vertebral load (100% body weight) was applied dorsal-ventral at the L3 spinous process. Mustard oil injection alone without vertebral loading (n = 7) increased mean splenic SND (60%), renal SND (30%), and heart rate (HR; 52 bpm). Mustard oil injection accompanied by the vertebral load (n = 7) increased mean splenic SND (55%), renal SND (16%), and HR (27 bpm). Blood pressure changes were biphasic and could not account for these changes. When the vertebral load accompanied mustard oil, the increases in splenic SND, renal SND, and HR remained elevated in a pattern significantly different from when the vertebral load was absent. Vehicle injection combined with the mechanical load (n = 3) did not change any of the autonomic responses. Similarly, mustard oil injection combined with a mechanical load did not change these responses when either the medial branches of the dorsal rami from T11-L5 had been cut (n = 4) or when the spinal cord had been transected between the second and third cervical vertebrae (n = 6). The results indicate that inflammatory stimulation of multifidus muscle in the low back evokes a somatosympathetic reflex integrated supraspinally in the upper cervical spinal cord or higher. The reflex's afferent arm travels in the medial branch of the dorsal ramus, and its efferent arm can affect sympathetic outflow to the spleen and the kidney as well as HR and BP. A static mechanical load applied to the lumbar spine accompanying the inflammatory stimulus appears to sustain the inflammatory-induced reflex activity.

We tested the hypothesis that intracerebroventricular (lateral ventricle) administration of interleukin-1beta (IL-1beta) antibody increases the level of sympathetic nerve discharge (SND) in alpha-chloralose-anesthetized rats. Mean arterial pressure (MAP), heart rate (HR), and SND (splenic and renal) were recorded before (Preinfusion), during (25 min), and for 45 min after infusion of IL-1beta antibody (15 microg, 50 microl icv) in baroreceptor-intact (intact) and sinoaortic-denervated (SAD) rats. The following observations were made. First, intracerebroventricular infusion of IL-1beta antibody (but not saline and IgG) significantly increased MAP and the pressor response was higher in SAD compared with intact rats. Second, renal and splenic SND were significantly increased during and after intracerebroventricular IL-1beta antibody infusion and sympathoexcitatory responses were higher in SAD compared with intact rats. Third, intracerebroventricular administration of a single dose of IL-1beta antibody (15 microg, 5 microl for 2 min) significantly increased splenic and renal SND in intact rats. These results suggest that under the conditions of the present experiments central neural IL-1beta plays a role in the tonic regulation of SND and arterial blood pressure.

Autospectral and coherence analyses were used to determine the role of and interactions between paraventricular nucleus (PVN) nitric oxide, gamma-aminobutyric acid (GABA), and the N-methyl-D-aspartic acid (NMDA)-glutamate receptor in regulation of sympathetic nerve discharge (SND) frequency components in anesthetized rats. Four observations were made. First, PVN microinjection of bicuculline (BIC) (GABA(A) receptor antagonist), but not single PVN injections of NMDA (excitatory amino acid) or N(G)-monomethyl-L-arginine (L-NMMA; a nitric oxide synthase inhibitor), altered SND frequency components. Second, combined PVN microinjections of L-NMMA and NMDA changed the SND bursting pattern; however, the observed pattern change was different from that produced by PVN BIC and not observed after sinoaortic denervation. Third, PVN microinjection of kynurenic acid prevented and reversed BIC-induced changes in the SND bursting pattern. Finally, vascular resistance (renal and splenic) was significantly increased after PVN BIC microinjection despite the lack of change in the level of renal and splenic SND. These data demonstrate that the PVN contains the neural substrate for altering SND frequency components and suggest complex interactions between specific PVN neurotransmitters and between PVN neurotransmitters and the arterial baroreceptor reflex in SND regulation.

Renal and splanchnic sympathetic nerve discharge (SND) responses to increased (38-41 degrees C) internal temperature were determined in anesthetized young (3-6 mo old), mature (12 mo old), and senescent (24 mo old) Fischer 344 (F344) rats. We hypothesized that SND responses would be altered in senescent and mature rats as demonstrated by attenuated sympathoexcitatory responses to heating and by the absence of hyperthermia-induced SND pattern changes. The following observations were made. 1) Renal and splanchnic SND responses were significantly increased during heating in young and mature but not in senescent rats. 2) At 41 degrees C, renal and splanchnic SND responses were higher in young compared with senescent rats, and renal SND was higher in mature than in senescent rats. 3) Heating changed the SND bursting pattern in young, but not in mature or senescent, rats. 4) SND responses to heating did not differ between baroreceptor-innervated (BRI) and sinoaortic-denervated (SAD) senescent rats but were higher in SAD compared with BRI young rats. These results demonstrate an attenuated responsiveness of sympathetic neural circuits to heating in senescent F344 rats.

We investigated the contributions of forebrain, brain stem, and spinal neural circuits to interleukin (IL)-1beta-induced sympathetic nerve discharge (SND) responses in alpha-chloralose-anesthetized rats. Lumbar and splenic SND responses were determined in spinal cord-transected (first cervical vertebra, C1), midbrain-transected (superior colliculus), and sham-transected rats before and for 60 min after intravenous IL-1beta (285 ng/kg). The observations made were the following: 1) lumbar and splenic SND were significantly increased after IL-1beta in sham C1-transected rats but were unchanged after IL-1beta in C1-transected rats; 2) intrathecal administration of DL-homocysteic acid (10 ng) increased SND in C1-transected rats; 3) lumbar and splenic SND were significantly increased after IL-1beta in sham- but not midbrain-transected rats; and 4) midbrain transection did not alter the pattern of lumbar and splenic SND, demonstrating the integrity of brain stem sympathetic neural circuits after decerebration. These results demonstrate that an intact forebrain is required for mediating lumbar and splenic sympathoexcitatory responses to intravenous IL-1beta, thereby providing new information about the organization of neural circuits responsible for mediating sympathetic-immune interactions.

Although interleukin-1beta (IL-1beta) administration produces nonuniform changes in the level of sympathetic nerve discharge (SND), the effect of IL-1beta on the frequency-domain relationships between discharges in different sympathetic nerves is not known. Autospectral and coherence analyses were used to determine the effect of IL-1beta and mild hypothermia (60 min after IL-1beta, colonic temperature from 38 degrees C to 36 degrees C) on the relationships between renal-interscapular brown adipose tissue (IBAT) and splenic-lumbar sympathetic nerve discharges in chloralose-anesthetized rats. The following observations were made. 1) IL-1beta did not alter renal-IBAT coherence values in the 0- to 2-Hz frequency band or at the cardiac frequency (CF). 2) Peak coherence values relating splenic-lumbar discharges at the CF were significantly increased after IL-1beta and during hypothermia. 3) Hypothermia after IL-1beta significantly reduced the coupling (0-2 Hz and CF) between renal-IBAT but not splenic-lumbar SND bursts. 4) Combining IL-1beta and mild hypothermia had a greater effect on renal-IBAT SND coherence values than did mild hypothermia alone. These data demonstrate functional plasticity in sympathetic neural circuits and suggest complex relationships between immune products and SND regulation.

Proinflammatory cytokines and acute physical stress influence sympathetic nerve discharge (SND). Because interleukin-1 beta (IL-1 beta) produces physiological responses that require central neural integration and because the sympathetic nervous system mediates physiological responses to environmental stress, we hypothesized that IL-1 beta modulates SND responses to acute physical stress. Therefore, this study examined the effects of IL-1 beta (290 ng/kg iv) and mild hypothermia on renal and interscapular brown adipose tissue (IBAT) SND regulation in chloralose-anesthetized rats. IBAT SND did not change after IL-1 beta administration but was significantly increased during acute mild hypothermia, which was induced 60 min after IL-1 beta treatment. Renal SND was unchanged after IL-1 beta administration and during hypothermia. Acute hypothermia, without prior IL-1 beta administration, did not alter IBAT and renal SND. Increases in IBAT SND during sustained (120 min) hypothermia were significantly higher in IL-1 beta-treated rats compared with saline-treated rats, whereas renal SND responses to sustained hypothermia did not differ among groups. Exposure to acute cold stress after sustained hypothermia produced greater increases in IBAT SND in IL-1 beta-treated rats compared with saline-treated controls. These data suggest that IL-1 beta alters IBAT SND responses to acute and sustained hypothermia.

Autospectral and coherence analyses were used to determine the effect of paraventricular nucleus (PVN) GABA(A) receptor antagonism [microinfusion or microinjections of bicuculline methiodide (BMI) 100 pmoles] on sympathetic nerve discharge (SND) frequency components (bursting pattern and relationships between discharges in regionally selective nerves) in alpha-chloralose-anesthetized rats. SND was recorded from the renal, splenic, and lumbar nerves. The following observations were made. First, PVN BMI microinjections, but not PVN saline or cortical BMI microinjections, transformed the cardiac-related SND bursting pattern in baroreceptor-innervated rats to one characterized by the presence of low-frequency bursts not synchronized to the cardiac cycle or phrenic nerve discharge bursts. Second, SND pattern changes were similar in the renal, splenic, and lumbar nerves, and peak coherence values relating low-frequency bursts in sympathetic nerve pairs (renal-splenic, renal-lumbar, and splenic-lumbar) were significantly increased from preinjection control after PVN BMI microinjection. Third, PVN BMI microinjections significantly increased the coupling between low-frequency SND bursts in baroreceptor-denervated rats. Finally, PVN BMI-induced changes in the SND bursting pattern were not observed after PVN pretreatment with muscimol (GABA agonist, 1 nmole). We conclude that PVN GABA(A) receptor antagonism profoundly alters the frequency components in sympathetic nerves.

Heart failure (HF) alters the regulation of basal sympathetic nerve discharge (SND); however, the effect of HF on SND responses to acute stress is not well established. In the present study, renal SND responses to hyperthermia were determined in chloralose-anesthetized HF rats and in sham controls. Whole body heating (colonic temperature increased from 38 to 41 degrees C) was used as an acute stressor because increased internal body temperature provides a potent stimulus to the sympathetic nervous system. Left ventricular end-diastolic pressure and the right ventricular wt-to-body wt ratio were increased (P < 0.05) in HF compared with sham rats. The following observations were made: 1) renal sympathoexcitatory responses to heating were significantly reduced in HF compared with sham rats, 2) renal blood flow remained unchanged from control levels during heating in HF rats but was significantly reduced in sham rats, and 3) renal SND responses to heating were significantly higher in HF rats with bilateral lesions of the hypothalamic paraventricular nucleus (PVN) compared with sham PVN-lesioned HF rats. These results demonstrate a marked attenuation in the responsiveness of renal SND to heating in HF rats and suggest that HF alters the organization of neural pathways mediating SND responses to heating.

In the present study, we investigated the contributions of forebrain, brain stem, and spinal neural circuits to heating-induced sympathetic nerve discharge (SND) responses in chloralose-anesthetized rats. Frequency characteristics of renal and splenic SND bursts and the level of activity in these nerves were determined in midbrain-transected (superior colliculus), spinal cord-transected [first cervical vertebra (C1)], and sham-transected (midbrain and spinal cord) rats during progressive increases in colonic temperature (T(c)) from 38 to 41.6-41.7 degrees C. The following observations were made. 1) Significant increases in renal and splenic SND were observed during hyperthermia in midbrain-transected, sham midbrain-transected, C1-transected, and sham C1-transected rats. 2) Heating changed the discharge pattern of renal and splenic SND bursts and was associated with prominent coupling between renal-splenic discharge bursts in midbrain-transected, sham midbrain-transected, and sham C1-transected rats. 3) The pattern of renal and splenic SND bursts remained unchanged from posttransection recovery levels during heating in C1-transected rats. We conclude that an intact forebrain is not required for the full expression of SND responses to increased T(c) and that spinal neural systems, in the absence of supraspinal circuits, are unable to markedly alter the frequency characteristics of SND in response to acute heat stress.

Frequency-domain analyses were used to determine the effect of cold stress on the relationships between the discharge bursts of sympathetic nerve pairs, sympathetic and aortic depressor nerve pairs, and sympathetic and phrenic nerve pairs in chloralose-anesthetized, baroreceptor-innervated rats. Sympathetic nerve discharge (SND) was recorded from the renal, lumbar, splanchnic, and adrenal nerves during decreases in core body temperature from 38 to 30 degrees C. The following observations were made. 1) Hypothermia produced nonuniform changes in the level of activity in regionally selective sympathetic nerves. Specifically, cold stress increased lumbar and decreased renal SND but did not significantly change the level of activity in splanchnic and adrenal nerves. 2) The cardiac-related pattern of renal, lumbar, and splanchnic SND bursts was transformed to a low-frequency (0-2 Hz) pattern during cooling, despite the presence of pulse-synchronous activity in arterial baroreceptor afferents. 3) Peak coherence values relating the discharges between sympathetic nerve pairs decreased at the cardiac frequency but were unchanged at low frequencies (0-2 Hz), indicating that the sources of low-frequency SND bursts remain prominently coupled during progressive reductions in core body temperature. 4) Coherence of discharge bursts in phrenic and renal sympathetic nerve pairs in the 0- to 2-Hz frequency band increased during mild hypothermia (36 degrees C) but decreased during deep hypothermia (30 degrees C). We conclude that hypothermia profoundly alters the organization of neural circuits involved in regulation of sympathetic nerve outflow to selected regional circulations.

Frequency-domain analyses were used to determine the effect of heat stress on the relationships between the discharge bursts of sympathetic nerve pairs and sympathetic and phrenic nerve pairs in chloralose-anesthetized rats. Sympathetic nerve discharge (SND) was recorded from the renal, splanchnic, splenic, and lumbar nerves during increases in core body temperature (Tc) from 38 to 41.4 +/- 0. 3 degreesC. The following observations were made: 1) hyperthermia transformed the cardiac-related bursting pattern of SND to a pattern that contained low-frequency, non-cardiac-related bursts, 2) the pattern transformation was uniform in regionally selective sympathetic nerves, 3) hyperthermia enhanced the frequency-domain coupling between SND and phrenic nerve bursts, and 4) low-frequency SND bursts recorded during hyperthermia contained significantly more activity than cardiac-related bursts. We conclude that acute heat stress profoundly affects the organization of neural circuits responsible for the frequency components in sympathetic nerve activity and that SND pattern transformation provides an important strategy for increasing the level of activity in sympathetic nerves during increased Tc.

We tested the hypothesis that sustained elevation in mean arterial pressure (MAP) alters the frequency-domain characteristics of efferent sympathetic nerve discharge (SND) after the return of MAP to control levels. Renal, lumbar, and splanchnic SND were recorded before, during, and after a 30-min increase in MAP produced by phenylephrine (PE) infusion in alpha-chloralose-anesthetized, spontaneously hypertensive (SH) rats. The following observations were made. 1) The basic cardiac-locked pattern of renal, lumbar, and splanchnic SND bursts was altered after sustained elevation in MAP, demonstrating prolonged effects on the neural circuits involved in entraining efferent SND to the cardiac cycle. Importantly, discharge bursts in afferent baroreceptor nerve activity remained pulse-synchronous after sustained increases in arterial pressure. 2) The frequency-domain relationships between the activity in sympathetic nerve pairs were altered after sustained elevation in MAP, suggesting a transformation from a system of tightly coupled neural circuits to one of multiple generators exerting selective control over SND. 3) The most prominent reduction in SND power after sustained elevation in MAP occurred in the frequency band containing the cardiac cycle, indicating that the prolonged suppression of SND after sustained increases in arterial pressure is due primarily to the selective inhibition of cardiac-related SND bursts. We conclude that sustained elevation in MAP profoundly affects the neural circuits responsible for the frequency components of basal SND in SH rats.

The present study examined the mechanisms responsible for the hindlimb vasodilation produced by elevating core body (colonic) temperature (Tco) of alpha-chloralose-anesthetized rats from 37 to 39 degrees C. Elevating Tco to 39 degrees C produced equivalent decreases in hindlimb vascular resistance in sham-operated (-48 +/- 2%) and sinoaortic baroreceptor-denervated rats (-44 +/- 3%) rats. There were no changes in mean arterial blood pressure, heart rate, or lumbar sympathetic nerve activity in either group. The prior administration of the alpha 1-adrenoceptor antagonist prazosin (100 micrograms/kg i.v.) did not prevent the heat-induced decrease in hindlimb resistance in sham-operated rats (-52 +/- 7% vs. baseline). In contrast, the fall in hindlimb resistance was markedly attenuated (-20 +/- 5% vs. baseline) in sham-operated rats that had received a prior injection of the nitric oxide synthase (NOS) inhibitor NG-nitro-L-arginine methyl ester (100 mumol/kg i.v.). Dexamethasone (1 mg/kg i.v.), administered to prevent the possible induction of inducible NOS, did not modify the heat-induced hindlimb vasodilation in sham-operated rats (-41 +/- 5%). These results demonstrate that the elevation of Tco to 39 degrees C in alpha-chloralose-anesthetized rats produces a relative vasodilation in the hindlimb that is not obviously linked to an alteration in lumbar sympathetic nerve activity. Because the vasodilation occurred in the presence of prazosin, it is unlikely that the decline in resistance is due to the loss of the vasoconstrictor potency of neurally derived catecholamines. The findings that NG-nitro-L-arginine methyl ester, but not dexamethasone, diminished the heat-induced hindlimb vasodilation suggests that the fall in resistance is due in part to constitutive NOS and supports a role for NOS as a mediator of thermoregulatory active vasodilation.

We tested the hypothesis that sustained increases in mean arterial pressure (MAP) produce nonuniform changes in regional sympathetic nerve discharges (SND) after the return of MAP to control levels. Renal, adrenal, splanchnic, and lumbar SND were recorded before, during, and after a 30-min elevation in MAP produced by phenylephrine (PE) infusion in alpha-chloralose-anesthetized spontaneously hypertensive (SH) rats. SND remained reduced from control values after PE infusion, despite the return of MAP to control levels. Importantly, the duration of poststimulus sympathoinhibition was significantly less in adrenal and splanchnic SND compared with renal and lumbar SND. In sinoaortic-denervated SH rats, SND remained at control levels during and after PE infusion. Simultaneous recordings of aortic depressor nerve (ADN) activity and SND demonstrated that prolonged renal and lumbar sympathoinhibition occurred even when ADN activity fell below control levels after PE infusion. We conclude that poststimulus responses of efferent SND in SH rats are regionally nonuniform and that renal and lumbar sympathoinhibitory responses are not mediated solely by prolonged increases in afferent baroreceptor nerve activity.

Recent studies have suggested that the interaction between the sympathetic nervous system and nitric oxide (NO) or nitrosyl factors may be an important means by which arterial blood pressure is regulated. We investigated whether NO synthase (NOS) inhibition modulates basal sympathetic nerve discharge (SND) in baroreceptor-innervated and -denervated, chloralose-anesthetized Sprague-Dawley rats. We recorded mean arterial pressure (MAP), renal SND, and lumbar SND before and after administration of the NOS inhibitor, NG-nitro-L-arginine methyl ester (L-NAME, 20 mg/kg iv). Two minutes after L-NAME administration in baroreceptor-innervated rats, MAP increased (+23 +/- 3 mmHg), whereas renal (-45 +/- 6%, n = 7) and lumbar (-35 +/- 2%, n = 6) SND significantly decreased from control levels. These changes persisted for up to 20 min after L-NAME administration. In baroreceptor-denervated rats, L-NAME increased MAP (+40 +/- 6 mmHg) and decreased lumbar SND (n = 7) (-37 +/- 10% from control at 20 min post-L-NAME). In contrast, renal SND progressively increased (+33 +/- 8% at 20 min post-L-NAME) from control after L-NAME administration in baroreceptor-denervated rats (n = 7). These results demonstrate that NOS inhibition can produce nonuniform changes in SND in baroreceptor-denervated rats and suggest that endogenous nitrosyl factors provide tonic excitation to lumbar SND, whereas they provide a tonic restraint to renal SND.

The aim of the present study was to characterize the sympathetic nerve responses to hyperthermia in chloralose-anesthetized rats. Discharges were recorded from the renal, lumbar, and splanchnic sympathetic nerves. Mean arterial pressure, heart rate, and sympathetic nerve discharge (SND) were recorded continuously during progressive increases in core body temperature (Tc) from 38.0 to 41.0 degrees C. The following observations were made: 1) significant increases in renal, lumbar, and splanchnic SND were observed during hyperthermia; 2) autospectral analysis of renal and lumbar SND revealed that the frequency distribution of SND can be altered during progressive increases in Tc; and 3) increases in splanchnic SND to acute heating were similar in baroreceptor-innervated and -denervated rats. We conclude that 1) hyperthermia is a potent stimulus to the sympathetic nervous system and increases the activity in three sympathetic nerves that innervate different regional arterial beds, 2) acute heating influences the neural circuits involved in generating SND as evidenced by changes in the basic pattern of renal and lumbar SND, and 3) the increase in splanchnic SND during hyperthermia is not opposed by the arterial and cardiopulmonary baroreceptors.

Frequency-domain analyses were used to characterize basal sympathetic nerve discharge (SND) and to determine the relationships between the activity recorded simultaneously from sympathetic nerves in chloralose-anesthetized, baroreceptor-innervated rats. Discharges were recorded from the splanchnic, renal, and lumbar nerves. The following observations were made. 1) Approximately 65-75% of total power in SND was contained below 9 Hz in either baroreceptor-innervated rats or baroreceptor-denervated rats. 2) Coherence values relating the activity of two nerves were significantly different from zero within this frequency band. 3) The interval between the discharges of two sympathetic nerves was either frequency dependent or constant within the coherent frequency band. 4) The frequency components of SND and the relationships between the activity in two sympathetic nerves could be altered during periods of acute stress. These results suggest that the system responsible for basal SND in rats is composed of either multiple sympathetic generators or multiple filters arranged in parallel, which are capable of producing different outputs. The dynamic nature of these circuits was revealed by the changes that occurred during periods of acute stress.

The purpose of this study was to determine whether the bradycardic response to peripheral hyperosmolality in conscious rats is dependent on afferent baroreceptor mechanisms and whether central histamine H2 receptors play a role in baroreflex-mediated changes in heart rate (HR). Mean arterial pressure (MAP) and HR were recorded continuously during a 30-min infusion of 2.5 M NaCl (10 microliters.100 g-1.min-1) hypertonic saline (HTS). HTS infusion significantly increased MAP (21 +/- 4 mmHg) and reduced HR (-62 +/- 10 beats/min) in rats with intact arterial baroreceptors. In sinoaortic-denervated rats, HR remained unchanged from control despite a significant increase in MAP. After intracerebroventricular (lateral ventricle) administration of cimetidine or ranitidine (H2-receptor antagonists) in intact rats, HTS infusion significantly increased MAP (19 +/- 2 and 17 +/- 2 mmHg, respectively) but the bradycardia was abolished (-12 +/- 10 and -10 +/- 10 beats/min, respectively). In contrast, central H2-receptor blockade did not alter reflex HR responses to the intravenous administration of phenylephrine and nitroprusside or to the central administration of histamine or angiotensin II. These results indicate that the bradycardic response to HTS infusion is mediated through the arterial baroreceptor reflex and involves in part a selective histaminergic pathway.

The relationships among the simultaneously recorded discharges of as many as four postganglionic sympathetic nerves were studied with coherence and phase spectral analyses in baroreceptor-denervated cats. Discharges were recorded from the inferior cardiac (CN), vertebral (VN), and renal (RN) nerves. Most of the power in sympathetic nerve discharge was less than 6 Hz, and the discharges of any two nerves coherred over a frequency band generally between 0 and 15 Hz. Peak coherence occurred between 2 and 6 Hz in most cases. Our most important observations are as follows. 1) Coherence values were significantly higher in chloralose-anesthetized than in unanesthetized decerebrate cats. 2) Coherence values were higher for near ipsilateral nerves (e.g., CN and VN) than for widely separated ipsilateral nerves (e.g., CN and RN). 3) Coherence values for most pairs were higher when the nerves were located on the same side (ipsilateral nerves) rather than on opposite sides (contralateral nerves) of the body. 4) Coherence values were higher for some functionally complementary nerves (e.g., CN and RN) than for noncomplementary nerves (e.g., VN and RN). These results support the hypothesis that the central circuits responsible for the basal discharges of different postganglionic sympathetic nerves are selectively coupled.

The coherence function and phase spectrum were used to study the relationships between the discharges of sets of two postganglionic or preganglionic sympathetic nerves in baroreceptor-denervated cats anesthetized with either 5,5-diallylbarbiturate-urethan or alpha-chloralose. Most of the power in sympathetic nerve discharge was contained between 2 and 6 Hz. The coherence values relating the activity of two nerves were significantly different from zero within this frequency band. The phase spectrum was either linear or complex (i.e., showed changes in slope) within the coherent frequency band. We observed three patterns of relationship. The first pattern was characterized by a constant interval between activity in different sympathetic nerves within the coherent frequency band. The second pattern was characterized by a frequency-dependent interval. The third pattern was characterized by uncoupling of the 2- to 6-Hz rhythms in the discharges of different nerves. Switching between these patterns was observed. We suggest that the three patterns reflect different functional states of the central system responsible for the 2- to 6-Hz rhythm. Two models of this system are entertained. The first model is one of a system of coupled oscillators while filtering circuits that receive common inputs are the elements of the second model.

Autospectral and coherence analyses were used to study the frequency-domain relationships between frontal-parietal cortical activity [electroencephalogram (EEG)] and the discharges of the interior cardiac and renal sympathetic nerves of baroreceptor-denervated and vagotomized cats anesthetized with either alpha-chloralose or pentobarbital sodium. Delta slow-wave activity in the EEG was correlated to sympathetic nerve discharge (SND) as shown by sharp peaks between 0.5 and 4 Hz in the coherence function. The relationship was stronger in chloralose- than in pentobarbital-anesthetized cats. Coherence of the two signals could be attributed to descending influences of forebrain delta slow-wave generators on sympathetic circuits, since midbrain transection preferentially reduced the power in SND at frequencies that cohered to the EEG before transection. In contrast, the power in the EEG was not reduced by midbrain transection. The relationship between cortical delta slow-wave activity and SND was stronger during than between cortical spindlelike events that lasted 1-3 s and recurred once every 5-10 s. These events were similar to cortical spindles observed during the early stages of sleep and under light barbiturate anesthesia. These observations raise the possibility that the influences of forebrain delta slow-wave generators on SND are gated by thalamic mechanisms normally involved in the sleep-wake cycle.

We tested the hypothesis that sustained elevations in mean arterial pressure (MAP) would produce an abbreviated suppression of renal sympathetic nerve activity (RSNA) in spontaneously hypertensive (SH) compared with Wistar-Kyoto (WKY) normotensive rats. For this purpose, we recorded RSNA during and after a 30-min elevation in MAP. Elevations in MAP (35-40 mmHg) induced by phenylephrine resulted in a suppression of RSNA in SH and WKY rats that persisted after MAP had returned to control levels. The prolonged suppression of RSNA was eliminated following sinoaortic denervation, indicating that this response was dependent on afferent baroreceptor mechanisms. The simultaneous recording of RSNA and aortic depressor nerve (ADN) activity provided additional evidence that baroreceptor afferents contribute to this prolonged suppression of RSNA in SH rats. Despite the return of MAP to control levels following the sustained pressure elevation in SH rats, there was a paradoxical increase in ADN activity (39 +/- 14%) compared with control values. In contrast, both ADN activity and MAP returned to control levels in WKY rats following the sustained pressure elevation. The sustained increase of ADN activity in SH rats did not account for the entire magnitude of the prolonged recovery of RSNA. In summary, both SH and WKY rats exhibit prolonged suppression of RSNA, whereas ADN activity is paradoxically elevated in only SH rats following a sustained elevation in MAP.

Seven female dogs (15-20 kg) were instrumented with seven bipolar electrodes sutured at 3-cm intervals to the serosal surface of the proximal jejunum and were exercised at different intensities and durations on a motor-driven treadmill. Slow-wave frequency increased (P less than 0.05) from preexercise control during prolonged (90 min) exercise and during recovery after short-term exercise (30 min) at 70% heart rate reserve (HRR). These changes were associated with an increase in core temperature. Spike-burst frequency (SBF) increased (P less than 0.05) with moderate exercise (50% HRR), but the magnitude was small. When exercise was extended beyond 30 min and during all recovery periods, SBF decreased significantly. Exercise produced migrating myoelectrical complexes in three experiments and less dramatic pattern changes characterized as "clustered contractions" (regular spike bursts preceded and followed by the absence of spike bursts) in at least nine other experiments. We conclude that exercise does alter jejunal myoelectrical activity, but myoelectrical patterns may be more important in explaining exercise-induced gastrointestinal symptoms than changes in spike-burst frequency or duration.