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-γ-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 M_r = ∼40,000 protein. Injection of the purified βγ-subunits of transducin enhanced the 5-HT response. The 5-HT and GTP-γ-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.