Glycine, like glutamate, microinjected into the nucleus tractus solitarii of rat decreases arterial pressure and heart rate

Glycinergic mechanisms have been implicated in central cardiovascular regulation. However, the inhibitory amino acid's role in the nucleus tractus solitarii (NTS), the site of termination of cardiovascular afferents, has not been clarified. Thus, we sought to determine if the microinjection of glycine into the NTS alters arterial pressure and heart rate. Microinjections of glycine, like glutamate, confined to the NTS decreased arterial pressure and heart rate in a neurally mediated, dose-dependent manner. The glycine antagonist strychnine completely blocked these effects of glycine but did not itself alter arterial pressure or heart rate, or interfere with the baroreceptor reflex. The acute hypotensive, bradycardic response to glycine was followed by a period during which glycine essentially eliminated the cardiovascular responses to the microinjection into the NTS of glutamate, an amino acid reputed to be a transmitter in the baroreceptor reflex arc. These data suggest that glycine is involved in cardiovascular regulation by the NTS but do not support its being an integral transmitter in the baroreceptor reflex.     

Microinjection of a 5-HT3 receptor agonist into the NTS of awake rats inhibits the bradycardic response to activation of the von Bezold-Jarisch reflex

In the present study we investigated the effects of bilateral microinjection into the lateral commissural nucleus tractus solitarius (NTS) of 2-methyl-5-HT, a 5-HT3 receptor agonist, on the bradycardic response of the von Bezold-Jarisch reflex of awake rats. We evaluated mainly the bradycardic response because in previous studies we documented that the hypotensive response of the von-Bezold-Jarisch reflex in awake rats is secondary to the intense bradycardic response. The Bezold-Jarisch reflex was activated by intravenous injection of serotonin (8 microg/kg) in awake rats before and 1, 3, 10, 20 and 60 min after bilateral microinjection of 2-methyl-5-HT (5 nmol/50 nl, n = 8) into the NTS. Microinjections of 2-methyl-5-HT into the NTS produced a significant increase in basal mean arterial pressure [(MAP), 97 +/- 4 vs. 114 +/- 4 mmHg), no changes in basal heart rate and a significant reduction in bradycardic (-78 +/- 19; -94 +/- 24 and -107 +/- 21 bpm) and hypotensive (-16 +/- 4; -10 +/- 5 and -17 +/- 4 mmHg) responses to activation of the von Bezold-Jarisch reflex at 3, 10 and 20 min, respectively, when compared with the control value (-231 +/- 13 bpm and -43 +/- 4 mmHg). The data of the present study suggest that serotonin acting on 5-HT3 receptors in the NTS may play an important inhibitory neuromodulatory role in the bradycardic response to activation of the von Bezold-Jarisch reflex.     

The effect of plasma volume expansion on the response to carotid occlusion in the non-human primate

The purpose of this study was to investigate the effects of plasma volume expansion on the cardiac and peripheral components of the baroreceptor reflex. Nine male Rhesus monkeys were chronically instrumented to measure arterial pressure and aortic blood flow. The aortic arch was denervated at the time of surgery. Bilateral carotid artery occlusion (BCO) was used to elicit the carotid sinus reflex both before and after 25% plasma volume expansion with an iso-osmotic dextran solution. The BCO elicited significant increases in heart rate (HR, 29.8 +/- 5.3 bpm) mean arterial pressure (MAP, 55.0 +/- 8.2 mm Hg) and total peripheral resistance (TPR, 0.076 +/- 0.01 mm Hg/ml/min) coupled with a significant reduction in mean aortic flow (AF, -246.9 +/- 55.3 ml/min) and stroke volume (SV, -2.86 +/- 0.36 ml). Volume expansion significantly attenuated the HR (15.2 +/- 5.8 bpm), MAP (30.4 +/- 4.4 mm Hg) and TPR (0.036 +/- 0.006 mm Hg/ml/min) response to carotid sinus hypotension. The changes in mean AF and SV elicited by BCO, however, were not significantly different between the control and volume expansion conditions. These data suggest that plasma volume expansion significantly attenuates the baroreceptor reflex control of circulation with a similar reduction in both the peripheral resistance and heart rate components of the response.     

Cardiovascular effects of microinjections of angiotensin II into the nucleus tractus solitarii

The role of the angiotensin II system within the nucleus tractus solitarii (NTS) in central cardiovascular control was investigated by local microinjections of angiotensin II and the angiotensin II receptor antagonist saralasin. Microinjections of 1 ng angiotensin II into the NTS resulted in a monophasic depressor response (-7.3 +/- 1.7 mm Hg), while higher doses were characterized by a biphasic response, with an initial decrease followed by a subsequent increase in blood pressure (10 ng: -4.7 +/- 1.3/+7.9 +/- 1.1 and 100 ng: -2.4 +/- 1.2/+7.5 +/- 1.2 mm Hg). Heart rate decreased significantly following microinjections of 1 and 10 ng angiotensin II (-27 +/- 5.0 and -15 +/- 5.9 bpm), while with 100 ng angiotensin II there was no significant effect on heart rate. Prior i.v. administration of atropine (1 mg/kg) abolished the bradycardia, but did not significantly affect the blood pressure response. Microinjections of saralasin into the NTS elicited a dose-dependent pressor response (10 ng: 6.0 +/- 1.5 mm Hg; 100 ng: 16.8 +/- 3.4 mm Hg) and tachycardia (10 ng: 5 +/- 3.2 bpm; 100 ng: 17 +/- 4.4 bpm). Our data support the hypothesis that angiotensin II acts on specific receptors within the NTS to modulate peripheral cardiovascular responses. The cardiovascular effects elicited by microinjections of the peptide exhibit complicated dose-response relationships. The effects on heart rate, but not on blood pressure, appear to be mediated by parasympathetic activation.     

Postnatal development of blood pressure and baroreflex in mice.

Postnatal development of blood pressure, heart rate and their regulation by arterial baroreceptor reflex in mice was examined. We first confirmed that simultaneous recordings of pulsatile blood pressure by the "servo null" method and the conventional catheter method gave almost identical tracings in halothane-anesthetized adult mice. We then measured blood pressure by servo null method together with electrocardiograph in mice of various ages from newborn to adult. Mean blood pressure increased progressively with age from 19 + 2 mm Hg in P0 newborn to 74+/-1 in adult mice, while heart rate initially increased from 365+/-12 bpm in newborn to 441+/-15 in infant (7 days old), and then decreased to 337+/-15 in adult mice. Between 1 and 2 weeks of age, gain of arterial baroreceptor reflex abruptly increased from a newborn value of 0.3 to a near adult value of 1.1 ms/mm Hg. On the other hand, sensitivity to anesthesia did not differ except for P1 and P2 newborns. We conclude that pulsatile blood pressure can be accurately measured by the servo null method even in the newborn mice and that baroreflex heart rate control mature at around 2 weeks after birth in the mice.     

Antagonism of the baroreceptor reflex by glutamate diethyl ester, an antagonist to L-glutamate

Recent reports have suggested that the excitatory amino acid L-glutamate is a neurotransmitter released by baroreceptor afferent nerves at their termination in the nucleus tractus solitarii (NTS). In this study we have examined the effect on arterial pressure, heart rate and baroreflex activity of the glutamate antagonist glutamate diethyl ester (GDEE) microinjected into the NTS of 50 rats anesthetized with Chloralose. Bilateral injections of GDEE produced dose dependent transient hypertension. The threshold dose was 2.5 micrograms/NTS and at a dose of 15 micrograms/NTS a maximal rise in arterial pressure and heart rate (from 95 +/- 8.7 mm Hg to 153 +/- 6.4 mm Hg and from 322 +/- 14.7 beats/min to 364 +/- 16.2 beats/min respectively, P less than 0.001, n = 6) occurred. A dose of 15 micrograms/NTS also completely blocked the baroreflex when injections were made bilaterally and it totally antagonized the cardiovascular effects of L-glutamate when injected immediately prior to L-glutamate. The hypertension, the antagonism of L-glutamate, and the blockade of the baroreflex persisted for 20-30 min. In that L-glutamate mediates a baroreflex-like response upon injection into NTS and in that GDEE blocks that response while at the same time blocking the naturally occurring baroreflex, the data are consistent with L-glutamate being a neurotransmitter of baroreflex afferents.     

Effect of atropine injected into the nucleus tractus solitarius on the regulation of blood pressure

Previous experiments have demonstrated that stimulation of muscarinic cholinergi receptors in the nucleus tractus solitarius (NTS) of the rat decreases arterial blood pressure and heart rate. The present studies were designed to examine the role of cholinergic mechanisms in the NTS in the tonic regulation of arterial pressure and the baroreceptor reflex. Atropine injected into the NTS of chloralose-anesthetized rats produced a dose-dependent inhibition of cardiovascular responses elicited by injection of acetylcholine into the same site; 240 pmol atropine eliminated acetylcholine-evoked responses. Atropine also increased arterial blood pressure but only at higher doses. Even larger doses of atropine were required to alter cardiovascular responses elicited by electrical stimulation of the aortic depressor nerve. Methylatropine injected into the NTS also blocked acetylcholine-evoked responses but, in contrast to the actions of atropine, did not increase arterial pressure in the dose range required to block acetylcholine-evoked responses. Furthermore, a dose of methylatropine (1 nmol) capable of blocking acetylcholine-evoked cardiovascular responses did not alter aortic depressor nerve-evoked cardiovascular responses. This lack of an effect of methylatropine on arterial pressure and aortic depressor nerve-evoked responses was not due to limited diffusion of the drug within the NTS since 1 nmol methylatropine completely blocked acetylcholine-evoked responses even when injected 0.5 mm distant from the site of acetylcholine injection. These results suggest that cholinergic mechanisms in the NTS are not involved in the tonic regulation of cardiovascular function or the baroreceptor reflex. Furthermore, these results highlight the importance of characterizing doses of drugs used in microinjection studies.     

Enhanced angiotensin-mediated responses in the nucleus tractus solitarii of spontaneously hypertensive rats

Studies using an AT(1) receptor antagonist, losartan, demonstrated that depressor and bradycardic responses to angiotensin II (Ang II) injection into the nucleus tractus solitarii (NTS) are mediated via those receptors. We further characterized Ang II-evoked cardiovascular responses in this nucleus in spontaneously hypertensive rats (SHR) using a new, selective AT(1) receptor antagonist, valsartan. In alpha-chloralose-anesthetized Sprague-Dawley (S-D) rats, Wistar-Kyoto (WKY) rats, and SHR, unilateral injection of Ang II into the NTS decreased arterial pressure (AP) and heart rate (HR). This response was eliminated by preinjection of valsartan. Depressor responses were much greater in SHR than in WKY rats. In normotensive rats, bilateral valsartan injection did not alter baseline AP or HR, or baroreceptor reflex index (BRI) calculated as the maximal change in HR (bpm) divided by phenylephrine- or nitroprusside-induced maximal change in mean AP (mmHg). In SHR, this treatment did not alter baseline HR and BRI, but significantly increased AP. Preinjection of valsartan did not alter injected glutamate effects in any strain. Thus, stimulation of AT(1) receptors within the NTS contributes to cardiovascular regulation independently of the baroreceptor reflex and the glutamatergic system. This angiotensinergic system in SHR acts tonically to reduce AP.     

Baroreflex control of renal sympathetic nerve activity and spontaneous rhythms at Mayer wave's frequency in rats

The effects of sedation with pentobarbital sodium (15 mg/kg followed by 15.9+/-1.2 mg/kg/h, i.v.) on arterial pressure (AP) Mayer waves and accompanying oscillations of renal sympathetic nerve activity (RSNA) were examined in rats (n=8). As compared with values observed in the conscious state, pentobarbital significantly (P<0.05) decreased AP (from 119+/-2 to 93+/-3 mm Hg), heart rate (HR; from 427+/-11 to 355+/-11 beats/min) and RSNA (from 1.20+/-0.27 to 0.62+/-0.13 microV). The baroreflex control of RSNA was analyzed by fitting a sigmoid logistic function to changes in RSNA and AP observed during nitroprusside and phenylephrine administrations. During pentobarbital infusion, the RSNA-AP relationship was reset towards lower AP values, but neither its maximum gain nor its gain at resting AP were significantly altered (from 6.3+/-1.0 to 5.8+/-1.4 and from 3.2+/-0.5 to 3.8+/-1.3 normalized units (n.u.)/mm Hg, respectively). Spectral power in the frequency band containing Mayer waves (0.29-0.73 Hz) was significantly decreased by pentobarbital for both AP (from 4.65+/-0.90 to 0.95+/-0.25 mm Hg2) and RSNA (from 1437+/-245 to 488+/-79 n.u.2). This effect was mainly secondary to the attenuation of strongly coherent oscillations of both variables at approximately 0.4 Hz. Although previous experimental evidence points to a major involvement of the sympathetic limb of the arterial baroreceptor reflex in the genesis of Mayer waves, the present study indicates that the amplitude of these oscillations cannot be used as a quantitative index of sympathetic baroreflex sensitivity.     

Involvement of NMDA receptors in the pressor response to microinjection of 5-HT3 agonist into the NTS of awake rats

Previous studies have shown that the activation of 5-HT3 receptors in the nucleus tractus solitarii (NTS) increases the baseline mean arterial pressure (MAP). In the present study, we evaluated the possible involvement of NMDA receptors in this pressor response. Four days before the experiments, under tribromoethanol anesthesia, rats received two guide cannulas in the direction of the NTS, and 1 day before the experiments, under tribromoethanol anesthesia, the femoral artery was cannulated for pulsatile arterial pressure (PAP), MAP, and heart rate (HR) measurements. On the day of the experiments, 2-methyl-serotonin, a 5-HT3 agonist, was microinjected into the NTS after microinjection of saline or AP-5, a selective NMDA receptor antagonist. Microinjection of 2-methyl-serotonin (5 nmol/50 nl) into the NTS after the vehicle (saline) produced a significant increase in MAP (+ 20 +/- 5 mm Hg, n = 8) while microinjection of 2-methyl-serotonin after microinjection of AP-5 (10 nmol/50 nl) produced no change in baseline MAP (-1 +/- 3 mm Hg, n = 11). Microinjection of AP-5 into the NTS produced no significant changes in the baseline MAP and HR. The data show that the increase in MAP in response to microinjection of a 5-HT3 agonist into the NTS is dependent on NMDA receptors.     

Effects of long-term angiotensin converting enzyme inhibition on cardiovascular variability in aging rats

We studied the effects of chronic (4 weeks) angiotensin converting enzyme inhibition with captopril on arterial pressure (AP) and heart rate (HR) variability, as well as on cardiac baroreflex sensitivity (BRS), in aged (20 months) rats. Series of basal RR interval (RRi) and systolic AP (SAP) were studied by autoregressive spectral analysis with oscillations quantified in low (LF: 0.2-0.8 Hz) and high frequency (HF: 0.8-2.5 Hz). BRS was measured by linear regression between HR and MAP changes. Captopril did not affect the spectra of RRi or SAP in young rats. Aged rats presented a reduction in variance (time domain) and in LF and HF oscillations of RRi and SAP. Captopril induced, in aged rats, a decrease in absolute and normalized LF oscillations and in LF/HF ratio of RRi. Captopril also reduced the variance, without changing its LF or HF components of SAP. Reflex tachycardia was reduced in aged as compared to young rats (-1.1+/-0.2 versus -3.4+/-0.5 bpm/mm Hg) and captopril did not affect it. Reflex bradycardia was also reduced in aged rats (-0.7+/-0.5 versus -2.0+/-0.4 bpm/mm Hg), but captopril prevented this attenuation in aged rats (-2.3+/-0.3 versus -0.7+/-0.5 bpm/mm Hg). These data indicate that there is a reduction in HR and SAP variability during aging, suggesting impairment of cardiovascular autonomic control. Captopril was able to change the power of oscillatory components of RRi, suggesting a shift in cardiac sympatho/vagal balance toward parasympathetic predominance. In addition, blockage of ACE improved the reflex bradycardia, but not the reflex tachycardia in aged rats.     

Effects of chronic lesions of the anteroventral third ventricle region on baroreceptor reflex function in conscious rats

This study determined baroreceptor reflex (BR) function in conscious rats which had received sham or electrolytic lesions of the anteroventral third ventricle (AV3V) 54-56 days previously. Resting mean arterial pressure (MAP) and heart rate (HR) values of the AV3V-lesion rats were similar to those of sham-lesion rats (P>0.05 for both comparisons). The sensitivity of the BR-mediated tachycardia in AV3V-lesion was greater than in sham-lesion rats (-9. 92+/-1.00 vs. -4.54+/-0.45 bpm/mmHg, P<0.05). The sensitivity of the BR-mediated bradycardia in AV3V-lesion rats was also greater than in rats with sham lesions (-3.56+/-0.38 vs. -2.06+/-0.42 bpm/mmHg, P<0. 05). The AV3V lesions did not affect other BR parameters. These findings demonstrate that chronic lesions of the AV3V region increase the sensitivity of the baroreceptor HR reflex in conscious rats.     

Role of carbon monoxide in L-glutamate-induced cardiovascular responses in nucleus tractus solitarius of conscious rats

Heme oxygenase degrades heme to form carbon monoxide. It has been reported that heme oxygenase-derived carbon monoxide may interact with L-glutamate (L-Glu) receptors in the nucleus tractus solitarius (NTS). Integrative studies suggest that heme oxygenase inhibitors raise blood pressure, in part, by inhibiting carbon monoxide formation in the NTS. The currents studies were designed to determine if heme oxygenase inhibitors affect the cardiovascular actions of L-Glu in the NTS. Accordingly, MAP and HR responses to unilateral microinjections of L-Glu (5 nmol/100 nl) into the NTS were measured before and after ipsilateral microinjections of zinc deuteroporphyrin 2,4-bis glycol (ZnDPBG, 4.5 nmol/100 nl) or chromium mesoporphyrin (CrMP, 1.5 nmol/100 nl) in awake rats chronically instrumented with NTS guide cannulaes and arterial catheters. With respect to non-treatment (+36+/-5 mmHg, -107 bpm, n=10), ZnDPBG pre-treatment attenuated the pressor and bradycardic responses to L-Glu (+7+/-3 mmHg, -10+/-6 bpm, P<0.05). CrMP similarly attenuated cardiovascular responses to L-Glu (+47+/-3 mmHg, -68+/-8 bpm vs. +20+/-5 mmHg, -40+/-9 bpm; before vs. after, n=10, P<0.05). Matched series yielded no vehicle- or time-related effects. Our findings suggest that a heme oxygenase product, such as carbon monoxide, may affect NTS glutamatergic neurotransmission to participate in cardiovascular control.     

Baroreflex actions of substance P microinjected into the nucleus tractus solitarii in rat: a consequence of local distortion

Biologically active substance P (SP) (1000 ng in 0.1 μl saline) microinjected into the nucleus tractus solitarii (NTS) of 25 rats did not affect arterial pressure, heart rate, or the baroreceptor reflex. However, microinjection of saline alone in volumes greater than 0.3 μl consistently elicited hypotension and bradycardia followed occasionally by transient hypertension. These data suggest that previously reported cardiovascular effects of SP microinjected into the NTS resulted from local distortion.     

Stimulation of alpha 2-adrenergic receptors in nucleus tractus solitarius is required for the baroreceptor reflex.

Bilateral injection into the nucleus tractus solitarius (NTS) of the alpha 2-adrenergic receptor antagonist yohimbine produced a dose-related (10-500 pmol) increase in arterial pressure, with a maximal response of approximately 60 mm Hg. Idazoxan, also an alpha 2-adrenergic receptor antagonist, produced a similar response although idazoxan was less potent than yohimbine. The pressor response elicited by these drugs was attenuated by stimulation of adrenergic receptors in the NTS by local administration of either clonidine or tyramine. Doses of yohimbine (200 pmol) or idazoxan (5 nmol) that maximally increased arterial pressure also completely inhibited the depressor and bradycardic responses to electrical stimulation of the aortic depressor nerve. These results indicate that tonic stimulation of alpha 2-adrenergic receptors in the NTS is required for baroreceptor reflex function.     

Glutamate stimulation of raphe pallidus attenuates the cardiopulmonary reflex in anaesthetised rats

Serotonin has been implicated as having a modulatory action on NTS neurones mediating vagal inputs controlling the cardiorespiratory system. Since raphe pallidus and raphe obscurus both send serotonergic projections to the NTS, we have investigated a neuromodulatory role for these structures on the cardiopulmonary reflex. A multibarrelled microelectrode positioned around the level of the area postrema was placed at varying depths into mid-line brainstem structures and the effect of glutamate stimulation on the cardiopulmonary reflex tested. Excitatory chemical stimulation in the region of raphe pallidus, but not raphe obscurus, attenuated significantly the respiratory and bradycardic components of the cardiopulmonary reflex. This attenuation was reversed by an NTS microinjection of RS-39604, a selective 5-HT, receptor antagonist. We propose that neurones in raphe pallidus that project to the NTS can release serotonin which acts via 5-HT4 receptors to attenuate the reflex phrenic nerve activity and heart rate components of the cardiopulmonary reflex.     

Facilitation of baroreflex-induced bradycardia by stimulation of specific hypothalamic sites in the rat

Hypothalamic stimulation generally inhibits baroreflex-induced bradycardia. However, we have noted discrete areas of the rat hypothalamus which facilitate reflex bradycardia. The effects of hypothalamic stimulation on baroreflex-induced changes in heart rate were investigated in urethane-anesthetized rats (1.2 g/kg, i.p.; n = 6) instrumented with femoral arterial and venous catheters. Bipolar electrodes (250 micron diameter) were implanted stereotaxically in the hypothalamus. Baroreflex-induced bradycardia was elicited by phenylephrine (PE) injection (8-20 micrograms/kg). Responses to stimulation (STIM) (50-150 microA, 80 Hz, 0.5 ms), PE, and Stim + PE were studied for 1 min. In the ventral medial and anterior hypothalamus, STIM caused transient increases in blood pressure and no changes in heart rate. Peak blood pressure was lower during STIM + PE than during PE (144 +/- 5 vs 164 +/- 3 mm Hg; P less than 0.05). However, STIM + PE resulted in a lower heart rate compared to PE (194 +/- 22 22 vs 270 +/- 17 bpm; P less than 0.05). At 1 min, the heart rate in STIM + PE rats remained lower than in PE rats (205 +/- 37 vs 319 +/- 16 bpm; P less than 0.05). Atropine administration indicated that the facilitation was primarily parasympathetic in nature. These results identify specific hypothalamic regions which facilitate baroreflex-induced bradycardia by parasympathetic mechanisms.     

Decreased baroreflex sensitivity in isoproterenol-treated mice with cardiac hypertrophy

The baroreflex has been shown to be impaired in rat models of cardiac hypertrophy. In the present study, we investigated the effects of beta-adrenoceptor-induced cardiac hypertrophy on the baroreflex in mice. Male Swiss Webster mice weighing 20-25 g were treated with the beta-adrenoceptor agonist isoproterenol (IPM; 15 microg/g/day, s.c.) for 7 days or with vehicle (control, CM). After treatment, IPM (n=9) and CM (n=9) were anesthetized with ketamine + xylazine (91.0: 9.1 mg/kg, i.p.) and had their carotid artery and jugular vein cannulated to test the arterial baroreflex. The baroreflex was evaluated by measuring changes in heart rate (HR) in response to acute increases and decreases in mean arterial pressure (MAP) induced by bolus injections of phenylephrine and sodium nitroprusside (1.5-24.0 microg/kg, i.v.) in conscious animals. IPM showed an increased cardiac weight/body weight (1.18 +/- 0.03 mg/g) ratio compared to CM (0.95 +/- 0.03 mg/g, p<0.05), but similar values of resting MAP (108 +/- 4 vs. 111 +/- 2 mm Hg) and HR (606 +/- 25 vs. 629 +/- 26 bpm). Sigmoidal barocurve analysis showed that isoproterenol treatment significantly reduced the following parameters: baroreflex sensitivity (IPM: -4.26 +/- 0.19 vs. CM: -5.92 +/- 0.54 bpm/mm Hg, p<0.05), reflex bradycardia plateau (IPM: 387 +/- 26 vs. CM: 318 +/- 19 bpm, p<0.05) and HR range (IPM: 369 +/- 30 vs. CM: 442 +/- 20 bpm, p<0.05). Linear regression analysis of baroreflex function also showed a diminished reflex bradycardia (CM: -8.92 +/- 0.87 bpm/mm Hg vs. IPM: -4.94 +/- 0.52 bpm/mm Hg, p<0.05), but similar reflex tachycardia (CM: -3.88 +/- 0.45 bpm/mm Hg vs. IPM: -3.52 +/- 0.43 bpm/mm Hg). In conclusion, beta-adrenoceptor-induced cardiac hypertrophy in mice led to impaired sensitivity of the cardiac baroreflex, which could be due to a diminished vagal activity to the heart.     

Inhibitory modulation of chemoreflex bradycardia by stimulation of the nucleus raphe obscurus is mediated by 5-HT3 receptors in the NTS of awake rats

Several studies demonstrated the involvement of 5-hydroxytryptamine (5-HT) and its different receptor subtypes in the modulation of neurotransmission of cardiovascular reflexes in the nucleus tractus solitarii (NTS). Moreover, anatomic evidence suggests that nucleus raphe obscurus (ROb) is a source of 5-HT-containing terminals within the NTS. In the present study we investigated the possible changes in the cardiovascular responses to peripheral chemoreceptor activation by potassium cyanide (KCN, i.v.) following ROb stimulation with L-glutamate (10 nmol/50 nL) and also whether 5-HT3 receptors in the caudal commissural NTS are involved in this neuromodulation. The results showed that stimulation of the ROb with L-glutamate in awake rats (n=15) produced a significant reduction in the bradycardic response 30 s after the microinjection (-182+/-19 vs -236+/-10 bpm; Wilcoxon test) but no changes in the pressor response to peripheral chemoreceptor activation (43+/-4 vs 51+/-3 mmHg; two-way ANOVA) in relation to the control. Microinjection of 5--HT3 receptors antagonist granisetron (500 pmol/50 nL), but not the vehicle, into the caudal commissural NTS bilaterally prevented the reduction of chemoreflex bradycardia in response to microinjection of L-glutamate into ROb. These data indicate that 5-HT-containing projections from ROb to the NTS play an inhibitory neuromodulatory role in the chemoreflex evoked bradycardia by releasing 5-HT and activating 5-HT3 receptors in the caudal NTS.     

Parasympathetic dysfunction is associated with baroreflex and chemoreflex impairment in streptozotocin-induced diabetes in rats

This study explored physiological mechanisms of diabetic dysfunction in baroreceptors and chemoreceptors-mediated hemodynamic responses, and cholinergic neurotransmission in 30-day diabetic rats (n = 14) and controls (n = 14). Basal hemodynamic data and vagal response to electrical stimulation and methacholine injection were also evaluated. Muscarinic receptors were characterized using a radioligand receptor binding assay ([3H]N methylscopolamine). Experimental diabetes (50 mg/kg of STZ, i.v.) decreased systolic, diastolic, and mean arterial pressure and basal heart rate. Heart rate (HR) responses to vagal electrical stimulation (16, 32, and 64 Hz) were 15%, 11%, and 14% higher in diabetics vs non-diabetics, as were HR responses to methacholine injection (-130+/-24, -172+/-18, -206+/-15 bpm vs. -48+/-15, -116+/-12, -151+/-18 bpm, P < 0.05). Muscarinic receptor density was higher (267.4+/-11 vs 193.5+/-22 fmol/mg/prot, P < 0.05) in the atria of diabetic rats than in those of controls; the affinity was similar between groups. Diabetes-induced reduction of reflex responses to baro- (reflex bradycardia: -3.4+/-0.3 and -2.7+/-0.2 bpm/mm Hg; reflex tachycardia: -1.6+/-0.1 and -1.4+/-0.07 bpm/mm Hg, in control and diabetics, P < 0.05) and chemoreceptor stimulation, enhancement of HR responsiveness to cardiac vagal electrical stimulation and methacholine stimulation, plus an increase in the number of atrial muscarinic receptors indicates reduced parasympathetic activity, which is probably derived from central nervous system derangement.