Differential regulation of N- and Q-type Ca2+ channels by cyclic nucleotides and G-proteins

Voltage-dependent Ca2+ channels play a central role in controlling neurotransmitter release at the synapse. They can be inhibited by certain G-protein-coupled receptors, acting by a pathway delimited to the membrane. In addition, modulation of Ca2+ channel activity by protein kinases also contributes to the dynamic regulation of neuronal physiology. Recently, differences in these modulations between Ca2+ channel subtypes have been shown in several neuronal preparations. Here we show that two types of presynaptic Ca2+ channel (N-type and Q-type) are differentially regulated by cAMP and G-proteins using a Xenopus oocyte expression system. Treatment to increase cytosolic cAMP concentration with forskolin and 3-isobutyl-1-methylxanthine (IBMX) markedly potentiated Q-type channel current, and the enhancement was reversed by protein kinase A inhibitors. Much smaller enhancement was observed in N-type channel current after the cAMP elevation. When large depolarizing prepulse was applied to the oocytes for evaluation of the tonic inhibition of Ca2+ channels by intrinsic G-protein activity, N-type channel current elicited a large prepulse facilitation but Q-type channels did not. The tonic inhibition of N-type channels was abolished by an intracellular perfusion with a 'cut-open' recording configuration, or by co-expression with G(alpha o). When kappa opioid receptors were co-expressed and stimulated with agonists, depolarization-resistant inhibition was more apparent in Q-type channels than in N-type channels. These results suggest that Q-type channels are more susceptible to the protein kinase A-mediated facilitation than N-type channels, and that activity of N-type channels can be more highly regulated in a voltage-dependent manner by G(betagamma) than that of Q-type channels. These differences may account for the selective regulation of neurotransmitter release by these Ca2+ channels.