GLYCINE AND GABA(A) ANTAGONISTS REDUCE THE INHIBITION OF PRIMATE SPINOTHALAMIC TRACT NEURONS PRODUCED BY STIMULATION IN PERIAQUEDUCTAL GRAY

Amino acids are demonstrated to be important neurotransmitters mediati ng the inhibitory transmission from nucleus raphe magnus to spinal noc iceptive dorsal horn neurons. In this study, the role of glycine and G ABA in the inhibitory processes evoked by stimulation in periaqueducta l gray (FAG) of responses of primate spinothalamic tract (STT) neurons to cutaneous mechanical and thermal stimuli was investigated by exami ning the effects of strychnine and bicuculline, antagonists of glycine and GABA(A) receptors, respectively, introduced into the dorsal horn through a microdialysis fiber. The inhibitory effects of iontophoretic application of glycine and GABA(A) agonists on STT cell activity evok ed by noxious mechanical stimulation of the skin were selectively bloc ked by their specific antagonist, strychnine or bicuculline, infused i nto the dorsal horn. Similarly, intra-spinal application of strychnine or bicuculline resulted in a significant reduction in the FAG stimula tion-induced inhibition of responses of STT cells to cutaneous stimuli . This reduction was mainly on the FAG-induced inhibition of the respo nses to noxious mechanical stimuli. Our results suggest that glycinerg ic and GABAergic inhibitory interneurons in the spinal cord dorsal hor n synapsing on STT cells are activated during stimulation in FAG and c ontribute to descending antinociceptive actions.