Abstract

We investigated the identity of the G protein mediating the muscarinic stimulation of adenylyl cyclase in rat olfactory bulb membranes by examining the sensitivity of this response to selective anti-G protein antisera. Preincubation of tissue membranes with the antisera AS/7 (anti-Gi1/2 alpha), EC/2 (anti-Gi3 alpha/Go alpha), and GO/1 (anti-G(o) alpha) but not with the antiserum QL (anti-Gq/11 alpha) significantly attenuated the carbachol-stimulated adenylyl cyclase activity. These antisera had no effect on the enzyme activity stimulated by the beta-adrenergic agonist L-isoproterenol. On the other hand, the anti-Gs alpha antiserum RM/1 markedly depressed both carbachol- and L-isoproterenol-stimulated adenylyl cyclase activities. This antiserum also reduced the basal enzyme activity to a similar extent. However, different than the anti-Gi/Go antisera, the RM/1 antiserum failed to affect the carbachol-stimulated [35S]guanosine 5'-O-(3-thiotriphosphate) binding to membrane G proteins, whereas it curtailed the [35S]guanosine 5'-O-(3-thiotriphosphate) binding stimulated by pituitary adenylate cyclase-activating peptide. Exposure to either pertussis toxin or the anti-Go alpha antiserum 9072 but not to cholera toxin or the anti-Gs alpha antiserum 1191 reduced the high-affinity binding of oxotremorine M to muscarinic receptors. Moreover, the labeling of a 45-kDa protein catalyzed by cholera toxin was markedly stimulated by pituitary adenylate cyclase-activating peptide but not by carbachol. These data indicate that in rat olfactory bulb membranes, muscarinic receptors interact with both Gi and Go and that these G proteins mediate the stimulation of adenylyl cyclase. Although this response appears to require Gs activity, no evidence was found for the direct coupling of muscarinic receptors to Gs.