Inhibition of Ca(2+) channel current by μ- and κ-opioid receptors coexpressed in Xenopus oocytes: desensitization dependence on Ca(2+) channel α(1) subunits

1. Desensitization of μ- and κ-opioid receptor-mediated inhibition of voltage-dependent Ca(2+) channels was studied in a Xenopus oocyte translation system. 2. In the oocytes coexpressing κ-opioid receptors with N- or Q-type Ca(2+) channel α(1) and β subunits, the κ-agonist, U50488H, inhibited both neuronal Ca(2+) channel current responses in a pertussis toxin-sensitive manner and the inhibition was reduced by prolonged agonist exposure. 3. More than 10 min was required to halve the inhibition of Q-type channels by the κ-agonist. However, the half-life for the inhibition of N-type channels was only 6±1 min. In addition, in the oocytes coexpressing μ-opioid receptors with N-type or Q-type channels, the uncoupling rate of the μ-receptor-mediated inhibition of N-channels was also faster than that of Q-type channels. 4. In the oocytes coexpressing both μ- and κ-receptors with N-type channels, stimulation of either receptor resulted in a cross-desensitization of the subsequent response to the other agonist. Treatment of oocytes with either H-8 (100 μM), staurosporine (400 nM), okadaic acid (200 nM), phorbol myristate acetate (5 nM) or forskolin (50 μM) plus phosphodiesterase inhibitor did not affect either the desensitization or the agonist-evoked inhibition of Ca(2+) channels. 5. These results suggest that the rate of rapid desensitization is dependent on the α(1) subtype of the neuronal Ca(2+) channel, and that a common phosphorylation-independent mechanism underlies the heterologous desensitization between opioid receptor subtypes