THE ALPHA-SUBUNIT OF G(Q) CONTRIBUTES TO MUSCARINIC INHIBITION OF THEM-TYPE POTASSIUM CURRENT IN SYMPATHETIC NEURONS

Rat superior cervical ganglion (SCG) neurons express low-threshold non inactivating M-type potassium channels (I-K(M)), which can be inhibite d by activation of M1 muscarinic receptors. This inhibition occurs via pertussis toxin-insensitive G-proteins belonging to the G alpha(q) fa mily (Caulfield et al., 1994). We have used DNA plasmids encoding anti sense sequences against the 3' untranslated regions of G alpha subunit s (antisense plasmids) to investigate the specific G-protein subunits involved in muscarinic inhibition of I-K(M). These antisense plasmids specifically reduced levels of the target G-protein 48 hr after intran uclear injection. In cells depleted of G alpha(q), muscarinic inhibiti on of I-K(M) was attenuated compared both with uninjected neurons and with neurons injected with an inappropriate G alpha(oA) antisense plas mid. In contrast, depletion of G alpha(11) protein did not alter I-K(M ) inhibition. To determine whether the alpha or beta gamma subunits of the G-protein mediated this inhibition, we have overexpressed the C t erminus of beta adrenergic receptor kinase 1 (beta ARK1), which binds free beta gamma subunits. beta ARK1 did not reduce muscarinic inhibiti on of I-K(M) at a concentration of plasmid that can reduce beta gamma- mediated inhibition of calcium current (Delmas et at., 1998a). Also, e xpression of beta(1) gamma(2) dimers did not alter the I-K(M) density in SCG neurons. In contrast, I-K(M) was virtually abolished in cells e xpressing GTPase-deficient, constitutively active forms of G alpha(q) and G alpha(11). These data suggest that G alpha(q) is the principal m ediator of muscarinic I-K(M) inhibition in rat SCG neurons and that th is more likely results from an effect of the alpha subunit than the be ta gamma subunits of the G(q) heterotrimer.