CONVERGENCE OF MULTIPLE G-PROTEIN-COUPLED RECEPTORS ONTO A SINGLE TYPE OF POTASSIUM CHANNEL

The light green cells (LGCs) in the central nervous system of the pond snail Lymnaea Stagnalis form a homogeneous group of neuroendocrine ce lls that are involved in the control of growth and metabolism. These c ells are inhibited by dopamine and the neuropeptides APGWamide, FMRFam ide and GGSLFRFamide. Thus, the LGCs form an endogenous system in whic h processing and integration of different inputs into a physiological response can be studied. In this study we characterize the current(s) that are responsible for the inhibition of the LGCs by dopamine, APGWa mide, FMRFamide and GGSLFRFamide. The responses are G-protein dependen t, as follows from experiments with GTP-gamma-S. Several experiments i ndicate that the four agonists activate a single type of potassium cha nnel. First, the currents evoked by the agonists have the same ion sel ectivity and voltage dependence. Potassium is the predominant charge c arrier and the responses are weakly voltage sensitive, with conductanc e decreasing at potentials below similar to-100 mV. Second, the curren ts activated by the four agonists display similar sensitivity towards several blockers. Internal and external TEA (10 mM), and extracellular Ba2+ (1 mM) cause a block of similar to 60-90%. External 4AP (1 mM) c auses similar to 30% block and external Cs+ (1 mM) causes a voltage se nsitive block. There is no sensitivity towards apamine and glibenclami de. Third, there is no summation of the responses to dopamine, APGWami de and GGSLFRFamide with maximal FMRFamide responses. Together, these data indicate that the responses induced by dopamine, APGWamide, FMRFa mide and GGSLFRFamide are G-protein mediated and converge onto a singl e type of potassium channel in the LCCs of Lymnaea stagnalis. (C) 1997 Elsevier Science B.V.