γ-Aminobutyric acid(A) receptor desensitization in mice spinal cord cultured neurons: Lack of involvement of protein kinases A and C

Desensitization of the γ-aminobutyric acid(A) (GABA(A)) receptor was studied in cultured mammalian spinal cord neurons, using a GABA-induced ³⁶Cl^- influx assay. GABA(A) receptor agonists such as GABA and muscimol produced desensitization of GABA(A) receptor-gated Cl^- channels. The ability of GABA to induce desensitization was time and concentration dependent and reversible. Involvement of protein kinase A in the desensitization phenomenon was studied by using activators of adenylate cyclase (forskolin analogs) and membrane-permeant analogs of cyclic AMP (8-bromo-cAMP and dibutyryl-cAMP). Both active forskolin and the inactive forskolin analog 1,9-dideoxyforskolin decreased GABA-induced ³⁶Cl^- influx alone, as well as when preincubated in conjunction with GABA. The effect of forskolin analogs appears to be nonspecific and unrelated to generation of cyclic AMP. GABA-induced ³⁶Cl^- influx was also inhibited directly by 8-bromo-cAMP, dibutyryl-cAMP, and cAMP. Furthermore, the protein kinase A inhibitor H-8 did not reverse the effect of cAMP analogs on the inhibition of GABA-induced ³⁶Cl^- influx. Taken together, these results suggest that cAMP analogs inhibit GABA-induced ³⁶Cl^- influx by acting via an extracellular site. The inability of the active phorbol ester to modify GABA-induced desensitization rules out the involvement of protein kinase C in the GABA receptor desensitization. These results suggest that protein kinases A and C are not involved in GABA(A) receptor desensitization in mouse spinal cord cultured neurons.