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.     

Skeletal muscle reflex-mediated changes in sympathetic nerve activity are abnormal in spontaneously hypertensive rats

In hypertension, the blood pressure response to exercise is exaggerated. We demonstrated previously that this heightened pressor response to physical activity is mediated by an overactive skeletal muscle exercise pressor reflex (EPR), with important contributions from its metaboreflex and mechanoreflex components. However, the mechanisms driving the abnormal blood pressure response to EPR activation are largely unknown. Recent evidence in humans suggests that the muscle metaboreflex partially mediates the enhanced EPR-induced pressor response via abnormally large changes in sympathetic nerve activity (SNA). Whether the muscle mechanoreflex induces similarly exaggerated alterations in SNA in hypertension remains unknown, as does the role of the mechanoreceptors mediating muscle reflex activity. To address these issues, the EPR was selectively activated by electrically inducing hindlimb muscle contraction in decerebrate normotensive Wistar-Kyoto (WKY) and spontaneously hypertensive (SHR) rats. Stimulation of the EPR evoked significantly larger increases in mean arterial pressure (MAP) and renal SNA (RSNA) in SHR compared with WKY (ΔRSNA from baseline: 140 ± 11 vs. 48 ± 8%). The mechanoreflex was stimulated by stretching hindlimb muscle which likewise elicited significantly greater elevations in MAP and RSNA in SHR than WKY (ΔRSNA from baseline: 105 ± 11 vs. 35 ± 7%). Blockade of mechanoreceptors in muscle with gadolinium significantly attenuated the MAP and RSNA responses to contraction and stretch in SHR. These data suggest that 1) the exaggerated pressor response to activation of the EPR and muscle mechanoreflex in hypertension is mediated by abnormally large reflex-induced augmentations in SNA and 2) this accentuated sympathetic responsiveness is evoked, in part, by stimulation of muscle mechanoreceptors.     

alpha(2)-Adrenergic receptors in NTS facilitate baroreflex function in adult spontaneously hypertensive rats

We examined the effect of alpha(2)-adrenoreceptor blockade in the nucleus of the solitary tract (NTS) on baroreflex responses elicited by electrical stimulation of the left aortic depressor nerve (ADN) in urethane-anesthetized spontaneously hypertensive rats (SHR, n = 11) and normotensive Wistar-Kyoto rats (WKY, n = 11). ADN stimulation produced a frequency-dependent decrease in mean arterial pressure (MAP), renal sympathetic nerve activity (RSNA), and heart rate (HR). In SHR, unilateral microinjection of idazoxan into the NTS markedly reduced baroreflex control of MAP, RSNA, and HR and had a disproportionately greater influence on baroreflex control of MAP than of RSNA. In WKY, idazoxan microinjections did not significantly alter baroreflex function relative to control vehicle injections. These results suggest that baroreflex regulation of arterial pressure in SHR is highly dependent on NTS adrenergic mechanisms. The reflex regulation of sympathetic outflow to the kidney is less influenced by the altered alpha(2)-adrenoreceptor mechanisms in SHR.     

Exercise training improves endogenous nitric oxide mechanisms within the paraventricular nucleus in rats with heart failure

Previously, we have demonstrated that an altered endogenous nitric oxide (NO) mechanism within the paraventricular nucleus (PVN) contributes to increased renal sympathetic nerve activity (RSNA) in heart failure (HF) rats. The goal of this study was to examine the effect of exercise training (ExT) in improving the endogenous NO mechanism within the PVN involved in the regulation of RSNA in rats with HF. ExT significantly restored the decreased number of neuronal NO synthase (nNOS)-positive neurons in the PVN (129 +/- 17 vs. 99 +/- 6). nNOS mRNA expression and protein levels in the PVN were also significantly increased in HF-ExT rats compared with HF-sedentary rats. To examine the functional role of NO within the PVN, an inhibitor of NOS, N(G)-monomethyl-L-arginine, was microinjected into the PVN. Dose-dependent increases in RSNA, arterial blood pressure (BP), and heart rate (HR) were produced in all rats. There was a blunted increase in these parameters in HF rats compared with the sham-operated rats. ExT significantly augmented RSNA responses in rats with HF (33% vs. 20% at the highest dose), thus normalizing the responses. The NO donor sodium nitroprusside, microinjected into the PVN, produced dose-dependent decreases in RSNA, BP, and HR in both sham and HF rats. ExT significantly improved the blunted decrease in RSNA in HF rats (36% vs. 17% at the highest dose). In conclusion, our data indicate that ExT improves the altered NO mechanism within the PVN and restores NO-mediated changes in RSNA in rats with HF.     

The effect of CNS opioid on autonomic nervous and cardiovascular responses in diet-induced obese rats

The intracerebroventricular (i.c.v.) infusion of beta-endorphin can cause either a decrease in blood pressure in normal rats or an increase in obese rats. Diet-induced obesity is associated with an increase of hypothalamic mu opioid receptors. Since beta-endorphins act by opioid receptors, we investigated the effect of CNS mu as well as kappa opioid receptor agonist and antagonist on mean blood pressure (MAP), heart rate (HR) and renal sympathetic nerve activity (RSNA) in male Wistar rats fed either a high fat (HF) (40% fat by weight) or a regular low fat (control) (4% fat by weight) diet. After a 12-week-feeding period the animals were implanted with i.c.v. cannulas and 3-5 days later they were anesthetized and instrumented to record MAP, HR and RSNA. HF rats have higher MAP and the i.c.v. injection of a mu opioid agonist (DAMGO) initially decreased the MAP and then increased MAP, HR and RSNA in the normal animals. The increase was greater in HF animals. The i.c.v. injection of the mu antagonist (beta-FNA) resulted in a significantly greater decrease in MAP in HF animals. beta-FNA increased the RSNA in the HF rats but decreased it in the normal rats. The kappa agonist (dynorphin) decreased MAP in normal rats followed by a return to baseline, but not in HF rats. The kappa antagonist, nor-binaltorphimine (N-BP), increased MAP and RSNA in normal rats and to a lesser extent in HF rats. These findings suggest that rats given a high fat diet have higher blood pressures and a greater mu opioid-mediated responsiveness with a greater mu opioid-mediated autonomic tone. Additionally there is a decreased kappa responsiveness and tone in the HF rats. Both these changes, increased mu and decreased kappa responsiveness could strongly contribute to the increased blood pressure in obese animals.     

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.     

Involvement of enhanced cardiac sympathetic afferent reflex in sympathetic activation in early stage of diabetes

Cardiac sympathetic afferent reflex (CSAR) is involved in sympathetic activation. The present study was designed to investigate the contribution of enhanced CSAR to sympathetic activation in the early stage of diabetes and the involvement of AT(1) receptors in the paraventricular nucleus (PVN). Diabetes was induced by a single intravenous injection of streptozotocin in rats. Acute experiments were carried out under anesthesia after 3 wk. The CSAR was evaluated by the responses of renal sympathetic nerve activity (RSNA) and mean arterial pressure (MAP) to epicardial application of capsaicin or bradykinin. Sympathetic activity and CSAR were enhanced in diabetic rats. Plasma norepinephrine and angiotensin II were increased, but the transient receptor potential vanilloid 1 (TRPV1) in the left ventricle wall was not significantly increased in diabetic rats. Pericardial injection of resiniferatoxin to desensitize cardiac afferents or PVN microinjection of lidocaine attenuated the CSAR and decreased the RSNA and MAP in diabetic rats. The AT(1) receptor expression in the PVN increased in diabetic rats. Angiotensin II in the PVN caused greater increases in the RSNA and MAP and enhancement in the CSAR in diabetic rats, which were abolished by the losartan pretreatment. Losartan decreased the RSNA and MAP and attenuated the CSAR in diabetic rats but not in control rats. These results indicate that the CSAR is enhanced in the early stage of diabetic rats, which contributes to the sympathetic activation. AT(1) receptors in the PVN are involved in the enhanced CSAR in diabetic rats.     

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

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

Cardiac sympathetic afferent stimulation augments the arterial chemoreceptor reflex in anesthetized rats.

Chronic heart failure (CHF) is well known to be associated with both an enhanced chemoreceptor reflex and an augmented cardiac "sympathetic afferent reflex" (CSAR). The augmentation of the CSAR may play an important role in the enhanced chemoreceptor reflex in the CHF state because the same central areas are involved in the sympathetic outputs of both reflexes. We determined whether chemical and electrical stimulation of the CSAR augments chemoreceptor reflex function in normal rats. Under anesthesia, renal sympathetic nerve activity (RSNA) and mean arterial pressure (MAP) were recorded. The chemoreceptor reflex was tested by unilateral intra-carotid artery bolus injection of potassium cyanide (KCN) and nicotine. We found that 1) left ventricular epicardial application of capsaicin increased the pressor responses and the RSNA responses to chemoreflex activation induced by both KCN and nicotine; 2) when the central end of the left cardiac sympathetic nerve was electrically stimulated, both the pressor and the RSNA responses to chemoreflex activation induced by KCN were increased; 3) pretreatment with intracerebroventricular injection of losartan (500 nmol) completely prevented the enhanced chemoreceptor reflex induced by electrical stimulation of the cardiac sympathetic nerve; and 4) bilateral microinjection of losartan (250 pmol) into the nucleus tractus solitarii (NTS) completely abolished the enhanced chemoreceptor reflex by epicardial application of capsaicin. These results suggest that both the chemical and electrical stimulation of the CSAR augments chemoreceptor reflex and that central ANG II, specially located in the NTS, plays a major role in these reflex interactions.     

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.     

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.     

Anandamide content and interaction of endocannabinoid/GABA modulatory effects in the NTS on baroreflex-evoked sympathoinhibition

Cannabinoids have been shown to modulate central autonomic regulation and baroreflex control of blood pressure (BP). The presence of cannabinoid CB(1) receptors on fibers in the nucleus tractus solitarius (NTS) suggests that some presynaptic modulation of transmitter release could occur in this region, which receives direct afferent projections from arterial baroreceptors and cardiac mechanoreceptors. This study, therefore, was performed to determine the mechanism(s) of effects of microinjection of an endocannabinoid, arachidonylethanolamide (anandamide, AEA), into the NTS on baroreflex sympathetic nerve responses produced by phenylephrine-induced pressure changes in anesthetized rats. AEA prolonged reflex inhibition of renal sympathetic nerve activity (RSNA), suggesting an increase in baroreflex sensitivity. This effect of AEA was blocked by prior microinjection of SR-141716 to block cannabinoid CB(1) receptors. To determine whether this baroreflex enhancement by AEA involved a GABA(A) mechanism, the baroreflex response to AEA was tested after prior blockade of postsynaptic GABA(A) receptors by bicuculline, which would eliminate any effects due to modulation of GABA activity. After bicuculline, which alone prolonged the baroreflex inhibition of RSNA, AEA shortened the duration of RSNA inhibition, suggesting a possible presynaptic inhibition of glutamate release previously obscured by a more dominant GABA(A) effect. To support a possible physiological role for AEA, AEA concentration in the NTS was measured after a phenylephrine-induced increase in BP. AEA content in the NTS was increased significantly over that in normotensive animals. These results support the hypothesis that AEA content is increased by brief periods of hypertension and suggest that AEA can modulate the baroreflex through activation of CB(1) receptors within the NTS, possibly modulating effectiveness of GABA and/or glutamate neurotransmission.     

Exercise training improves aortic depressor nerve sensitivity in rats with ischemia-induced heart failure

Exercise training improves arterial baroreflex control in heart failure (HF) rabbits. However, the mechanisms involved in the amelioration of baroreflex control are unknown. We tested the hypothesis that exercise training would increase the afferent aortic depressor nerve activity (AODN) sensitivity in ischemic-induced HF rats. Twenty ischemic-induced HF rats were divided into trained (n = 11) and untrained (n = 9) groups. Nine normal control rats were also studied. Power spectral analysis of pulse interval, systolic blood pressure, renal sympathetic nerve activity (RSNA), and AODN were analyzed by means of autoregressive parametric spectral and cross-spectral algorithms. Spontaneous baroreflex sensitivity of heart rate (HR) and RSNA were analyzed during spontaneous variation of systolic blood pressure. Left ventricular end-diastolic pressure was higher in HF rats compared with that in the normal control group (P = 0.0001). Trained HF rats had a peak oxygen uptake higher than untrained rats and similar to normal controls (P = 0.01). Trained HF rats had lower low-frequency [1.8 +/- 0.2 vs. 14.6 +/- 3 normalized units (nu), P = 0.0003] and higher high-frequency (97.9 +/- 0.2 vs. 85.0 +/- 3 nu, P = 0.0005) components of pulse interval than untrained rats. Trained HF rats had higher spontaneous baroreceptor sensitivity of HR (1.19 +/- 0.2 vs. 0.51 +/- 0.1 ms/mmHg, P = 0.003) and RSNA [2.69 +/- 0.4 vs. 1.29 +/- 0.3 arbitrary units (au)/mmHg, P = 0.04] than untrained rats. In HF rats, exercise training increased spontaneous AODN sensitivity toward normal levels (trained HF rats, 1,791 +/- 215; untrained HF rats, 1,150 +/- 158; and normal control rats, 2,064 +/- 327 au/mmHg, P = 0.05). In conclusion, exercise training improves AODN sensitivity in HF rats.     

Exercise training normalizes enhanced sympathetic activation from the paraventricular nucleus in chronic heart failure: role of angiotensin II

Exercise training (ExT) normalizes the increased sympathetic outflow in heart failure (HF), but the underlying mechanisms are not known. We hypothesized ExT would normalize the augmented activation of the paraventricular nucleus (PVN) via an angiotensinergic mechanism during HF. Four groups of rats used were the following: 1) sham-sedentary (Sed); 2) sham-ExT; 3) HF-Sed, and 4) HF-ExT. HF was induced by left coronary artery ligation. Four weeks after surgery, 3 wk of treadmill running was performed in ExT groups. The number of FosB-positive cells in the PVN was significantly increased in HF-Sed group compared with the sham-Sed group. ExT normalized (negated) this increase in the rats with HF. In anesthetized condition, the increases in renal sympathetic nerve activity (RSNA), mean arterial pressure (MAP), and heart rate (HR) in response to microinjection of angiotensin (ANG) II (50～200 pmol) in the PVN of HF-Sed group were significantly greater than of the sham-Sed group. In the HF-ExT group the responses to microinjection of ANG II were not different from sham-Sed or sham-ExT groups. Blockade of ANG II type 1 (AT(1)) receptors with losartan in the PVN produced a significantly greater decrease in RSNA, MAP, and HR in HF-Sed group compared with sham-Sed group. ExT prevented the difference between HF and sham groups. AT(1) receptor protein expression was increased 50% in HF-Sed group compared with sham-Sed group. In the HF-ExT group, AT(1) receptor protein expression was not significantly different from sham-Sed or sham-ExT groups. In conclusion, one mechanism by which ExT alleviates elevated sympathetic outflow in HF may be through normalization of angiotensinergic mechanisms within the PVN.     

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.     

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.     

Glutamatergic inputs to the CVLM independent of the NTS promote tonic inhibition of sympathetic vasomotor tone in rats

GABAergic neurons in the caudal ventrolateral medulla (CVLM) are driven by baroreceptor inputs relayed via the nucleus tractus solitarius (NTS), and they inhibit neurons in rostral ventrolateral medulla to reduce sympathetic nerve activity (SNA) and arterial pressure (AP). After arterial baroreceptor denervation or lesions of the NTS, inhibition of the CVLM continues to increase AP, suggesting additional inputs also tonically activate the CVLM. This study examined whether the NTS contributes to baroreceptor-independent drive to the CVLM and whether glutamate promotes baroreceptor- and NTS-independent activation of the CVLM to tonically reduce SNA. In addition, we evaluated whether altering central respiratory drive, a baroreceptor-independent regulator of CVLM neurons, influences glutamatergic inputs to the CVLM. Splanchnic SNA and AP were measured in chloralose-anesthetized, ventilated, paralyzed rats. The infusion of nitroprusside decreased AP below threshold for baroreceptor afferent firing (<50 mmHg) and increased SNA to 209+/-22% (P<0.05), but the subsequent inhibition of the NTS by microinjection of the GABA(A) agonist muscimol did not further increase SNA. In contrast, after inhibition of the NTS, blockade of glutamatergic inputs to CVLM by microinjection of kynurenate increased SNA (274+/-54%; P<0.05; n=7). In vagotomized rats with baroreceptors unloaded, inhibition of glutamatergic inputs to CVLM evoked a larger rise in SNA when central respiratory drive was increased (219+/-16% vs. 271+/-17%; n=5; P<0.05). These data suggest that baroreceptor inputs provide the major drive for the NTS-mediated excitation of the CVLM. Furthermore, glutamate tonically activates the CVLM to reduce SNA independent of the NTS, and this excitatory input appears to be affected by the strength of central respiratory drive.     

Altered nitric oxide mechanism within the paraventricular nucleus contributes to the augmented carotid body chemoreflex in heart failure

Our previous study demonstrated a contribution of the paraventricular nucleus (PVN) of the hypothalamus in the processing of the carotid body (CB) chemoreflex. Nitric oxide (NO) (within the PVN), known to modulate autonomic function, is altered in rats with heart failure (HF). Therefore, the goal of the present study was to examine the influence of endogenous and exogenous NO within the PVN on the sympathoexcitatory component of the peripheral chemoreflex in normal and HF states. We measured mean arterial blood pressure, heart rate, renal sympathetic nerve activity (RSNA), and phrenic nerve activity (PNA) in sham-operated and HF rats (6-8 wk after coronary artery ligation) after incremental doses of potassium cyanide (25-100 mug/kg iv). There was potentiation of the reflex responses in HF compared with sham-operated rats. Bilateral microinjection of an inhibitor of NO synthase, N(G)-monomethyl-l-arginine (50 pmol), into the PVN augmented the RSNA and PNA response to peripheral chemoreceptor stimulation in sham-operated rats but had no effect in HF rats. Conversely, bilateral microinjection of a NO donor, sodium nitroprusside (50 nmol), into the PVN attenuated the RSNA response of the peripheral chemoreflex in sham-operated rats but to a smaller extent in HF rats. These data indicate that 1) NO within the PVN plays an important role in the processing of the CB chemoreflex and 2) there is an impairment of the NO function within the PVN of HF rats, which contributes to an augmented peripheral chemoreflex and subsequent elevation of sympathetic activity in HF.     

Overexpression of eNOS in brain stem reduces enhanced sympathetic drive in mice with myocardial infarction

Reduced nitric oxide (NO) in the brain might contribute to enhanced sympathetic drive in heart failure (HF). The aim of this study was to determine whether increased NO production induced by local overexpression of endothelial NO synthase (eNOS) in the nucleus tractus solitarius (NTS) of the brain stem reduces the enhanced sympathetic drive in mice with HF. Myocardial infarction (MI) was induced in mice by ligating the left coronary artery. MI mice exhibited left ventricular dilatation and a reduced left ventricular ejection fraction. Urinary norepinephrine excretion in MI mice was greater than that in sham-operated mice, indicating that sympathetic drive was enhanced in this model. Thus this model has features that are typical of HF. Western blot analysis and immunohistochemical staining for neuronal NOS (nNOS) indicated that nNOS protein expression was significantly reduced in the brain stem of MI mice. MI mice had a significantly smaller increase in blood pressure evoked by intracisternal injection of N(G)-monomethyl-L-arginine than sham-operated mice. Adenoviral vectors encoding either eNOS (AdeNOS) or beta-galactosidase (Adbeta gal) were transfected into the NTS to examine the effect of increased NO production in the NTS on the enhanced sympathetic drive in HF. After the gene transfer, urinary norepinephrine excretion was reduced in AdeNOS-transfected MI mice but not in Adbeta gal-transfected MI mice. These results indicate that nNOS expression in the brain stem, especially in the NTS, is reduced in the MI mouse model of HF, and increased NO production induced by overexpression of eNOS in the NTS attenuates the enhanced sympathetic drive in this model.     

Differential roles for glutamate receptor subtypes within commissural NTS in cardiac-sympathetic reflex

Ischemic stimulation of cardiac receptors evokes excitatory sympathetic reflexes. Although the nucleus of the solitary tract (NTS) is an important site for integration of visceral afferents, its involvement in the cardiac-renal sympathetic reflex remains to be fully defined. This study examined the role of glutamate receptor subtypes in the commissural NTS in the sympathetic responses to stimulation of cardiac receptors. Renal sympathetic nerve activity (RSNA) was recorded in anesthetized rats. Cardiac receptors were stimulated by epicardial application of bradykinin (BK; 10 microg/ml). Application of BK significantly increased the mean arterial pressure from 78.2 +/- 2.2 to 97.5 +/- 2.9 mmHg and augmented RSNA by 38.5 +/- 2.5% (P < 0.05). Bilateral microinjection of 10 pmol of 6-cyano-7-nitroquinoxaline-2,3-dione, a non-N-methyl-D-aspartate (NMDA) antagonist, into the commissural NTS eliminated the pressor and RSNA responses to BK application in 10 rats. However, microinjection of 2-amino-5-phosphonopentanoic acid (0.1 and 1 nmol, n = 8), an NMDA- receptor antagonist, or alpha-methyl-4-carboxyphenylglycine (0.1 and 1 nmol, n = 5), a glutamate metabotropic receptor antagonist, failed to attenuate significantly the pressor and RSNA responses to stimulation of cardiac receptors with BK. Thus this study suggests that non-NMDA, but not NMDA and glutamate metabotropic, receptors in the commissural NTS play an important role in the sympathoexcitatory reflex response to activation of cardiac receptors during myocardial ischemia.