INHIBITORY EFFECTS EVOKED FROM BOTH THE LATERAL AND VENTROLATERAL PERIAQUEDUCTAL GREY ARE SELECTIVE FOR THE NOCICEPTIVE RESPONSES OF RAT DORSAL HORN NEURONS

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 presser and depressor sites in the lateral and ventrolateral periaqueductal grey (PAG). Stimulating micro pipettes were inserted stereotaxically into the lateral or ventrolater al PAG at sites where microinjection of DL-homocysteic acid (DLH) evok ed 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 stimul i applied to their cutaneous receptive fields. Single unit extracellul ar 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 low er lumbar spinal cord. The responses of Class 1 neurones to innocuous prodding of their receptive fields were unaffected by neuronal activat ion in either the lateral or ventrolateral PAG. The nociceptive (noxio us pinch/heat) responses of most Class 2 neurones were strongly inhibi ted by chemical stimulation in either sector of the PAG. The low thres hold (prod) responses of the same neurones were generally unaffected o r only weakly inhibited by identical stimulation, regardless of stimul ation site. No significant differences were found between the effects of lateral vs. ventrolateral PAG stimulation on the responses of dorsa l horn neurones. These results do not support the view that dorsal hor n neurones may be inhibited with different selectivities by hyper- and hypotensive regions of the PAG. (C) 1997 Elsevier Science B.V.