Medullary pathways mediating the parasubthalamic nucleus depressor response

The parasubthalamic nucleus (PSTN) projects extensively to the nucleus of the solitary tract (NTS); however, the function of PSTN in cardiovascular regulation is unknown. Experiments were done in alpha-chloralose anesthetized, paralyzed, and artificially ventilated rats to investigate the effect of glutamate (10 nl, 0.25 M) activation of PSTN neurons on mean arterial pressure (MAP), heart rate (HR), and renal sympathetic nerve activity (RSNA). Glutamate stimulation of PSTN elicited depressor (-20.4 +/- 0.7 mmHg) and bradycardia (-26.0 +/- 1.0 beats/min) responses and decreases in RSNA (67 +/- 17%). Administration (intravenous) of atropine methyl bromide attenuated the bradycardia response (46%), but had no effect on the MAP response. Subsequent intravenous administration of hexamethonium bromide blocked both the remaining bradycardia and depressor responses. Bilateral microinjection of the synaptic blocker CoCl(2) into the caudal NTS region attenuated the PSTN depressor and bradycardia responses by 92% and 94%, respectively. Additionally, prior glutamate activation of neurons in the ipsilateral NTS did not alter the magnitude of the MAP response to stimulation of PSTN, but potentiated HR response by 35%. Finally, PSTN stimulation increased the magnitude of the reflex bradycardia to activation of arterial baroreceptors. These data indicate that activation of neurons in the PSTN elicits a decrease in MAP due to sympathoinhibition and a cardiac slowing that involves both vagal excitation and sympathoinhibition. In addition, these data suggest that the PSTN depressor effects on circulation are mediated in part through activation of NTS neurons involved in baroreflex function.     

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.     

NO and endogenous angiotensin II interact in the generation of renal sympathetic nerve activity in conscious rats

Nitric oxide (NO) appears to inhibit sympathetic tone in anesthetized rats. However, whether NO tonically inhibits sympathetic outflow, or whether endogenous angiotensin II (ANG II) promotes NO-mediated sympathoinhibition in conscious rats is unknown. To address these questions, we determined the effects of NO synthase (NOS) inhibition on renal sympathetic nerve activity (RSNA) and heart rate (HR) in conscious, unrestrained rats on normal (NS), high-(HS), and low-sodium (LS) diets, in the presence and absence of an ANG II receptor antagonist (AIIRA). When arterial pressure was kept at baseline with intravenous hydralazine, NOS inhibition with l-NAME (10 mg/kg i.v.) resulted in a profound decline in RSNA, to 42 +/- 11% of control (P < 0.01), in NS animals. This effect was not sustained, and RSNA returned to control levels by 45 min postinfusion. l-NAME also caused bradycardia, from 432 +/- 23 to 372 +/- 11 beats/min postinfusion (P < 0.01), an effect, which, in contrast, was sustained 60 min postdrug. The effects of NOS inhibition on RSNA and HR did not differ between NS, HS, and LS rats. However, when LS and HS rats were pretreated with AIIRA, the initial decrease in RSNA after l-NAME infusion was absent in the LS rats, while the response in the HS group was unchanged by AIIRA. These findings indicate that, in contrast to our hypotheses, NOS activity provides a stimulatory input to RSNA in conscious rats, and that in LS animals, but not HS animals, this sympathoexcitatory effect of NO is dependent on the action of endogenous ANG II.     

Roles of adrenomedullin 2 in regulating the cardiovascular and sympathetic nervous systems in conscious rats

Recently, a new member of the calcitonin gene-related peptide (CGRP) family, adrenomedullin 2 (AM2) or intermedin (IMD), was identified. AM2/IMD has been shown to have a vasodilator effect in mice and rats and an effect on urine formation in rats. In the present study, we investigated the effects of intravenously infused rat AM2 (rAM2) on blood pressure (BP), heart rate (HR), renal sympathetic nerve activity (RSNA), and renal blood flow (RBF) in conscious unrestrained rats relative to the effects of rat adrenomedullin (rAM) and proadrenomedullin NH2-terminal 20 peptide (rPAMP). Intravenous infusion of rAM2 (5 nmol/kg) significantly decreased BP and increased HR, RSNA, and RBF. These hypotensive and sympathoexcitatory effects diminished after 20 min, and HR returned to control levels 30 min after cessation of the infusion. In contrast, a significant increase in RBF was still evident 60 min after cessation of the peptide infusion. The duration of BP, HR, and RSNA responses was longer with rAM (5 nmol/kg) than with rAM2 infusion, whereas the increases in RBF induced by rAM2 and rAM were similar in their amplitude and duration. Infusion of rPAMP (200 nmol/kg) increased HR and RSNA but had no effect on RBF. Baroreceptor denervation suppressed, but did not diminish, the increases in HR and RSNA to rAM2. These findings indicate that the physiological roles of rAM2 and rAM are similar and that rAM2 also has a long-lasting vasodilator action on the renal vascular bed.     

Coactivation of renal sympathetic neurons and somatic motor neurons by chemical stimulation of the midbrain ventral tegmental area

We examined whether neurons in the midbrain ventral tegmental area (VTA) play a role in generating central command responsible for autonomic control of the cardiovascular system in anesthetized rats and unanesthetized, decerebrated rats with muscle paralysis. Small volumes (60 nl) of an N-methyl-D-aspartate receptor agonist (L-homocysteic acid) and a GABAergic receptor antagonist (bicuculline) were injected into the VTA and substantia nigra (SN). In anesthetized rats, L-homocysteic acid into the VTA induced short-lasting increases in renal sympathetic nerve activity (RSNA; 66 ± 21%), mean arterial pressure (MAP; 5 ± 2 mmHg), and heart rate (HR; 7 ± 2 beats/min), whereas bicuculline into the VTA produced long-lasting increases in RSNA (130 ± 45%), MAP (26 ± 2 mmHg), and HR (66 ± 6 beats/min). Bicuculline into the VTA increased blood flow and vascular conductance of the hindlimb triceps surae muscle, suggesting skeletal muscle vasodilatation. However, neither drug injected into the SN affected all variables. Renal sympathetic nerve and cardiovascular responses to chemical stimulation of the VTA were not essentially affected by decerebration at the premammillary-precollicular level, indicating that the ascending projection to the forebrain from the VTA was not responsible for evoking the sympathetic and cardiovascular responses. Furthermore, bicuculline into the VTA in decerebrate rats produced long-lasting rhythmic bursts of RSNA and tibial motor nerve discharge, which occurred in good synchrony. It is likely that the activation of neurons in the VTA is capable of eliciting synchronized stimulation of the renal sympathetic and tibial motor nerves without any muscular feedback signal.     

Nitric oxide modulation of glutamatergic, baroreflex, and cardiopulmonary transmission in the nucleus of the solitary tract

The neuromodulatory effect of NO on glutamatergic transmission has been studied in several brain areas. Our previous single-cell studies suggested that NO facilitates glutamatergic transmission in the nucleus of the solitary tract (NTS). In this study, we examined the effect of the nitric oxide synthase (NOS) inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME) on glutamatergic and reflex transmission in the NTS. We measured mean arterial pressure (MAP), heart rate (HR), and renal sympathetic nerve activity (RSNA) from Inactin-anesthetized Sprague-Dawley rats. Bilateral microinjections of L-NAME (10 nmol/100 nl) into the NTS did not cause significant changes in basal MAP, HR, or RSNA. Unilateral microinjection of (RS)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA, 1 pmol/100 nl) into the NTS decreased MAP and RSNA. Fifteen minutes after L-NAME microinjections, AMPA-evoked cardiovascular changes were significantly reduced. N-methyl-D-aspartate (NMDA, 0.5 pmol/100 nl) microinjection into the NTS decreased MAP, HR, and RSNA. NMDA-evoked falls in MAP, HR, and RSNA were significantly reduced 30 min after L-NAME. To examine baroreceptor and cardiopulmonary reflex function, L-NAME was microinjected at multiple sites within the rostro-caudal extent of the NTS. Baroreflex function was tested with phenylephrine (PE, 25 microg iv) before and after L-NAME. Five minutes after L-NAME the decrease in RSNA caused by PE was significantly reduced. To examine cardiopulmonary reflex function, phenylbiguanide (PBG, 8 microg/kg) was injected into the right atrium. PBG-evoked hypotension, bradycardia, and RSNA reduction were significantly attenuated 5 min after L-NAME. Our results indicate that inhibition of NOS within the NTS attenuates baro- and cardiopulmonary reflexes, suggesting that NO plays a physiologically significant neuromodulatory role in cardiovascular regulation.     

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.     

Activation of NTS A1 adenosine receptors inhibits regional sympathetic responses evoked by activation of cardiopulmonary chemoreflex

Previously we have shown that adenosine operating via the A(1) receptor subtype may inhibit glutamatergic transmission in the baroreflex arc within the nucleus of the solitary tract (NTS) and differentially increase renal (RSNA), preganglionic adrenal (pre-ASNA), and lumbar (LSNA) sympathetic nerve activity (ASNA>RSNA≥LSNA). Since the cardiopulmonary chemoreflex and the arterial baroreflex are mediated via similar medullary pathways, and glutamate is a primary transmitter in both pathways, it is likely that adenosine operating via A(1) receptors in the NTS may differentially inhibit regional sympathetic responses evoked by activation of cardiopulmonary chemoreceptors. Therefore, in urethane-chloralose-anesthetized rats (n = 37) we compared regional sympathoinhibition evoked by the cardiopulmonary chemoreflex (activated with right atrial injections of serotonin 5HT(3) receptor agonist phenylbiguanide, PBG, 1-8 μg/kg) before and after selective stimulation of NTS A(1) adenosine receptors [microinjections of N(6)-cyclopentyl adenosine (CPA), 0.033-330 pmol/50 nl]. Activation of cardiopulmonary chemoreceptors evoked differential, dose-dependent sympathoinhibition (RSNA>ASNA>LSNA), and decreases in arterial pressure and heart rate. These differential sympathetic responses were uniformly attenuated in dose-dependent manner by microinjections of CPA into the NTS. Volume control (n = 11) and blockade of adenosine receptor subtypes in the NTS via 8-(p-sulfophenyl)theophylline (8-SPT, 1 nmol in 100 nl) (n = 9) did not affect the reflex responses. We conclude that activation of NTS A(1) adenosine receptors uniformly inhibits neural and cardiovascular cardiopulmonary chemoreflex responses. A(1) adenosine receptors have no tonic modulatory effect on this reflex under normal conditions. However, when adenosine is released into the NTS (i.e., during stress or severe hypotension/ischemia), it may serve as negative feedback regulator for depressor and sympathoinhibitory reflexes integrated in the NTS.     

Differential role of nitric oxide in regional sympathetic responses to stimulation of NTS A2a adenosine receptors

Our previous studies showed that preganglionic adrenal (pre-ASNA), renal (RSNA), lumbar, and postganglionic adrenal sympathetic nerve activities (post-ASNA) are inhibited after stimulation of arterial baroreceptors, nucleus of the solitary tract (NTS), and glutamatergic and P2x receptors and are activated after stimulation of adenosine A1 receptors. However, stimulation of adenosine A2a receptors inhibited RSNA and post-ASNA, whereas it activated pre-ASNA. Because the effects evoked by NTS A2a receptors may be mediated via activation of nitric oxide (NO) mechanisms in NTS neurons, we tested the hypothesis that NO synthase (NOS) inhibitors would attenuate regional sympathetic responses to NTS A2a receptor stimulation, whereas NO donors would evoke contrasting responses from pre-ASNA versus RSNA and post-ASNA. Therefore, in chloralose/urethane-anesthetized rats, we compared hemodynamic and regional sympathetic responses to microinjections of selective A2a receptor agonist (CGS-21680, 20 pmol/50 nl) after pretreatment with NOS inhibitors Nomega-nitro-L-arginine methyl ester (10 nmol/100 nl) and 1-[2-(trifluoromethyl)phenyl]imidazole (100 pmol/100 nl) versus pretreatment with vehicle (100 nl). In addition, responses to microinjections into the NTS of different NO donors [40 and 400 pmol/50 nl sodium nitroprusside (SNP); 0.5 and 5 nmol/50 nl 3,3-bis(aminoethyl)-1-hydroxy-2-oxo-1-triazene (DETA NONOate, also known as NOC-18), and 2 nmol/50 nl 3-(2-hydroxy-2-nitroso-1-propylhydrazino)-1-propanamine (PAPA NONOate, also known as NOC-15)], the NO precursor L-arginine (10-50 nmol/50 nl), and sodium glutamate (500 pmol/50 nl) were evaluated. SNP, DETA NONOate, and PAPA NONOate activated pre-ASNA and inhibited RSNA and post-ASNA, whereas l-arginine and glutamate microinjected into the same site of the NTS inhibited all these sympathetic outputs. Decreases in heart rate and depressor or biphasic responses accompanied the neural responses. Pretreatment with NOS inhibitors reversed the normal depressor and sympathoinhibitory responses to stimulation of NTS A2a receptors into pressor and sympathoactivatory responses and attenuated the heart rate decreases; however, it did not change the increases in pre-ASNA. We conclude that NTS NO mechanisms differentially affect regional sympathetic outputs and differentially contribute to the pattern of regional sympathetic responses evoked by stimulation of NTS A2a receptors.     

Effect of cardiac receptor stimulation on renal vascular resistance in the pregnant rat

Stimulation of cardiac receptors (CR) evokes blunted reflex reductions in mean arterial pressure (MAP) in pregnant compared with virgin rats. Because CR-mediated sympathoinhibition has preferential effects on the kidney, we tested whether, during pregnancy, renal vascular resistance (RVR) changes less in response to CR stimulation and investigated possible mechanisms. MAP, right atrial pressure, renal sympathetic nerve activity (RSNA), renal blood flow (RBF), and RVR were measured in anesthetized animals in response to CR stimulation by graded atrial injections of saline. Baseline MAP and RVR and reflex changes in these variables during CR stimulation were reduced in late-pregnant vs. virgin rats (P<0.05). Reflex changes in RSNA were attenuated in pregnant rats, but changes in RBF as a function of RSNA were similar in both groups. ANG II AT(1)-receptor blockade increased basal RBF more in virgin rats (P<0.05), but between-group differences in reflex changes in MAP, RSNA, and RVR were maintained after AT(1) blockade. Thus during CR simulation, reflex changes in RVR were reduced in pregnant versus virgin rats. This difference does not appear to involve differential effects of ANG II.     

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 alpha-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 integralDelta%), virtually abolished the decreases in HR (-258.3 +/- 54.0 vs. 18.9 +/- 57.8 integralDeltabeats/min) and RSNA (-239.3 +/- 47.4 vs. 15.9 +/- 36.1 integralDelta%), and did not affect the increases in pre-ASNA (279.7 +/- 48.3 vs. 233.1 +/- 54.1 integralDelta%) evoked by A2a receptor stimulation. The responses partially returned toward normal values approximately 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.     

Impact of L-NAME on the cardiopulmonary reflex in cardiac hypertrophy

There is evidence that in cardiac failure, there is defective baroreceptor reflex control of sympathetic nerve activity. Often, cardiac failure is preceded by a state of cardiac hypertrophy in which there may be enhanced performance of the heart. This study investigated whether in two different models of cardiac hypertrophy, there was an increased contribution of nitric oxide (NO) to the low-pressure baroreceptor regulation of renal sympathetic nerve activity (RSNA) and nerve-dependent excretory function. Administration of a volume load, 0.25* body wt/min saline for 30 min, in normal rats decreased RSNA by 40* and increased urine flow by some 9-fold. Following nitro-L-arginine methyl ester (L-NAME) administration, 10 μg·kg(-1)·min(-1) for 60 min, which had no effect on blood pressure, heart rate, or RSNA, the volume load-induced renal sympathoinhibitory and excretory responses were markedly enhanced. In cardiac hypertrophy states induced by 2 wk of isoprenaline/caffeine or 1 wk thyroxine administration, the volume challenge failed to suppress RSNA, and there were blunted increases in urine flow in the innervated kidneys, but following L-NAME infusion, the volume load decreased RSNA by 30-40* and increased urine flow by some 20-fold in the innervated kidneys, roughly to the same extent as observed in normal rats. These findings suggest that the blunted renal sympathoinhibition and nerve-dependent diuresis to the volume load in cardiac hypertrophy are related to a heightened production or activity of NO within either the afferent or central arms of the reflex.     

Differential role of ionotropic glutamatergic mechanisms in responses to NTS P(2x) and A(2a) receptor stimulation

Activation of ATP P(2x) receptors in the subpostremal nucleus tractus solitarii (NTS) via microinjection of alpha,beta-methylene ATP (alpha,beta-MeATP) elicits fast initial depressor and sympathoinhibitory responses that are followed by slow, long-lasting inhibitory effects. Activation of NTS adenosine A(2a) receptors via microinjection of CGS-21680 elicits slow, long-lasting decreases in arterial pressure and renal sympathetic nerve activity (RSNA) and an increase in preganglionic adrenal sympathetic nerve activity (pre-ASNA). Both P(2x) and A(2a) receptors may operate via modulation of glutamate release from central neurons. We investigated whether intact glutamatergic transmission is necessary to mediate the responses to NTS P(2x) and A(2a) receptor stimulation. The hemodynamic and neural (RSNA and pre-ASNA) responses to microinjections of alpha,beta-MeATP (25 pmol/50 nl) and CGS-21680 (20 pmol/50 nl) were compared before and after pretreatment with kynurenate sodium (KYN; 4.4 nmol/100 nl) in chloralose-urethan-anesthetized male Sprague-Dawley rats. KYN virtually abolished the fast responses to alpha,beta-MeATP and tended to enhance the slow component of the neural responses. The depressor responses to CGS-21680 were mostly preserved after pretreatment with KYN, although the increase in pre-ASNA was reduced by one-half following the glutamatergic blockade. We conclude that the fast responses to stimulation of NTS P(2x) receptors are mediated via glutamatergic ionotropic mechanisms, whereas the slow responses to stimulation of NTS P(2x) and A(2a) receptors are mediated mostly via other neuromodulatory mechanisms.     

Leptin in nucleus of the solitary tract alters the cardiovascular responses to aortic baroreceptor activation

Recent data suggests that neurons expressing the long form of the leptin receptor form at least two distinct groups within the caudal nucleus of the solitary tract (NTS): a group within the lateral NTS (Slt) and one within the medial (Sm) and gelantinosa (Sg) NTS. Discrete injections of leptin into Sm and Sg, a region that receives chemoreceptor input, elicit increases in arterial pressure (AP) and renal sympathetic nerve activity (RSNA). However, the effect of microinjections of leptin into Slt, a region that receives baroreceptor input is unknown. Experiments were done in the urethane-chloralose anesthetized, paralyzed and artificially ventilated Wistar or Zucker obese rat to determine leptin's effect in Slt on heart rate (HR), AP and RSNA during electrical stimulation of the aortic depressor nerve (ADN). Depressor sites within Slt were first identified by the microinjection of l-glutamate (Glu; 0.25 M; 10 nl) followed by leptin microinjections. In the Wistar rat leptin microinjection (50 ng; 20 nl) into depressor sites within the lateral Slt elicited increases in HR and RSNA, but no changes in AP. Additionally, leptin injections into Slt prior to Glu injections at the same site or to stimulation of the ADN were found to attenuate the decreases in HR, AP and RSNA to both the Glu injection and ADN stimulation. In Zucker obese rats, leptin injections into NTS depressor sites did not elicit cardiovascular responses, nor altered the cardiovascular responses elicited by stimulation of ADN. Those data suggest that leptin acts at the level of NTS to alter the activity of neurons that mediate the cardiovascular responses to activation of the aortic baroreceptor reflex.     

Glucocorticoids reduce responses to AMPA receptor activation and blockade in nucleus tractus solitarius

We tested the hypothesis that glucocorticoids attenuate changes in arterial pressure and renal sympathetic nerve activity (RSNA) in response to activation and blockade of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors within the nucleus of the solitary tract (NTS). Experiments were performed in Inactin-anesthetized male Sprague-Dawley rats treated for 7 +/- 1 days with a subcutaneous corticosterone (Cort) pellet or in control rats. Baseline mean arterial pressure (MAP) was significantly higher in Cort-treated rats (109 +/- 2 mmHg, n = 39) than in control rats (101 +/- 1 mmHg, n = 48, P < 0.05). In control rats, microinjection of AMPA (0.03, 0.1, and 0.3 pmol/100 nl) into the NTS significantly decreased MAP at all doses and decreased RSNA at 0.1 and 0.3 pmol/100 nl. Responses to AMPA in Cort-treated rats were attenuated at all doses of AMPA (P < 0.05). Responses to the AMPA-kainate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) were also significantly reduced in Cort-treated rats relative to control rats. Blockade of glucocorticoid type II receptors with mifepristone significantly enhanced responses to CNQX in both control and Cort rats. We conclude that glucocorticoids attenuate MAP and RSNA responses to activation and blockade of AMPA receptors in the NTS.     

AT1 receptors in the nucleus tractus solitarii mediate the interaction between the baroreflex and the cardiac sympathetic afferent reflex in anesthetized rats

The cardiac "sympathetic afferent" reflex (CSAR) has been reported to increase sympathetic outflow and depress baroreflex function via a central angiotensin II (ANG II) mechanism. In the present study, we examined the role of ANG II type 1 (AT(1)) receptors in the nucleus tractus solitarii (NTS) in mediating the interaction between the CSAR and the baroreflex in anesthetized rats. We examined the effects of bilateral microinjection of AT(1) receptor antagonist losartan (100 pmol) into the NTS on baroreflex control of renal sympathetic nerve activity (RSNA) before and after CSAR activation by epicardial application of capsaicin (0.4 microg). Using single-unit extracellular recording, we further examined the effects of CSAR activation on the barosensitivity of barosensitive NTS neurons and the effects of intravenous losartan (2 mg/kg) on CSAR-induced changes in activity of NTS barosensitive neurons. Bilateral NTS microinjection of losartan significantly attenuated the increases in arterial pressure, heart rate, and RSNA evoked by capsaicin but also markedly (P < 0.01) reversed the CSAR-induced blunted baroreflex control of RSNA (Gain(max) from 1.65 +/- 0.10 to 2.22 +/- 0.11%/mmHg). In 17 of 24 (70.8%) NTS barosensitive neurons, CSAR activation significantly (P < 0.01) inhibited the baseline neuronal activity and attenuated the neuronal barosensitivity. In 11 NTS barosensitive neurons, intravenous losartan effectively (P < 0.01) normalized the decreased neuronal barosensitivity induced by CSAR activation. In conclusion, blockade of NTS AT(1) receptors improved the blunted baroreflex during CSAR activation, suggesting that the NTS plays an important role in processing the interaction between the baroreflex and the CSAR via an AT(1) receptor-dependent mechanism.     

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.     

Central nitric oxide modulates hindquarter vasodilation elicited by AMPA receptor stimulation in the NTS of conscious rats

Microinjection of S-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) in the nucleus of the solitary tract (NTS) of conscious rats causes hypertension, bradycardia, and vasoconstriction in the renal, mesenteric, and hindquarter vascular beds. In the hindquarter, the initial vasoconstriction is followed by vasodilation with AMPA doses >5 pmol/100 nl. To test the hypothesis that this vasodilation is caused by activation of a nitroxidergic pathway in the NTS, we examined the effect of pretreatment with the nitric oxide synthase inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME, 10 nmol/100 nl, microinjected into the NTS) on changes in mean arterial pressure, heart rate, and regional vascular conductance (VC) induced by microinjection of AMPA (10 pmol/100 nl in the NTS) in conscious rats. AMPA increased hindquarter VC by 18 +/- 4%, but after pretreatment with L-NAME, AMPA reduced hindquarter VC by 16 +/- 7% and 17 +/- 9% (5 and 15 min after pretreatment, P < 0.05 compared with before pretreatment). Pretreatment with L-NAME reduced AMPA-induced bradycardia from 122 +/- 40 to 92 +/- 32 beats/min but did not alter the hypertension induced by AMPA (35 +/- 5 mmHg before pretreatment, 43 +/- 6 mmHg after pretreatment). Control injections with D-NAME did not affect resting values or the response to AMPA. The present study shows that stimulation of AMPA receptors in the NTS activates both vasodilatatory and vasoconstrictor mechanisms and that the vasodilatatory mechanism depends on production of nitric oxide in the NTS.     

Stimulation of NTS A1 adenosine receptors differentially resets baroreflex control of regional sympathetic outputs

Previously we showed that pressor and differential regional sympathoexcitatory responses (adrenal > renal >/= lumbar) evoked by stimulation of A(1) adenosine receptors located in the nucleus of the solitary tract (NTS) were attenuated/abolished by baroreceptor denervation or blockade of glutamatergic transmission in the NTS, suggesting A(1) receptor-elicited inhibition of glutamatergic transmission in baroreflex pathways. Therefore we tested the hypothesis that stimulation of NTS A(1) adenosine receptors differentially inhibits/resets baroreflex responses of preganglionic adrenal (pre-ASNA), renal (RSNA), and lumbar (LSNA) sympathetic nerve activity. In urethane-chloralose-anesthetized male Sprague-Dawley rats (n = 65) we compared baroreflex-response curves (iv nitroprusside and phenylephrine) evoked before and after bilateral microinjections into the NTS of A(1) adenosine receptor agonist (N(6)-cyclopentyladenosine, CPA; 0.033-330 pmol/50 nl). CPA evoked typical dose-dependent pressor and differential sympathoexcitatory responses and similarly shifted baroreflex curves for pre-ASNA, RSNA, and LSNA toward higher mean arterial pressure (MAP) in a dose-dependent manner; the maximal shifts were 52.6 +/- 2.8, 48.0 +/- 3.6, and 56.8 +/- 6.7 mmHg for pre-ASNA, RSNA, and LSNA, respectively. These shifts were not a result of simple baroreceptor resetting because they were two to three times greater than respective increases in baseline MAP evoked by CPA. Baroreflex curves for pre-ASNA were additionally shifted upward: the maximal increases of upper and lower plateaus were 41.8 +/- 16.4% and 45.3 +/- 8.7%, respectively. Maximal gain (%/mmHg) measured before vs. after CPA increased for pre-ASNA (3.0 +/- 0.6 vs. 4.9 +/- 1.3), decreased for RSNA (4.1 +/- 0.6 vs. 2.3 +/- 0.3), and remained unaltered for LSNA (2.1 +/- 0.2 vs. 2.0 +/- 0.1). Vehicle control did not alter the baroreflex curves. We conclude that the activation of NTS A(1) adenosine receptors differentially inhibits/resets baroreflex control of regional sympathetic outputs.     

Relationship between renal sympathetic nerve activity and renal blood flow during natural behavior in rats

The purpose of the present study was to determine the relationship between renal sympathetic nerve activity (RSNA) and renal blood flow (RBF) during normal daily activity in conscious, chronically instrumented Wistar rats (n = 8). The animal's behavior was classified as rapid eye movement (REM) sleep, non-REM (NREM) sleep, quiet awake, moving, and grooming states. On average RSNA was lowest during REM sleep, which was decreased by 39.0 +/- 3.2% (P < 0.05) relative to NREM sleep, and rose linearly with an increase in activity level in the order of quiet awake (by 10.9 +/- 1.8%, P < 0.05), moving (by 29.4 +/- 2.9%, P < 0.05), and grooming (by 65.3 +/- 3.9%, P < 0.05) relative to NREM sleep. By contrast, RBF was highest during REM sleep, which was increased by 4.8 +/- 0.7% (P < 0.05) relative to NREM sleep and decreased significantly (P < 0.05) by 5.5 +/- 0.6 and 6.6 +/- 0.5% during moving and grooming states, respectively, relative to NREM sleep. There was a significant (P < 0.05) inverse linear relationship between the percent changes in RSNA and RBF and between those in RSNA and renal vascular conductance. Furthermore, renal denervation (n = 8) abolished the changes in RBF induced by different natural behavioral activities. These results suggest that the changes in RSNA induced by natural behavioral activities had a significant influence on RBF.