MUSCARINIC AND BETA-ADRENERGIC DEPRESSION OF THE SLOW CA-2+-ACTIVATED POTASSIUM CONDUCTANCE IN HIPPOCAMPAL CA3 PYRAMIDAL CELLS IS NOT MEDIATED BY A REDUCTION OF DEPOLARIZATION-INDUCED CYTOSOLIC CA-2+-TRANSIENTS

Combined intracellular and microfluorometric recording techniques were used to evaluate whether the inhibition by cholinergic or adrenergic transmitters of the Ca2+-activated potassium current (I(AHP)) in hippocampal CA3 pyramidal cells was mediated by an alteration of depolarization-induced change in cytosolic free Ca2+ concentration ([Ca2+](i)). Low concentrations of isoproterenol (1-10 μM) and muscarine (0.25-1 μM) reversibly abolished I(AHP) without affecting concomitant Ca2+ transients or the steady-state [Ca2+][(i). Only after application of higher concentrations of muscarine, [Ca2+](i) increased; in the presence of potassium channel blockers, muscarine depressed Ca2+ currents and concomitant Ca2+ transients. These observations provide direct evidence that the inhibition of I(AHP) by isoproterenol and muscarine are not mediated by an alteration of Ca2+ dynamics.