Pasteurella multocida toxin (PMT) is a bacterial toxin that mediates at least some of its effects via actions on the G-protein, Gq (Baldwin et al, 2003) and shows selectivity for Gq over G11 (Orth et al, 2004). Inhibition of M-current by M1 muscarinic receptor activation in rat superior cervical ganglion (SCG) neurons has been shown to be primarily mediated by Gq, rather than G11 (Haley et al, 1998). In this study we have investigated the acute and chronic effects of PMT and a non toxic mutant PMT(C1165S) on muscarinic receptor-induced M-current inhibition. SCGs (isolated from rats killed by approved Schedule 1 methods) were cultured for 2-4 days on laminin coated plastic dishes (for electrophysiology) or glass coverslips (for immunocytochemistry). Control and PMT (Calbiochem) treated cells were exposed to rabbit anti rat Gq and G11 antibodies overnight at 4^oC followed by a donkey anti rabbit FITC secondary antibody for 1 hour. Amphotericin-perforated patch recording were performed on cells superfused at 23^oC with a solution containing (mM): NaCl 144, KCl 2.5, MgCl₂ 0.5, CaCl₂ 2, Hepes 5, glucose 10, TTX 0.0005, pH 7.4. Patch pipettes were filled with a solution containing (mM): K acetate 80, KCl 30, Hepes 40, MgCl₂ 3, EGTA 3, CaCl₂ 1, pH 7.2. Cells were clamped at -20 mV and deactivating M-current tails evoked by 1 second hyperpolarizing steps to -50 mV every 30 seconds. Gq antibody labelling was more intense than that for G11 in all cells tested. There was no qualitative difference in the labelling of Gq or G11 between control or PMT treated (500 ng ml^-1, 24 hours at 37^oC) cells. Acute applications (1-10 min) of PMT (100 ng ml^-1) had no effect on the amplitude of the M-current (n=4) and the amplitude or kinetics of the M-current was not altered when cells were pre-incubated in PMT (500 ng ml^-1) for 18-24 hours. Concentration inhibition curves to the muscarinic agonist oxotremorine-M were generated in control, PMT and PMT(C1165S) mutant pretreated cells. IC₅₀s were (geometric mean and 95% confidence limits) 1.32 (0.65-2.67) μM, 0.07 (0.02-0.22)μM and 0.30 (0.14-0.67) μM respectively. The slopes were (mean ± sem) 0.75 ± 0.16 (n=6) for control, 0.90 ± 0.15 (n=9) for PMT and 0.84 ± 0.15 (n=6) for PMT (C1165S) pre-treated cells. Voltage dependent norepinephrine inhibition of N-type (Cav2.2) calcium channel current was not altered by pre-treatment with PMT. These data support the idea that one of the actions of PMT is to enhance the action of Gq, in part by phosphorylation.