Long-term increases in excitability in the CA1 region of rat hippocampus induced by beta-adrenergic stimulation: possible mediation by cAMP.

The cellular mechanisms underlying beta-adrenergic potentiation in the CA1 region of the rat hippocampus were examined. A 10 min treatment with isoproterenol (ISO) induced a long-term depolarization of the pyramidal neurons that persisted for at least 30 min of washout; the ISO-induced decrease in the calcium-activated potassium conductance (afterhyperpolarization, or AHP) was similarly prolonged. The long-term excitability changes induced by ISO did not depend upon the calcium concentration of the medium and could be elicited in medium containing as little as 240 microM calcium. The persistent increase in population spike induced by ISO was mimicked by superfusion with several cAMP analogs and by forskolin (which directly activates adenylate cyclase), but not by the inactive dideoxyforskolin. Forskolin and cAMP analogs also induced decreases in AHPs that could be quite prolonged, but did not depolarize pyramidal neurons as consistently as did ISO. We hypothesize that activation of beta-adrenergic receptors in the CA1 region of hippocampus may induce an alteration of the hippocampal "state" that can persist for as long as several hours, during which the induction of other forms of plasticity may be enhanced.