Differential control of adrenal and renal sympathetic nerve activity during hemorrhagic hypotension in rats

ENDOCANNABINOID SYSTEM IN THE PARAVENTRICULAR NUCLEUS OF THE HYPOTHALAMUS MODULATES AUTONOMIC AND CARDIOVASCULAR CHANGES BUT NOT VASOPRESSIN RESPONSE IN A RAT HEMORRHAGIC SHOCK MODEL

ABSTRACT

We evaluated the participation of the endocannabinoid system in the paraventricular nucleus of the hypothalamus (PVN) on the cardiovascular, autonomic, and plasma vasopressin (AVP) responses evoked by hemorrhagic shock in rats. For this, the PVN was bilaterally treated with either vehicle, the selective cannabinoid receptor type 1 antagonist AM251, the selective fatty acid amide hydrolase amide enzyme inhibitor URB597, the selective monoacylglycerol-lipase enzyme inhibitor JZL184, or the selective transient receptor potential vanilloid type 1 antagonist capsazepine. We evaluated changes on arterial pressure, heart rate, tail skin temperature (ST), and plasma AVP responses induced by bleeding, which started 10 min after PVN treatment. We observed that bilateral microinjection of AM251 into the PVN reduced the hypotension during the hemorrhage and prevented the return of blood pressure to baseline values in the posthemorrhagic period. Inhibition of local 2-arachidonoylglycerol metabolism by PVN treatment with JZL184 induced similar effects in relation to those observed in AM251-treated animals. Inhibition of local anandamide metabolism via PVN treatment with URB597 decreased the depressor effect and ST drop induced by the hemorrhagic stimulus. Bilateral microinjection of capsazepine mitigated the fall in blood pressure and ST. None of the PVN treatments altered the increased plasma concentration of AVP and tachycardia induced by hemorrhage. Taken together, present results suggest that endocannabinoid neurotransmission within the PVN plays a prominent role in cardiovascular and autonomic, but not neuroendocrine, responses evoked by hemorrhage.

A Critical Role for the Paraventricular Nucleus of the Hypothalamus in the Regulation of the Volume Reflex in Normal and Various Cardiovascular Disease States

PURPOSE OF REVIEW

This review focuses on studies implicating forebrain neural pathways and neuromodulator systems, particularly, the nitric oxide system within the paraventricular nucleus of the hypothalamus in regulating neurohumoral drive, autonomic pathways, and fluid balance.

RECENT FINDINGS

Accumulating evidence from animals with experimental models of hypertension and heart failure as well as humans with hypertension suggests that alterations in central neural pathways, particularly, within the PVN neuromodulated by neuronal nitric oxide, are involved in regulating sympathetic outflow particularly to the kidney resulting in alterations in fluid balance commonly observed in hypertension and heart failure states. The characteristics of the hypertensive and heart failure states include alterations in neuronal nitric oxide within the PVN to cause an increase in renal sympathetic nerve activity to result in sodium and fluid retention in these diseases. A comprehensive understanding of these mechanisms will enhance our ability to treat hypertensive and heart failure conditions and their cardiovascular complications more efficiently.

Adrenergic C1 neurons monitor arterial blood pressure and determine the sympathetic response to hemorrhage

Hemorrhage initially triggers a rise in sympathetic nerve activity (SNA) that maintains blood pressure (BP); however, SNA is suppressed following severe blood loss causing hypotension. We hypothesized that adrenergic C1 neurons in the rostral ventrolateral medulla (C1^RVLM) drive the increase in SNA during compensated hemorrhage, and a reduction in C1^RVLM contributes to hypotension during decompensated hemorrhage. Using fiber photometry, we demonstrate that C1^RVLM activity increases during compensated hemorrhage and falls at the onset of decompensated hemorrhage. Using optogenetics combined with direct recordings of SNA, we show that C1^RVLM activation mediates the rise in SNA and contributes to BP stability during compensated hemorrhage, whereas a suppression of C1^RVLM activity is associated with cardiovascular collapse during decompensated hemorrhage. Notably, re-activating C1^RVLM during decompensated hemorrhage restores BP to normal levels. In conclusion, C1 neurons are a nodal point for the sympathetic response to blood loss.

N-Methyl-D-aspartate Glutamate Receptor Modulates Cardiovascular and Neuroendocrine Responses Evoked by Hemorrhagic Shock in Rats

Here, we report the participation of N-methyl-D-aspartate (NMDA) glutamate receptor in the mediation of cardiovascular and circulating vasopressin responses evoked by a hemorrhagic stimulus. In addition, once NMDA receptor activation is a prominent mechanism involved in nitric oxide (NO) synthesis in the brain, we investigated whether control of hemorrhagic shock by NMDA glutamate receptor was followed by changes in NO synthesis in brain supramedullary structures involved in cardiovascular and neuroendocrine control. Thus, we observed that intraperitoneal administration of the selective NMDA glutamate receptor antagonist dizocilpine maleate (MK801, 0.3 mg/kg) delayed and reduced the magnitude of hemorrhage-induced hypotension. Besides, hemorrhage induced a tachycardia response in the posthemorrhage period (i.e., recovery period) in control animals, and systemic treatment with MK801 caused a bradycardia response during hemorrhagic shock. Hemorrhagic stimulus increased plasma vasopressin levels during the recovery period and NMDA receptor antagonism increased concentration of this hormone during both the hemorrhage and postbleeding periods in relation to control animals. Moreover, hemorrhagic shock caused a decrease in NOx levels in the paraventricular nucleus of the hypothalamus (PVN), amygdala, bed nucleus of the stria terminalis (BNST), and ventral periaqueductal gray matter (vPAG). Nevertheless, treatment with MK801 did not affect these effects. Taken together, these results indicate that the NMDA glutamate receptor is involved in the hemorrhagic shock by inhibiting circulating vasopressin release. Our data also suggest a role of the NMDA receptor in tachycardia, but not in the decreased NO synthesis in the brain evoked by hemorrhage.

AT II Receptor Blockade and Renal Denervation: Different Interventions with Comparable Renal Effects?

BACKGROUND

Angiotensin II (Ang II) and the renal sympathetic nervous system exert a strong influence on renal sodium and water excretion. We tested the hypothesis that already low doses of an Ang II inhibitor (candesartan) will result in similar effects on tubular sodium and water reabsorption in congestive heart failure (CHF) as seen after renal denervation (DNX).

METHODS

Measurement of arterial blood pressure, heart rate (HR), renal sympathetic nerve activity (RSNA), glomerular filtration rate (GFR), renal plasma flow (RPF), urine volume, and urinary sodium. To assess neural control of volume homeostasis, 21 days after the induction of CHF via myocardial infarction rats underwent volume expansion (0.9% NaCL; 10% body weight) to decrease RSNA. CHF rat and controls with or without DNX or pretreated with the Ang II type-1 receptor antagonist candesartan (0.5 ug i.v.) were studied.

RESULTS

CHF rats excreted only 68 + 10.2% of the volume load (10% body weight) in 90 min. CHF rats pretreated with candesartan or after DNX excreted from 92 to 103% like controls. Decreases of RSNA induced by volume expansion were impaired in CHF rats but unaffected by candesartan pointing to an intrarenal drug effect. GFR and RPF were not significantly different in controls or CHF.

CONCLUSION

The prominent function of increased RSNA - retaining salt and water - could no longer be observed after renal Ang II receptor blockade in CHF rats.

Somatostatin 2 Receptor Activation in the Rostral Ventrolateral Medulla Does Not Mediate the Decompensatory Phase of Haemorrhage

Decompensation, a critical phase in the response to hemorrhage, is characterized by profound sympathoinhibition and the overriding of baroreflex mediated compensation. As sympathoexcitatory neurons of the rostral ventrolateral medulla (RVLM) maintain vasomotor tone and are essential for sympathetic baroreceptor reflex function, the RVLM is the likely mediator. However, how decompensation occurs is a mystery. Our previous work demonstrated that the inhibitory neuropeptide somatostatin (SST), evokes potent sympathoinhibition. Here we test the hypothesis that, in response to hypovolemia, SST in the RVLM evokes sympathoinhibition, driving decompensation and suppressing baroreflex compensation. We evaluated neuronal activation at sites that contain SST mRNA and project to the RVLM and, in SST2A expressing neurons in the RVLM. We determined the effects on cardiovascular and sympathetic responses to haemorrhage, of bilateral blockade of SST2 receptors in both the RVLM and A1 regions. Haemorrhage in conscious rats evoked c-Fos immunoreactivity in the amygdala, periaqueductal gray, and parabrachial nuclei, regions previously associated with hemorrhage, shown to contain SST and project to the RVLM. Although c-Fos labeling was found throughout the ventrolateral medulla, only a small subset of RVLM SST2A receptor expressing neurons were activated, consistent with the idea that these neurons are inhibited during hemorrhage. However, SST2 receptor antagonists bilaterally injected in the RVLM or the A1 region did not affect the decompensation response to hemorrhage. Thus somatostatin in the RVLM does not mediate decompensation. The physiological role associated with somatostatin-induced sympathoinhibition in the RVLM together with the central mechanisms responsible for decompensation remain elusive.

Typical vasovagal syncope as a "defense mechanism" for the heart by contrasting sympathetic overactivity

Many observations suggest that typical (emotional or orthostatic) vasovagal syncope (VVS) is not a disease, but rather a manifestation of a non-pathological trait. Some authors have hypothesized this type of syncope as a "defense mechanism" for the organism and a few theories have been postulated. Under the human violent conflicts theory, the VVS evolved during the Paleolithic era only in the human lineage. In this evolutionary period, a predominant cause of death was wounding by a sharp object. This theory could explain the occurrence of emotional VVS, but not of the orthostatic one. The clot production theory suggests that the vasovagal reflex is a defense mechanism against hemorrhage in mammals. This theory could explain orthostatic VVS, but not emotional VVS. The brain self-preservation theory is mainly based on the observation that during tilt testing a decrease in cerebral blood flow often precedes the drop in blood pressure and heart rate. The faint causes the body to take on a gravitationally neutral position, and thereby provides a better chance of restoring brain blood supply. However, a decrease in cerebral blood flow has not been demonstrated during negative emotions, which trigger emotional VVS. Under the heart defense theory, the vasovagal reflex seems to be a protective mechanism against sympathetic overactivity and the heart is the most vulnerable organ during this condition. This appears to be the only unifying theory able to explain the occurrence of the vasovagal reflex and its associated selective advantage, during both orthostatic and emotional stress.

Biphasic Renal Sympathetic Response to Hemorrhagic Hypotension in Mice

AIM

The inhibitory responses of renal sympathetic nerve activity (RSNA) and heart rate (HR) to sustained hemorrhagic shock occurred in anesthetized rats, but have not yet been determined in mice. Here, we investigated the responses of RSNA and HR to hemorrhagic hypotension in anesthetized mice, with an emphasis on the molecule-based mechanism for roles of afferent vagal nerves.

METHODS

RSNA, HR, and mean systemic arterial pressure were continuously measured in male pentobarbital-anesthetized C57BL/6N mice. Hemorrhagic hypotension of 50 mmHg was evoked and maintained for 10 min.

RESULTS

During hemorrhagic hypotension, RSNA initially increased and then sustainedly decreased, while HR progressively decreased. Vagotomy eliminated the second-phase sympathoinhibition and bradycardia, and carotid sinus denervation with vagotomy abolished the initial renal sympathoexcitation. The renal sympathoinihibition during hemorrhagic hypotension of 50 mmHg was eliminated in mice pretreated with a transient receptor potential vanilloid type 1 channel (TRPV1) inhibitor, capsazepine, and in TRPV1 knockout (TRPV1) mice, but not in TRPV4 knockout mice. The bradycardia response to hemorrhagic hypotension was also absent in TRPV1 mice and mice pretreated with capsazepine.

CONCLUSION

Hemorrhagic hypotension in anesthetized mice causes biphasic responses of RSNA with an initial increase, followed by a sustained decrease, and a progressive decrease in HR. The initial sympathoexcitation is mediated by carotid sinus baroreceptors, while the later sympathoinhibition and bradycardia are mediated via the TRPV1 signals of vagal afferents.

Altered Differential Control of Sympathetic Outflow Following Sedentary Conditions: Role of Subregional Neuroplasticity in the RVLM

Despite the classically held belief of an “all-or-none” activation of the sympathetic nervous system, differential responses in sympathetic nerve activity (SNA) can occur acutely at varying magnitudes and in opposing directions. Sympathetic nerves also appear to contribute differentially to various disease states including hypertension and heart failure. Previously we have reported that sedentary conditions enhanced responses of splanchnic SNA (SSNA) but not lumbar SNA (LSNA) to activation of the rostral ventrolateral medulla (RVLM) in rats. Bulbospinal RVLM neurons from sedentary rats also exhibit increased dendritic branching in rostral regions of the RVLM. We hypothesized that regionally specific structural neuroplasticity would manifest as enhanced SSNA but not LSNA following activation of the rostral RVLM. To test this hypothesis, groups of physically active (10–12 weeks on running wheels) or sedentary, male Sprague-Dawley rats were instrumented to record mean arterial pressure, LSNA and SSNA under Inactin anesthesia and during microinjections of glutamate (30 nl, 10 mM) into multiple sites within the RVLM. Sedentary conditions enhanced SSNA but not LSNA responses and SSNA responses were enhanced at more central and rostral sites. Results suggest that enhanced SSNA responses in rostral RVLM coincide with enhanced dendritic branching in rostral RVLM observed previously. Identifying structural and functional neuroplasticity in specific populations of RVLM neurons may help identify new treatments for cardiovascular diseases, known to be more prevalent in sedentary individuals.

NMDA and non-NMDA glutamate receptors in the paraventricular nucleus of the hypothalamus modulate different stages of hemorrhage-evoked cardiovascular responses in rats

Here we report the involvement of N-Methyl-d-Aspartate (NMDA) and non-NMDA glutamate receptors from the paraventricular nucleus of the hypothalamus (PVN) in the mediation of cardiovascular changes observed during hemorrhage and post-bleeding periods. In addition, the present study provides further evidence of the involvement of circulating vasopressin and cardiac sympathetic activity in cardiovascular responses to hemorrhage. Systemic treatment with the V1-vasopressin receptor antagonist dTyr(CH2)5(Me)AVP (50 μg/kg, i.v.) increased the latency to the onset of hypotension during hemorrhage and slowed post-bleeding recovery of blood pressure. Systemic treatment with the β1-adrenergic receptor antagonist atenolol (1 mg/kg, i.v.) also increased the latency to the onset of hypotension during hemorrhage. Moreover, atenolol reversed the hemorrhage-induced tachycardia into bradycardia. Bilateral microinjection of the selective NMDA glutamate receptor antagonist LY235959 (2 nmol/100 nL) into the PVN blocked the hypotensive response to hemorrhage and reduced the tachycardia during the post-hemorrhage period. Systemic treatment with dTyr(CH2)5(Me)AVP inhibited the effect of LY235959 on hemorrhage-induced hypotension, without affecting the post-bleeding tachycardia. PVN treatment with the selective non-NMDA receptor antagonist NBQX (2 nmol/100 nL) reduced the recovery of blood pressure to normal levels in the post-bleeding phase and reduced hemorrhage-induced tachycardia. Combined blockade of both NMDA and non-NMDA glutamate receptors in the PVN completely abolished the hypotensive response in the hemorrhage period and reduced the tachycardiac response in the post-hemorrhage period. These results indicate that local PVN glutamate neurotransmission is involved in the neural pathway mediating cardiovascular responses to hemorrhage, via an integrated control involving autonomic nervous system activity and vasopressin release into the circulation.

The hemodynamic response to blood loss in the conscious rat: contributions of cardiac vagal and cardiac spinal signals

The hemodynamic response to progressive blood loss passes through three distinct phases: an initial normotensive compensatory phase, a secondary hypotensive decompensatory phase, and a posthemorrhage recompensatory phase. The role of cardiac vagal and cardiac spinal signals in triggering the different phases of the response to hemorrhage was evaluated in the unanesthetized, freely moving rat by observing the effects on the response to 30% blood loss of prior cardiac vagal deafferentation (bilateral vagal rhizotomy) or prior cardiac spinal deafferentation (bilateral stellate ganglionectomy). In comparison to control animals, it was found that (i) cardiac spinal deafferentation significantly delayed the onset of the decompensatory phase, and (ii) cardiac vagal deafferentation slightly potentiated the decompensatory phase and impaired the recompensatory phase. These results indicate that it is cardiac spinal signals, rather than cardiac vagal signals, which in the conscious rat contribute to the triggering and progression of the decompensatory response to blood loss.

Vasovagal Syncope As A Manifestation Of An Evolutionary Selected Trait

Some observations suggest that typical (emotional or orthostatic) vasovagal syncope (VVS) is not a disease, but rather a manifestation of a non-pathological trait. We conducted an extensive bibliographic research on the vasovagal reactions in animals, including humans, in order to investigate the possible factors that may explain the origin and evolution of VVS. We found two processes which appear relevant for the investigation of VVS evolution: fear/threat bradycardia (alarm bradycardia) in animals, mainly during tonic immobility and vasovagal reflex during hemorrhagic shock (thoracic hypovolemia) both in animals and humans. The available data suggest that VVS in humans, alarm bradycardia in animals and the vasovagal reflex during hemorrhagic shock share the same physiological mechanisms and that is indicative of a common evolutionary root. However, during the vasovagal reflex loss of consciousness occurs in humans, but it is absent (or extremely rare) in animals. That can be explained as a by-product due to the erect position and the large brain evolved in our species. If the vasovagal reflex persisted for millions of years along the vertebrates evolutionary history, we can reasonably assume that it has a function and it is not harmful. It could be neutral or beneficial, but the available data suggest it is beneficial; likely, it evolved as an advantageous response to stressful and possibly dangerous heart conditions. Emotional or orthostatic vasovagal reflex is preceded by enhanced sympathetic activity, which is harmful and possibly dangerous. The transient inhibition of the sympathetic system, together with activation of the vagal tone , characterizes VVS. The consequent slowing of the heart rate induced by the vasovagal reflex may constitute a beneficial break of the cardiac pump, thereby reducing myocardial oxygen consumption. We suggest that typical VVS should be regarded as a selected response, which probably evolved in the ancient past as a defense mechanism of the organism within some ancestral group(s) of vertebrates.

Severe hemorrhage attenuates cardiopulmonary chemoreflex control of regional sympathetic outputs via NTS adenosine receptors

Selective stimulation of inhibitory A1 and facilitatory A2a adenosine receptor subtypes located in the nucleus of the solitary tract (NTS) powerfully inhibits cardiopulmonary chemoreflex (CCR) control of regional sympathetic outputs via different mechanisms: direct inhibition of glutamate release and facilitation of an inhibitory neurotransmitter release, respectively. However, it remains unknown whether adenosine naturally released into the NTS has similar inhibitory effects on the CCR as the exogenous agonists do. Our previous study showed that adenosine is released into the NTS during severe hemorrhage and contributes to reciprocal changes of renal (decreases) and adrenal (increases) sympathetic nerve activity observed in this setting. Both A1 and A2a adenosine receptors are involved. Therefore, we tested the hypothesis that, during severe hemorrhage, CCR control of the two sympathetic outputs is attenuated by adenosine naturally released into the NTS. We compared renal and adrenal sympathoinhibitory responses evoked by right atrial injections of 5HT3 receptor agonist phenylbiguanide (2-8 μg/kg) under control conditions, during hemorrhage, and during hemorrhage preceded by blockade of NTS adenosine receptors with bilateral microinjections of 8-(p-sulfophenyl) theophylline (1 nmol/100 nl) in urethane/chloralose anesthetized rats. CCR-mediated inhibition of renal and adrenal sympathetic activity was significantly attenuated during severe hemorrhage despite reciprocal changes in the baseline activity levels, and this attenuation was removed by bilateral blockade of adenosine receptors in the caudal NTS. This confirmed that adenosine endogenously released into the NTS has a similar modulatory effect on integration of cardiovascular reflexes as stimulation of NTS adenosine receptors with exogenous agonists.

Validation of lower body negative pressure as an experimental model of hemorrhage

Lower body negative pressure (LBNP), a model of hemorrhage (Hem), shifts blood to the legs and elicits central hypovolemia. This study compared responses to LBNP and actual Hem in sedated baboons. Arterial pressure, pulse pressure (PP), central venous pressure (CVP), heart rate, stroke volume (SV), and +dP/dt were measured. Hem steps were 6.25%, 12.5%, 18.75%, and 25% of total estimated blood volume. Shed blood was returned, and 4 wk after Hem, the same animals were subjected to four LBNP levels which elicited equivalent changes in PP and CVP observed during Hem. Blood gases, hematocrit (Hct), hemoglobin (Hb), plasma renin activity (PRA), vasopressin (AVP), epinephrine (EPI), and norepinephrine (NE) were measured at baseline and maximum Hem or LBNP. LBNP levels matched with 6.25%, 12.5%, 18.75%, and 25% hemorrhage were -22 ± 6, -41 ± 7, -54 ± 10, and -71 ± 7 mmHg, respectively (mean ± SD). Hemodynamic responses to Hem and LBNP were similar. SV decreased linearly such that 25% Hem and matching LBNP caused a 50% reduction in SV. Hem caused a decrease in Hct, Hb, and central venous oxygen saturation (ScvO2). In contrast, LBNP increased Hct and Hb, while ScvO2 remained unchanged. Hem caused greater elevations in AVP and NE than LBNP, while PRA, EPI, and other hematologic indexes did not differ between studies. These results indicate that while LBNP does not elicit the same effect on blood cell loss as Hem, LBNP mimics the integrative cardiovascular response to Hem, and validates the use of LBNP as an experimental model of central hypovolemia associated with Hem.

Centrally acting leptin induces a resuscitating effect in haemorrhagic shock in rats

Centrally acting leptin induces the activation of the sympathetic nervous system with a pressor effect in normotensive rats. The purpose of the study was to examine central leptin-evoked action in critical haemorrhagic hypotension. In anaesthetized male Wistar rats subjected for irreversible haemorrhagic shock with mean arterial pressure (MAP) 20-25 mmHg haemodynamic parameters and plasma concentrations of adrenaline and noradrenaline were measured. Leptin given intracerebroventricularly (20 μg) evoked long-lasting rises in MAP and heart rate (HR), with a subsequent increase in renal, mesenteric and hindquarters blood flows and a 100% survival at 2 h. MAP and peripheral blood flow changes were inhibited by a pre-treatment with α(1)- and α(2)-adrenoceptor antagonists prazosin (0.5 mg/kg) and yohimbine (1 mg/kg), while β-adrenoceptor antagonist propranolol (1 mg/kg) blocked leptin-induced HR changes, without influence on MAP, peripheral blood flows and survival. Twenty min after leptin treatment, there were higher plasma concentrations of noradrenaline, but not adrenaline, in comparison with the saline-treated control group. In conclusion, centrally acting leptin induces a long-lasting pressor effect with an improvement in the survival rate in haemorrhage-shocked rats. The effect may be associated with the activation of the sympathetic nervous system.

Impaired blood pressure recovery to hemorrhage in obese Zucker rats with orthopedic trauma

We have shown that obese Zucker rats with orthopedic trauma (OZT) exhibit a loss of arteriolar tone in skeletal muscle. We hypothesize that the loss of arteriolar tone in OZT blunts vasoconstrictor responses to hemorrhage, resulting in an impaired blood pressure recovery. Orthopedic trauma was induced with soft tissue injury and local injection of bone components in both hindlimbs in lean (LZT) and OZT (11-13 wk). One day after the orthopedic trauma, blood pressure responses following hemorrhage were measured in conscious control lean, control obese, LZT, and OZT. In another set of experiments, the spinotrapezius muscle of control and trauma animals was prepared for microcirculatory observation. Arteriolar responses to phenylephrine (PE) or hemorrhage were determined. Hemorrhage resulted in similar blood pressure responses in control animals and LZT, but the blood pressure recovery following hemorrhage was blunted in the OZT. In the spinotrapezius, OZT exhibited decreased arteriolar tone and blunted vasoconstrictor responses to PE and hemorrhage. Treatment with glibenclamide improved the blood pressure recovery in the conscious OZT and improved the arteriolar tone, and PE induced vasoconstriction in the spinotrapezius of the OZT. Thus, ATP-dependent K(+) channel-mediated loss of arteriolar tone in OZT blunts the arteriolar constriction to hemorrhage, resulting in impaired blood pressure recovery.

Differential activation of adrenal, renal, and lumbar sympathetic nerves following stimulation of the rostral ventrolateral medulla of the rat

Under acute and chronic conditions, the sympathetic nervous system can be activated in a differential and even selective manner. Activation of the rostral ventrolateral medulla (RVLM) has been implicated in differential control of sympathetic outputs based on evidence primarily in the cat. Although several studies indicate that differential control of sympathetic outflow occurs in other species, only a few studies have addressed whether the RVLM is capable of producing varying patterns of sympathetic activation in the rat. Therefore, the purpose of the present study was to determine whether activation of the RVLM results in simultaneous and differential increases in preganglionic adrenal (pre-ASNA), renal (RSNA), and lumbar (LSNA) sympathetic nerve activities. In urethane-chloralose anesthetized rats, pre-ASNA, RSNA, and LSNA were recorded simultaneously in all animals. Microinjections of selected concentrations and volumes of glutamate increased pre-ASNA, RSNA, and LSNA concurrently and differentially. Pre-ASNA and RSNA (in most cases) exhibited greater increases compared with LSNA on a percentage basis. By varying the volume or location of the glutamate microinjections, we also identified individual examples of differential and selective activation of these nerves. Decreases in arterial pressure or bilateral blockade of RVLM GABA(A) receptors also revealed differential activation, with the latter having a 3- to 4-fold greater effect on sympathetic activity. Our data provide evidence that activation of the rat RVLM increases renal, lumbar, and preganglionic adrenal sympathetic nerve activities concurrently, differentially, and, in some cases, selectively.

Hypertonic sodium resuscitation after hemorrhage improves hemodynamic function by stimulating cardiac, but not renal, sympathetic nerve activity

Small volume hypertonic saline resuscitation can be beneficial for treating hemorrhagic shock, but the mechanism remains poorly defined. We investigated the effects of hemorrhagic resuscitation with hypertonic saline on cardiac (CSNA) and renal sympathetic nerve activity (RSNA) and the resulting cardiovascular consequences. Studies were performed on conscious sheep instrumented with cardiac ( n = 7) and renal ( n = 6) sympathetic nerve recording electrodes and a pulmonary artery flow probe. Hemorrhage (20 ml/kg over 20 min) caused hypotension and tachycardia followed by bradycardia, reduced cardiac output, and abolition of CSNA and RSNA. Resuscitation with intravenous hypertonic saline (1.2 mol/l at 2 ml/kg) caused rapid, dramatic increases in mean arterial pressure, heart rate, and CSNA, but had no effect on RSNA. In contrast, isotonic saline resuscitation (12 ml/kg) had a much delayed and smaller effect on CSNA, less effect on mean arterial pressure, no effect on heart rate, but stimulated RSNA, although the plasma volume expansion was similar. Intracarotid infusion of hypertonic saline (1 ml/min bilaterally, n = 5) caused similar changes to intravenous administration, indicating a cerebral component to the effects of hypertonic saline. In further experiments, contractility (maximum change in pressure over time), heart rate, and cardiac output increased significantly more with intravenous hypertonic saline (2 ml/kg) than with Gelofusine (6 ml/kg) after hemorrhage; the effects of hypertonic saline were attenuated by the β-receptor antagonist propranolol ( n = 6). These results demonstrate a novel neural mechanism for the effects of hypertonic saline resuscitation, comprising cerebral stimulation of CSNA by sodium chloride to improve cardiac output by increasing cardiac contractility and rate and inhibition of RSNA.

Modulation of heart rate variability during severe hemorrhage at different rates in conscious rats

This study was undertaken to evaluate heart rate (HR) regulation during severe hemorrhage (HEM) at different rates of blood loss. Chronically instrumented male rats underwent HEM at one of three rates: slow (0.5 ml/min/kg; S-HEM), intermediate (1.0 ml/min/kg I-HEM), or 2.0 ml/min/kg (fast; F-HEM) until 30% of the estimated total blood volume (ETBV) was withdrawn. Heart rate variability analysis was performed and the absolute power within the low frequency (LF; 0.16-0.6 Hz) and high frequency (HF; 0.6-3 Hz) ranges was evaluated. During the first 15% of ETBV loss, arterial pressure (AP) was maintained while HR increased. The increase in HR was greatest in the S-HEM and I-HEM groups and was associated with a significant reduction in HF power in the S-HEM group only. As blood loss progressed, AP and HR declined in all treatment groups. The decrease in HR was associated with a significant increase in HF power in the F-HEM and I-HEM groups only. Parasympathetic blockade with atropine methyl bromide eliminated all decreases in HR, independent of the rate of hemorrhage. Blockade of parasympathetic activity also significantly increased the AP at ETBV losses > or =20% independent of the rate of hemorrhage. The effect of atropine on AP was most noticeable in the S-HEM and F-HEM groups. These results demonstrate that rate of blood loss has an important impact on autonomic regulation during severe HEM and support previous findings that neural strategies underlying autonomic control may vary depending on the rate of blood loss.

The origin of vasovagal syncope: to protect the heart or to escape predation?

Major lines of evidence suggest that classical (emotional and orthostatic) vasovagal syncope (VVS) is not a disease, but rather a manifestation of a non-pathological trait. It is, therefore, reasonable to investigate the possible factors that may explain its origin and evolution. We reviewed the data available in the literature on the vasovagal reaction in humans and animals in order to identify possible similarities that might provide insight into the evolution of VVS. We found two processes which appear relevant to the investigation of VVS evolution: fear and threat bradycardia in animals, and the vasovagal reflex during hemorrhagic shock in humans and animals. We suggest that VVS in humans involves physiological mechanisms similar to those found in other vertebrates, and that this may indicate a common evolutionary root. The available data seem to suggest that VVS evolved as an advantageous response to inescapable predators or to stressful and possibly dangerous heart conditions. The inhibition of the sympathetic system, together with activation of the vagal system, characterizes VVS. The consequent slowing of the heart rate induced by VVS may constitute a beneficial break of the cardiac pump, thereby reducing myocardial oxygen consumption. We suggest that classical VVS did not evolve recently in the modern human lineage; rather, it should be regarded as a selected response, which probably evolved in the ancient past as a "defense mechanism" of the organism within some ancestral group(s) of vertebrates.

Centrally administered N-methyl-d-aspartate evokes the adrenal secretion of noradrenaline and adrenaline by brain thromboxane A2-mediated mechanisms in rats

Plasma adrenaline mainly originated from adrenaline-containing cells in the adrenal medulla, while plasma noradrenaline reflects the release from sympathetic nerves in addition to the secretion from noradrenaline-containing cells in the adrenal medulla. The present study was undertaken to characterize the source of plasma catecholamines induced by centrally administered N-methyl-d-aspartate with regard to the brain prostanoid, using urethane-anesthetized rats. Intracerebroventricularly (i.c.v.) administered N-methyl-d-aspartate (1.0, 5.0, 10.0 nmol/animal) dose-dependently elevated plasma levels of noradrenaline and adrenaline. The N-methyl-d-aspartate (5.0 nmol/animal, i.c.v.)-induced elevation of both catecholamines was reduced by dizocilpine maleate (5 nmol/animal, i.c.v.), a non-competitive N-methyl-d-aspartate receptor antagonist. Indomethacin (0.6 and 1.2 micromol/animal, i.c.v.), an inhibitor of cyclooxygenase, dose-dependently reduced the N-methyl-d-aspartate (5.0 nmol/animal, i.c.v.)-induced elevation of both catecholamines. The N-methyl-d-aspartate-induced response was dose-dependently attenuated by furegrelate (0.9 and 1.8 micromol/animal, i.c.v.), an inhibitor of thromboxane A2 synthase. Furthermore, the acute bilateral adrenalectomy abolished the N-methyl-d-aspartate-induced responses, indicating that the source of increase in plasma noradrenaline evoked by N-methyl-d-aspartate is due to secretion from the adrenal gland and not due to release from sympathetic nerve terminals. These results suggest that centrally administered N-methyl-d-aspartate induces the secretion of noradrenaline and adrenaline from adrenal medulla by the brain thromboxane A2-mediated mechanisms in rats.

Hypovolemic shock: critical involvement of a projection from the ventrolateral periaqueductal gray to the caudal midline medulla

Previous research has suggested that the ventrolateral column of the periaqueductal gray (vlPAG) plays a crucial role in triggering a decompensatory response (sympathoinhibition, hypotension, bradycardia) to severe blood loss. vlPAG excitation triggers also quiescence, decreased vigilance and decreased reactivity, the behavioral response which usually accompanies hypovolemic shock. The aim of this study was to identify, in unanesthetized rats, the main descending pathway(s) via which vlPAG neurons trigger sympathoinhibition and bradycardia in response to severe blood loss. Firstly, immediate early gene (c-Fos) expression was used to identify vlPAG neurons selectively activated by severe blood loss. Subsequently, the specific medullary projections of these vlPAG neurons were defined by combined c-Fos, retrograde tracing (double-label) experiments. It was found that vlPAG neurons selectively activated by severe hemorrhage project overwhelmingly to the vasodepressor portion of the caudal midline medulla (CMM). Previous studies indicate that this CMM region mediates behaviorally-coupled cardiovascular adjustments and the findings described here fit with the idea that CMM neurons are uniquely recruited by salient challenges, the adaptive responses to which require more than reflexive homeostatic cardiovascular adjustments.

Predictor of a negative tilt-table test result based on early heart rate variation in patients with a history of syncope

Lack of early sympathetic overreactivity measured by early increase in heart rate has been shown to predict a negative tilt-table (TT) test result. The goal of this study was to evaluate this observation in patients with a history of syncope. The author retrospectively identified 27 patients with a history of syncope undergoing TT testing. The difference between maximal heart rate during the first 10 minutes of the tilting and baseline heart rate was used for early heart rate change. There was a significant association between the probability of a negative TT test result and failure to increase early heart rate over 19 beats per minute. The sensitivity of a negative TT test result was 93.3%, with a specificity of 66.7% (odds ratio, 0.1; confidence interval, 0.14-0.69; P=.001). This study confirms previous findings that failure to increase the heart rate early during a TT test can predict low probability of syncope.

Renal sympathetic nerves modulate erythropoietin plasma levels after transient hemorrhage in rats

In contrast to other sympathetic outflow tracts, renal sympathetic nerve activity (RSNA) decreases in response to hypotensive hemorrhage. The functional significance of this "paradox" is not known. We tested the hypothesis that RSNA modulates renal perfusion and thus erythropoietin (EPO) release after transient hypotensive hemorrhage in anesthetized rats. Plasma EPO was measured before and after 30 min of transient hypotensive hemorrhage (i.e., -40 mmHg from mean baseline blood pressure, followed by reinfusion of shed blood) and 120 min thereafter in sham-denervated rats, and after renal denervation (DNX) or bilateral cervical vagotomy (VX) to abolish/blunt the RSNA decrease mediated by a cardiopulmonary reflex. RSNA, renal Doppler flow, renal vascular resistance (RVR), resistance index, and oxygen delivery/uptake (Do(2)/Vo(2)) were measured. RSNA decreased in intact animals (-40 +/- 5% from baseline, P < 0.05). This was blunted by VX. With intact nerves, EPO level did not increase. In DNX rats, EPO was increased at minute 120 (49 +/- 3 vs. 74 +/- 2 mU/ml; P < 0.05), in VX rats this (47 +/- 2 vs. 62 +/- 4 mU/ml; P < 0.05) was less pronounced. Do(2) in DNX rats was lower compared with intact and VX rats (0.25 +/- 0.04 vs. 0.51 +/- 0.06 and 0.54 +/- 0.05 ml O(2)/min; P < 0.05) due to lower Doppler flow and increased RVR. RVR and Do(2) were similar in intact and VX rats, but resistance index differed between all groups (0.70 +/- 0.02 vs. 0.78 +/- 0.02 vs. 0.85 +/- 0.02; P < 0.05, intact vs. VX vs. DNX), indicating differential reactivity of renal vasculature. Vo(2) was unaffected by VX and DNX. Renal sympathoinhibition during hypotensive hemorrhage might help to preserve sufficient oxygenation of renal tissue by modulation of hemodynamic mechanisms that act to adapt renal oxygen availability to demand.

Intravenous 6-hydroxydopamine attenuates vasopressin and oxytocin secretion stimulated by hemorrhage and hypotension but not hyperosmolality in rats

The present study sought to determine whether chemical destruction of peripheral catecholaminergic fibers with 6-hydroxydopamine (6OHDA) attenuates vasopressin (VP) and oxytocin (OT) secretion stimulated by hemorrhage, hypotension, and hyperosmolality. Rats received 6OHDA (100 mg/kg iv) or vehicle (1 ml/kg iv) on days 1 and 7, and experiments were performed on day 8. Serial hemorrhage (4 samples of 2 ml per 300 g body wt at 10-min intervals) increased plasma VP and OT levels in both groups; however, the increase in plasma VP and OT levels was significantly attenuated in 6OHDA-treated vs. control rats despite a significantly lower mean arterial blood pressure. Similarly, the increase in plasma VP and OT levels in response to hypotension produced by the selective arteriolar vasodilator diazoxide was significantly attenuated in 6OHDA-treated rats. In marked contrast to hemorrhage and hypotension, hyperosmolality produced by an infusion of 1 M NaCl (2 ml/h iv) stimulated increases in plasma VP and OT levels that were not different between 6OHDA-treated and control rats. In a parallel set of experiments, intravenous 6OHDA treatment reduced dopamine--hydroxylase immunoreactivity in the posterior pituitary but had no substantial effect in the hypothalamic paraventricular and supraoptic nuclei. In each experiment, the pressor response to tyramine (250 microg/kg iv) was significantly attenuated in 6OHDA-treated rats, thereby confirming that 6OHDA treatment destroyed sympathetic catecholaminergic fibers. Collectively, these findings suggest that catecholaminergic fibers located outside the blood-brain barrier contribute to VP and OT secretion during hemorrhage and arterial hypotension.

Adenosine receptors located in the NTS contribute to renal sympathoinhibition during hypotensive phase of severe hemorrhage in anesthetized rats

Stimulation of nucleus of the solitary tract (NTS) A(2a)-adenosine receptors elicits cardiovascular responses quite similar to those observed with rapid, severe hemorrhage, including bradycardia, hypotension, and inhibition of renal but activation of preganglionic adrenal sympathetic nerve activity (RSNA and pre-ASNA, respectively). Because adenosine levels in the central nervous system increase during severe hemorrhage, we investigated to what extent these responses to hemorrhage may be due to activation of NTS adenosine receptors. In urethane- and alpha-chloralose-anesthetized male Sprague-Dawley rats, rapid hemorrhage was performed before and after bilateral nonselective or selective blockade of NTS adenosine-receptor subtypes [A(1)- and A(2a)-adenosine-receptor antagonist 8-(p-sulfophenyl)theophylline (1 nmol/100 nl) and A(2a)-receptor antagonist ZM-241385 (40 pmol/100 nl)]. The nonselective blockade reversed the response in RSNA (-21.0 +/- 9.6 Delta% vs. +7.3 +/- 5.7 Delta%) (where Delta% is averaged percent change from baseline) and attenuated the average heart rate response (change of -14.8 +/- 4.8 vs. -4.4 +/- 3.4 beats/min). The selective blockade attenuated the RSNA response (-30.4 +/- 5.2 Delta% vs. -11.1 +/- 7.7 Delta%) and tended to attenuate heart rate response (change of -27.5 +/- 5.3 vs. -15.8 +/- 8.2 beats/min). Microinjection of vehicle (100 nl) had no significant effect on the responses. The hemorrhage-induced increases in pre-ASNA remained unchanged with either adenosine-receptor antagonist. We conclude that adenosine operating in the NTS via A(2a) and possibly A(1) receptors may contribute to posthemorrhagic sympathoinhibition of RSNA but not to the sympathoactivation of pre-ASNA. The differential effects of NTS adenosine receptors on RSNA vs. pre-ASNA responses to hemorrhage supports the hypothesis that these receptors are differentially located/expressed on NTS neurons/synaptic terminals controlling different sympathetic outputs.

Vasopressin V1 receptors contribute to hemodynamic and sympathoinhibitory responses evoked by stimulation of adenosine A2a receptors in NTS

Activation of adenosine A2a receptors in the nucleus of the solitary tract (NTS) decreases mean arterial pressure (MAP), heart rate (HR), and renal sympathetic nerve activity (RSNA), whereas increases in preganglionic adrenal sympathetic nerve activity (pre-ASNA) occur, a pattern similar to that observed during hypotensive hemorrhage. Central vasopressin V1 receptors may contribute to posthemorrhagic hypotension and bradycardia. Both V1 and A2a receptors are densely expressed in the NTS, and both of these receptors are involved in cardiovascular control; thus they may interact. The responses elicited by NTS A2a receptors are mediated mostly via nonglutamatergic mechanisms, possibly via release of vasopressin. Therefore, we investigated whether blockade of NTS V1 receptors alters the autonomic response patterns evoked by stimulation of NTS A2a receptors (CGS-21680, 20 pmol/50 nl) in α-chloralose-urethane anesthetized male Sprague-Dawley rats. In addition, we compared the regional sympathetic responses to microinjections of vasopressin (0.1–100 ng/50 nl) into the NTS. Blockade of V1 receptors reversed the normal decreases in MAP into increases (−95.6 ± 28.3 vs. 51.4 ± 15.7 ∫Δ%), virtually abolished the decreases in HR (−258.3 ± 54.0 vs. 18.9 ± 57.8 ∫Δbeats/min) and RSNA (−239.3 ± 47.4 vs. 15.9 ± 36.1 ∫Δ%), and did not affect the increases in pre-ASNA (279.7 ± 48.3 vs. 233.1 ± 54.1 ∫Δ%) evoked by A2a receptor stimulation. The responses partially returned toward normal values ∼90 min after the blockade. Microinjections of vasopressin into the NTS evoked dose-dependent decreases in HR and RSNA and variable MAP and pre-ASNA responses with a tendency toward increases. We conclude that the decreases in MAP, HR, and RSNA in response to NTS A2a receptor stimulation may be mediated via release of vasopressin from neural terminals in the NTS. The differential effects of NTS V1 and A2a receptors on RSNA versus pre-ASNA support the hypothesis that these receptor subtypes are differentially located/expressed on NTS neurons/neural terminals controlling different sympathetic outputs.

Influence of short-term versus prolonged cardiopulmonary receptor stimulation on renal and preganglionic adrenal sympathetic nerve activity in rats

Renal and preganglionic adrenal sympathetic nerve activities (RSNA, ASNA) are regulated differentially. Various cardiopulmonary receptor (CPR) stimulation procedures were performed to distinguish short-term and prolonged as well as mechanical and chemical stimulatory effects on RSNA and ASNA. In anesthetized male Sprague-Dawley rats blood pressure, heart rate, left ventricular end-diastolic pressure (LVEDP), RSNA and ASNA were recorded. CPRs were stimulated as follows: Short-term mechanical: LVEDP changes (+/-4, +/-6, +/-8 mmHg) via aortic and caval vein occlusion; Short-term chemical: phenylbiguanide (PBG-bolus, 0.1, 1, 10 microg IV); Prolonged mechanical (15 min): volume expansion (0.9% NaCl, 5% body weight) and hemorrhage, to modulate LVEDP; Prolonged chemical: PBG infusion (32 microg/min IV, for 15 min); Stimulations were done with 1) all afferents intact, 2) bilateral cervical vagotomy (VX), 3) VX + SAD (sino-aortic denervation; short-term protocols and hemorrhage).1) Short-term mechanical stimuli decreased RSNA (-52 +/- 12%) and ASNA (-37 +/- 13%). 2) PBG-bolus decreased RSNA (-54 +/- 12%) but increased ASNA (+40 +/- 13%). 3) Volume expansion decreased RSNA (-55 +/- 7%), ASNA was unaffected. 4) PBG infusion persistently decreased RSNA (-60 +/- 6%) but just shortly increased ASNA (+120 +/- 15%); VX abolished all responses. 5) Hypotensive hemorrhage decreased RSNA (-39 +/- 9%) but increased ASNA (+42 +/- 9%). VX abolished RSNA response; ASNA response only disappeared with VX + SAD.Short-term mechanical CPR stimulation uniformly decreased sympathetic activities, whereas chemical stimulation had opposing effects on renal and adrenal sympathetic responses. All prolonged stimuli decreased RSNA, whereas ASNA was virtually unaffected: Sympathetic out.ow is differentially controlled not only with regard to target organs or afferent receptors but also stimulus time pattern.

Evidence of differential control of renal and lumbar sympathetic nerve activity in conscious rabbits

We have explored the possibility that renal sympathetic nerve activity (RSNA) and vasomotor sympathetic nerve activity are differentially regulated. We measured sympathetic nerve activity (SNA) to the kidney and the hind limb vasculature in seven conscious rabbits 6-8 days after the implantation of recording electrodes. Acute infusion of N(G)-nitro-L-arginine methyl ester (L-NAME) (6 mg.kg(-1).min(-1) for 5 min) led to an increase in blood pressure (from 66 +/- 1 to 82 +/- 3 mmHg) and a decrease in heart rate (from 214 +/- 15 to 160 +/- 13 bpm). L-NAME administration caused a significantly greater decrease in RSNA than lumbar sympathetic nerve activity (LSNA) (to 68 +/- 14% vs. 84 +/- 4% of control values, respectively). Volume expansion (1.5 ml.kg(-1).min(-1)) resulted in a significant decrease in RSNA to 66 +/- 7% of control levels but no change in LSNA (127 +/- 20%). There was no difference in the gain of the baroreflex curves between the LSNA and RSNA [maximum gain of -7.6 +/- 0.4 normalized units (nu)/mmHg for LSNA vs. -7.9 +/- 0.75 nu/mmHg for RSNA]. A hypoxic stimulus (10% O2 and 3% CO2) led to identical increases in both RSNA and LSNA (195 +/- 40% and 158 +/- 21% of control values, respectively). Our results indicate tailored differential control of RSNA and LSNA in response to acute stimuli.

Neuronal activity within the ventrolateral periaqueductal gray during simulated hemorrhage in conscious rabbits

The ventrolateral (vl) periaqueductal gray (PAG) has been proposed as a site responsible for the active process triggering the onset of hypotension (i.e., phase 2) during blood loss in conscious animals (Cavun S and Millington WR. Am J Physiol Regul Integr Comp Physiol 281: R747-R752, 2001). We recorded the extracellular activity of PAG neurons in conscious rabbits to test the hypothesis that vlPAG neurons change their firing frequency before the onset of hypotension during simulated hemorrhage. Arterial and venous catheters, an intrathoracic vena caval occluder, and midbrain microelectrodes on a microdrive were implanted in 10 rabbits. During simulated hemorrhage, the occluder was inflated until arterial pressure < or = 40 mmHg. We compared changes in neuronal activity during simulated hemorrhage with those during a similar length control period for 64 vlPAG and 29 dorsolateral (dl) PAG neurons. Arterial pressure pulse modulation of neuronal activity was present in 45 and 76% of vlPAG and dlPAG neurons, respectively. When we evaluated the absolute change in activity, thus accounting for both increases and decreases, simulated hemorrhage had a significant effect on activity of vlPAG but not dlPAG neurons. The majority (56%) of vlPAG neurons did not appear to respond to simulated hemorrhage. Of the 28 responsive vlPAG neurons, 11 showed an abrupt change in firing frequency during the time interval preceding the onset of hypotension; 13 responded after the onset of hypotension; and 4 showed a consistent direction of change across the entire simulated hemorrhage. Thus 24 (38%) of the vlPAG neurons recorded responded at a time consistent with a contribution to the hypotension associated with simulated hemorrhage.

Purinergic mechanisms of the nucleus of the solitary tract and neural cardiovascular control

OBJECTIVES

This review addresses the role of central purinergic receptors in the operation of the cardiovascular reflexes.

METHODS

Potential physiological role of purinergic receptors operating in the nucleus of the solitary tract (NTS) was assessed via comparison of the regional patterns of hemodynamic and sympathetic responses evoked by selective stimulation/inhibition of NTS purinergic receptor subtypes, with the patterns evoked by stimulation and unloading of arterial baroreceptors, and other known patterns of autonomic responses. The effects of sino-aortic denervation plus vagotomy and ionotropic glutamatergic blockade of NTS mechanisms on the patterns of the responses were also considered.

RESULTS

Selective stimulation of NTS A1 receptors with CPA evoked a pattern of regional autonomic responses consistent with inhibition of baroreflex mechanisms and facilitation/ disinhibition of chemoreflex mechanisms. Selective stimulation of NTS A(2a) receptors with CGS 21680-evoked pattern of the responses different than that evoked by stimulation of baroreflex afferents what remains in contrast to previous reports suggesting that NTS A2a receptors facilitate baroreflex transmission. The pattern of the responses was similar to that observed during hypotensive hemorrhage. Preferential, b -adrenergic iliac vasodilation evoked by stimulation of adenosine A2a receptors and preferential activation of sympathetic output to the adrenal medulla by both adenosine A1 and A2a receptors are consistent with contribution of these receptors to the defense response, stress and exercise. These observations support previous findings that NTS A1 receptors contribute to the hypothalamic defense response. The effects of stimulation and blockade of NTS P2x receptors with alpha, beta-methylene ATP and suramin, respectively, suggested that neuronally-released ATP operating via P2x receptors may be a crucial co-transmitter with glutamate in mediating baroreflex responses.

DISCUSSION

The above observations strongly suggest that purinergic receptor subtypes operating in NTS circuitry are linked to specific afferent and descending mechanisms primarily integrated in the NTS.

Blockade of brain mineralocorticoid receptors or Na+ channels prevents sympathetic hyperactivity and improves cardiac function in rats post-MI

In rats post-myocardial infarction (MI), sympathetic hyperactivity can be prevented by blockade of brain mineralocorticoid receptors (MR). Stimulatory responses to central infusion of aldosterone can be blocked by benzamil and therefore appear to be mediated via Na+ channels, presumably epithelial Na+ channels (ENaC), in the brain. To evaluate this concept of endogenous mineralocorticoids in Wistar rats post-MI, we examined effects of blockade of MR and Na+ channels in the brain. At 3 days after coronary artery ligation, intracerebroventricular infusions were started with spironolactone (400 ng·kg−1·h−1) or its vehicle, or with benzamil (4 μg·kg−1·h−1) or its vehicle, using osmotic minipumps. Rats with sham ligation served as control. After 4 wk, in conscious rats, mean arterial pressure, heart rate, and renal sympathetic nerve activity were recorded at rest and in response to air-jet stress, intracerebroventricular injection of the α2-adrenoceptor agonist guanabenz, and intravenous infusion of phenylephrine and nitroprusside for baroreflex function. MI size was similar among the four groups of rats (∼31%). In rats treated post-MI with vehicles, cardiac function was decreased, sympathetic reactivity was enhanced, and baroreflex function was impaired. Blockade of brain Na+ channels or brain MR similarly prevented sympathetic hyperactivity and impairment of baroreflex function and improved cardiac function. These findings suggest that in rats post-MI, increased binding of endogenous agonists to MR increases ENaC activity in the brain and thereby leads to sympathetic hyperactivity and progressive left ventricular dysfunction.

Haemorrhage-evoked decompensation and recompensation mediated by distinct projections from rostral and caudal midline medulla in the rat

The haemodynamic response to blood loss consists of three phases: (i) an initial compensatory phase during which resting arterial pressure is maintained; (ii) a decompensatory phase characterized by a sudden, life-threatening hypotension and bradycardia; and (iii) if blood loss ceases, a recompensatory phase during which arterial pressure returns to normal. Previous research indicates that topographically distinct, rostral and caudal parts of the caudal midline medulla (CMM) contain neurons that differentially regulate the timing and magnitude of each of the three phases. Specifically, decompensation depends critically on the integrity of the rostral CMM; whereas compensation and recompensation depend upon the integrity of the caudal CMM. This study aimed to determine, using retrograde and anterograde tracing techniques, if the rostral and caudal CMM gave rise to different sets of projections to the major cardiovascular region of the ventrolateral medulla (VLM) and spinal cord. It was found that rostral and caudal CMM each have projections of varying density to the region containing bulbospinal (presympathetic) motor neurons in the rostral VLM and preganglionic sympathetic motor neurons in the intermediolateral cell column of the spinal cord. Via these projections vasomotor tone and hence arterial pressure can be regulated. More strikingly: (i) consistent with a role in mediating bradycardia during decompensation, the rostral CMM projects uniquely to VLM regions containing vagal cardiac motor neurons; and (ii) consistent with its role in mediating recompensation, the caudal CMM projects uniquely onto tyrosine hydroxylase-containing, caudal VLM (A1) neurons whose activity mediates vasopressin release, on which recompensation depends.

Evidence that venoconstriction reverses the phase II sympathoinhibitory and bradycardic response to haemorrhage

Severe hypotensive haemorrhage results in a biphasic response, characterized by an initial increase in heart rate and sympathetic vasomotor activity (phase I) followed by a life-threatening hypotension, accompanied by profound sympathoinhibition and bradycardia (phase II). The phase II response is believed to be dependent on inputs from cardiopulmonary receptors, and may be triggered by the reduction in venous return and cardiac filling associated with severe haemorrhage. In this study, we tested the hypothesis that the phase II response could be reversed by venoconstriction, which is known to enhance venous return and cardiac filling, by comparing the effects of phenylephrine (which constricts veins as well as arterioles) with that of vasopressin (which constricts arterioles but not veins). In sodium pentobarbitone-anaesthetised rats, haemorrhage evoked an initial increase in heart rate (HR) and renal sympathetic activity (RSNA) followed by a large decrease in both variables to levels below the pre-haemorrhage baseline levels (phase II response). During the phase II response, an intravenous injection of phenylephrine, sufficient to restore mean arterial pressure to the pre-haemorrhage level, resulted in a gradually developing increase (over 3-4 min) in HR and RSNA back to the baseline levels. In contrast, intravenous injection of an equipressor dose of vasopressin did not result in any increase in RSNA and only a transient increase in HR. Injection of phenylephrine, but not vasopressin, also increased the pulsatile component of central venous pressure, indicative of reduced venous capacitance. The findings indicate that venoconstriction reverses the phase II sympathoinhibition and bradycardia.

Hemorrhagic sympathoinhibition mediated through the periaqueductal gray in the rat

In response to hemorrhage in the anesthetized rat, an initial renal sympathoexcitation is followed by profound sympathoinhibition and hypotension with increasing blood loss. Microinjection of the gamma-aminobutyric acid(A) agonist muscimol to block neurotransmission through the sympathoinhibitory region of the ventrolateral periaqueductal gray matter (vlPAG) did not alter resting sympathetic nerve activity or blood pressure. However, the response to hemorrhage was converted to a maintained renal sympathoexcitation with a delayed and attenuated accompanying hypotension. These data indicate that neurons in the vlPAG mediate the sympathetic and cardiovascular responses to severe hemorrhage.

Haemodynamic response to haemorrhage: distinct contributions of midbrain and forebrain structures

The haemodynamic response to a fixed volume haemorrhage passes through three distinct phases: a normotensive, compensatory phase; a hypotensive, decompensatory phase; and a post-haemorrhage, recompensatory phase. The role of the forebrain and midbrain in regulating the triphasic response to a 'fast' (1.5%/min) or 'slow' (0.75%/min) rate of blood withdrawal (30% haemorrhage) was evaluated by comparing, in unanaesthetised rats, the effects of pre-collicular (PCD) vs. pre-trigeminal decerebrations (PTD). It was found that pre-trigeminal decerebration attenuated the decompensatory (hypotensive) phase to either a fast or slow haemorrhage. In contrast, pre-collicular decerebration attenuated the compensatory and recompensatory phases of the response to a 'fast' (but not a slow) haemorrhage. These results suggest that the integrity of (i) forebrain structure(s) are critical for compensatory and recompensatory responses to 'rapid' blood loss; and (ii) midbrain structure(s) are critical for the decompensatory response to progressive blood loss irrespective of rate.

Role of the paraventricular nucleus in renal excretory responses to acute volume expansion: role of nitric oxide

Acute volume expansion (VE) produces a suppression of renal sympathetic nerve discharge (RSND) resulting in diuresis and natriuresis. Recently, we have demonstrated that the endogenous nitric oxide (NO) system within the paraventricular nucleus (PVN) produces a decrease in RSND. We hypothesized that endogenous NO in the PVN is involved in the suppression of RSND leading to diuretic and natriuretic responses to acute VE. To test this hypothesis, we first measured the VE-induced increase in renal sodium excretion and urine flow with and without blockade of NO, with microinjection of NG-monomethyl-L-arginine (L-NMMA; 200 pmol in 200 nl), within the PVN of Inactin-anesthetized male Sprague-Dawley rats. Acute VE produced significant increases in urine flow and sodium excretion, which were diminished in rats treated with L-NMMA within the PVN. This effect of NO blockade within the PVN on VE-induced diuresis and natriuresis was abolished by renal denervation. Consistent with these data, acute VE induced a decrease in RSND (52% of the baseline level), which was significantly blunted by prior administration of L-NMMA into the PVN (28% of the baseline level) induced by a comparable level of acute VE. Using the push-pull perfusion technique, we found that acute VE induced a significant increase in NOx concentration in the perfusate from the PVN region. Taken together, these results suggest that acute VE induces an increase in NO production within the PVN that leads to renal sympathoinhibition, resulting in diuresis and natriuresis. We conclude that NO within the PVN plays an important role in regulation of sodium and water excretions in the volume reflex via modulating renal sympathetic outflow.

Haemorrhage-evoked compensation and decompensation are mediated by distinct caudal midline medullary regions in the urethane-anaesthetised rat

Previous research using microinjections of excitatory amino acids suggested that the caudal midline medulla (including nucleus raphe obscurus and nucleus raphe pallidus) contained a mixed population of sympathoexcitatory and sympathoinhibitory neurones. The results of this study indicate that different anaesthetic regimes (urethane versus halothane) determine whether sympathoexcitatory (urethane only) or sympathoinhibitory (halothane only) responses are evoked by stimulation within distinct caudal midline medullary regions. In addition, anaesthetic regimes also affect the caudal midline medullary-mediated response to haemorrhage. Specifically, under conditions of urethane anaesthesia, inactivation (lignocaine) of the midline medullary region immediately caudal to the obex, prematurely triggered and dramatically potentiated the hypotension and bradycardia evoked by 15% haemorrhage; whereas under halothane anaesthesia, inactivation of the same region had no effect. In contrast, under urethane anaesthesia, inactivation of the midline medullary region immediately rostral to the obex, delayed the onset of the hypotension and bradycardia to 15% haemorrhage; inactivation of the same region under halothane anaesthesia blocked haemorrhage-evoked hypotension and bradycardia. Our findings indicate that topographically distinct parts of the caudal midline medulla contain neurones (i) that differentially regulate the timing and magnitude of the compensatory (normotensive) versus decompensatory (hypotensive) phases of the response to haemorrhage; and (ii) whose activity is altered by urethane versus halothane anaesthesia.

Mechanisms mediating regional sympathoactivatory responses to stimulation of NTS A(1) adenosine receptors

Selective activation of adenosine A(1) and A(2a) receptors in the subpostremal nucleus tractus solitarius (NTS) increases and decreases mean arterial pressure (MAP), respectively, and decreases heart rate (HR). We have previously shown that the decreases in MAP evoked by NTS A(2a) receptor stimulation were accompanied with differential sympathetic responses in renal (RSNA), lumbar (LSNA), and preganglionic adrenal sympathetic nerve activity (pre-ASNA). Therefore, now we investigated whether stimulation of NTS A(1) receptors via unilateral microinjection of N(6)-cyclopentyladenosine (CPA) elicits differential activation of the same sympathetic outputs in alpha-chloralose-urethane-anesthetized male Sprague-Dawley rats. CPA (0.33-330.0 pmol in 50 nl) evoked dose-dependent increases in MAP, variable decreases in HR, and differential increases in all recorded sympathetic outputs: upward arrow pre-ASNA >> upward arrow RSNA > or = upward arrow LSNA. Sinoaortic denervation + vagotomy abolished the MAP and LSNA responses, reversed the normal increases in RSNA into decreases, and significantly attenuated increases in pre-ASNA. NTS ionotropic glutamatergic receptor blockade with kynurenate sodium (4.4 nmol/100 nl) reversed the responses in MAP, LSNA, and RSNA and attenuated the responses in pre-ASNA. We conclude that afferent inputs and intact glutamatergic transmission in the NTS are necessary to mediate the pressor and differential sympathoactivatory responses to stimulation of NTS A(1) receptors.

Decompensated hemorrhage activates serotonergic neurons in the subependymal parapyramidal region of the rat medulla

According to prior evidence opioid and serotonin release by lower brain stem neurons may contribute to hemorrhage-induced sympathoinhibition (HISI). Here we seek direct evidence for the activation of opioidergic, GABAergic, or serotonergic neurons by severe hemorrhage in the medulla oblongata. Blood was withdrawn from awake rats (40-50% total volume) causing hypotension and profound initial bradycardia. Other rats received the vasodilator hydralazine, causing tachycardia and hypotension. Neuronal activation was gauged by the presence of Fos-immunoreactive (ir) nuclei after 2 h. Serotonergic, enkephalinergic, and GABAergic neurons were identified by the presence of a diagnostic enzyme or mRNA. Hemorrhaged rats had 30% fewer non-GABAergic Fos-ir neurons in the rostral ventrolateral medulla (RVLM) than hydralazine-treated rats, but they had six times more Fos-ir neurons within the subependymal parapyramidal nucleus (SEPPN). Fos-labeled SEPPN neurons were serotonergic (40-60%), GABAergic (31%), enkephalinergic (15%), or had mixed phenotypes. The data suggest that a reduced sympathoexcitatory drive from RVLM may contribute to HISI. SEPPN neuronal activation may also contribute to HISI or could mediate defensive thermoregulatory mechanisms triggered by hemorrhage-induced hypothermia.

Nitric oxide-independent effects of tempol on sympathetic nerve activity and blood pressure in DOCA-salt rats

The role of sympathetic nerves and nitric oxide (NO) in tempol-induced cardiovascular responses was evaluated in urethane-anesthetized sham and deoxycorticosterone acetate (DOCA)-salt-treated (DOCA-salt) rats. Tempol (30-300 micromol/kg iv), a superoxide (O) scavenger, decreased renal sympathetic nerve activity (RSNA), mean arterial pressure (MAP), and heart rate (HR) in DOCA-salt and sham rats. The antioxidants tiron and ascorbate did not alter MAP, HR, or RSNA in any rat. Tempol responses were unaffected after sham rats were treated with N(G)-nitro-L-arginine (L-NNA, 13 mg/kg). In DOCA-salt rats, L-NNA reduced tempol-induced depressor responses but not the inhibition of HR or RSNA. Tempol did not significantly decrease MAP, HR, or RSNA after hexamethonium (30 mg/kg iv) treatment in any rat. Dihydroethidine histochemistry revealed increased O levels in arteries and veins from DOCA-salt rats. Tempol treatment in vitro reduced O levels in arteries and veins from DOCA-salt rats. In conclusion, tempol-induced depressor responses are mediated largely by NO-independent sympathoinhibition in sham and DOCA-salt rats. There is an additional interaction between NO and tempol that contributes to depressor responses in DOCA-salt rats.

Altered frequency responses of sympathetic nerve discharge bursts after IL-1beta and mild hypothermia

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.

Renal sympathoinhibition mediated by 5-HT(1A) receptors in the RVLM during severe hemorrhage in rats

The role of 5-hydroxytryptamine type 1A (5-HT(1A)) receptors in the rostral ventrolateral medulla (RVLM) in the mediation of the sympathoinhibitory and hypotensive responses to severe hemorrhage was examined in pentobarbital sodium-anesthetized rats. The control response to hemorrhage (1 ml/min to 50 mmHg) consisted of a fall in arterial blood pressure and an initial baroreflex increase in renal sympathetic nerve activity followed after 2 min by a rapid decline in blood pressure accompanied by a decrease in renal sympathetic nerve activity. In response to hemorrhage in animals in which the specific 5-HT(1A) receptor antagonist WAY-100635 was microinjected into the pressor area of the RVLM, the fall in blood pressure was delayed and attenuated while renal sympathetic nerve activity was increased and maintained above baseline. In barodenervated animals with blockade of RVLM 5-HT(1A) receptors, there was no change in renal sympathetic nerve activity in response to hemorrhage. These data suggest that renal sympathoinhibition elicited in response to severe hemorrhage is mediated by 5-HT(1A) receptors in the RVLM.

Paraventricular nucleus bicuculline alters frequency components of sympathetic nerve discharge bursts

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.

Differential control of sympathetic outflow

With advances in experimental techniques, the early views of the sympathetic nervous system as a monolithic effector activated globally in situations requiring a rapid and aggressive response to life-threatening danger have been eclipsed by an organizational model featuring an extensive array of functionally specific output channels that can be simultaneously activated or inhibited in combinations that result in the patterns of autonomic activity supporting behavior and mediating homeostatic reflexes. With this perspective, the defense response is but one of the many activational states of the central autonomic network. This review summarizes evidence for the existence of tissue-specific sympathetic output pathways, which are likely to include distinct populations of premotor neurons whose target specificity could be assessed using the functional fingerprints developed from characterizations of postganglionic efferents to known targets. The differential responses in sympathetic outflows to stimulation of reflex inputs suggest that the circuits regulating the activity of sympathetic premotor neurons must have parallel access to groups of premotor neurons controlling different functions but that these connections vary in their ability to influence different sympathetic outputs. Understanding the structural and physiological substrates antecedent to premotor neurons that mediate the differential control of sympathetic outflows, including those to noncardiovascular targets, represents a challenge to our current technical and analytic approaches.

Nitric oxide-independent effects of tempol on sympathetic nerve activity and blood pressure in normotensive rats

The role of the sympathetic nervous system in 4-hydroxy-2,2,6,6-tetramethyl piperidinoxyl (tempol)-induced cardiovascular responses in urethane-anesthetized, normotensive rats was evaluated. Tempol caused dose-dependent (30-300 micromol/kg iv) decreases in renal sympathetic nerve activity (RSNA), mean arterial blood pressure (MAP), and heart rate (HR). Similar responses were obtained after sinoaortic denervation and cervical vagotomy. These responses were not blocked following treatment with the nitric oxide synthase inhibitor NG-nitro-L-arginine (2.6 mg x kg(-1) x min(-1) iv for 5 min) or the alpha2-adrenergic receptor antagonist idazoxan (0.3 mg/kg iv bolus). Idazoxan blocked the effects of clonidine (10 miccrog/kg iv) on HR, MAP, and RSNA. Hexamethonium (30 mg/kg iv) inhibited RSNA, and tempol did not decrease RSNA after hexamethonium. The effects of tempol on HR and MAP were reduced by hexamethonium. In conclusion, depressor responses caused by tempol are mediated, partly, by sympathoinhibition in urethane-anesthetized, normotensive rats. Nitric oxide does not contribute to this response, and the sympathoinhibitory effect of tempol is not mediated via alpha2-adrenergic receptors. Finally, tempol directly decreases HR, which may contribute to the MAP decrease.