Shaun F. Morrison

RVLM and raphe differentially regulate sympathetic outflows to splanchnic and brown adipose tissue

To determine whether neurons in the rostral raphe pallidus (RPa) specifically control the sympathetic nerve activity to brown adipose tissue (BAT SNA), thereby regulating adipocyte metabolism and BAT thermogenesis, the responses in BAT SNA to disinhibition of RPa neurons and to disinhibition of neurons in the vasomotor region of the rostral ventrolateral medulla (RVLM) were compared with those in splanchnic (Spl) SNA, which primarily regulates visceral vasoconstriction. In urethan-chloralose-anesthetized ventilated rats, both acute hypothermia and microinjection of bicuculline into RPa produced significantly larger increases in BAT SNA (542 and 1,949% of control) than in Spl SNA (19 and 24% of control). The enhanced burst discharge in BAT SNA was not coherent with that in Spl SNA or with the arterial pressure (AP) at any frequency except the central respiratory frequency. Microinjections of bicuculline into RVLM evoked increases in Spl SNA (86% of control) and AP (32 mmHg), but reduced BAT SNA to low, normothermic levels. Microinjections of muscimol into RVLM reduced Spl SNA (-82% of control) and AP (-59 mmHg), but did not prevent the increase in BAT SNA after disinhibition of RPa neurons. These results indicate that the neural networks generating BAT SNA in response to disinhibition of RPa neurons are independent of those generating basal Spl SNA and support a model in which sympathetic outflow to tissues involved in thermoregulation and metabolism is regulated by central pathways, including neurons in RPa, that are distinct from those involved in the sympathetic control of the cardiovascular system.To determine whether neurons in the rostral raphe pallidus (RPa) specifically control the sympathetic nerve activity to brown adipose tissue (BAT SNA), thereby regulating adipocyte metabolism and BAT thermogenesis, the responses in BAT SNA to disinhibition of RPa neurons and to disinhibition of neurons in the vasomotor region of the rostral ventrolateral medulla (RVLM) were compared with those in splanchnic (Spl) SNA, which primarily regulates visceral vasoconstriction. In urethan-chloralose-anesthetized ventilated rats, both acute hypothermia and microinjection of bicuculline into RPa produced significantly larger increases in BAT SNA (542 and 1,949% of control) than in Spl SNA (19 and 24% of control). The enhanced burst discharge in BAT SNA was not coherent with that in Spl SNA or with the arterial pressure (AP) at any frequency except the central respiratory frequency. Microinjections of bicuculline into RVLM evoked increases in Spl SNA (86% of control) and AP (32 mmHg), but reduced BAT SNA to low, normothermic levels. Microinjections of muscimol into RVLM reduced Spl SNA (-82% of control) and AP (-59 mmHg), but did not prevent the increase in BAT SNA after disinhibition of RPa neurons. These results indicate that the neural networks generating BAT SNA in response to disinhibition of RPa neurons are independent of those generating basal Spl SNA and support a model in which sympathetic outflow to tissues involved in thermoregulation and metabolism is regulated by central pathways, including neurons in RPa, that are distinct from those involved in the sympathetic control of the cardiovascular system.

GABA-mediated inhibition of raphe pallidus neurons regulates sympathetic outflow to brown adipose tissue

Sympathetic nerve activity to brown adipose tissue (BAT) regulates adipocyte metabolism of its stored lipid fuel and thus the thermogenesis in BAT. To determine if the discharge of neurons in the rostral raphe pallidus (RPa) can influence BAT thermogenesis, changes in sympathetic nerve activity to BAT were recorded after microinjection (60 nl) of the GABAA receptor antagonist bicuculline (500 μM) into the RPa in chloralose-urethan-anesthetized, ventilated rats. Bicuculline caused a large, rapid rise in the sympathetic nerve activity to BAT (which had also increased during acute hypothermia) from very low, normothermic control levels to maximum values (mean: 1,949 ± 604% control; n = 13) after 4-6 min. The sympathetic nerve discharge to BAT had a mean burst frequency (3.5 ± 0.3 Hz) that was significantly less than the heart rate (7.3 ± 0.2 beats/min), and it was not inhibited during baroreceptor reflex activation. Bicuculline-stimulated increases in the sympathetic nerve activity to BAT and cold-evoked increases in neuronal fos expression were localized to the RPa at the level of the caudal half of the facial nucleus. This dramatic increase in sympathetic nerve activity to BAT after disinhibition of neurons in rostral RPa is consistent with a major role for RPa neurons, perhaps as sympathetic premotoneurons for BAT, in medullary control of BAT thermogenesis.

The caudal ventrolateral medulla is a source of tonic sympathoinhibition

The caudal ventrolateral medulla (CVLM) contains neurons that are essential for the elaboration of the sympathoinhibitory effects of baroreceptor afferent stimulation. To determine if neurons in the CVLM also mediate a tonic inhibition of sympathetic nerve activity (SNA), we examined the effects of lesioning CVLM neurons in baroreceptor-denervated rats. The sustained increases in both arterial pressure (AP; 40 mmHg) and splanchnic SNA (200%) indicate that the discharge of neurons in the CVLM maintains a tonic sympathoinhibition that contributes significantly to the maintenance of normotensive levels of SNA and AP.

Differential Regulation of Sympathetic Outflows to Vasoconstrictor and Thermoregulatory Effectors

Abstract: The medullary premotor neurons determining the sympathetic outflow regulating cardiac function and vasoconstriction are located in the rostral ventrolateral medulla (RVLM). The present study sought evidence for differential characteristics and baroreceptor reflex sensitivities between the sympathetic nerve activity (SNA) controlling brown adipose tissue (BAT) metabolism and thermogenesis and cardiovascular SNA such as that controlling mesenteric vasoconstriction via the splanchnic (SPL) nerve. The tonic discharge of sympathetic nerves is determined by the inputs to functionally specific sympathetic preganglionic neurons from supraspinal populations of premotor neurons. Under normothermic conditions, BAT SNA was nearly silent, while SPL SNA exhibited sustained, large‐amplitude bursts. Disinhibition of neurons in the rostral raphe pallidus (RPa), a potential site of sympathetic premotor neurons controlling BAT SNA, or icv injection of prostaglandin E2, a pyrogenic stimulus, elicited a dramatic increase in BAT SNA. SPL SNA was strongly influenced by the baroreceptor reflex as indicated by a high coherence to the arterial pressure, while activated BAT SNA exhibited no correlation with the arterial pressure. Since these characteristics and reflex responses in sympathetic outflow have been shown to arise from the ongoing or altered discharge of sympathetic premotor neurons, the marked differences between SPL SNA and BAT SNA provide strong evidence supporting the hypothesis that vasoconstriction and thermogenesis (metabolism) are controlled by distinct populations of sympathetic premotor neurons, the former in the RVLM and strongly baroreceptor‐modulated and the latter potentially in the RPa exhibiting little influence of baroreceptor reflex activation.

Pontine lesions produce apneusis in the rat

Although lesion experiments in the vagotomized cat have indicated that neurons in the parabrachial region of the dorsolateral pons contribute to the mechanisms involved in terminating inspiration, a similar role for pontine structures in the rat has been questioned since pontomedullary transections in the anesthetized rat failed to prolong inspiration. In the present study, lesions of the parabrachial pons of the decerebrate, unanesthetized rat produced an increase in the duration of inspiration to 400% of control and a doubling of the duration of expiration, suggesting a role for this pontine area in the regulation of the timing of the phases of respiration.

Experimental Biology 2000 Symposium on Differential Control of Sympathetic Outflow DIFFERENTIAL REGULATION OF BROWN ADIPOSE AND SPLANCHNIC SYMPATHETIC OUTFLOWS IN RAT: ROLES OF RAPHE AND ROSTRAL VENTROLATERAL MEDULLA NEURONS

1. The medullary premotor neurons determining the sympathetic outflow regulating cardiac function and vasoconstriction are located in the rostral ventrolateral medulla (RVLM). The present study sought evidence for an alternative location for the sympathetic premotor neurons determining the sympathetic nerve activity (SNA) controlling brown adipose tissue (BAT) metabolism and thermogenesis.

Responses of adrenal sympathetic preganglionic neurons to stimulation of cardiopulmonary receptors.

The current study examined whether or not the activation of Bezold-Jarisch reflex with administration of phenylbiguanide (PBG, 100 microg/kg) into right atrium elicits differential responses in the two populations of adrenal sympathetic preganglionic neurons (SPNs) regulating the release of epinephrine (EPI ADR SPNs) and norepinephrine (NE ADR SPNs), respectively, from adrenal medullary chromaffin cells. Extracellular activity of 48 adrenal SPNs in the intermediolateral cell column (IML) were recorded in urethane/chloralose-anesthetized rats. Twenty-three EPI ADR SPNs and 25 NE ADR SPNs were antidromically activated by stimulation of left adrenal nerve and orthodromically activated by rostral ventrolateral medulla (RVLM) stimulation. At a mean arterial pressure (MAP) of 99. 6+/-2.8 mmHg, the mean spontaneous discharge rates of EPI ADR SPNs and NE ADR SPNs were 6.2+/-0.5 and 4.3+/-0.5 spikes/s, respectively. Intra-atrial PBG markedly inhibited 96% of EPI ADR SPNs (by 3.8+/-0. 4 spikes/s; n=22) and 76% of NE ADR SPNs (by 2.9+/-0.5 spikes/s; n=19) with hypotensive responses (DeltaMAP=33.2+/-5.3 and 26.4+/-5.0 mmHg, respectively). The remaining SPNs were weakly excited or unaffected. We conclude that both groups of SPNs regulating catecholamine release are primarily inhibited by stimulation of cardiopulmonary receptors and that these responses parallel the sympathoinhibitory and hypotensive components of the Bezold-Jarisch reflex.

Adrenal epinephrine secretion is not regulated by sympathoinhibitory neurons in the caudal ventrolateral medulla.

By providing the principal inhibitory regulation of the discharge of sympathetic premotor neurons in the rostral ventrolateral medulla (RVLM), neurons in the caudal ventrolateral medulla (CVLM) play a major role in regulating the level of sympathetic nerve activity (SNA) to cardiovascular targets. To determine whether adrenal medullary secretion of epinephrine (EPI) is also regulated by sympathoinhibitory inputs from the CVLM to the RVLM, we compared levels of plasma EPI obtained after disinhibition of RVLM neurons with levels obtained after inhibition of CVLM neurons, both of which result in sustained elevations in arterial blood pressure (AP), SNA, and heart rate (HR). Plasma norepinephrine (NE) concentrations were significantly elevated following bilateral microinjection either of bicuculline (BIC) into the RVLM or of muscimol into the CVLM of urethane/chloralose-anesthetized, artificially-ventilated rats. In sharp contrast, although plasma EPI concentrations were significantly elevated following disinhibition of neurons in the RVLM, they were unchanged by inhibition of neurons in the CVLM. These results demonstrate that the discharge of sympathetic premotor neurons in the RVLM regulating adrenal secretion of EPI is modulated by a tonic, GABA-ergic inhibition that arises from a source that is different from the sympathoinhibitory neurons in the CVLM that project to RVLM sympathetic premotor neurons controlling vasoconstrictor and cardiac targets.

Effect of ETA Receptor Antagonists on Cardiovascular Responses Induced by Centrally Administered Sarafotoxin 6b: Role of Sympathetic Nervous System

The present study was carried out to investigate the cardiovascular effects of centrally administered SRT6b in saline, BQ123 and BMS182874 pretreated male Sprague-Dawley rats, using a radioactive microsphere technique. SRT6b (100 ng, ICV) produced a transient increase (40%) in blood pressure at 5 min followed by a sustained decrease (-42%) at 30 and 60 min in control rats. Total peripheral resistance and heart rate were not significantly altered. Cardiac output increased (16%) at 5 min and decreased 30 and 60 min following SRT6b administration. Central venous pressure was not affected by SRT6b. Regional blood flow and vascular resistance did not change at 5 min following administration of SRT6b. However, a significant decrease in blood flow to the brain, heart, kidneys, liver, spleen, gastrointestinal tract and mesentery and pancreas was observed 30 and 60 min following administration of SRT6b in control (saline treated) rats. Pretreatment with ETA selective receptor antagonists, BQ123 (10 micrograms, ICV) or BMS182874 (50 micrograms, ICV) significantly attenuated the pressor and depressor effects of centrally administered SRT6b. SRT6b induced decrease in blood flow was completely blocked by pretreatment with BQ123 or BMS182874. ET-1 (100 ng, ICV) produced an increase followed by a decrease similar to SRT6b. Reserpine (5 mg/kg, IP) pretreatment attenuated the cardiovascular effects of ET-1. Role of sympathetic nervous system was determined by measuring splanchnic nerve activity. SRT6b when administered in the lateral cerebral ventricle did not produce any significant effect at 5 min, however, a significant decrease in sympathetic nerve activity was observed 30 min after its administration. It is concluded that centrally administered SRT6b produces significant changes in systematic and regional blood circulation which can be completely blocked by ETA receptor antagonist. The cardiovascular effects of centrally administered SRT6b appear to be mediated through the sympathetic nervous system.

Effect of centrally administered endothelin agonists on systemic and regional blood circulation in the rat: role of sympathetic nervous system

The aims of the present study were to determine (1) the hypotensive and regional circulatory effects of centrally administered endothelin (ET) ETA and ETB agonists, and (2) the role of the sympathetic nervous system in the mediation of hypotensive effects due to centrally administered ET-1. The systemic haemodynamics and regional blood circulation in urethane anaesthetized rats following intracerebroventricular (i.c.v.) administration of ET-1, ET-2, SRT6b, ET-3 and SRT6c (10, 30 and 90 ng) were determined by a radioactive microsphere technique. The effect of centrally administered ET-1 on sympathetic nerve activity was also analysed. Systemic haemodynamics and regional blood circulation were determined before (baseline) and 30 min after administration of ET agonists. Cumulative administration of three doses of saline (5 microliters, i.c.v. at 30 min intervals) did not produce any significant cardiovascular effects. ET-1, ET-2 and SRT6b produced a decrease in blood pressure (51%, 47% and 41%, respectively) along with a decrease in cardiac output (58%, 60% and 45%, respectively) and stroke volume. Heart rate and total peripheral resistance were not affected. ET-1, ET-2 and SRT6b also produced a significant reduction in blood flow to the brain, kidneys, heart, portal, mesentery and pancreas, gastrointestinal tract (GIT) and musculoskeletal system. The effect of ET-2 on the cardiovascular system was less intense in comparison with ET-1 and SRT6b. Centrally administered specific ETB receptor agonists ET-3 and SRT6c did not produce any change in systemic haemodynamics and regional blood flow. Centrally administered ET-1 (90 ng) produced a significant decrease (61%) in sympathetic nerve activity 30 min after drug administration, along with a fall in blood pressure. It is concluded that centrally administered ETA agonists produce significant cardiovascular effects mediating through the sympathetic nervous system.