Involvement of a GTP-binding protein in mediation of serotonin and acetylcholine responses in Xenopus oocytes injected with rat brain messenger RNA.

Injection of poly(A)+ RNA from rat brain into Xenopus oocytes caused the appearance of Cl currents in response to serotonin (5-HT) and acetylcholine (ACh). Both neurotransmitters evoked two-component currents similar in their time course to the oocyte's endogenous cholinergic muscarinic response, which was shown in previous studies to be mediated by IP3 synthesis leading to Ca release from intracellular stores. The responses to ACh and 5-HT exhibited self- and cross-desensitization, i.e., application of either ACh or 5-HT inhibited the subsequent response to either one of the two transmitters. Intracellular injection of guanosine 5'-O-(3-thiotriphosphate) (GTP-gamma-S) mimicked the 5-HT and ACh response, and also completely suppressed the response to the subsequent application of either ACh or 5-HT. Treatment of the oocytes with pertussis toxin (PTX) caused a 50% attenuation of ACh and 5-HT responses. In the membranes of both control and mRNA-injected oocytes, PTX catalyzed the ADP-ribosylation of a single Mr = approximately 40,000 protein. Injection of the purified beta gamma-subunits of transducin enhanced the 5-HT response. The 5-HT and GTP-gamma-S responses were inhibited by intracellular injection of the Ca2+ chelator, EGTA, as previously shown for the ACh response. These data suggest that ACh and 5-HT receptors, synthesized in the oocytes on the template of brain mRNA, act through a common pathway that involves (a) a guanine nucleotide binding protein and (b) IP3 production leading to Ca mobilization.