Vasopressin receptors in the area postrema differentially modulate baroreceptor responses in rats

Role of vasopressin V1 antagonist in the action of vasopressin on the cooling-evoked hemodynamic perturbations of rats

We aimed to investigate the role of arginine vasopressin (AVP) acting via the AVPV1 receptor in the autonomic cardiovascular responses to cold stress (CS). The study was conducted on adult male Sprague-Dawley rats with telemetry transmitters implanted to monitor heart rate (HR) and systolic blood pressure (SBP) throughout the experiment course. Rats were divided into four groups and were given, respectively, saline (control group), AVPV1 antagonist (V1880) alone, and V1880 following the removal of sympathetic outflows using hexamethonium (HEX+V1880) or guanethidine (GUA + V1880). Rats were subjected to the CS stimuli (rapid immersion of the rat's limbs into 4 °C water). Hemodynamic responses were recorded at baseline (PreCS), during CS, and after CS. Data analysis was performed using descriptive methods and spectral and cross-spectral analysis of blood pressure variability (BPV) and heart rate variability (HRV). Key results showed that at PreCS, inhibition of AVPV1 increases SBP and HR as well as very-low-frequency BPV and low-frequency BPV, which is attenuated by hexamethonium (effect on SBP only) and guanethidine (effect on both SBP and HR). HEX+V1880 results in increased high-frequency BPV and attenuated very-low-frequency HRV, while GUA + V1880 results in increased high-frequency HRV and attenuated very-low-frequency HRV. During CS, we observed that SBP and HR, as well as very-low-frequency BPV and low-frequency BPV, were similar in the control group and the group with AVPV1 inhibition, while AVPV1 inhibition results in attenuated high-frequency BPV. Furthermore, we observed that changes produced by AVPV1 inhibition alone were affected differently by HEX+V1880 and GUA + V1880, particularly in low-frequency HRV and very-low-frequency HRV. The results support that AVPV1 mediates autonomic cardiovascular responses at both baseline and CS stimuli conditions are associated with central mechanism engagement.

Effects of a single terlipressin administration on cardiac function and perfusion in cirrhosis

BACKGROUND

The vasoconstrictor terlipressin is widely used in the treatment of the hepatorenal syndrome and variceal bleeding. However, terlipressin may compromise cardiac function and induce ischemia.

AIM

Therefore, we aimed to assess the effects of terlipressin on cardiac function and perfusion.

METHODS

Twenty-four patients with cirrhosis and ascites participated, including nine with refractory ascites. Gated myocardial perfusion imaging, mean arterial blood pressure (MAP), cardiac output (CO), ejection fraction (EF), end-diastolic volume (EDV), perfusion, and motion of the myocardium were determined before and after a bolus injection of 2 mg terlipressin.

RESULTS

MAP increased after terlipressin (P value of less than 0.001). EF and CO fell by -16 and -17%, respectively in the terlipressin group versus 1 and -2%, respectively in the placebo group (P value of less than 0.001 and P value of less than 0.01). In the terlipressin group, EDV increased by 18 versus -4% in the placebo group (P value of less than 0.01). Wall motion in the anterior and posterior walls fell by -18 and -22%, respectively after terlipressin treatment versus 0 and 0% in the placebo group (P value of less than 0.01). In contrast, myocardial perfusion and stroke volume were unaltered in both the groups. The change in EF during terlipressin treatment correlated significantly with the change in MAP (r=-0.60, P value <0.002). Patients with refractory ascites had a higher EF and lower EDV and ESV than the patients with nonrefractory ascites, both at baseline and after terlipressin treatment. The decrease in the left ventricular wall thickening and wall motion correlated with the Child--Pugh score, r=-0.59, P=0.005 and r=-0.48, P=0.03.

CONCLUSION

In advanced cirrhosis, the increase in afterload and EDV after terlipressin treatment result in a decrease in left ventricular wall motion, resulting in reduced CO and EF, but myocardial perfusion is preserved. Alteration in cardiac function at baseline and after terlipressin treatment relates to the stage of decompensation.

Fos-Related Antigen Immunoreactivity After Acute and Chronic Angiotensin II–Induced Hypertension in the Rabbit Brain

Several brain regions are proposed as contributing to chronic sympatho-excitatory effects of elevated circulating angiotensin II. However, earlier c-Fos studies have been limited to acute angiotensin II exposure. This study aims to determine brain regions responding with chronic elevated angiotensin II. Rabbits were administered angiotensin II (50 ng/kg per minute) or saline for 3 hours, 3 days, or 14 days. Basal mean arterial pressure was 71±2 mm Hg and increased 23±2 mm Hg, 32±4 mm Hg, and 22±2 mm Hg for 3 hours, 3 days, and 14 days, respectively, with angiotensin II infusion. Neuronal activation was detected using Fos-related antigens, which recognizes all of the known members of the Fos family. Neurons located in the amygdala and area postrema were activated transiently after acute infusion of angiotensin II but were not responsive by days 3 or 14. Neurons located in the nucleus of the solitary tract, caudal ventrolateral medulla, and lateral parabrachial nucleus were activated for ≤3 days after infusion of angiotensin II but were not responsive by day 14, which is consistent with their role in response to baroreceptor pathways that reset with sustained hypertension. The vascular organ of the lamina terminalis and subfornical organ showed sustained but diminishing activation over the 14-day period. However, the downstream hypothalamic nuclei that receive inputs from these nuclei, the paraventricular, supraoptic, and arcuate nuclei, showed marked sustained activation. These findings suggest that there is desensitization of circumventricular organs but sensitization of neurons in hypothalamic regions to long-term angiotensin II infusion.

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.

Effects of ethanol consumption on vasopressin and neuropeptide Y immunoreactivity and mRNA expression in peripheral and central areas related to cardiovascular regulation

Results from previous studies have demonstrated that ethanol influences central neural mechanisms involved in the control of blood pressure. We studied the effects of ethanol consumption on vasopressin and neuropeptide Y immunoreactivity and mRNA expression in the nucleus tractus solitarius and paraventricular hypothalamic nucleus, as well as in the petrosal and nodose ganglia of rats. The ethanol-fed rats received liquid diet ad libitum containing 37.5% ethanol-derived calories (6.7% volume/volume), and the pair-fed rats received the same volume of diet containing isocaloric amounts of maltose-dextrin substituted for ethanol for 3 or 28 days. Arterial blood pressure was evaluated in a separate group of rats, which was unchanged by 3 days, but elevated by 21% after 28 days of ethanol consumption. Vasopressin immunoreactivity and mRNA signal were not detected in the ganglia, nor were they changed in the nucleus tractus solitarius and paraventricular hypothalamic nucleus, by 3 days of ethanol consumption. However, after 28 days of ethanol liquid diet consumption, vasopressin-positive terminals were decreased in the nucleus tractus solitarius and vasopressin immunoreactivity cell bodies and mRNA signal were decreased in the paraventricular hypothalamic nucleus. Neuropeptide Y-immunoreactive terminals were increased in the nucleus tractus solitarius only after 28 days of ethanol liquid diet consumption, but they were decreased in the paraventricular hypothalamic nucleus in rats treated with ethanol for 3 or 28 days. We concluded that the levels of both vasopressin and neuropeptide Y neurotransmitters are changed by long-term ethanol consumption in the neuronal pathways related to control of blood pressure.

P2X(2) receptor immunoreactivity in the dorsal vagal complex and area postrema of the rat

Adenosine 5'-triphosphate (ATP) can function as a fast synaptic transmitter through its actions on ionotropic (P2X) and metabotropic (P2Y) receptors in neuronal tissue. The ionotropic receptors have been classified into seven subtypes (P2X(1)-P2X(7)) by molecular cloning. However, they are difficult to distinguish pharmacologically owing to an absence of specific agonists and antagonists. In this study we used neuroanatomical methods to determine the origin and neurochemical phenotype of the P2X(2) subtype of purinoceptor in the dorsal medulla of the rat. Using immunohistochemistry we observed dense networks of P2X(2) receptor immunoreactive labelled fibres and terminals in the dorsal vagal complex and area postrema, as well as labelled cell bodies in the dorsal vagal nucleus and the area postrema. The P2X(2) receptor was localized presynaptically in vagal afferent fibres and terminals in the nucleus tractus solitarius at the ultrastructural level by combining injections of an anterograde tracer (biotin dextran amine) into the nodose ganglion with pre-embedding immunogold visualization of P2X(2) immunoreactivity. Terminals immunoreactive for the P2X(2) receptor in the nucleus tractus solitarius were found to contain glutamate, but not GABA immunoreactivity by post-embedding immunogold-labelling techniques. In cell bodies in the area postrema, dual immunofluorescence also indicated that P2X(2) receptor immunoreactive cells are glutamatergic but not GABAergic. The P2X(2) receptor was localized to vagal preganglionic neurons in the dorsal vagal nucleus that were identified by prior intraperitoneal injections of the retrograde tracer FluoroGold. No cells immunoreactive for the P2X(2) receptor were observed in the nucleus tractus solitarius. The localization of P2X(2) receptor immunoreactivity presynaptically in vagal afferent terminals indicates that the receptor may be involved in modulating transmitter release from vagal afferent fibres. Furthermore, the presence of the P2X(2) receptor in vagal preganglionic cells and in glutamatergic cells of the area postrema implies that it may, respectively, play a role in regulation of vagal efferent cell activity and modulation of excitatory outputs from the area postrema to other brain regions.

Stimulation of the ventral tegmental area enhances the effect of vasopressin on blood pressure in conscious rats

The mesolimbic dopamine system projects to a large number of forebrain areas and plays an important role in the regulation of locomotor activity, cognition and reward. We previously found evidence for a functional interaction between the mesolimbic dopamine system and circulating vasopressin and the present study was performed to test the hypothesis that mesolimbic dopamine stimulation modulates the cardiovascular effects of vasopressin. Sprague-Dawley rats were stereotaxically implanted with a guide cannula into the region of origin of the mesolimbic system, the ventral tegmental area, and instrumented with catheters into the abdominal aorta and jugular vein. One week later, separate groups of conscious rats were injected intravenously with 1, 3 or 10 ng kg(-1) of arginine-vasopressin or other vasopressor drugs before and after intra-ventral tegmental area injection of 10 nmol of neurotensin. Intra-ventral tegmental area injections of neurotensin had no significant effect on mean arterial pressure and heart rate but significantly potentiated the pressor response to intravenous administration of vasopressin when compared to saline-injections. However, the vasopressin-induced bradycardia was unaffected. Intravenous pretreatment with raclopride blocked the ability of neurotensin, injected into the ventral tegmental area, to potentiate the vasopressin-induced pressor response. Intra ventral tegmental area injections of neurotensin had no effect on the pressor response and bradycardia induced by intravenous angiotensin II or methoxamine. In conclusion, these results suggest that the mesolimbic dopamine system, in addition to its well-known role in the regulation of behaviour, modulates cardiovascular control by potentiating the effects of vasopressin on mean arterial pressure. British Journal of Pharmacology (2000) 129, 29 - 36

Participation of arterial baroreceptors input and peripheral vasopressin in the suppression of renal sympathetic nerve activity induced by central salt loading in conscious rats

We examined whether renal sympathetic nerve activity (RSNA) is suppressed in response to intracerebroventricular (i.c.v.) administration of hypertonic saline (HS) in conscious rats. RSNA was suppressed by i.c.v. administration of HS (0.3 M, 0.67 M, and 1.0 M, 1 microl/min for 20 min) in a concentration-dependent manner, which was attenuated under pentobarbital anesthesia. To elucidate mechanisms responsible for central HS-induced decrease in RSNA, possible involvement of arterial baroreceptors and peripheral arginine vasopressin (AVP) secreted from the posterior pituitary gland was examined using sinoaortic denervated (SAD) rats and non-peptide vasopressin receptor antagonists. The maximum suppression of RSNA (-81.5 +/- 5.5%) in control rats was significantly attenuated to -32.5 +/- 6.7% in SAD rats and to -55.8 +/- 5.7% in rats pretreated with intravenous vasopressin V1 receptor antagonist, OPC-21268 (5 mg/kg, i.v.). However, in SAD rats, pretreatment with vasopressin V1 receptor antagonist did not further affect the RSNA inhibition induced by central salt loading. The results suggest that the suppression of RSNA during central salt loading is mainly dependent on the arterial baroreceptors input and the 'additive' role of peripheral vasopressin.

Enhanced renal vasoconstrictor responsiveness to vasopressin after renal denervation

Previous studies from our laboratory and others demonstrated a relative insensitivity of the renal vasculature to the vasoconstrictor influences of arginine vasopressin (AVP). Experiments were therefore conducted to identify the possible roles of renal sympathetic withdrawal and V2-vasopressinergic receptor-mediated vasodilation in this response. Renal hemodynamic responses to AVP or saline vehicle were examined in the presence or absence of either selective V1-vasopressinergic-receptor blockade or nonselective vasopressinergic-receptor antagonism in conscious intact and renal-denervated rats. Our results indicate that renal denervation profoundly augmented AVP-induced renal vasoconstriction, whereas no differences were observed in the responses to V1-receptor blockade versus combined V1, V2-receptor antagonism. These findings are consistent with a role for renal sympathoinhibition in mediating the relative insensitivity of the renal bed to the vasoconstrictor effects of AVP in conscious rats but do not support a modulatory influence of V2-receptor activity in this response.

Non-NMDA and NMDA receptors transmit area postrema input to aortic baroreceptor neurons in NTS

We sought to determine whether glutamate acting at both N-methyl-D-aspartate (NMDA) and non-NMDA receptors transmits area postrema (AP) excitatory inputs to nucleus tractus solitarii (NTS) neurons in the aortic baroreceptor or vagal afferent pathways in vivo. In alpha-chloralose-anesthetized rabbits, we recorded extracellular NTS neuronal responses to low-frequency aortic depressor nerve (ADN), vagus nerve, and AP stimulation and to iontophoresis of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid and NMDA during control, iontophoresis of 2, 3-dihdroxy-6-nitro-7-sulfamoylbenzo(f)quinoxaline (NBQX), DL-2-amino-5-phosphonovaleric acid (AP5), or both, and recovery conditions. In neurons receiving AP and ADN inputs, NBQX attenuated AP- and ADN-evoked responses by 46 (P = 0.0206) and 49% (P = 0.0042). AP5 attenuated AP- and ADN-evoked responses by 39 (P = 0.0270) and 40% (P = 0.0157). NBQX + AP5 attenuated AP- and ADN-evoked responses by 74 (P = 0.0040) and 75% (P = 0.0028). In neurons receiving AP and vagal inputs, AP transmission was attenuated by 58, 60, and 98%; vagal transmission was attenuated by 62, 35, and 83% during NBQX, AP5, and both antagonists, respectively. These data suggest that both non-NMDA and NMDA receptors transmit AP input to NTS neurons in aortic baroreceptor or vagal afferent pathways.

Ovariectomy abolishes ethanol-induced impairment of baroreflex control of heart rate in conscious rats

Our previous studies have shown that ethanol attenuates baroreflex control of heart rate in male rats. The present study investigated whether this effect of ethanol is gender-related, and whether it involves hormonal factors. The effect of intragastric administration of ethanol or equal volume of water on baroreflex-mediated decreases in heart rate in response to increments in blood pressure evoked by phenylephrine were evaluated in conscious age-matched male and female Sprague-Dawley rats as well as in ovariectomized rats. Baroreflex curves relating changes in blood pressure and associated heart rate responses were constructed, and the slopes of the regression lines were taken as a measure of baroreflex sensitivity. Phenylephrine (1-16 microg kg(-1), i.v.) elicited dose-dependent pressor responses that were similar in all groups of rats. However, the associated reflex bradycardic responses depended on the rat preparation and the dose of ethanol employed. In water-treated (control) animals, significantly (P < 0.05) lesser reflex bradycardic responses were observed in female compared with male rats (baroreflex sensitivity, -1.21 +/- 0.12 vs. -1.67 +/- 0.12 beats min(-1) mmHg(-1)). Ovariectomy resulted in a further reduction in baroreflex sensitivity (-0.82 +/- 0.06 beats min(-1) mmHg(-1)), suggesting a favorable role for ovarian hormones in baroreflex modulation. In male rats, ethanol (0.25, 0.5, or 1 g kg(-1), intragastric) elicited dose-related decreases in reflex bradycardic responses. The reduction in the regression coefficient obtained by the two higher doses (0.5 and 1 g kg(-1)) of ethanol was statistically significant compared with control values. The ability of ethanol to reduce baroreflex sensitivity appears to be gender-independent as it was similarly demonstrated in intact female rats. In contrast, ethanol had no effect on reflex bradycardic responses in ovariectomized rats at any of the doses tested. The data suggest that ethanol reduces baroreflex control of heart rate irrespective of the rat gender. Further, the lack of an effect of ethanol on baroreflex sensitivity in ovariectomized rats may suggest a role for ovarian hormones in ethanol-evoked baroreflex attenuation.

Interactions between vasopressin and baroreflex control of the sympathetic nervous system

1. In addition to its effects at the renal tubules to influence water retention and at vascular smooth muscle to cause vasoconstriction, the hormone arginine vasopressin also appears to modulate cardiovascular reflex control of the sympathetic nervous system. Infusion or endogenous release of vasopressin results in enhanced baroreflex sympatho-inhibitory responses compared with other pressor agents. In addition, when changes in arterial pressure are imposed on an elevated background level of circulating vasopressin, due either to infusion or endogenous release, the arterial baroreflex response is shifted to lower pressures, and the maximum sympatho-excitation to a decrease in pressure is reduced. 2. Evidence suggests that vasopressin may influence cardiovascular reflex function at multiple sites. Nevertheless, the primary site involved in the effects of circulating vasopressin on baroreflex function appears to be in the central nervous system, specifically in the area postrema. Lesion of the area postrema abolishes the ability of circulating vasopressin to modulate arterial baroreflex and cardiopulmonary reflex function and electrical or chemical stimulation of this circumventricular organ mimics the effects of vasopressin. In addition, vasopressin has been shown to influence the activity of area postrema neurons in vivo and in vitro. Although not all studies agree, the effects of the area postrema and vasopressin on cardiovascular reflex function appear to be dependent on afferent input from peripheral baroreceptors. 3. Most evidence suggests that vasopressin exerts its effects on baroreflex function through a V1 vasopressin receptor mechanism. Systemic administration or microinjection into the area postrema of a specific V1 receptor antagonist abolishes the action of arginine vasopressin on arterial baroreflex and cardiopulmonary reflex control of the sympathetic nervous system. 4. The ability of vasopressin and the area postrema to influence baroreflex function appears to be dependent on an alpha 2-adrenoceptor mechanism at the level of the nucleus tractus solitarius (NTS). Blockade of alpha 2-adrenoceptors in the NTS abolishes the effects of vasopressin and the area postrema on the sympathetic nervous system. Facilitation of NTS processing of baroreceptor afferent inputs by the area postrema could contribute to the enhanced sympatho-inhibition and shift of the baroreflex curve to lower pressures during elevations in circulating vasopressin.

Ethanol microinjection into the area postrema selectively attenuates baroreflex sensitivity measured by vasopressin in conscious rats

Microinjection of ethanol (10 microg) into the area postrema (AP) of conscious rats attenuated baroreflex sensitivity (BRS) measured by arginine vasopressin (AVP) (-1.73 +/- 0.13 versus -2.47 +/- 0.16 bpm/mmHg), but not by phenylephrine (PE) (-1.94 +/- 0.26 versus -1.82 +/- 0.20 bpm/mmHg). Intra-AP injection of the V1 receptor antagonist D(CH2)5Tyr(Me)AVP replicated the differential effects of ethanol on BRS measured by AVP (-1.89 +/- 0.11 versus -2.52 +/- 0.10 bpm/mmHg) and PE (-2.10 +/- 0.12 versus -2.09 +/- 0.19 bpm/mmHg). Intra-AP artificial cerebrospinal fluid (ACSF) did not change BRS measured by AVP or PE. These data suggest that ethanol attenuates the facilitatory action of AVP on baroreflexes via its interaction with AVP neurons in the AP.

Neurochemical modulation of cardiovascular control in the nucleus tractus solitarius

The central control of cardiovascular function has been keenly studied for a number of decades. Of particular interest are the homeostatic control mechanisms, such as the baroreceptor heart-rate reflex, the chemoreceptor reflex, the Bezold-Jarisch reflex and the Breuer-Hering reflex. These neurally-mediated reflexes share a common termination point for their respective centrally-projecting sensory afferents, namely the nucleus tractus solitarius (NTS). Thus, the NTS clearly plays a critical role in the integration of peripherally initiated sensory information regarding the status of blood pressure, heart rate and respiratory function. Many endogenous neurochemicals, from simple amino acids through biogenic amines to complex peptides have the ability to modulate blood pressure and heart rate at the level of the NTS. This review will attempt to collate the current knowledge regarding the roles of neuromodulators in the NTS, the receptor types involved in mediating observed responses and the degree of importance of such neurochemicals in the tonic regulation of the cardiovascular system. The neural pathway that controls the baroreceptor heart-rate reflex will be the main focus of attention, including discussion of the identity of the neurotransmitter(s) thought to act at baroafferent terminals within the NTS. In addition, this review will provide a timely update on the use of recently developed molecular biological techniques that have been employed in the study of the NTS, complementing more classical research.