Effects of the chlorotriazine herbicide, cyanazine, on GABA(A) receptors in cortical tissue from rat brain

Chlorotriazine herbicides disrupt luteinizing hormone (LH) release in femal e rats following in vivo exposure. Although the mechanism of action is unkn own, significant evidence suggests that inhibition of LH release by chlorot riazines may be mediated by effects in the central nervous system. GABA, re ceptors are important for neuronal regulation of gonadotropin releasing hor mone and LH release. The ability of chlorotriazine herbicides to interact w ith GABA, receptors was examined by measuring their effects on [H-3]muscimo l, [H-3]Ro15-4513 and [S-35]tert-butylbicyclophosphorothionate (TBPs) bindi ng to rat cortical membranes. Cyanazine (1-400 mu M) inhibited [H-3]Ro15-45 13 binding with an IC50 of approximately 105 mu M (n = 4). Atrazine (1-400 mu M) also inhibited [H-3]Ro15-4513 binding, but was less potent than cyana zine (IC50 = 305 mu M). However, the chlorotriazine metabolites diaminochlo rotriazine, 2-amino-4-chloro-6-ethylamino-s-triazine and 2-amino-4-chloro-6 -isopropylamino-s-triazine were without significant effect on [H-3]Ro15-451 3 binding. Cyanazine and the other chlorotriazines were without effect on [ H-3]muscimol or [S-35]TBPS binding. To examine whether cyanazine altered GA BA(A) receptor function, GABA-stimulated Cl-36(-) flux into synaptoneurosom es was examined. Cyanazine (50-100 mu M) alone did not significantly decrea se GABA-stimulated Cl-36(-) flux. Diazepam (10 mu M) and pentobarbital (100 mu M) potentiated GABA-stimulated Cl-36(-) flux to 126 and 166% of control , respectively. At concentrations of 50 and 100 mu M, cyanazine decreased p otentiation by diazepam to 112 and 97% of control, respectively, and decrea sed potentiation by pentobarbital to 158 and 137% of control (n = 6). Inter estingly, at lower concentrations (5 mu M), cyanazine shifted the EC50 for GABA-stimulated Cl-36(-) flux into synaptoneurosomes from 28.9 to 19.4 mu M , respectively (n = 5). These results suggest that cyanazine modulates benz odiazepine, but not the muscimol (GABA receptor site) or TBPS (Cl- channel) , binding sites on GABA, receptors. Furthermore, at low concentrations, cya nazine may slightly enhance function of GABA, receptors, but at higher conc entrations, cyanazine antagonizes GABA, receptor function and in particular antagonizes the positive modulatory effects of diazepam and pentobarbital. (C) 1999 Elsevier Science Ireland Ltd. All rights reserved.