Disruption of GABA-dependent chloride flux by cyclodienes and hexachlorocyclohexanes in primary cultures of cortical neurons

The effect of convulsant and nonconvulsant hexachlorocyclohexane (HCH) isomers and cyclodienes on GABA-induced Cl- flux was studied in primary cultures of neocortical neurons by measuring the GABA-stimulated 36Cl- uptake. GABA induced a dose-dependent chloride uptake. The convulsant agents gamma-HCH and cyclodienes alpha-endosulfan, dieldrin and aldrin blocked this 36Cl- uptake. A total or partial inhibition of GABA-induced 36Cl- uptake was produced by the noncompetitive GABAA antagonists picrotoxinin (PTX) and pentylenetetrazol, respectively. The inhibitory potencies of 36Cl- uptake by the organochlorine compounds (alpha-endosulfan > dieldrin > gamma-HCH > aldrin) were well correlated with their inhibitory potencies of [35S]TBPS binding. Positive modulators of GABAergic function (flunitrazepam and phenobarbital) prevented the blocking of GABA-induced chloride uptake by PTX but not that induced by alpha-endosulfan. The depressant beta- and delta-HCH isomers produced a biphasic response, increasing or decreasing the GABA-stimulated chloride uptake, depending on the HCH isomer and GABA concentrations used. The present results support the idea of cyclodienes and gamma-HCH action at the GABAA receptor by interacting with the TBPS binding site. A different interaction of PTX and alpha-endosulfan in the same recognition site is also suggested. An increase of GABA-induced 36Cl- flux by beta- and delta-HCH can account for the depressant activity of these compounds. This work also demonstrates the usefulness of primary neuronal cultures to perform functional studies of the GABAA receptor, taking into account allosteric interactions between the different recognition sites of the GABAA receptor