GABA-IMMUNOREACTIVE BOUTONS CONTACT IDENTIFIED OFF AND ON CELLS IN THE NUCLEUS RAPHE MAGNUS

The pontomedullary raphe magnus (RM) contains two physiologically defi ned types of neurons that participate in the opioid-induced modulation of dorsal horn nociceptive messages: OFF cells, which decrease, and O N cells, which increase their discharge rates when reflex behavior is evoked by noxious pinch or heat. Because both types of neuron have inh ibitory inputs and because there is evidence that gamma-aminobutyric a cid (GABA inhibitory mechanisms within RM contribute to the antinocice ptive action of opioids, we have sought anatomical evidence for a dire ct GABAergic input to OFF and ON cells. Ln this study, cells of each t ype located in the RM were electrophysiologically defined and intracel lularly filled with horseradish peroxidase or Neurobiotin. One cell of each type was labeled in the cat, and 2-3 cells of each type were lab eled in the rat. Thin sections were labeled by a postembedding immunog old procedure by using an antibody directed against glutaraldehyde-con jugated GABA. GABA-immunoreactive (GABA-ir) boutons contained small, r ound, clear vesicles and made symmetrical synapses with identified den drites. GABA ir boutons were apposed to soma and to proximal and dista l dendrites of both cell types in both species. These findings demonst rate direct GABAergic input to identified OFF and ON cells in the RM. (C) 1997 Wiley-Liss, Inc.