Inhibitory modulation of the cardiovascular defence response by the ventrolateral periaqueductal grey matter in rats

In rats anaesthetised with alphaxalone/alphadolone, electrical stimulation in the dorsal periaqueductal grey matter (PAG) evoked a pressor response with tachycardia, vasodilatation in the hindlimb and hyperpnoea: a pattern of response known as the defence reaction. Microinjection of the synaptic excitant, D,L-homocysteic acid (DLH), but not saline, into the ventrolateral PAG at the level of the decussation of the superior cerebellar peduncle (approximately 7.3-8.3 mm caudal to bregma) produced a reduction in the size of the cardiovascular components of the defence reaction evoked by electrical stimulation in the dorsal PAG. Injections of DLH made outside this region had no effect on the defence response. Injection of DLH into the "defence inhibition area" had no effect on the pressor response evoked distally in the efferent pathway for the defence reaction, by electrical stimulation in the rostral ventrolateral medulla (RVLM). Activation of neurones in a restricted portion of the caudal ventrolateral PAG appears to modulate activity in the descending pathway for the defence response evoked from the dorsal PAG. It is argued that the inhibitory interaction probably occurs at the level of synapses in the RVLM.