EVIDENCE OF SYNAPTIC CONTACTS OF NOCICEPTIVE PRIMARY AFFERENT CENTRALTERMINALS ON GABAERGIC INTERNEURONS IN THE SUBSTANTIA-GELATINOSA

Recently, we showed that capsaicin induced the degeneration of not onl y glomerular CI terminals but also of non-glomerular CI terminals maki ng presynaptic contact with interneuronal soma. Studies of the nature of interneurons making contact by nonglomerular CI terminals should pr ovide important information to facilitate our understanding of the pro cessing of nociceptive impulses in the substantia gelatinosa. The most likely candidate molecule involved in this process in these interneur ons is gamma-aminobutylic acid (GABA). Therefore, ultrastructural rela tionships between nonglomerular CI terminals and GABAergic interneuron al soma in the mouse substatia gelatinosa were examined by an immunocy tochemical method using an antibody to GABA. Terminals with the same p rofiles as the CI terminals, i.e., slender, sinuous and scalloped term inals filled with clear synaptic vesicles, were found to make synaptic contacts with GABA-immunoreactive somata. Thus, nociceptive primary a fferents are suggested to modulate pain transmission by themselves via GABAergic neurons in the substantia gelatinosa.