Protein kinase A-mediated phosphorylation reduces only the fast desensitizing glycine current in acutely dissociated ventromedial hypothalamic neurons.

Modulation of glycine receptor-ionophore complex by internally perfused cyclic AMP was investigated and compared to that of GABA in the acutely dissociated ventromedial hypothalamic neurons using whole-cell and outside-out patch-clamp techniques. Cyclic AMP significantly reduced both GABA- and glycine-gated chloride currents. The reduction in glycine-induced chloride current was specific in that only the fast-desensitizing one gated by high concentrations of glycine (30-100 microM) was affected. Cyclic AMP did not modulate the non-desensitizing current induced by lower concentrations (6-10 microM). Addition of N-[-2-(methylamino)ethyl]-5-isoquinolinesulfonamide dihydrochloride, a protein kinase A inhibitor, did not have a significant effect on its own but prevented the attenuation of fast desensitizing glycine current induced by cyclic AMP. Both the reversal potential and inactivation kinetics of glycine current were not affected by the activation of protein kinase A, suggesting that cyclic AMP-mediated attenuation is not due to an enhancement of desensitization. In outside-out patch studies intracellular perfusion of cyclic AMP reduced the open probability of the 100 microM glycine-activated channels without affecting that of the 6 microM glycine-activated channels. In conclusion, cyclic AMP selectively modulates the channel open frequency of the glycine receptor when activated at higher concentrations through a protein kinase A-mediated phosphorylation.