INHIBITORY EFFECT OF ADENOSINE ON ELECTRICAL-ACTIVITY OF FROG MELANOTROPHS MEDIATED THROUGH A(1) PURINERGIC RECEPTORS

1. The effects of adenosine were studied in cultured frog melanotrophs by the patch-clamp technique. 2. In cell-attached experiments, most c ells responded to adenosine (50 mu M) by a reversible inhibition of ac tion current discharges without any apparent desensitization. 3. In wh ole-cell experiments, adenosine provoked a hyperpolarization accompani ed by a depression of spontaneous action potentials and a decrease in membrane resistance. When adenosine was repeatedly applied, tachyphyla xis was observed. Addition of GTP (100 mu M) in the intracellular solu tion augmented the percentage of cells hyperpolarized by adenosine, an d the duration and amplitude of the hyperpolarization, and prevented t he tachyphylaxis. 4. Pretreatment with pertussis toxin (1 mu g ml(-1)) blocked adenosine-induced inhibition. 5. In cells dialysed with the n on-hydrolysable GTP analogue GTP gamma S (100 mu M), adenosine caused a sustained, strong hyperpolarization and an irreversible inhibition o f spikes. 6. The effect of adenosine was mimicked by the A(1) receptor agonist R-PIA (R-N-6-phenylisopropyl-adenosine; 50 mu M) and blocked by the A(1) receptor antagonist CPDPX (8-cyclopentyl-1,3-dipropylxanth ine, 50 mu M). The A(1) receptor antagonist CGS15943 (9-chloro-2-(2-fu ranyl)-5,6-dihydro 1,2,4-triazolo [1,5-c] quinazoline-5-imine; 50 mu M ) did not affect the adenosine-induced response. 7. The results sugges t that, in frog melanotrophs, adenosine exerts a direct hyperpolarizin g effect accompanied by blockage of spontaneous action potentials. The effect of adenosine is mediated through A(1) receptors coupled to a G (1/0), protein.