Maintenance of GABA receptor function of small-diameter cockroach neurons by adenine nucleotides

Small diameter (<20 microm) neurons from the sixth abdominal ganglion of the American cockroach, Periplaneta americana, were enzymatically isolated and responses to exogenously applied gamma-aminobutyric acid (GABA) were recorded using the whole-cell patch clamp technique. With a minimal intracellular medium, responses to repeated applications of GABA decreased to zero within a few minutes. The rate of rundown of GABA responses was decreased by the intracellular inclusion of the phosphatase inhibitors microcystin and okadaic acid, suggesting that phosphorylation is necessary for the maintenance of cockroach GABA receptor function. ATP (5 mM) prevented GABA response rundown. ADP (5 mM) also slowed GABA response rundown, but responses stabilized at a level about half that seen with ATP. In the presence of protein kinase A inhibitory peptide (PKI), ATP was only as efficacious as ADP in slowing rundown. PKI had no effect on the ability of ADP to slow rundown, suggesting that the beta-phosphate of ADP is not involved in PKA-dependent phosphorylation of the GABA receptor. These results suggest that in cockroach neurons, GABA receptor function is maintained intracellularly by adenine nucleotides, not only by phosphorylation, but also possibly by an interaction with a nucleotide recognition site unrelated to PKA-dependent phosphorylation.