The GABA(A) receptor complex in the developing chick optic tectum: characterization of [(3)H]muscimol binding sites

As part of an ongoing study on the GABA(A) receptor complex in the developing chick optic tectum we describe some properties of the agonist site, as labeled by [(3)H]muscimol, including methodological, kinetic and pharmacological aspects. 16-day embryos and 10-day chicks have been selected as representative age points for the initial characterization of the receptor, prior to more detailed developmental studies. Our data indicate the existence, in both embryos and young birds, of a single class of statistically equivalent, high-affinity, saturable binding sites, with a dissociation constant (K(d)) of 80-90 nM in freeze-thawed/washed membranes, and about 8 nM in membranes additionally extracted with low concentrations of Triton X-100. Maximal densities of binding sites are nearly identical in both membrane preparations, ranging from ?2 to ?3 pmol/mg for the two age points considered. The pharmacological profiles suggest that avian receptors for [(3)H]muscimol are generally similar to the corresponding mammalian sites, behaving as typical bicuculline-sensitive, baclofen-insensitive type A GABA receptor sites. However, bicuculline and its derivatives are less efficient displacers of [(3)H]muscimol in detergent-extracted membrane preparations, being in all cases, as usually, much less effective displacers than GABA agonists. The effect of Triton X-100 on the muscimol site in the GABA(A) receptor, increasing the affinity for the radioligand by a factor of 10, and diminishing the efficiency of antagonists, is considered here in terms of structural changes in the receptor, induced by the action of the detergent on the membrane microenvironment.