G-PROTEINS AND G-PROTEIN SUBUNITS MEDIATING CHOLINERGIC INHIBITION OFN-TYPE CALCIUM CURRENTS IN SYMPATHETIC NEURONS

One postsynaptic action of the transmitter acetylcholine in sympatheti c ganglia is to inhibit somatic N-type Ca2+ currents: this reduces Ca2 +-activated K+ currents and facilitates high-frequency spiking. Previo us experiments on rat superior cervical ganglion neurons have revealed two distinct pathways for this inhibitory action: a rapid, voltage-de pendent inhibition through activation of M-4 muscarinic acetylcholine receptors (mAChRs), and a slower, voltage-independent inhibition via M -1 mAChRs [Hille (1994) Trends in Neurosci., 17, 531-536]. We have ana lysed the mechanistic basis for this divergence at the level of the in dividual G-proteins and their alpha and beta gamma subunits, using a c ombination of site-directed antibody injection, plasmid-driven antisen se RNA expression, overexpression of selected constitutively active su bunits, and antagonism of endogenously liberated beta gamma subunits b y over-expression of beta gamma-binding beta-adrenergic receptor kinas e 1 (beta ARK1) peptide. The results indicate that: (i) M-4 mAChR-indu ced inhibition is mediated by G(oA): (ii) alpha and beta gamma subunit s released from the activated G(oA) heterotrimer produce separate volt age-insensitive and voltage-sensitive components of inhibition, respec tively; and (iii) voltage-insensitive M-1 mAChR-induced inhibition is likely to be mediated by the a subunit of G(q). Hence, Ca2+ current in hibition results from the concerted, but independent actions of three different G-protein subunits.