COEXISTENCE OF GAMMA-AMINOBUTYRIC-ACID TYPE-A AND TYPE-B RECEPTORS IN TESTICULAR INTERSTITIAL-CELLS

The existence of specific gamma-aminobutyric acid (GABA)ergic receptors in testicular interstitial cells was investigated in the present study. Specific binding of [H-3]GABA to interstitial cell membranes was found to be time- and temperature-dependent and varied according to Ca2+ concentration present in the incubation medium. We analyzed the ability of different GABAergic agonists and antagonists to displace the bound radioactivity. In the absence of Ca2+ (1 mM EDTA), GABA and the GABAergic agonist isoguvacine displaced the bound radioactivity. When the radioligand assay was performed in the presence of 2.5 mM CaCl2, the [H-3]GABA specifically bound increased twofold. Under such conditions, the specific GABAergic agonist baclofen, as well as GABA and isoguvacine, displaced the [H-3]GABA bound. Saturation analysis revealed the presence of a population of GABA(A) binding sites with a K(D) value of 45.2 nM and a maximal number of binding sites of 57.4 fmol/mg of protein. The maximal binding increased on addition of 2.5 mM CaCl2 to 102 fmol/mg of protein, indicating the existence of a second population of GABAergic receptors, i.e., type B, with essentially the same affinity. In addition, the incubation of testicular interstitial cells with GABA and baclofen resulted in an increase in androgen production. These results support a functional role of GABA in the neuroendocrine control of the male gonad.