下丘脑室旁核内GABA在中枢高渗刺激诱发的心血管反应中的作用

目的：探讨下丘脑室旁核（PVN）内的γ-氨基丁酸（GABA）在中枢高渗刺激诱发的应激性心血管反应中的作用及其机制。方法：在清醒自由活动大鼠,用脑部微量透析法和高效液相色谱法观察中枢高渗刺激对PVN区域GABA含量的影响,并同时记录血压和心率的变化;用GABAA受体阻断剂Bicuculline或GABAB受体阻断剂Saelofen直接灌流PVN区并给予中枢高渗刺激,进一步探讨PVN区GABA在中枢高渗刺激诱发的应激性心血管反应中的作用。结果：①PVN局部灌流0．6mol／L盐水时,血压和心率都显著增加（均为P〈0．01）,同时,PVN区细胞外液中GABA水平也明显增加到刺激前的561．96％±173．96％（P〈0．05）;②PVN局部灌流Bicuculline或Saclofen的同时,给予0．6mol／L盐水的刺激,可使高渗刺激引起的血压增加幅度明显降低（均为P〈0．01）,而心率的增加幅度未受明显影响（均为P〈0．05）。结论：中枢高渗刺激可引起PVN内GABA的分泌,而后者可通过GABAA和GABAB受体产生血压的升高反应。     

Interaction between glutamate and GABA systems in the integration of sympathetic outflow by the paraventricular nucleus of the hypothalamus

The paraventricular nucleus (PVN) of the hypothalamus is a central site known to modulate sympathetic outflow. Excitatory and inhibitory neurotransmitters within the PVN dictate final outflow. The goal of the present study was to examine the role of the interaction between the excitatory neurotransmitter glutamate and the inhibitory neurotransmitter GABA in the regulation of sympathetic activity. In alpha-chloralose- and urethane-anesthetized rats, microinjection of glutamate and N-methyl-D-aspartate (NMDA; 50, 100, and 200 pmol) into the PVN produced dose-dependent increases in renal sympathetic nerve activity, blood pressure, and heart rate. These responses were blocked by the NMDA receptor antagonist DL-2-amino-5-phosphonovaleric acid (AP-5). Microinjection of bicuculline, a GABA(A) receptor antagonist, into the PVN (50, 100, and 200 pmol) also produced significant, dose-dependent increases in renal sympathetic nerve activity, blood pressure, and heart rate; AP-5 also blocked these responses. Using microdialysis and HPLC/electrochemical detection techniques, we observed that bicuculline infusion into the PVN increased glutamate release. Using an in vitro hypothalamic slice preparation, we found that bicuculline increased the frequency of glutamate-mediated excitatory postsynaptic currents in PVN-rostral ventrolateral medullary projecting neurons, supporting a GABA(A)-mediated tonic inhibition of this excitatory input into these neurons. Together, these data indicate that 1) glutamate, via NMDA receptors, excites the presympathetic neurons within the PVN and increases sympathetic outflow and 2) this glutamate excitatory input is tonically inhibited by a GABA(A)-mediated mechanism.     

AT1 receptors in the paraventricular nucleus mediate the hyperthermia-induced reflex reduction of renal blood flow in rats

Increasing body core temperature reflexly decreases renal blood flow (RBF), and the hypothalamic paraventricular nucleus (PVN) plays an essential role in this response. ANG II in the brain is involved in the cardiovascular responses to hyperthermia, and ANG II receptors are highly concentrated in the PVN. The present study investigated whether ANG II in the PVN contributes to the cardiovascular responses elicited by hyperthermia. Rats anesthetized with urethane (1-1.4 g/kg iv) were microinjected bilaterally into the PVN (100 nl/side) with saline (n = 5) or losartan (1 nmol/100 nl) (n = 7), an AT1 receptor antagonist. Body core temperature was then elevated from 37°C to 41°C and blood pressure (BP), heart rate (HR), RBF, and renal vascular conductance (RVC) were monitored. In separate groups losartan (n = 4) or saline (n = 4) was microinjected into the PVN, but body core temperature was not elevated. Increasing body core temperature in control rats elicited significant decreases in RBF (-48 ± 5% from a resting level of 14.3 ± 1.4 ml/min) and MVC (-40 ± 4% from a resting level of 0.128 ± 0.013 ml/min·mmHg), and these effects were entirely prevented by pretreatment with losartan. In rats in which body core temperature was not altered, losartan microinjected into the PVN had no significant effects on these variables. The results suggest that endogenous ANG II acts on AT1 receptors in the PVN to mediate the reduction in RBF induced by hyperthermia.     

Stimulation of cardiac sympathetic nerve activity by central angiotensinergic mechanisms in conscious sheep

Central actions of angiotensin play an important role in cardiovascular control and have been implicated in the pathogenesis of hypertension and heart failure. One feature of centrally or peripherally administered angiotensin is that the bradycardia in response to an acute pressor effect is blunted. It is unknown whether after central angiotensin this is due partly to increased cardiac sympathetic nerve activity (CSNA). We recorded CSNA and arterial pressure in conscious sheep, at least 3 days after electrode implantation. The effects of intracerebroventricular infusions of ANG II (3 nmol/h for 30 min) and artificial cerebrospinal fluid (CSF) (1 ml/h) were determined. The response to intracerebroventricular hypertonic saline (0.6 M NaCl in CSF at 1 ml/h) was examined as there is evidence that hypertonic saline acts via angiotensinergic pathways. Intracerebroventricular angiotensin increased CSNA by 23 +/- 7% (P < 0.001) and mean arterial pressure (MAP) by 7.6 +/- 1.2 mmHg (P < 0.001) but did not significantly change heart rate (n = 5). During intracerebroventricular ANG II the reflex relation between CSNA and diastolic blood pressure was significantly shifted to the right (P < 0.01). Intracerebroventricular hypertonic saline increased CSNA (+9.4 +/- 6.6%, P < 0.05) and MAP but did not alter heart rate. The responses to angiotensin and hypertonic saline were prevented by intracerebroventricular losartan (1 mg/h). In conclusion, in conscious sheep angiotensin acts within the brain to increase CSNA, despite increased MAP. The increase in CSNA may account partly for the lack of bradycardia in response to the increased arterial pressure. The responses to angiotensin and hypertonic saline were losartan sensitive, indicating they were mediated by angiotensin AT-1 receptors.     

Responses to GABA-A receptor blockade in the hypothalamic PVN are attenuated by local AT1 receptor antagonism

Blockade of GABA-A receptors in the hypothalamic paraventricular nucleus (PVN) has been repeatedly shown to increase arterial blood pressure (ABP), heart rate (HR), and sympathetic nerve activity (SNA), but the mechanism(s) that underlies this response has not been determined. Here, we tested whether full expression of the response requires activation of local ANG II AT1 receptors. ABP, HR, and renal SNA responses to PVN microinjection of bicuculline methobromide (BIC; 0.1 nmol) were recorded before and after microinjection of vehicle (saline); losartan (or L-158809), to block local AT1 receptors; or PD123319, to block AT2 receptors. After PVN microinjection of vehicle or PD123319 (10 nmol), BIC significantly (P < 0.05) increased mean arterial pressure (MAP), HR, and renal SNA. However, PVN microinjection of 2 and 20 nmol of losartan dose dependently reduced responses to PVN-injected BIC, with the 20-nmol dose nearly abolishing MAP (P < 0.005), HR (P < 0.05), and renal SNA (P < 0.005) responses. Another AT1 receptor antagonist, L-158809 (10 nmol), produced similar effects. Neither losartan nor L-158809 altered baseline parameters. Responses to PVN injection of BIC were unchanged by losartan (20 nmol) given intravenously or into the PVN on the opposite side. MAP, HR, and renal SNA responses to PVN microinjection of l-glutamate (10 nmol) were unaffected by PVN injection of losartan (20 nmol), indicating that effects of losartan were not due to nonspecific depression of neuronal excitability. We conclude that pressor, tachycardic, and renal sympathoexcitatory responses to acute blockade of GABA-A receptors in the PVN depend on activation of local AT1 receptors.     

Cardiac baroreflex facilitation evoked by hypothalamus and prefrontal cortex stimulation: role of the nucleus tractus solitarius 5-HT2A receptors

We previously showed that serotonin (5-HT2) receptor activation in the nucleus of the tractus solitarius (NTS) produced hypotension, bradycardia, and facilitation of the baroreflex bradycardia. Activation of the preoptic area (POA) of the hypothalamus, which is involved in shock-evoked passive behaviors, induces similar modifications. In addition, previous studies showed that blockade of the infralimbic (IL) part of the medial prefrontal cortex, which sends projections to POA, produced an inhibitory influence on the baroreflex cardiac response. Thus, to assess the possible implication of NTS 5-HT2 receptors in passive cardiovascular responses, we analyzed in anesthetized rats the effects of NTS inhibition and NTS 5-HT2 receptor blockade on the cardiovascular modifications induced by chemical (0.3 M D,L-homocysteic acid) and electrical (50 Hz, 150-200 microA) stimulation of IL or POA. Intra-NTS microinjections of muscimol, a GABAA receptor agonist, prevented the decreases in blood pressure and heart rate normally evoked by IL or POA activation. In addition, we found that intra-NTS microinjection of R(+)-alpha-(2,3-dimethoxyphenyl)-1-[2-(4-fluorophenylethyl)]-4-piperidine-methanol, a specific 5-HT2A receptor antagonist, did not affect the decreases in cardiovascular baseline parameters induced by IL or POA stimulation but prevented the facilitation of the aortic baroreflex bradycardia normally observed during IL (+65 and +60%) or POA (+70 and +69%) electrical and chemical stimulation, respectively. These results show that NTS 5-HT2A receptors play a key role in the enhancement of the cardiac response of the baroreflex but not in the changes in basal heart rate and blood pressure induced by IL or POA stimulation.     

Effect of Barodenervation on Cardiovascular Responses Elicited from the Hypothalamic Arcuate Nucleus of the Rat

We have previously reported that chemical stimulation of the hypothalamic arcuate nucleus (ARCN) in the rat elicited increases as well as decreases in blood pressure (BP) and sympathetic nerve activity (SNA). The type of response elicited from the ARCN (i.e., increase or decrease in BP and SNA) depended on the level of baroreceptor activity which, in turn, was determined by baseline BP in rats with intact baroreceptors. Based on this information, it was hypothesized that baroreceptor unloading may play a role in the type of response elicited from the ARCN. Therefore, the effect of barodenervation on the ARCN-induced cardiovascular and sympathetic responses and the neurotransmitters in the hypothalamic paraventricular nucleus (PVN) mediating the excitatory responses elicited from the ARCN were investigated in urethane-anesthetized adult male Wistar rats. Bilateral barodenervation converted decreases in mean arterial pressure (MAP) and greater splanchnic nerve activity (GSNA) elicited by chemical stimulation of the ARCN with microinjections of N-methyl-D-aspartic acid to increases in MAP and GSNA and exaggerated the increases in heart rate (HR). Combined microinjections of NBQX and D-AP7 (ionotropic glutamate receptor antagonists) into the PVN in barodenervated rats converted increases in MAP and GSNA elicited by the ARCN stimulation to decreases in MAP and GSNA and attenuated increases in HR. Microinjections of SHU9119 (a melanocortin 3/4 receptor antagonist) into the PVN in barodenervated rats attenuated increases in MAP, GSNA and HR elicited by the ARCN stimulation. ARCN neurons projecting to the PVN were immunoreactive for proopiomelanocortin, alpha-melanocyte stimulating hormone (alpha-MSH) and adrenocorticotropic hormone (ACTH). It was concluded that increases in MAP and GSNA and exaggeration of tachycardia elicited by the ARCN stimulation in barodenervated rats may be mediated via release of alpha-MSH and/or ACTH and glutamate from the ARCN neurons projecting to the PVN.     

Role of Muscarinic Acetylcholine Receptor-2 in the Cerebellar Cortex in Cardiovascular Modulation in Anaesthetized Rats

Our previous investigations have demonstrated that microinjection of acetylcholine (ACh) or muscarinic ACh receptor activation in the cerebellar cortex induces a systemic blood pressure depressor response. This study aimed to determine the role of muscarinic ACh receptor-2 (M2 receptor) in the cerebellar cortex in cardiovascular function regulation in rats. A nonselective muscarinic receptor agonist (oxotremorine M, OXO; 30 mM), a selective M2 receptor agonist (arecaidine but-2-ynyl ester tosylate, ABET; 3, 10, and 30 mM), 30 mM OXO mixed with a selective M2 receptor antagonist (methoctramine hydrate, MCT; 0.3, 1, and 3 mM), and normal saline (0.9 % NaCl) were separately microinjected (0.5 µl/5 s) into the cerebellar cortex (lobule VI) of anaesthetized rats. We measured the mean arterial pressure (MAP), maximum change in MAP, and reactive time (RT; the duration required for the blood pressure to return to basal levels), heart rate (HR) and the maximum change in HR during the RT in response to drug activation. The results demonstrated that ABET dose-dependently decreased MAP and HR, increased the maximum change in MAP and the maximum change in HR, and prolonged the RT. Furthermore, MCT dose-dependently blocked the OXO-mediated cardiovascular depressor response. This study provides the first evidence that M2 receptors in the cerebellar cortex are involved in cardiovascular regulation, the activation of which evokes significant depressor and bradycardic responses.     

Hemodynamic effects of hypertonic saline in the conscious rat

The present study examines the role of vasopressin and the sympathetic nervous system on the hemodynamic effects of an infusion of hypertonic saline (NaCl 1.5 M) in conscious rats. The cardiovascular response to hypertonic saline was similar in both untreated and hexamethonium-pretreated rats. Mean arterial pressure increased by 15 mmHg as a consequence of the elevation of total peripheral resistance, while cardiac index was decreased. The administration of an antagonist to the pressor activity of vasopressin in rats with intact reflexes, partially decreased mean arterial pressure and total peripheral resistance and increased cardiac index toward basal values. In contrast, the hemodynamic response to hypertonic saline was totally reverted when the vasopressin antagonist was injected in the hexamethonium-pretreated rats. The results of the present study indicate that the hypertensive response induced by hypertonic saline in conscious rats is due to the vasoconstrictor effects of both vasopressin and the sympathetic nervous system.     

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.     

Role of the hypothalamic PVN in the reflex reduction in mesenteric blood flow elicited by hyperthermia

The hypothalamic paraventricular nucleus (PVN) is an important integrative center in the brain. In the present study, we investigated whether the PVN is a key region in the mesenteric vasoconstriction that normally accompanies an increase in core body temperature. Anesthetized rats were monitored for blood pressure, heart rate, mesenteric blood flow, and vascular conductance. In control rats, elevation of core body temperature to 41 degrees C had no significant effect on blood pressure, increased heart rate, and reduced mesenteric blood flow by 21%. In a separate group of rats, muscimol was microinjected bilaterally (1 nmol/side) into the PVN. Compared with the control group, there was no significant difference in the blood pressure and heart rate responses elicited by the increase in core body temperature. In contrast to control animals, however, mesenteric blood flow did not fall in the muscimol-treated rats in response to the elevation in core body temperature. In a separate group, in which muscimol was microinjected into regions outside the PVN, elevating core body temperature elicited the normal reduction in mesenteric blood flow. The results suggest that the PVN may play a key role in the reflex decrease in mesenteric blood flow elicited by hyperthermia.     

Role of L-glutamate in systemic AVP-induced hypothermia.

It has been reported that systemic injection of arginine vasopressin (AVP) induces a drop in body core temperature (T(c)), but little is known about the mechanisms involved. Because glutamate is an important excitatory neurotransmitter involved in a number of thermoregulatory actions, in the present study, we tested the hypothesis that glutamate plays a role in systemic AVP-induced hypothermia. Wistar rats were pretreated intracerebroventricularly (icv) with kynurenic acid, an antagonist of l-glutamate ionotropic receptors, alpha-methyl-(4-carboxyphenyl)glycine (MCPG), an antagonist of l-glutamate metabotropic receptors, or saline 15 min before intravenous injection of AVP (2 microg/kg) or saline. T(c), brown adipose tissue (BAT) temperature, blood pressure, heart rate, and tail skin temperature were measured continuously. Administration of saline icv followed by intravenous AVP caused a significant drop in T(c) brought about by a reduction in BAT thermogenesis and an increase in heat loss through the tail. MCPG treatment (icv) did not affect the fall in T(c) induced by AVP. Treatment with kynurenic acid (icv) abolished AVP-induced hypothermia but did not affect the AVP-evoked rise in blood pressure or drop in heart rate and BAT temperature. Heat loss through the tail was significantly reduced in animals injected with AVP and pretrated with kynurenic acid. These data indicate that ionotropic receptors of l-glutamate in the central nervous system participate in peripheral AVP-induced hypothermia by affecting heat loss through the tail.     

Activation of 5-hydroxytryptamine-1A receptors suppresses cardiovascular responses evoked from the paraventricular nucleus

Activation of central 5-hydroxytryptamine-1A (5-HT(1A)) receptors powerfully inhibits stress-evoked cardiovascular responses mediated by the dorsomedial hypothalamus (DMH), as well as responses evoked by direct activation of neurons within the DMH. The hypothalamic paraventricular nucleus (PVN) also has a crucial role in cardiovascular regulation and is believed to regulate heart rate and renal sympathetic activity via pathways that are independent of the DMH. In this study, we determined whether cardiovascular responses evoked from the PVN are also modulated by activation of central 5-HT(1A) receptors. In anesthetized rats, the increases in heart rate and renal sympathetic nerve activity evoked by bicuculline injection into the PVN were greatly reduced (by 54% and 61%, respectively) by intravenous administration of (±)-8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), an agonist of 5-HT(1A) receptors, but were then completely restored by subsequent administration of WAY-100635, a selective antagonist of 5-HT(1A) receptors. Microinjection of 8-OH-DPAT directly into the PVN did not significantly affect the responses to bicuculline injection into the PVN, nor did systemic administration of WAY-100635 alone. In control experiments, a large renal sympathoexcitatory response was evoked from both the PVN and DMH but not from the intermediate region in between; thus the evoked responses from the PVN were not due to activation of neurons in the DMH. The results indicate that activation of central 5-HT(1A) receptors located outside the PVN powerfully inhibits the tachycardia and renal sympathoexcitation evoked by stimulation of neurons in the PVN.     

Increased dietary sodium inhibits baroreflex-induced bradycardia during acute sodium loading

The present study investigated the effects of increased dietary sodium on the modification of cardiac baroreflex responses induced by acute sodium loading. Changes in blood pressure and heart rate during intravenous phenylephrine and nitroprusside administration were compared using a four-parameter sigmoid logistic function before and after a 30-min infusion of 0.6 or 1.0 M NaCl in conscious male Sprague-Dawley rats consuming only tap water (Tap) or isotonic saline (Iso) for 2-3 wk. In Tap animals, infusion of 1.0 M NaCl increased the baroreflex-induced heart rate minimum, reduced heart rate range, and increased the operating blood pressure. In contrast, infusion of 0.6 M NaCl in Tap rats reduced both heart rate minimum and maximum. However, infusion of 0.6 M NaCl in Iso animals produced responses similar to that shown in Tap rats infused with 1.0 M NaCl. In addition, the decreased heart rate minimum in Tap rats after infusion of 0.6 M NaCl was prevented by intravenous administration of a vasopressin V1-receptor antagonist. Furthermore, cardiac parasympathetic responses were similar in Tap and Iso rats before and after 0.6 M NaCl infusion. However, in animals receiving intravenous atropine, 0.6 M NaCl decreased heart rate minimum and maximum in Tap but did not alter the response parameters in Iso rats. These results demonstrate that the facilitation of cardiac baroreflex responses normally observed during moderate sodium loading is mediated by vasopressin and that increased dietary sodium ingestion reverses this facilitation by reducing sympathetic nervous system withdrawal.     

室旁核在刺激中脑防御反应区诱发防御性升压反应中的作用

雄性ＳＤ大鼠,用乌拉坦（７００ｍｇ／ｋｇ）和氯醛糖（３０ｍｇ／ｋｇ）腹腔麻醉。实验结果：（１）每隔５ｍｉｎ电刺激中脑导水管周围灰质背侧部“防御反应区”（ｄＰＡＧ）,持续观察５０ｍｉｎ,可见恒定的升压反应。若电解毁单侧室旁核（ＰＶＮ）区。１ｈ后,电刺激中脑ｄＰＡＧ区诱发的升压反应幅度部分减小。而损毁穹隆部、下丘脑前部、下丘脑背内侧核、下丘脑腹内侧核则无上述效应。（２）电刺激或微量注射高半胱胺酸（ＤＬ     

Melanocortin receptors mediate the excitatory effects of blood-borne murine leptin on hypothalamic paraventricular neurons in rat

The central pathways and mediators involved in sympathoexcitatory responses to circulating leptin are not well understood, although the arcuate-paraventricular nucleus (ARC-PVN) pathway likely plays a critical role. In urethane-anesthetized rats, ipsilateral intracarotid artery (ICA) injection of murine leptin (100 microg/kg) activated most PVN neurons tested. These responses were reduced by intracerebroventricular injection of the melanocortin subtype 3 and 4 receptor (MC3/4-R) antagonist SHU-9119 (0.6 nmol). The MC3/4-R agonist MTII (0.6 nmol icv) activated PVN neurons. Some PVN neurons that were excited by ICA leptin were inhibited by local application of neuropeptide Y (NPY, 2.5 ng). ICA leptin (100 microg/kg) excited presympathetic rostral ventrolateral medulla neurons and renal sympathetic nerve activity without significant change in blood pressure or heart rate; these effects were mimicked by intracerebroventricular injection of MTII (0.6 nmol). These data provide in vivo electrophysiological evidence to support the hypothesis that circulating leptin activates the sympathetic nervous system by stimulating the release of alpha-melanocyte-stimulating hormone in the vicinity of PVN neurons that are inhibited by the orexogenic peptide NPY.     

Cardiovascular responses to exercise in the rat: role of corticotropin-releasing factor

The effects of intracerebroventricular (icv) administration of a corticotropin-releasing factor (CRF) receptor antagonist, alpha-helical CRF, on systemic and regional hemodynamic adjustments to exercise were studied in conscious rats. On consecutive days, rats received saline icv, alpha-helical CRF icv, and no treatment 30 min before treadmill exercise (TMX). Increases in heart rate (HR) and mean arterial pressure (MAP) in response to TMX (16.1-28.6 m/min) were similar after icv administration of saline or no treatment. In rats receiving saline icv or no treatment, estimated vascular resistance increased in the mesenteric and renal regions and declined in the iliac (hindlimb) region. After icv administration of alpha-helical CRF9-41, HR and MAP responses during TMX were significantly attenuated. In addition, TMX-induced elevations of estimated mesenteric vascular resistance and iliac blood flow velocity were blunted after CRF receptor blockade. These altered cardiovascular and hemodynamic responses were ultimately reflected in the animals' compromised ability to run. The results suggest that the central nervous system actions of endogenous CRF are necessary for the full expression of the cardiovascular adjustments to TMX in the conscious rat.     

Role of glutamate in a visceral sympathoexcitatory reflex in rostral ventrolateral medulla of cats

The rostral ventrolateral medulla (rVLM) is involved in processing visceral sympathetic reflexes. However, there is little information on specific neurotransmitters in this brain stem region involved in this reflex. The present study investigated the importance of glutamate and glutamatergic receptors in the rVLM during gallbladder stimulation with bradykinin (BK), because glutamate is thought to function as an excitatory neurotransmitter in this region. Stimulation of visceral afferents activated glutamatergic neurons in the rVLM, as noted by double-labeling with c-Fos and the cellular vesicular glutamate transporter 3 (VGLUT3). Visceral reflex activation significantly increased arterial blood pressure as well as extracellular glutamate concentrations in the rVLM as determined by microdialysis. Barodenervation did not alter the release of glutamate in the rVLM evoked by visceral reflex stimulation. Iontophoresis of glutamate into the rVLM enhanced the activity of sympathetic premotor cardiovascular rVLM neurons. Also, the responses of these neurons to visceral afferent stimulation with BK were attenuated significantly (70%) by blockade of glutamatergic receptors with kynurenic acid. Microinjection of either an N-methyl-D-aspartate (NMDA) receptor antagonist 2-amino-5-phosphonopentanate (25 mM, 30 nl) or an dl-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (2 mM, 30 nl) into the rVLM significantly attenuated the visceral sympathoexcitatory reflex responses. These results suggest that glutamate in the rVLM serves as an excitatory neurotransmitter through a baroreflex-independent mechanism and that both NMDA and AMPA receptors mediate the visceral sympathoexcitatory reflex responses.     

Enhanced activation of RVLM-projecting PVN neurons in rats with chronic heart failure

Previous studies have indicated that there is increased activation of the paraventricular nucleus (PVN) in rats with chronic heart failure (CHF); however, it is not clear if the preautonomic neurons within the PVN are specifically overactive. Also, it is not known if these neurons have altered responses to baroreceptor or osmotic challenges. Experiments were conducted in rats with CHF (6-8 wk after coronary artery ligation). Spontaneously active neurons were recorded in the PVN, of which 36% were antidromically activated from the rostral ventrolateral medulla (RVLM). The baseline discharge rate in RVLM-projecting PVN (PVN-RVLM) neurons from CHF rats was significantly greater than in sham-operated (sham) rats (6.0 ± 0.6 vs. 2.6 ± 0.3 spikes/s, P < 0.05). Picoinjection of the N-methyl-D-aspartate (NMDA) receptor antagonist D,L-2-amino-5-phosphonovaleric acid significantly decreased the basal discharge of PVN-RVLM neurons by 80% in CHF rats compared with 37% in sham rats. Fifty-two percent of spontaneously active PVN-RVLM neurons responded to changes in the mean arterial pressure (MAP). The changes in discharge rate in PVN-RVLM neurons after a reduction in MAP (+52 ± 7% vs. +184 ± 61%) or an increase in MAP (-42 ± 8% vs. -71 ± 6%) were significantly attenuated in rats with CHF compared with sham rats. Most PVN-RVLM neurons (63%), including all barosensitive PVN-RVLM neurons, were excited by an internal carotid artery injection of hypertonic NaCl (2.1 osmol/l), whereas a smaller number (7%) were inhibited. The increase in discharge rate in PVN-RVLM neurons to hypertonic stimulation was significantly enhanced in rats with CHF compared with sham rats (134 ± 15% vs. 92 ± 13%). Taken together, these data suggest that PVN-RVLM neurons are more active under basal conditions and this overactivation is mediated by an enhanced glutamatergic tone in rats with CHF. Furthermore, this enhanced activation of PVN-RVLM neurons may contribute to the altered responses to baroreceptor and osmotic challenges observed during CHF.     

Role of paraventricular nucleus in the cardiogenic sympathetic reflex in rats

Myocardial ischemia stimulates cardiac spinal afferents to initiate a sympathoexcitatory reflex. However, the pathways responsible for generation of increased sympathetic outflow in this reflex are not fully known. In this study, we determined the role of the paraventricular nucleus (PVN) in the cardiogenic sympathetic reflex. Renal sympathetic nerve activity (RSNA) and blood pressure were recorded in anesthetized rats during epicardial application of 10 microg/ml bradykinin. Bilateral microinjection of muscimol (0.5 nmol), a GABA(A) receptor agonist, was performed to inhibit the PVN. In 10 vehicle-injected rats, epicardial bradykinin significantly increased RSNA 178.4 +/- 48.5% from baseline, and mean arterial pressure from 76.9 +/- 2.0 to 102.3 +/- 3.3 mmHg. Microinjection of muscimol into the PVN significantly reduced the basal blood pressure and RSNA (n = 12). After muscimol injection, the bradykinin-induced increases in RSNA (111.6 +/- 35.9% from baseline) and mean arterial pressure (61.2 +/- 1.3 to 74.5 +/- 2.7 mmHg) were significantly reduced compared with control responses. The response remained attenuated even when the basal blood pressure was restored to the control. In a separate group of rats (n = 9), bilateral microinjection of the ionotropic glutamate antagonist kynurenic acid (4.82 or 48.2 nmol in 50 nl) had no significant effect on the RSNA and blood pressure responses to bradykinin compared with controls. These results suggest that the tonic PVN activity is important for the full manifestation of the cardiogenic sympathoexcitatory response. However, ionotropic glutamate receptors in the PVN are not directly involved in this reflex response.