An Intrinsic γ-Aminobutyric Acid (GABA)ergic System in the Adrenal Cortex: Findings from Human and Rat Adrenal Glands and the NCI-H295R Cell Line

gamma-Aminobutyric acid (GABA), a major neurotransmitter in the central nervous system, also acts as a paracrine or autocrine signaling molecule in endocrine tissues such as the pancreatic islets, adenohypophysis, and testis. In the present study, we describe local GABA production and functional GABA(B) receptors in the adrenal cortex, possibly forming an auto- or paracrine GABAergic system. Using immunohistochemistry and RT-PCR, we localized the GABA-synthesizing enzyme glutamate decarboxylase 67 and the vesicular GABA transporter in steroid-producing cells of the human and rat adrenal cortex. Immunocytochemistry, Western blots, and RT-PCR experiments demonstrated the presence of glutamate decarboxylase 67 in the human adrenocortical cell line NCI-H295R. Measurements of glutamate decarboxylase activity confirmed that, in these cells and in rat adrenals, glutamate is decarboxylated to form GABA. In addition, we found expression of the GABA(B(1a)), GABA(B(1e)), and GABA(B(2)) subunits of the heterodimeric GABA(B) receptor in NCI-H295R cells as shown by RT-PCR. GABA(B(1a)) and its truncated splice variant GABA(B(1e)) were also found in human and rat adrenal glands. Immunostaining for the GABA(B(2)) subunit revealed its presence in the human and rat adrenal cortex and in NCI-H295R cells. The GABA(B) receptors we identified were functional because the GABA(B) agonist baclofen inhibited T-type Ca(2+) currents in whole-cell patch clamp experiments on NCI-H295R cells. This effect was blocked by pertussis toxin. Furthermore, the alpha(2)-, alpha(3)-, beta(2)-, beta(3)- gamma(2)-, and epsilon-subunits of the GABA(A) receptor were detected in this cell line by RT-PCR. Hence, we conclude that GABA is synthesized and stored by steroid-producing cells of the adrenal cortex and may influence these cells in a paracrine or autocrine manner.