Richard J. Fels

Effect of cervical vagotomy on sympathetic nerve responses to peripheral interleukin-1β

Although the vagus nerve is an important neural pathway mediating immune-to-brain communication, the role of the vagus in mediating sympathetic nerve discharge (SND) responses to peripheral cytokines is not well established. In the present study we determined renal, interscapular brown adipose tissue (IBAT), splenic, and lumbar SND responses before and for 60 min after the intravenous administration of interleukin-1beta (IL-1beta, 100 ng) in chloralose-anesthetized, sham-vagotomized and cervical-vagotomized (bilateral) rats. In sham-vagotomized rats, IL-1beta administration increased (P<0.05) splenic and lumbar SND while renal and IBAT SND remained unchanged from control levels. Renal, splenic, and lumbar SND were increased (P<0.05) whereas IBAT SND remained unchanged from control after IL-1beta in vagotomized rats. Renal, splenic, and lumbar SND responses were significantly higher after IL-1beta in vagotomized compared with sham-vagotomized rats. These results demonstrate that regionally-selective SND (renal, splenic, and lumbar) responses to IL-1beta can occur in the absence of the vagus nerve and suggest that the vagus nerve provides a tonic inhibition to the discharges in these nerves in response to peripheral IL-1beta.

Regulation of the sympathetic nerve discharge bursting pattern during heat stress

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°C. 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.

Inhibition of RVLM Synaptic Activation at Peak Hyperthermia Reduces Visceral Sympathetic Nerve Discharge

Hyperthermia is an environmental stressor that produces marked increases in visceral sympathetic nerve discharge (SND) in young rats. The brainstem in rats contains the essential neural circuitry for mediating visceral sympathetic activation; however, specific brainstem sites involved remain virtually unknown. The rostral ventral lateral medulla (RVLM) is a key central nervous system region involved in the maintenance of basal SND and in mediating sympathetic nerve responses evoked from supraspinal sites. In the present study we tested the hypothesis that inhibition of RVLM synaptic activation at peak hyperthermia (internal body temperature, Tc, increased to 41.5 degrees C) would affect heating-induced visceral sympathetic activation. Experiments were completed in chloralose-urethane anesthetized, baroreceptor-intact and sinoaortic-denervated, 3-6 month-old Sprague-Dawley rats. Bilateral inhibition of RVLM synaptic activation produced by muscimol microinjections (400 and 800 pmol) at 41.5 degrees C resulted in immediate and significant reductions in peak heating-induced renal and splenic sympathoexcitation. Interruption of RVLM synaptic activation and axonal transmission by lidocaine microinjections (40 nmol) at 41.5 degrees C produced significant reductions in hyperthermia-induced sympathetic activation to similar levels produced by RVLM muscimol microinjections. The total amount of SND inhibited by RVLM muscimol and lidocaine microinjections was significantly more during hyperthermia (41.5 degrees C) than normothermia (38 degrees C). These findings demonstrate that maintenance of sympathetic activation at peak hyperthermia is dependent on the integrity of RVLM neural circuits.

Central nervous system administration of interleukin-6 produces splenic sympathoexcitation.

Interleukin-6 (IL-6) is a multifunctional cytokine that has been shown to play a pivotal role in centrally-mediated physiological responses including activation of the hypothalamic-pituitary-adrenal axis. Cerebral spinal fluid (CSF) concentrations of IL-6 are elevated in multiple pathophysiological conditions including Alzheimers disease, autoimmune disease, and meningitis. Despite this, the effect of IL-6 on central regulation of sympathetic nerve discharge (SND) remains unknown which limits understanding of sympathetic-immune interactions in health and disease. In the present study we determined the effect of intracerebroventricular (i.c.v, lateral ventricle) administration of IL-6 on splenic SND in urethane-chloralose-anesthetized rats. A second goal was to determine if icv injected IL-6 enters the brain parenchyma and acts as a volume transmission signal to access areas of the brain involved in regulation of sympathetic nerve outflow. i.c.v administration of IL-6 (10 ng, 100 ng, and 400 ng) significantly and progressively increased splenic SND from control levels in baroreceptor-denervated Sprague-Dawley rats. Administration of 100-ng and 400-ng IL-6 resulted in significantly higher SND responses when compared to those elicited with a 10-ng dose. Sixty minutes following icv administration, fluorescently labeled IL-6 was not distributed throughout the parenchyma of the brain but was localized to the periventricular areas of the ventricular system. Brain sections counter-stained for the IL-6 receptor (IL-6R) revealed that IL-6 and the IL-6R were co-localized in periventricular areas adjoining the third ventricle. These results demonstrate that icv IL-6 administration increases splenic SND, an effect likely achieved via signaling mechanisms originating in the periventricular cells.

Altered frequency characteristics of sympathetic nerve activity after sustained elevation in arterial pressure.

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 α-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.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.

Neuronal nitric oxide synthase inhibition and regional sympathetic nerve discharge: implications for peripheral vascular control.

Neuronal nitric oxide (NO) synthase (nNOS) inhibition with systemically administered S-methyl-l-thiocitrulline (SMTC) elevates mean arterial pressure (MAP) and reduces rat hindlimb skeletal muscle and renal blood flow. We tested the hypothesis that those SMTC-induced cardiovascular effects resulted, in part, from increased sympathetic nerve discharge (SND). MAP, HR, and lumbar and renal SND (direct nerve recordings) were measured in 9 baroreceptor (sino-aortic)-denervated rats for 20min each following both saline and SMTC (0.56mg/kg i.v.). SMTC increased MAP (peak ΔMAP: 50±8mmHg, p<0.01) compared to saline. Lumbar and renal SND were not different between saline and SMTC conditions at any time (p>0.05). The ΔSND between saline and SMTC conditions for the lumbar and renal nerves were not different from zero (peak ΔSND, lumbar: 2.0±6.8%; renal: 9.7±9.0%, p>0.05 versus zero for both). These data support that SMTC-induced reductions in skeletal muscle and renal blood flow reported previously reflect peripheral nNOS-derived NO vascular control as opposed to increased sympathetic vasoconstriction.

Is visceral sympathoexcitation to heat stress dependent on activation of ionotropic excitatory amino acid receptors in the rostral ventrolateral medulla

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.

Bacillus anthracis lethal toxin alters regulation of visceral sympathetic nerve discharge

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.

Disinhibition of RVLM neural circuits and regulation of sympathetic nerve discharge at peak hyperthermia

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.

Dexmedetomidine and regulation of splenic sympathetic nerve discharge

Recent lines of inquiry indicate that sedatives can influence the immune system, leading to the concept of sedative-induced immunomodulation. It has been hypothesized that sedatives may alter immune responses by modulating the sympathetic nervous system, however, little information is known regarding the effects of sedatives on regulation of splenic sympathetic nerve discharge (SND), a significant omission based on the functional role that changes in splenic SND exert on splenic cytokine gene expression. The present investigation determined the effect of systemic Dexmedetomidine (Dex) administration on the level of directly-recorded splenic SND and tested the hypothesis that the intravenous administration of Dex would inhibit splenic SND in anesthetized rats. The present results demonstrate for the first time that intravenous Dex administration significantly reduces splenic sympathetic nerve outflow in baroreceptor-intact and sinoaortic-denervated rats, indicating that Dex administration alters the central regulation of splenic SND. The present results provide new information regarding the effect of a centrally-acting alpha2-adrenergic agonist on the level of sympathetic nerve outflow to a secondary lymphoid organ that plays a critical role in peripheral immune responses.