OPIOID-PEPTIDES MODULATE GABA(A) RECEPTOR RESPONSES IN NEURONS OF BULLFROG DORSAL-ROOT GANGLIA

Effects of enkephalin and selective opioid-receptor agonists on GABA-i nduced current were examined in dissociated neurons of bullfrog dorsal root ganglia (DRG) by using whole-cell patch-clamp method. Leucine-(L eu)-enkephalin and methionine-(Met)-enkephalin depressed GABA(A) recep tor-mediated currents. DPDPE, DAMGO and dynorphin-A (Dyn-A) also depre ssed the inward current produced by GABA; the order of agonist potency was DPDPE greater than or equal to DAMGO > Dyn-A. Naloxone blocked th e inhibitory effects of enkephalins and other opioid agonists on the G ABA current. Naltrindole (NTI), a delta-receptor antagonist, prevented the DPDPE-induced depression of the GABA current. beta-Funaltrexamine (beta-FNA), a mu-receptor antagonist, reduced the DAMGO-induced depre ssion of GABA currents. Nor-binaltorphimine (nor-BNI), a kappa-recepto r antagonist, reduced the effects of Dyn-A in depressing the GABA curr ent. The results suggest that enkephalin down-regulates GABA(A) recept or function through mainly delta- and mu-opioid receptors in bullfrog DRG neurons.