Noradrenergic potentiation of cerebellar Purkinje cell responses to GABA: Cyclic AMP-AS intracellular intermediary

Norepinephrine and the beta-adrenergic receptor agonist, isoproterenol; have been shown to potentiate the amplitude of GABA(A) receptor-mediated whole-cell current responses in Purkinje cells acutely dissociated from the rat cerebellum. However, the steps leading from the activation of beta-adrenergic receptors to the modulation of GABA(A) receptor remain to be delineated. This study tested the hypothesis that a sequelae of intracellular intermediaries involving the cyclic AMP second messenger system serves as the subcellular link to promote this heteroreceptor interaction. Exposure to cholera toxin, but not to pertussis toxin, increased the amplitude of GABA-activated current responses in acutely dissociated Purkinje cells. Intracellular dialysis with guanosine 5'-O-(3-thiotriphosphate) also resulted in a time- and dose-dependent augmentation of the response to GABA, while guanosine 5'-O-(2-thiodiphosphate) blocked the norepinephrine-mediated facilitation. A positive modulation of the current response to GABA was observed following intracellular delivery of cyclic AMP or the catalytic subunit of the cyclic AMP-dependent protein kinase. Furthermore, the norepinephrine-induced potentiation of the GABA-activated current response was prevented in the presence of the Rp isomer of cyclic AM, the regulatory subunit of cyclic AMP-dependent protein kinase and an inhibitor of cyclic AMP-dependent protein kinase. These findings led to the formulation of a working model in which activation of the beta-adrenergic receptor triggers a G(S)-protein-mediated transduction cascade in cerebellar Purkinje cells which activates adenylate cyclase, resulting in a rise in intracellular levels of cyclic AMP, increased phosphorylating activity by cyclic AMP-dependent protein kinase and, ultimately, a potentiation of GABA(A) receptor function. Copyright (C) 1996 IBRO.