Inhibitory effects evoked from both the lateral and ventrolateral periaqueductal grey are selective for the nociceptive responses of rat dorsal horn neurones.

In rats anaesthetized with alphaxalone/alphadolone a comparative study was made of the inhibitory effects on dorsal horn neurones evoked by chemical stimulation at identified pressor and depressor sites in the lateral and ventrolateral periaqueductal grey (PAG). Stimulating micropipettes were inserted stereotaxically into the lateral or ventrolateral PAG at sites where microinjection of DL-homocysteic acid (DLH) evoked increases or decreases respectively in mean arterial blood pressure. The effects of DLH microinjection at these sites were tested against the responses of dorsal horn neurones to noxious and innocuous stimuli applied to their cutaneous receptive fields. Single unit extracellular recordings were made from 15 Class 1 (low-threshold) and 37 Class 2 (wide dynamic range) dorsal horn neurones in laminae II-VI of the lower lumbar spinal cord. The responses of Class 1 neurones to innocuous prodding of their receptive fields were unaffected by neuronal activation in either the lateral or ventrolateral PAG. The nociceptive (noxious pinch/heat) responses of most Class 2 neurones were strongly inhibited by chemical stimulation in either sector of the PAG. The low threshold (prod) responses of the same neurones were generally unaffected or only weakly inhibited by identical stimulation, regardless of stimulation site. No significant differences were found between the effects of lateral vs. ventrolateral PAG stimulation on the responses of dorsal horn neurones. These results do not support the view that dorsal horn neurones may be inhibited with different selectivities by hyper- and hypotensive regions of the PAG.