ACTIVATION OF A BULBOSPINAL OPIOIDERGIC PROJECTION BY PAIN STIMULI INTHE AWAKE RAT

WE examined the expression of preproenkephalin mRNA in pain-modulating regions of the rat's brainstem using in situ hybridization histochemi stry. We found that neurones in the Kolliker-Fuse nucleus, which recei ves projections from spinal nociceptive-specific cells, express enkeph alin mRNA, that this expression is increased by noxious pinch applied to the skin of awake animals, and that these enkephalinergic neurones are part of a descending system that terminates in the rostroventral m edulla and the spinal cord. These findings show that natural noxious s timuli activate opioidergic bulbospinal neurones that could directly m odulate spinal nociceptive transmission.