Inhibition of cardiac baroreflex by noxious thermal stimuli: a key role for lateral paragigantocellular serotonergic cells.

The present study was designed to identify the neuronal mechanisms causing cardiac baroreflex inhibition associated with thermal nociception in rats. Under urethane-anesthesia, noxious thermal stimuli > or = 48 degrees C were found to inhibit the cardiac baroreflex, whereas noxious stimuli < or = 46 degrees C had no effect. Using double immunohistochemical labeling, noxious stimuli > or = 48 degrees C were found to evoke primarily a strong expression of Fos protein (Fos) encoded by c-fos gene in serotonergic neurons of lateral paragigantocellular reticular nucleus (LPGi). Noxious stimuli < or = 46 degrees C did not evoke Fos expression in any serotonergic neurons of the brainstem. Local blockade of neuronal activity by bilateral microinjections of fluorescent muscimol (a GABA(A) receptor agonist tagged with a fluorophore that allowed visualization of the injections) into both the LPGi and the raphe magnus nucleus prevented the inhibitory effect of noxious stimuli > or = 48 degrees C on the cardiac baroreflex. Bilateral microinjections of granisetron (a 5-HT(3) antagonist) within the nucleus tractus solitarius also prevented the inhibition of cardiac baroreflex elicited by noxious stimuli > or = 48 degrees C. These results show that activation of serotonergic cells in the LPGi is critical to trigger nucleus tractus solitarius-mediated cardiac baroreflex inhibition elicited by intense thermal noxious stimuli.

Inhibition of the bradycardic component of the von Bezold-Jarisch reflex and carotid chemoreceptor reflex by periaqueductal gray stimulation: involvement of medullary receptors.

Stimulation of the dorsolateral periaqueductal gray matter (dlPAG) and the B3 cell group inhibits the cardiovagal component of the baroreflex in rats. Our aim was to determine whether the defence reaction induces similar modulatory effects on the cardiac response of the von Bezold-Jarisch reflex and the carotid chemoreceptor reflex. We examined the effects of dlPAG stimulation on the reflex bradycardia triggered by systemic administration of phenylbiguanide or potassium cyanide. Electrical and chemical stimulation of the dlPAG produced marked inhibition of the cardiovagal components of the von Bezold-Jarisch and the carotid chemoreceptor reflexes. In addition, as 5-HT(3), NK(1) and GABA(A) receptor activation blocks cardiac reflex responses, we studied whether these receptors were involved in the dlPAG-induced inhibitory effects. We found that, after microinjection of granisetron (a 5-HT(3) receptor antagonist), bicuculline (a GABA(A) receptor antagonist) and GR-205171 (an NK(1) receptor antagonist) into the nucleus of the solitary tract (NTS), reflex bradycardic responses were preserved during dlPAG stimulation. Finally, activation of the B3 region also inhibited both reflex bradycardic responses, and these effects were prevented by prior blockade of 5-HT(3) receptors in the NTS. The inhibitory effect of dlPAG stimulation on the cardiac reflex responses was prevented by inhibition of neurons in the medullary B3 region. In conclusion, 5-HT(3), GABA(A) and NK(1) receptors in the NTS appear to be involved in the inhibition of the von Bezold-Jarisch reflex and the carotid chemoreceptor reflex bradycardia evoked by activation of neurons in the dlPAG and the raphé magnus.

Critical role of B3 serotonergic cells in baroreflex inhibition during the defense reaction triggered by dorsal periaqueductal gray stimulation.

The present study was designed to identify the serotonergic pathway causing baroreflex inhibition associated with the defense reaction in rats. Under conditions that produce physiological responses typical of the defense reaction, electrical stimulation of the dorsal periaqueductal gray (dPAG) was found to double c-Fos immunoreactive serotonergic neurons within the mid-rostrocaudal extent of the B3 group (which comprises the raphe magnus and the lateral paragigantocellular reticular nuclei) in anesthetized rats. Local blockade of neuronal activity by microinjection of muscimol (a GABA(A) receptor agonist) directly into the B3 region prevented the inhibitory effect of dPAG activation on the cardiac baroreflex. Conversely, neuron activation by local application of D,L-homocysteic acid into B3 region caused baroreflex inhibition that was suppressed by microinjection of granisetron (a 5-HT(3) antagonist) into the nucleus tractus solitarius. These results show that activation of serotonergic cells in the mid-portion of B3 group is critical to trigger baroreflex inhibition occurring during the defense reaction evoked by dPAG stimulation.

5-HT-mediated inhibition of cardiovagal baroreceptor reflex response during defense reaction in the rat.

Previous studies showed that the cardiac response of the baroreceptor reflex (bradycardia) is inhibited during the defense reaction evoked by direct electrical or chemical stimulation of the periaqueductal gray (dPAG) in the rat. Whether central serotonin and nucleus tractus solitarius (NTS) serotonin(3) (5-HT(3)) receptors might participate in this inhibition was investigated in urethane-anesthetized and atenolol-pretreated rats. Our results showed that both electrical and chemical stimulation of the dPAG produced a drastic reduction of the cardiovagal component of the baroreceptor reflex triggered by either intravenous administration of phenylephrine or aortic nerve stimulation. This inhibitory effect of dPAG stimulation on the baroreflex bradycardia was not observed in rats that had been pretreated with p-chlorophenylalanine (300 mg/kg ip daily for 3 days) to inhibit serotonin synthesis. Subsequent 5-hydroxytryptophan administration (60 mg/kg ip), which was used to restore serotonin synthesis, allowed the inhibitory effect of dPAG stimulation on both aortic and phenylephrine-induced cardiac reflex responses to be recovered in p-chlorophenylalanine-pretreated rats. On the other hand, in nonpretreated rats, the inhibitory effect of dPAG stimulation on the cardiac baroreflex response could be markedly reduced by prior intra-NTS microinjection of granisetron, a 5-HT(3) receptor antagonist, or bicuculline, a GABA(A) receptor antagonist. These results show that serotonin plays a key role in the dPAG stimulation-induced inhibition of the cardiovagal baroreceptor reflex response. Moreover, they support the idea that 5-HT(3) and GABA(A) receptors in the NTS contribute downstream to the inhibition of the baroreflex response caused by dPAG stimulation.

Antinociceptive effect of cardiopulmonary chemoreceptor and baroreceptor reflex activation in the rat.

The effect on the nociceptive tail-flick (TF) reflex of cardiopulmonary chemoreceptor and arterial baroreceptor activation, producing Bezold-Jarisch like- and baro-reflex responses, respectively, was analysed in lightly halothane-anaesthetized rats. Intra-cardiac administration of phenylbiguanide (5-100 microg/kg, into the right atrium) or veratrine (30-150 microg/kg, into the left ventricle), which both elicited the characteristic Bezold-Jarisch-like cardiovascular reflex responses (hypotension and bradycardia), produced a dose-dependent increase in TF latency. A similar inhibitory influence on the TF reflex was noted upon baroreflex activation by acute administration of phenylephrine (15-50 microg/kg i.v.) or aortic depressor nerve stimulation (100-400 microA). As expected from the involvement of local excitatory amino acid receptors in both vagally mediated cardiovascular reflex responses and inhibition of the TF reflex, microinjections of kynurenic acid (3 nmol/0.1 microl), an N-methyl-D-aspartate (NMDA) and non-NMDA receptor antagonist, into the nucleus tractus solitarius, prevented the cardiovascular responses as well as the concomitant increase in TF latency produced by cardiopulmonary chemoreceptor and baroreceptor stimulations. The present data show that induction of the cardiopulmonary chemoreceptor and baroreceptor reflexes produces an antinociceptive effect which can be assessed using the TF test, and that glutamate ionotropic receptors within the nucleus tractus solitarius mediate this effect.

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.

Glutamate and NO mediation of the pressor response to 5-HT3 receptor stimulation in the nucleus tractus solitarii.

The possible participation of glutamate and NO/cGMP in the pressor response to 5-HT3 receptor activation in the nucleus tractus solitarii (NTS) was investigated using selective antagonists in urethane-anaesthetized rats. Intra-NTS administration of NMDA and non-NMDA receptor antagonists, but not metabotropic glutamate receptor antagonists, markedly reduced (70%) the increase in blood pressure caused by local application of the potent 5-HT3 receptor agonist, 1-(m-chlorophenyl)-biguanide. The 5-HT3 receptor-mediated pressor response was also significantly attenuated by the local blockade of nitric oxide synthase and soluble guanylyl cyclase. These data suggest that ionotropic glutamate receptors and the associated NO/cGMP transduction mechanism contribute downstream to the pressor effect elicited by 5-HT3 receptor stimulation in the NTS.