INHIBITION OF SPINAL NEURONAL RESPONSES TO NOXIOUS SKIN HEATING BY STIMULATION OF MESENCEPHALIC PERIAQUEDUCTAL GRAY IN THE CAT

Discharges of lumbar dorsal horn neurons were evoked by noxious radiant skin heating, and inhibition of the heat-evoked responses by stimulation of the mesencephalic periaqueductal gray was investigated in N2O-anesthesized cats. 37 units selected on the basis of receiving afferent C-fiber input from the posterior tibial and/or superficial peroneal nerves responded vigorously to 50°C heating of the plantar surface of the ipsilateral hind-paw. All discharges were inhibited by periaqueductal gray stimulation (PAGS) at current strengths of 300-900 μA; the mean threshold for inhibition was 167 μA. The mean frequency of the inhibited discharge was 39% of the control response. Effective PAGS sites were distributed throughout the ventral PAG bilaterally. Stimulus current-distance estimates indicate that small (0.5-1.2 mm diameter) volumes of tissue within the PAG were stimulated. A monotonic relationship between temperature and unitary discharge was found for skin heating from threshold to about 50°C. PAGS resulted in a decrease in the slope of the curve plotting discharge against temperature, without altering the threshold. Inhibition of the heat-evoked discharges rarely outlasted the PAGS. Possible neural substrates for descending inhibition and correlates with neural mechanisms of analgesia are discussed.