The major neurotransmitter of the central nervous system, gamma-aminobutyric acid (GABA), exerts its actions through GABA_A, GABA_B and GABA_C receptors. GABA and GABA receptors are, however, also present in several non-neural tissues, including the endocrine organs pituitary, pancreas and testis. In the case of the rat testis, GABA appears to be linked to the regulation of steroid synthesis by Leydig cells via GABA_A receptors, but neither testicular sources of GABA, nor the precise nature of testicular GABA receptors are fully known. We examined these points in rat, mouse, hamster and human testicular samples. RT-PCR followed by sequencing showed that the GABA-synthesizing enzymes glutamate decarboxylase (GAD) 65 and/or GAD67, as well as the vesicular GABA transporter vesicular inhibitory amino acid transporter (VIAAT/VGAT) are expressed. Testicular GAD in the rat was shown to be functionally active by using a GAD assay, and Western blot analysis confirmed the presence of GAD65 and GAD67. Interstitial cells, most of which are Leydig cells according to their location and morphological characteristics, showed positive immunoreaction for GAD and VIAAT/VGAT proteins. In addition, several GABA_A receptor subunits (α1–3, β1–3, γ1–3), as well as GABA_B receptor subunits R1 and R2, were detected by RT-PCR. Western blot analysis confirmed the results for GABA_A receptor subunits β2/3 in the rat, and immunohistochemistry identified interstitial Leydig cells to possess immunoreactive GABA_A receptor subunits β2/3 and α1. The presence of GABA_A receptor subunit α1 mRNA in interstitial cells of the rat testis was further shown after laser microdissection followed by RT-PCR analysis. In summary, these results describe molecular details of the components of an intratesticular GABAergic system expressed in the endocrine compartment of rodent and human testes. While the physiological significance of this peripheral neuroendocrine system conserved throughout species remains to be elucidated, its mere presence in humans suggests the possibility that clinically used drugs might be able to interfere with testicular function.

Gamma-aminobutyric acid (GABA) is a major inhibitory neurotransmitter in the brain. Furthermore, by acting through pituitary GABA A -, GABA B -, and GABA C receptors, it also plays an important role in the regulation of pituitary function. Although it has generally been assumed that the source of this pituitary GABA is the hypothalamus, here we provide evidence that GABA synthesis also occurs within the pituitary gland itself. Using RT-PCR, in situ hybridization histochemistry, immunohistochemistry, and immunoelectron microscopy, were detected the GABA synthesizing enzyme glutamic acid decarboxylase (GAD 67), the vesicular GABA transporter (VIAAT/VGAT), and GABA in the rat and rhesus monkey pituitary. Although these proteins were found in all of the endocrine cells of the intermediate lobe, we found these proteins in the growth hormone (GH) producing endocrine cells in the anterior lobe only, as well as in a rat GH producing cell line (GH3). In addition, GAD enzyme activity was readily detectable in the rat pituitary and GH3 cells. Many endocrine cells in the anterior pituitary, including GH-cells as well as PRL-, TSH-, ACTH-, and LH/FSH-cells were found to contain GABA A and or GABA B receptors, as shown by double-immunofluorescence staining. Therefore the identification of a novel site of synthesis of GABA within the pituitary, namely POMC-cells of the intermediate lobe and GH cells in the anterior lobe and the presence of pituitary GABA receptors, imply unexpected auto/paracrine actions of the neurotransmitter GABA to occur within the pituitary. These results suggest a new basic mechanism in the regulation of pituitary function by GABA.

There is increasing evidence suggesting that the neurotransmitter γ-aminobutyric acid (GABA) is a local factor involved in the regulation of endocrine organs. Examples of such functions are documented in the pancreas, but recent results suggest that GABA may act in a similar way in the pituitary, in which GABA receptors are expressed and pituitary growth hormone (GH) cells provide a source of GABA. We hypothesised that GABA secreted in somatotropes may act as an autoregulatory signaling molecule. To test this hypothesis we first examined the nature of GABA receptors expressed by GH cells. RT-PCR analysis demonstrated that GABA-B receptor subunits R1 and R2 are present in the whole rat pituitary. Laser microdissection of immunostained GH cells, followed by RT-PCR as well as immunoelectron microscopy, showed that GABA-B receptors are expressed on somatotropes. To investigate GABA-B receptor function in somatotropes, we used rat GH3 adenoma cells, which, like pituitary GH cells, express GABA-B R1 and R2 (as assessed by RT-PCR and immunoelectron microscopy) and produce GABA (checked by high performance liquid chromatography). After inhibition of endogenous GABA synthesis, GH production was stimulated by baclofen, a chromatography). After inhibition of endogenous GABA synthesis, GH production was stimulated by baclofen, a GABA-B receptor agonist. By contrast, blocking GABA-B receptors by an antagonist, phaclofen, decreased GH levels. We conclude that in GH-producing cells, GABA acts as an autocrine factor via GABA-B receptors to control GH levels.

The neurotransmitter ␥-aminobutyric acid (GABA), released by hypothalamic neurons as well as by growth hormone-(GH) and adrenocorticotropin-producing cells, is a regulator of pituitary endocrine functions. Different classes of GABA receptors may be involved. In this study, we report that GH cells, isolated by laser microdissection from rat pituitary slices, possess the GABA-C receptor subunit 2 . We also demonstrate that in the GH adenoma cell line, GH3, GABA-C receptor subunits are not only expressed but also form functional channels. GABA-induced Cl ؊ currents were recorded using the whole cell patch clamp technique; these currents were insensitive to bicuculline (a GABA-A antagonist) but could be induced by the GABA-C agonist cis-4-aminocrotonic acid. In contrast to typical GABA-C mediated currents in neurons, they quickly desensitized. Ca

gamma-Aminobutyric acid (GABA) is an important regulatory factor of pituitary gland function, which in addition to hypothalamic neurons, can be derived from intrapituitary sources, ie, growth hormone (GH) cells of rat and monkey. We report that human pituitary glands also express 2 isoforms of the GABA-synthesizing enzyme glutamate decarboxylase (GAD 65; GAD 67), the vesicular GABA transporter (VGAT), and multiple subunits of GABA (A, B, and C) receptors. GABA production and storage occurs in GH cells, as demonstrated by cellular colocalization of immunoreactive GAD and VGAT in GH cells and by reverse transcription-polymerase chain reaction analysis of laser capture-microdissected immunostained GH cells. It is interesting that human pituitary GH adenomas share expression of VGAT and GABA receptors with normal pituitary glands but lack GAD 65. We propose that GABA, synthesized by GH cells, might act as a paracrine or autocrine regulating factor in the human pituitary gland and in human GH adenoma. Because many drugs interfere with GABA function, the identification of GABA system components might have clinical implications.