MOLECULAR AND PHARMACOLOGICAL CHARACTERIZATION OF GABA(A) RECEPTORS IN THE RAT PITUITARY

Levels of mRNA for the major subunits of the GABA(A) receptor were ass ayed in the rat pituitary anterior and neurointermediate lobes by ribo nuclease protection assay. alpha 1, beta 1, beta 2, beta 3, and gamma2 s were found to be the predominant subunits in the anterior lobe, wher eas alpha 2, alpha 3, beta 1, beta 3, gamma 2s, and gamma 1 were the p redominant subunits expressed in the neurointermediate lobe. alpha 5, alpha 6, and delta subunits were not detectable. Hill and Scatchard an alysis of [H-3]muscimol binding to anterior and neurointermediate lobe membranes showed high-affinity binding sites with dissociation consta nts of 5.6 and 4.5 nM, respectively, and Hill coefficients near 1. Mus cimol sites were present at a maximum of 126 fmol/mg in the anterior l obe and 138 fmol/mg in the neurointermediate robe. The central-type be nzodiazepine antagonist [H-3]Ro 15-1788 bound to a high-affinity site with a dissociation constant of 1.5 nM in both tissues, at a maximum o f 60 fmol/mg in anterior pituitary and 72 fmol/mg in neurointermediate lobe. A Hill coefficient of 1 was measured for this site in both tiss ues. Assays of CL 218 872 displacement of Ro 15-1788 were consistent w ith a pure type I benzodiazepine site in the anterior robe and a pure type II site in the intermediate robe. These results are consistent wi th both tissue-specific expression of particular GABA(A) receptor subu nits and receptor heterogeneity within individual cells in the pituita ry.