Catecholamine distribution in adult rat testis

Biogenic amines in the testis: sources, receptors and actions

Biogenic amines are signaling molecules with multiple roles in the central nervous system and in peripheral organs, including the gonads. A series of studies indicated that these molecules, their biosynthetic enzymes and their receptors are present in the testis and that they are involved in the regulation of male reproductive physiology and/or pathology. This mini-review aims to summarize the current knowledge in this field and to pinpoint existing research gaps. We suggest that the widespread clinical use of pharmacological agonists/antagonists of these signaling molecules, calls for new investigations in this area. They are necessary to evaluate the relevance of biogenic amines for human male fertility and infertility, as well as the potential value of at least one of them as an anti-aging compound in the testis.

Physiopharmacological properties of the testicular capsule: A concise review

The testicular capsules of different mammalian species exhibit spontaneous motor activity. In addition, contractions can be mediated by neuronal stimulation or exogenous drug administration. However, the physiological role of testicular capsule motor activity is still not well understood. Nevertheless, there is evidence for putative roles in spermatozoa transport from the testis to the caput epididymis, control of interstitial/intratesticular pressure and testicular blood flow. In this review, we have collated information about the agents that regulate testicular capsule motor activity, their receptors and second messengers as well as the impact of altered testicular capsule function on the male reproductive system. Furthermore, we highlight the knowledge gaps in the physiology and pharmacology of the testicular capsule as indicators of future research directions that may lead to a better understanding of the physiological role of testicular capsule motor activity and its importance in male fertility.

β-adrenergic receptors in the up-regulation of COX2 expression and prostaglandin production in testicular macrophages: Possible relevance to male idiopathic infertility

Catecholaminergic neuronal elements (CNE) and macrophages (MACs) are increased in testes of patients with idiopathic infertility. Now, we describe an anatomical proximity between CNE and MACs, expression of specific α- and β-adrenergic receptors (ADRs) subtypes in MACs, and a positive correlation between the number of MACs and cyclooxygenase (COX2) expression - key enzyme in prostaglandin (PG) synthesis and an inflammatory marker - in testes of infertile men. To examine a potential effect of adrenergic input on COX2 expression, we used two additional experimental models: non-testicular human MACs (THP1 cell line) and non-human testicular MACs purified from adult Syrian hamsters. We found that epinephrine and norepinephrine up-regulate COX2 expression and PGD2 production through β1-and β2-ADRs. Our results demonstrate the existence of a yet unknown link between CNE and MACs in the human testis that could trigger inflammation and tissue homeostatic dysregulation associated with pathogenesis or maintenance of infertility states.

Alpha 1 adrenergic receptor-mediated inflammatory responses in human testicular peritubular cells

Stress activates the sympathetic nervous system and is linked to impaired fertility in man. We hypothesized that catecholamines by acting on testicular cells have a role in these events, possibly by fostering an inflammatory environment. The cells of the wall of seminiferous tubules, human testicular peritubular cells (HTPCs), express adrenergic receptors (ADRs) α1B, α1D, β1 and β2. A selective α1-ADR agonist, phenylephrine, increased intracellular Ca²⁺-levels in cultured HTPCs and induced COX-2, IL-6 and MCP-1 mRNA expression without affecting IL-1β mRNA. These changes were paralleled by a significant increase in the secretion of IL-6 and MCP-1. Epinephrine was also effective, but salbutamol, a selective β2-ADR agonist was not. Our results suggest that stress-associated elevation of catecholamines may be able to promote inflammatory events by targeting peritubular cells in the human testis. Blockage of α1-ADRs may therefore be a novel way to interfere with stress-related impairment of male reproductive functions.

Involvement of the mesenteric ganglia on androstenedione, noradrenaline and nitrite release using a testis ex vivo system

The autonomic nerve fibres converge to the testis along two major pathways, the superior spermatic nerve (SSN) and the inferior spermatic nerve (ISN). The object of this work was to evaluate whether the addition of noradrenaline (NA) in the ganglionic compartment of two ex vivo systems: superior mesenteric ganglion (SMG)-SSN-testis, inferior mesenteric ganglion (IMG)-ISN-testis modulate androstenedione (A₂), NA and nitrite release and to determine whether there are secretory differences between the right and the left testis. Each gonad with its respective ganglion was transferred into a cuvette with two compartments and incubated in a Dubnoff metabolic shaker. The testis incubation liquids were collected and analysed for NA by HPLC, A₂ by RIA and nitrites by the Griess method. When NA is added to the IMG, A₂ and NA release diminishes and nitrite increases in the left testis, while in the right gonad, A₂ and NA increase and nitrite decreases. When NA was administered to the SMG, A₂ and NA increase and nitrite diminishes in the left gonad, but they show opposite fluctuations in the right testis. These ex vivo systems appear to be excellent models for studying the sympathetic ganglionic control of the testis though A₂, NA and nitrite release from the male gonad. It is evident that a better knowledge about the role of catecholamines and nitric oxide in the testis physiology may facilitate the understanding of some reproductive diseases.

Peptidergic not monoaminergic fibers profusely innervate the young adult human testis

Numerous studies have reported that intratesticular nerves exert important regulatory effects on the functions of the male gonad; however, as yet little is known about their distribution in the young adult human testis. The purpose of this study was to explore whether peptidergic and adrenergic nerves occur in the male gonad of this age, and, if present, to depict their distribution further. Thirty testes were collected from 15 reproductively healthy donors aged 21-32 years. Antibodies against protein gene product 9.5 (PGP 9.5), neuropeptide Y (NPY), C-terminal flanking peptide of NPY (CPON) and vasoactive intestinal peptide (VIP) were employed for immunohistochemical detection of intratesticular peptidergic nerves, and those against dopamine-beta-hydroxylase (DBH) and 5-hydroxytryptamine (5-HT) for monoaminergic ones. The testicular parenchyma exhibited a rich innervation by PGP 9.5-positive fibers, mainly associated with Leydig cell nests, blood vessels, and seminiferous tubules. Numerous NPY- and CPON-immunoreactive (IR) nerves also appeared in the gonads, but the vast majority were confined to blood vessels. A small number of VIP-IR fibers were detected in some arterioles. By contrast, however, no fibers displaying DBH or 5-HT immunoreactivity were observed within the testis. Additionally, expression of PGP-9.5, NPY, CPON, VIP, DBH and 5-HT was found in Leydig cells, PGP 9.5 in spermatogonia, and NPY and CPON in peritubular myoid cells. Our results suggest that the young adult human testis is devoid of monoaminergic nerves but profusely innervated by peptidergic fibers, which may serve as major neuronal regulators for testicular functions at this age.

Novel identification of peripheral dopaminergic D2 receptor in male germ cells

Dopamine is a recognized modulator in the central nervous system (CNS) and peripheral organ functions. The presence of peripheral dopamine receptors outside the CNS has suggested an intriguing interaction between the nervous system and other functional systems, such as the reproductive system. In the present study we analyzed the expression of D2R receptors in rat testis, rat spermatogenic cells and spermatozoa, in different mammals. The RT-PCR analysis of rat testis mRNA showed specific bands corresponding to the two dopamine receptor D2R (L and S) isoforms previously described in the brain. Using Western blot analysis, we confirmed that the protein is present in rat testis, isolated spermatogenic cells and also in spermatozoa of a range of different mammals, such as rat, mouse, bull, and human. The immunohistochemistry analysis of rat adult testis showed that the receptor was expressed in all germ cells (pre- and post-meiotic phase) of the tubule with staining predominant in spermatogonia. Confocal analysis by indirect immunofluorescence revealed that in non-capacitated spermatozoa of rat, mouse, bull, and human, D2R is mainly localized in the flagellum, and is also observed in the acrosomal region of the sperm head (except in human spermatozoa). Our findings demonstrate that the two D2 receptor isoforms are expressed in rat testis and that the receptor protein is present in different mammalian spermatozoa. The presence of D2R receptors in male germ cells implies new and unsuspected roles for dopamine signaling in testicular and sperm physiology.

Sympathetic neurotransmission in the rat testicular capsule: functional characterization and identification of mRNA encoding alpha1-adrenoceptor subtypes

The rat testicular capsule is a thin tissue surrounding the testis, whose precise function is still unknown. We have studied the contractile effects of electrical field stimulation, noradrenaline, and the blockade by antagonists of adrenergic receptors, in order to characterize sympathetic neurotransmission, and adrenoceptor subtypes. In addition, reverse transcription polymerase chain reaction (RT-PCR) assays were made to check for the expression of the three known subtypes of alpha(1)-adrenoceptors. The effects of electrical field stimulation (2 to 20 Hz, 1 ms, 60 V) were almost totally abolished by depletion of neuronal noradrenaline storage with reserpine (10 mg/Kg), but not by the purinergic receptor antagonist suramin (10(-5) M), indicating that noradrenaline, but not ATP, was involved in contractions. The selective alpha(1)-adrenoceptor antagonist prazosin (10(-7) M) was more effective than the selective alpha(2)-adrenoceptor antagonist idazoxan (10(-7) M) to inhibit contractions induced by electrical field stimulation, pointing out a major involvement of alpha(1)-adrenoceptor. When noradrenaline was used instead of electrical field stimulation, it showed a high potency (pD(2)=7.9). Noradrenaline-induced contractions were competitively blocked by the selective alpha(1A)-adrenoceptor antagonists WB 4101 (pA(2)=8.88), phentolamine (pA(2)=8.39) and by the alpha(1B)-adrenoceptor antagonist spiperone (pA(2)=8.57), indicating the presence of functional alpha(1A)- and alpha(1B)-adrenoceptors. In addition, contractions were not blocked by the selective alpha(1D)-adrenoceptor antagonist BMY 7378 (up to 10(-6) M), while selective alpha(2)-adrenoceptor antagonists showed low pA(2) values (yohimbine, 7.25 and idazoxan, 7.49), suggesting a minor role, if any, for alpha(1D)- and alpha(2)-adrenoceptors. To check the proportionate role of alpha(1A)- and alpha(1B)-adrenoceptors, we blocked alpha(1B)-adrenoceptors with chloroethylclonidine (CEC, 30 microM, 45 min), that reduced the maximal effect of noradrenaline by about 60%. The remnant CEC-insensitive noradrenaline contraction was assumed to be unrelated to alpha(1B)-adrenoceptor, and was inhibited by 5-methyl-urapidil (pA(2)=8.94) and by the Ca(2+) channel blocker nifedipine (3 microM), confirming the involvement of alpha(1A)-adrenoceptors. The presence of mRNA encoding alpha(1A)- and alpha(1B)-adrenoceptor was also shown on RT-PCR assays. Unexpectedly, alpha(1D)-transcripts were also detected in these assays. Taken together, our results show that ATP co-transmission could not be detected, and that neurotransmission involves the interaction of noradrenaline with both alpha(1A)- and alpha(1B)-, but not with alpha(1D)- or alpha(2)-adrenoceptor. The fact that the functional alpha(1D)-adrenoceptor could not be detected in spite of the presence of the corresponding mRNA, remains to be investigated.

Catecholamine-synthesizing enzymes in the adult and prenatal human testis

Catecholamines play functional roles in the mature and developing mammalian testis but the cell types responsible for their local synthesis are still controversially discussed. Here, we demonstrate that four enzymes involved in the biosynthesis of catecholamines, namely, tyrosine hydroxylase (TH), aromatic amino acid decarboxylase (AADC), dopamine beta-hydroxylase (DBH) and phenylethanolamine- N-methyltransferase (PNMT), are expressed in Leydig cells of the human testis. Tyrosine hydroxylase, the key enzyme of the biosynthesis of catecholamines, was localized to Leydig cells both at the transcript level (by RT-PCR analyses and by in situ hybridization assays) and at the protein level (by immunoblotting and by immunohistochemistry). The other enzymes were also demonstrated in Leydig cells by RT-PCR and immunohistochemical analyses. The presence of TH, AADC, DBH, and PNMT in human Leydig cells was found, in addition, by immunohistochemical approaches carried out on sections from prenatal human testes. Thus, the present study identifies the Leydig cells as the presumed sites of catecholamine production in both the mature and fetal human testes and further supports the previously recognized neuroendocrine characteristics of this cell type.

Administration of acetylcholine to the spermatic nerve plexus inhibits testosterone secretion in an in vitro isolated rat testis-nerve plexus system

Strong evidence indicated that spermatic nerves are involved in the regulation of testosterone secretion. Our previous work showed that the inferior spermatic nerves play a more significant role than the superior ones in the regulation of testosterone secretion. However, it is unknown whether traditional neurotransmitters are involved in this regulation. In order to evaluate this point, the present experiments were carried out in an in vitro system where an isolated testis-spermatic nerve plexus preparation was incubated in two separate containers, one for the testis and the other for the nerve plexus and both interconnected by the inferior spermatic nerves. Both tissues were maintained in the same environmental conditions except for the neurotransmitter treatment, applied only to the nerve plexus. Acetylcholine can significantly inhibit the secretion of testosterone until the end of incubation. The present experiments suggest that the secretion of testosterone could be regulated, at least in part, by acetylcholine through the inferior spermatic nerves.

Immunohistochemical investigations of the autonomous nerve distribution in the testis of the camel (Camelus dromedarius)

The distribution of autonomous nerves in the testis of the camel was studied by immunohistochemical methods. A total of 26 testes was collected during the different seasons of the year. As pan-neuronal markers, antibodies to protein gene product 9.5 and to neurofilaments are superior to antibodies against neuron-specific enolase and acetylcholinesterase histochemistry for the description of the nerves in the camel testis. Testicular nerves reach the camel testis by three access-routes as (1) funicular contribution, (2) mesorchial contribution and (3) as caudal contribution. The main target for testicular nerves is the arterial vascular tree of the organ, whereas all veins of testis and pampiniform plexus are devoid of any innervation in the camel. In the wall of the arteries, the nerves form a plexus at the media-adventitia border. The density of the arterial plexuses increases along the vascular tree: smaller septal and mediastinal arteries are better innervated than albugineal arteries and the latter better than the A. testicularis. The nerves in the septula testis, in the mediastinum and between the Leydig cells show clear seasonal changes, being particularly abundant in autumn and particularly scarce in spring. The nerves that reach the camel testis are unmyelinated and represent in the vast majority postjunctional sympathetic neurons. Cholinergic fibers are absent in the camel testis. Neuropeptide Y is the dominating peptidergic transmitter in the testicular nerves and colocalized with noradrenaline in the same axons. Vasoactive intestinal polypeptide-containing fibers reach the camel testis exclusively as parts of the caudal nervous contribution via the ligamentous bridge between testis and epididymal tail and are restricted to the caudal pole of the testis. Calcitonin gene-related peptide-positive axons are not frequent in the camel testis; nevertheless, they seem to be the most important sensory pathway of this organ.

Peptidergic innervation of blood vessels and interstitial cells in the testis of the cat

We studied the innervation of the cat testis using a panel of antisera against the following neuronal markers: protein gene product 9.5 (PGP), neuropeptide Y, C-terminal peptide of neuropeptide Y, galanin, vasoactive intestinal peptide (VIP), calcitonin gene-related peptide, and substance P. Immunoreactivity against PGP, a general neuronal label, demonstrated the arrangement of fibers from the superior spermatic nerve (SSN) in the testicular pedicle and the cephalic testicular pole, and those of the inferior spermatic nerve (ISN) along the vas deferens and the inferior testicular ligament. The testicular parenchyma exhibited a very rich innervation, mainly distributed to blood vessels and Leydig cell nests, but also in close association with seminiferous tubules. Numerous peptidergic fibers were present in the SSN and ISN, albeit in different proportions. Thus, VIP-immunoreactive fibers were almost absent in the SSN, but were the most abundant subpopulation of the ISN. The testicular interstitium contained numerous peptidergic fibers, associated with blood vessels, interstitial Leydig cells, and seminiferous tubules. Similar fibers were related to the rete testis. Parenchymatous VIP-immunoreactive nerves disappeared after bilateral vasectomy. Stimulation of the ISN under experimental conditions was associated with an increase of blood flow, and induced a large release of VIP into the spermatic vein. The extensive and selective distribution of nerve fibers within the cat testicular parenchyma supports the importance of spermatic nerves for testicular function. Furthermore, the differences in the fiber composition of the SSN and ISN can be correlated with their opposing effects on testosterone secretion and testicular blood flow.

Has active immunization against gonadotrophin-releasing hormone any effect on testis innervation in the pig? An immunohistochemical study

The innervation of porcine testes was studied in intact animals and in boars undergoing active immunization against gonadotrophin-releasing hormone (GnRH) by means of immunohistochemistry using antibodies to tyrosine hydroxylase (TH), dopamine beta-hydroxylase (D beta H), vasoactive intestinal polypolypeptide (VIP), neuropeptide Y (NPY), synaptosome-associated protein of 25 kDa (SNAP-25) and protein gene product 9.5 (PGP 9.5). Moreover, the distribution of luteinizing hormone (LH) receptors in clusters of Leydig cells was also investigated. To identify these cells easily, either the NADPH-diaphorase histochemical technique or the Mayer counter-staining procedure was applied. Differences in the distribution pattern and relative density of particular subsets of intratesticular nerve fibres were observed in immunized boars as compared to those found in the intact animals. In the testes of non-treated animals, only single TH-immunoreactive (TH-IR) nerve fibres were observed. However, many D beta H-IR nerve terminals surrounded blood vessels in the tunica albuginea and parenchyma. Very scarce VIP-IR nerves occurred only in the tunica albuginea, mainly in close vicinity to blood vessels. Immunoreactivity to NPY occurred in single nerve fibres. Immunoreactivity to SNAP-25 and PGP 9.5 was found in single nerve fibres distributed mainly in the tunica albuginea. The interstitial cells were heavily stained for LH-receptors and NADPH-diaphorase. In the testes of immunized animals, only single TH-IR nerve fibres, scattered mainly in the tunica albuginea, were observed. Some TH-IR nerve terminals were also encountered in the parenchyma of the organ, where they were always associated with blood vessels. D beta H-IR nerve fibres formed a dense network distributed throughout the testis in association with the capsule, vasculature and interstitium. Some fibres were observed to run between seminiferous tubules. VIP-IR nerve fibres were located in the neighbourhood of blood vessels in the tunica albuginea and parenchyma. Only single VIP-IR nerves were found between seminiferous tubules. Numerous NPY-IR nerve fibres occurred in the tunica albuginea and parenchyma of the organ. SNAP-25-IR and PGP 9.5-IR nerve terminals formed a dense network distributed throughout the testis and many fibres were observed between seminiferous tubules. Interstitial cells were very weakly stained for LH receptors or NADPH-diaphorase.

Noradrenaline and nitrite-nitrate concentrations in the contralateral testes during ipsilateral spermatic cord torsion in the presence or absence of a testis and epididymis

OBJECTIVE

To determine the changes occurring during ipsilateral spermatic cord torsion either in the presence or absence of the ipsilateral testis and epididymis, by evaluating noradrenaline and nitrite-nitrate concentrations in the contralateral testes.

MATERIALS AND METHODS

Forty male albino rats were allocated randomly to one of four equal groups undergoing: group 1, a sham operation; group 2, ipsilateral spermatic cord torsion; group 3, epididymo-orchidectomy only; and group 4, spermatic cord torsion after epididymo-orchidectomy. The contralateral testes were harvested after 24 h and the noradrenaline and nitrite-nitrate contents determined. The levels in each group were compared using the Kruskal-Wallis and Mann-Whitney U-tests.

RESULTS

The noradrenaline content of testes from group 2 was significantly lower than in those of groups 1 and 3, but there were no significant differences in content between groups 1 and 3, 1 and 4, and 2 and 4. The content in group 4 was significantly less than that in group 3. There were no significant differences in nitrite-nitrate contents among any of the groups.

CONCLUSION

Spermatic cord torsion for 24 h, either in the presence or absence of a testis and epididymis, significantly decreased the noradrenaline content in the contralateral testis. This finding supports the suggestion that the sympathetic system is activated by exposure to noradrenaline in the contralateral testis during ipsilateral spermatic cord torsion, with no dependency on the presence of a testis and epididymis. As the nitrite-nitrate concentrations were unaffected, nitric oxide seems to have no role in contralateral testicular deterioration.

Beta2 adrenergic receptors mediate cAMP, tissue-type plasminogen activator and transferrin production in rat Sertoli cells

FSH is the main regulator of Sertoli cell function. Nevertheless, several other effectors such as catecholamines can also stimulate these cells through the adenylyl cyclase transduction pathway. However, the expression of beta adrenergic receptors in Sertoli cells is a subject of controversy. The aim of the present study was to determine if there are physiologically functional beta adrenergic receptors in Sertoli cells and to which subtype(s) they belong. In freshly isolated Sertoli cells, isoproterenol, a non selective beta-adrenergic agonist, was found to stimulate cAMP production and tissue-type plasminogen activator secretion. Specific transcripts for the beta1 and beta2, but not beta3, subtypes were detected by RT-PCR analysis. Beta2 transcripts were the form expressed predominantly in Sertoli cells. Binding experiments carried out on freshly isolated and on cytospined Sertoli cells indicated that in both conditions, [125I]iodocyanopindolol binding was inhibited by a non-selective and a 2 selective antagonist, whereas a beta1 selective antagonist had no effect. Scatchard analysis of beta2 specific inhibition revealed a dissociation constant of 0.3 nM and a receptor density of 14000 sites per cell. In freshly isolated Sertoli cells, we observed that cAMP and tissue-type plasminogen activator were stimulated by isoproterenol and a beta2 selective agonist, but not by beta1 or beta3 selective agonists. Accordingly, the isoproterenol-stimulated tissue-type plasminogen activator responses were abolished by the beta2 selective antagonist only. In cultured Sertoli cells, the trend was the same: tissue-type plasminogen activator and transferrin secretions were increased by isoproterenol and beta2 but not by beta1 or beta3 selective agonists. We conclude that freshly isolated Sertoli cells express beta2 adrenergic receptors which are functionally coupled to adenylyl cyclase and that these characteristics are preserved in cell culture. For the tested parameters, catecholamines and FSH effects were similar, but response magnitudes were systematically lower with beta agonists than with FSH. As norepinephrine is normally present in physiologically-relevant amounts in the interstitial fluid, it can be suspected to play a role in the regulation of Sertoli cell function.

Neuropeptide Y1 receptors in the rat genital tract

Using in situ hybridization and immunohistochemistry, the expression of type 1 neuropeptide Y (NPY) receptors (Y1-Rs) has been demonstrated in the rat genital tract. In the male Y1-R mRNA and Y1-R-like immunoreactivity (LI) were found in smooth muscles of predominantly arterioles and small arteries inside testis. Fibers showing NPY-LI could not be detected within testis but only in the tunica albuginea. These Y1-Rs are suggested to mediate vasoconstriction, possibly activated by NPY released from nerves in the tunica albuginea. In the female rat Y1-R mRNA, but not Y1-R-LI was found in vascular smooth muscles of arteries in the ovary and oviduct. In the oviduct Y1-R mRNA was also detected in the non-vascular smooth muscle layer. Fibers showing NPY-LI were found around blood vessels both in the ovary and oviduct. In the female genital tract also Y1-Rs may thus be involved in regulatory mechanisms mediating, for example, vasoconstriction.

Visualization and ablation of phenylethanolamine N-methyltransferase producing cells in transgenic mice

We cloned and sequenced the mouse phenylethanolamine N-methyltransferase (PNMT) gene which encodes the enzyme that catalyses the conversion of norepinephrine to epinephrine. The ability of various length sequences flanking the mouse or human PNMT genes to direct expression of reporter genes in transgenic mice was examined. We show that 9 kb of 5' flanking sequences from the cloned mouse PNMT gene can direct expression of the Escherichia coli beta-galactosidase (lacZ) gene to predicted regions of the adrenal, eye and brain in the adult transgenic mouse. The transgene was also expressed during development, in the myelencephalon, adrenal medulla and dorsal root ganglia. PNMT-producing cells were ablated by expression of the diphtheria toxin (DT-A) gene driven by the human PNMT promoter, resulting in abnormalities in the adrenal medulla, eye and testis. The hPNMT8 kb-DT-A line presents a model with which to examine the developmental ramifications of deletion of PNMT-producing cell populations from the adrenal medulla and retina.

Inhibitory role of cholinergic agonists on testosterone secretion by purified rat Leydig cells

The effects of cholinometics on basal or hCG-induced testosterone (T) release by Percoll-purified Leydig cells of the rat were studied. Acetylcholine and carbachol as well as nicotine decreased basal and hCG-induced T secretion. The ganglionic nicotine antagonist hexamethonium promoted a partial reversal of the inhibitory effect of nicotine on basal or hCG-stimulated T secretion. Atropine also reduced the inhibitory effect of carbachol on basal or stimulated androgen release. These data indicate that, in short-term incubations, testosterone released by purified Leydig cells is inhibited by nicotinic and muscarinic cholinergic agonists, thus supporting the hypothesis that parasympathetic autonomic system may be involved in the negative regulation of testicular androgen secretion.

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 [3H]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 [3H]GABA specifically bound increased twofold. Under such conditions, the specific GABAergic agonist baclofen, as well as GABA and isoguvacine, displaced the [3H]GABA bound. Saturation analysis revealed the presence of a population of GABAA binding sites with a KD 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.