The effects of stimulation of the ventrolateral part of the midbrain periaqueductal grey matter (PAG) on the arterial baroreflex were investigated in urethane-chloralose anaesthetized and artificially ventilated rats. 2. Both electrical and chemical stimulation of the ventrolateral PAG provoked hypotension, vagal bradycardia and marked facilitation of baroreflex vagal bradycardia (BVB), which was induced by stimulation of the aortic depressor nerve. The magnitude of ventrolateral PAG facilitation of BVB was 328 +/- 193% (n = 34) for electrical stimulation and 243 +/- 224% (n = 13) for chemical stimulation. Baroreflex hypotension was slightly augmented during either electrical or chemical stimulation of the ventrolateral PAG in vagotomized rats. 3. Stimulation of the nucleus raphe magnus (NRM) also provoked hypotension, vagal bradycardia and facilitation of BVB. The magnitude of BVB facilitation was 234 +/- 132% (n = 8) for electrical stimulation and 328 +/- 170% (n = 7) for chemical stimulation. After microinjection of kainic acid into the NRM region, baroreflex facilitation, as well as hypotension and vagal bradycardia, produced by ventrolateral PAG stimulation, was almost abolished. 4. In conclusion, the ventrolateral PAG, besides producing hypotension and bradycardia, facilitates arterial baroreflexes. These effects are exerted via the NRM, sharply contrasting with effects of the dorsal PAG.
