KAPPA-OPIOID AGONIST U50488 INHIBITS P-TYPE CA2+ CHANNELS BY 2 MECHANISMS

The effects of U50488, kappa-opioid agonist on P-type Ca2+ channels, w ere studied. U50488 inhibited depolarization-induced Ca2+ uptake into rat brain synaptosomes, which was sensitive to omega-Agatoxin IVA (ome ga-AgaIVA; P-type Ca2+ channel blocker) and inhibited P-type Ca2+ chan nel currents recorded from rat cerebellar Purkinje neurons by the whol e-cell patch clamp method. Dynorphin A also inhibited P-type Ca2+ chan nel currents. The inhibition by U50488 was biphasic; high affinity com ponent (21%, IC50 = 8.9 X 10(-8) M) and low affinity component (79%, I C50 = 1.1 X 10(-5) M). At low concentrations of U50488 (10(-6) M), P-t ype Ca2+ channel current inhibition was attenuated by norbinartorphimi ne (nor-BNI), kappa-opioid antagonist, and by dialysis of cells with a pipette solution containing guanosine 5'-O-(2-thiodiphosphate) (GDP-b eta S). At high concentrations of U50488 (10 -5 M), P-type Ca2+ channe l current inhibition was frequency-dependent. Thus U50488-induced curr ent inhibition is mediated by two mechanisms. Its high affinity compon ent is produced by activation of kappa-opioid receptors, whereas the l ow affinity component is due to its direct action on the P-type Ca2+ c hannel.