Inhibitory effect of adenosine on electrical activity of frog melanotrophs mediated through A1 purinergic receptors.

1. The effects of adenosine were studied in cultured frog melanotrophs by the patch-clamp technique. 2. In cell-attached experiments, most cells responded to adenosine (50 microM) by a reversible inhibition of action current discharges without any apparent desensitization. 3. In whole-cell experiments, adenosine provoked a hyperpolarization accompanied by a depression of spontaneous action potentials and a decrease in membrane resistance. When adenosine was repeatedly applied, tachyphylaxis was observed. Addition of GTP (100 microM) in the intracellular solution augmented the percentage of cells hyperpolarized by adenosine, and the duration and amplitude of the hyperpolarization, and prevented the tachyphylaxis. 4. Pretreatment with pertussis toxin (1 microgram ml-1) blocked adenosine-induced inhibition. 5. In cells dialysed with the non-hydrolysable GTP analogue GTP gamma S (100 microM), adenosine caused a sustained, strong hyperpolarization and an irreversible inhibition of spikes. 6. The effect of adenosine was mimicked by the A1 receptor agonist R-PIA (R-N6-phenylisopropyl-adenosine; 50 microM) and blocked by the A1 receptor antagonist CPDPX (8-cyclopentyl-1,3-dipropylxanthine, 50 microM). The A2 receptor antagonist CGS15943 (9-chloro-2-(2-furanyl)-5,6-dihydro-1,2,4-triazolo[1,5-c] quinazoline-5-imine; 50 microM) did not affect the adenosine-induced response. 7. The results suggest that, in frog melanotrophs, adenosine exerts a direct hyperpolarizing effect accompanied by blockage of spontaneous action potentials. The effect of adenosine is mediated through A1 receptors coupled to a Gi/o protein.