Interactions between loreclezole, chlormethiazole and pentobarbitone at GABA(A) receptors: functional and binding studies.

1. Interactions were investigated between loreclezole, chlormethiazole and pentobarbitone as potentiators of depolarization responses mediated by gamma-aminobutyric acid(A) (GABA(A)) receptors on afferent nerve terminals in the rat cuneate nucleus in vitro. These drugs were also compared as modulators of [3H]-flunitrazepam (FNZ) binding to synaptic membranes prepared from rat whole brain homogenate. 2. In rat cuneate nucleus slices, the drugs shifted muscimol log dose response lines to the left in an approximately parallel fashion with the result that 200 microM chlormethiazole potentiated muscimol responses by 0.567 +/- 0.037 log unit (mean +/- s.e.mean, n = 4) while loreclezole gave a maximal potentiation at 10 microM of only 0.121 +/- 0.037 (n=6) log unit and 0.071 +/- 0.039 (n=22) at 50 microM. 3. While 50 microM chlormethiazole and 30 microM pentobarbitone showed no significant interactions between each other when potentiating muscimol responses in combination, 50 microM loreclezole in combination with either chlormethiazole or pentobarbitone attenuated their potentiating effects, possibly by inducing desensitization of GABA(A) receptors. 4. In the [3H]-FNZ binding studies on well-washed membranes, loreclezole enhanced binding to a maximum of 47.3 +/- 2.83% of control (mean +/- s.e.mean, n = 3) at 300 microM. Scatchard analysis revealed no change in Bmax but a decrease in K(D) for [3H]-FNZ from 3.9 +/- 0.29 nM to 2.7 +/- 0.10 nM (mean +/- s.e.mean, n=4) in the presence of 100 microM loreclezole. In contrast, 100 microM chlormethiazole caused no potentiation. A small component of the enhancement by loreclezole could be blocked by 100 microM bicuculline and could also be blocked by 100 microM chlormethiazole. It seems likely that the effects on [3H]-FNZ binding are due predominantly to direct actions of the drugs on the GABA(A) receptor and are separate from the GABA-potentiating effects. 5. The results indicate distinctly different profiles of action for loreclezole, chlormethiazole and pentobarbitone on GABA(A) receptors.