Abstract

Previous studies have found that ethanol (EtOH) will consistently potentiate γ-aminobutyric acid (GABA) receptor function in the cerebellum during beta adrenergic receptor activation. One consequence of beta adrenergic receptor stimulation is to increase cAMP levels, which, in turn, activate protein kinase A (PKA)-mediated phosphorylation of intracellular protein sites. In the present study, we investigated three cAMP analogues, two activators and one inhibitor of PKA to determine whether this cAMP-mediated second-messenger system may be one mechanism involved in the previously observed beta adrenergic interaction of EtOH with the GABA_A receptor. Furthermore, because the phosphorylation state of the GABA_A receptor may be an important determinant of function, we investigated the effect of the block of phosphatase activity on EtOH/GABA receptor interactions. We found that similar to the beta adrenergic agonist isoproterenol, local applications of the membrane-permeable cAMP analogues 8-bromo-cAMP and Sp-cAMP could modulate responses to iontophoretically applied GABA and that these modulated GABA responses were sensitized to the potentiative effects of EtOH. EtOH did not facilitate unmodulated GABA effects or GABA responses that were maximally modulated by 8-bromo-cAMP, suggesting that the cAMP mechanism mediates the observed EtOH interaction with GABA mechanisms. Furthermore, the PKA inhibitor Rp-cAMP reversed the EtOH-induced potentiation of the isoproterenol-modulated GABA responses. Finally, microcystin-LR and okadaic acid, which are type I and IIa phosphatase inhibitors, could also modulate and sensitize GABA responses to EtOH. These data suggest that beta adrenergic sensitization of GABA_Areceptors to EtOH involves the intracellular cAMP/PKA second-messenger cascade.