ALLOSTERIC MODULATION OF [S-35] TBPS-BINDING IN THE CEREBRAL-CORTEX OF THE RAT DURING POSTNATAL-DEVELOPMENT

The ontogenesis of the GABA-gated Cl- channel was investigated in the cerebral cortex of the rat by monitoring the binding parameters of [S- 35]t-butylbicyclophosphorothionate ([S-35]TBPS) at intervals after bir th (1-90 days). To investigate the influence of the developmental chan ges in the content of GABA on [S-35]TBPS-binding, the assays were carr ied out in unwashed membranes, in which the concentration of GABA was dependent on its content in vivo, and in repeatedly washed membranes i n the presence of defined concentrations of exogenous GABA. At birth, the density (B-max) of [S-35]TBPS-binding sites in unwashed membranes was similar to that found in well-washed membranes. However, in unwash ed membranes, the number of [S-35]TBPS-binding sites increased by two- fold within 10 days after birth whereas in washed membranes it increas ed by four-fold during the same period. The higher density of [S-35]TB PS-binding sites in washed membranes as compared with the unwashed cou nterparts persisted throughout development. In unwashed membranes, the apparent K-d for [S-35]TBPS-binding increased with age whereas in was hed membranes the affinity of [S-35]TBPS for its binding sites remaine d constant throughout development. The binding of [S-35]TBPS to the GA BA-gated Cl- channel is allosterically modulated by drugs acting on di fferent sites of the GABA(A) receptor complex. Thus, GABA and diazepam decrease [S-35]TBPS-binding whereas the GABA(A) receptor antagonist, bicuculline, and the inverse agonist for benzodiazepine receptors, 6,7 -dimethoxy-4-ethyl-beta-carboline-3-carboxylic acid methyl ester, incr ease it. The magnitude of the effects of positive and negative modulat ors of the GABA(A) receptor on [S-35]TBPS-binding did not change signi ficantly from birth through adulthood, provided the concentration of G ABA in the incubation medium was kept constant at 3 mu M. These result s reflect the early development of the functional interactions between the different components of the GABA(A) receptor.