Résumé
Helix aspersa neuronal cell membranes contain distinct serotonin (5-HT) and dopamine (DA) sensitive adenylate cyclases. We have taken advantage of the fact that in this system, both in vitro (enzymatic assays) and in vivo (electrophysiological measurements) experiments can be used to explore the GTP dependence and the pharmacological properties of this neurotransmitter-sensitive enzyme system. The first property was studied using non-hydrolysable GTP analogs (guanosine 5′-O-(3-thio-triphosphate) or GTPγS, and guanosine 5′-imido diphosphate or Gpp(NH)p). In vitro, these two components stimulate the enzyme activity but with different potencies (K
apparent
= 10
−8 to 5 × 10
−8 M for GTPγS, and 10
−5 M for Gpp(NH)p). Intracellular injections of GTPγS, but not of Gpp(NH)p, produced an electrophysiological response similar to the one elicited by 5-HT and DA. These results imply that, even in the presence of the high endogenous GTP concentration normally present in the cell (10
−3 M), GTPγS may bind to the GTP-binding protein. Such an interpretation is consistent with the in vitro competition experiments between GTP and GTPγS for adenylate cyclase activation. The pharmacology of 5-HT and DA receptors involved in adenylate cyclase stimulation and electrophysiological responses was studied. Serotoninergic antagonists and neuroleptics inhibited the 5-HT-sensitive adenylate cyclase in a stereospecific manner. However, their inhibition was not simply competitive. Our results suggest that they irreversibly bind a component localized on the cytoplasmic side of the membrane. Unexpectedly, the DA receptor coupled with adenylate cyclase was insensitive to any of the several antagonists tested.