κ-Opioid agonist U50488 inhibits P-type Ca 2+ channels by two mechanisms

Abstract

The effects of U50488, κ-opioid agonist on P-type Ca 2+ channels, were studied. U50488 inhibited depolarization-induced Ca 2+ uptake into rat brain synaptosomes, which was sensitive to ω-Agatoxin IVA (ω-AgaIVA; P-type Ca 2+ channel blocker) and inhibited P-type Ca 2+ channel currents recorded from rat cerebellar Purkinje neurons by the whole-cell patch clamp method. Dynorphin A also inhibited P-type Ca 2+ channel currents. The inhibition by U50488 was biphasic; high affinity component (21%, ICS, = 8.9 × 10 −8 M) and low affinity component (79%, IC 50 = 1.1 × 10 −5 M). At low concentrations of U50488 (10 −6 M), P-type Ca 2+ channel current inhibition was attenuated by norbinartorphimine (nor-BNI), κ-opioid antagonist, and by dialysis of cells with a pipette solution containing guanosine 5′- O-(2-thiodiphosphate) (GDP-βS). At high concentrations of U50488 (10 −5 M), P-type Ca 2+ channel current inhibition was frequency-dependent. Thus U50488-induced current inhibition is mediated by two mechanisms. Its high affinity component is produced by activation of κ-opioid receptors, whereas the low affinity component is due to its direct action on the P-type Ca 2+ channel.