| Abstract: | The molecular mechanism underlying the signal transduction from muscarinic and serotonergic receptors to Cl- channels were investigated in Xenopus oocyte microinjected with rat brain poly(A)+ mRNA. Transient Cl- current responses of the mRNA-injected oocytes to acetylcholine (ACh) and serotonin (5-HT) were similar in amplitude and onset. Although pharmacological characterization indicated that distinct M1-like and S1-like receptors of rat brain are involved in the ACh and 5-HT responses, respectively, these responses cross-desensitized each other completely. A common involvement of the GTP-binding proteins coupled to phosphoinositide breakdown was suggested by the findings that intracellular application of guanosine 5'-O-(2-thio)bisphosphate (GDP beta S) or neomycin greatly suppressed both ACh and 5-HT responses. These responses were not affected by exposure of the mRNA-injected cells to cholera toxin, but they were inhibited by pertussis toxin. The increase in inositol trisphosphate (IP3) responsive both to ACh and 5-HT coincided with the expression of Cl- current responses. However, only 5-HT but not ACh slightly increased the cyclic AMP (cAMP) content of the mRNA-injected cells. Intracellular injection of either IP3 or Ca2+ produced a transient Cl- current in the mRNA-injected cells as well as in non-injected cells, while 1-oleoyl-2-acetylglycerol (OAG), cAMP or cyclic GMP (cGMP) never elicited chloride current responses. It was proposed that muscarinic and serotonergic receptors are commonly linked to phosphoinositide breakdown through the mediation of GTP-binding proteins Ni and/or No in mRNA-injected oocytes. |
