Cocaine inhibition of GABAA current: Role of dephosphorylation

Acute cocaine toxicity is frequently associated with seizures. The mechanisms underlying the convulsant effect of cocaine are not well understood. Previously, we have shown that cocaine depresses whole-cell current evoked by γ-aminobutyric acid (GABA) in hippocampal neurons freshly isolated from rats. Cocaine's effect was voltage-independent and concentration-dependent. In the present study, using whole-cell patch-clamp recording on rat neurons freshly isolated from hippocampus, we examined the intracellular mechanisms involved in cocaine's action. Increasing intracellular Ca2+ concentration ([Ca]i) from 0.01 to 5 μM strongly increased the depressant effect of cocaine. By contrast, 1-[N, O-bis (5-isoquinolinesulfonyl)-N-methyl-L-tyrosyl]-4-phenylpiperazine (KN-62), a specific antagonist of Ca/calmodulin-depen-dent protein kinase (CaMKII), attenuated or enhanced cocaine's action in different neurons: in three out of nine neurons dialysed with 5 μM KN-62,1 mM cocaine depressed GABA current by only 33%, but in another three out of nine neurons, cocaine depressed GABA current by as much as 83%. Chelerythrine (a specific CaCa2+/phospholipid- dependent protein kinase C [PKC] antagonist) had minimal effect on cocaine's action. We suggest that cocaine induces an increase in [Ca]i, which stimulates phosphatase activity and thus leads to dephosphorylation of GABA receptors. This dephosphorylation-mediated disinhibitory action may play a role in cocaine-induced convulsant states.