RAPHE MAGNUS STIMULATION-INDUCED ANTINOCICEPTION IN THE CAT IS ASSOCIATED WITH RELEASE OF AMINO-ACIDS AS WELL AS SEROTONIN IN THE LUMBAR DORSAL HORN

Stimulation in the nucleus raphe magnus (NRM) inhibits transmission of nociceptive information within the spinal cord through activation of bulbospinal pathways. This study used microdialysis in combination wit h high pressure liquid chromatography to measure the release of seroto nin (5HT) and several amino acids, including glutamate, aspartate and glycine, from the lumbar dorsal horn during electrical stimulation wit hin the NRM in the a-chloralose anesthetized cat. Observed release of putative neurotransmitters was correlated with inhibition of nocicepti ve projection neurons recorded from sites within 800 mum rostral or ca udal to the dialysis fiber. NRM stimulus parameters considered to pref erentially activate myelinated fibers caused inhibition of nociceptive evoked activity, and increased the release of excitatory amino acids and glycine within the spinal cord, with no detectable release of 5HT. When pulse widths were lengthened and unmyelinated fibers were also a ctivated, increases in 5HT in the spinal dialysate were observed as we ll. Strychnine administered through the dialysis fiber (0.02-1 mM) ant agonized NRM-induced inhibition when 5HT release was not detected. Inh ibition produced by stimulation that increased 5HT concentrations was relatively strychnine resistant. These results point to a raphe-spinal inhibitory pathway that is not dependent on 5HT, the activation of wh ich results in the spinal release of glycine.