The GABA(A) receptor complex in the developing chick optic tectum: characterization of benzodiazepine and ionophore-linked convulsant/barbiturate recognition sites.

By use of membrane preparations and incubation conditions optimized for each binding site, we have characterized the benzodiazepine and ionophore-linked-convulsant/barbiturate modulatory sites within the chick tectal GABA(A) receptor complex. Using [(3)H]flunitrazepam (FNZ) and [(35)S]t-butylbicyclophosphorothionate (TBPS), respectively, as specific radioligand probes for the two sites, we have found in each case one single population of high-affinity, saturable, specific binding sites. The apparent dissociation constants (K(d)) show no change during tectal development (9 nM for [(3)H]FNZ, and 25-28 nM for [(35)S]TBPS) while the respective densities of binding sites at saturation (B(max)) experience in both cases a twofold increase between embryonic day 16 and postnatal day 10. Ligand-specific pharmacological profiles and allosteric interactions between the transmitter and modulatory sites appear to be well preserved in the chick tectal membrane preparations employed in this study.