MECHANISM OF CHLORDIAZEPOXIDE ENHANCEMENT OF 4-AMINOBUTYRETE-RECEPTORDESENSITIZATION INVESTIGATED WITH BR-82(-) RADIOTRACER

Rates of desensitization of a rapidly desensitizing 4-aminobutyrate (4 -Abu) receptor (t(1/2) = 33 ms at saturation) were measured in a previ ously characterized experimental system from rat cerebral cortex. Dese nsitization was measured at times between 100 ms and several s. Measur ements with 300-ms or 1-s Br-82(-) influx assays after prior incubatio n of the membrane with 4-Abu covered the whole 4-Abu concentration ran ge of response. Desensitization was not affected by the low Br- concen trations (less than or equal to 1 mM) present. The:presence of the tra nquilizing drug chlordiazepoxide (Librium) in the prior incubation inc reased the desensitization rate at 4-Abu concentrations below saturati on. This drug changed the dependence of desensitization rate on 4-Abu concentration from the sigmoid response, previously reported with 4-Ab u alone, to a hyperbolic dependence. In the presence of chlordiazepoxi de the measurements approximated linear Eadie-Hofstee and Lineweaver-B urk plots. This extended the range of response to lower 4-Abu concentr ations, so that the relative increase of desensitization rate became l arger with decreasing 4-Abu concentration and was about sixfold at 1 m u M 4-Abu. The maximum rate, at saturation with 4-Abu was unchanged by chlordiazepoxide. This observation parallels the change in the mechan ism of channel opening due to chlordiazepoxide previously reported. Al l the measurements could be fitted to a simple minimal kinetic model, in which the effect of chlordiazepoxide could be attributed to a chang e in the receptor with one bound 4-Abu molecule. This could be an incr ease in desensitization rate or a decrease in dissociation constant of the receptor with one bound 4-Abu molecule and no other change. The e ffects of the tranquilizer, chlordiazepoxide with this receptor are to extend the 4-Abu concentration range of response 10-fold, to lower 4- Abu concentrations, so that the major part of the response occurs betw een 0.3 mu M and 1000 mu M 4-Abu. In addition to the effect on channel opening, the receptor desensitization rate is affected in the same wa y.