GABA activation of benzodiazepine receptors in native and Triton-treated membranes

Abstract

Saturable binding of 3H-flunitrazepam ( 3H-FNZ) to native and Triton X-100-treated membranes of cerebral cortex was examined. GABA stimulated 3H-FNZ binding to the native membranes by increasing the affinity for radioligand from a dissociation constant, Kd (± SEM), of 1.54 ± 0.04 to 0.90 ± 0.03 nM ( p<0.001) with a small but significant increase in density of sites (± SEM) from 2.30 ± 0.03 to 2.47 ± 0.04 pmole/ mg protein ( p<0.025). The stimulation by GABA persisted after a preincubation with 10 μM GABA at 4° or 37°. GABA reduced the rate of dissociation of the bound radioligand from a half-life t ( 1 2 ) of 15.5 ± 0.3 to 26.4 ± 0.6 min ( p<0.001) without changing the rate of association within the first min. The activation by GABA appeared 2–3 min later and continued up to 20 min of incubation. The delay in the activation may be due to the decrease in rate of dissociation. GABA activated 3H-FNZ binding in the Triton-treated membranes in a similar manner. With the native membranes, however, GABA caused a biphasic effect on the dissociation of bound 3H-FNZ suggesting two classes of benzodiazepine receptors with different sensitivities toward GABA.