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-HT₃, NK₁ 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-HT₃ receptor antagonist), bicuculline (a GABA_A receptor antagonist) and GR-205171 (an NK₁ 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-HT₃ 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-HT₃, GABA_A and NK₁ 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.