Ventrolateral medullary lesions block the antinociceptive and cardiovascular responses elicited by stimulating the dorsal periaqueductal grey matter in rats.

In rats anaesthetised with Saffan (Glaxovet), inhibition of the tail flick reflex evoked by electrical stimulation in the dorsal periaqueductal grey matter (PAG) was accompanied by an increase in blood pressure, tachycardia, vasodilatation in hind limb muscle, an increase in respiration, pupillodilatation and widening of the palpebral fissure. Stimulation deeper in the PAG and in the tegmentum ventral to it produced analgesia but without this pattern of autonomic changes. The antinociceptive, cardiovascular and respiratory effects of PAG stimulation were abolished by bilateral lesions in the ventrolateral medulla in the area which lies ventromedial to the facial nucleus (i.e., in nucleus paragigantocellularis lateralis, RPGL). Lesions in nucleus raphe magnus (NRM), or bilateral lesions of nucleus reticularis gigantocellularis (RG) and paragigantocellularis (RPG) or a combination of a lesion in RPGL with one in NRM or the contralateral RG or RPG did not block the effects of stimulating in the dorsal PAG. These findings are discussed in relation to the role of the PAG in mediating behavioural responses to stress.