Seasonal changes in testicular steroidogenesis in the toad Bufo arenarum H

Reconsidering reproductive patterns in a model dissociated species, the red-sided garter snake: Sex-specific and seasonal changes in gonadal steroidogenic gene expression

Sex steroid hormones are powerful regulators of reproductive behavior and physiology in vertebrates, and steroidogenesis has distinct sex- and season-specific patterns ultimately dictated by the expression of key enzymes. Most comparative endocrinology studies, however, focus only on circulating levels of sex steroids to determine their temporal association with life-history events in what are termed associated reproductive patterns. The red-sided garter snake (Thamnophis sirtalis parietalis) is a notable exception; this species exhibits maximal sex behavior decoupled from maximal sex steroid production and gametogenesis in what is termed a dissociated reproductive pattern. And while this is true for male red-sided garter snakes and their production of testosterone, females have maximal estradiol production during peak breeding (spring) but only immediately after mating. Here, we demonstrate that expression of ovarian aromatase (conversion of androgens to estrogens) matches the established seasonal hormone pattern in females. Additionally, steroidogenic gene expression in the ovary is broadly reduced if not suppressed compared to the testis throughout the active year. Bizarrely, male red-sided garter snakes demonstrate an unexplained pattern of steroidogenic gene expression in the testis. StAR (import of cholesterol to steroidogenesis) is maximally expressed in spring, yet Hsd17b3 expression (conversion of androstenedione to testosterone) is highest in summer, with the latter matching the established summer peak in male testosterone. The function of elevated StAR in spring is unknown, but our results suggest a decoupling between maximal StAR expression and testosterone biosynthesis (Hsd17b3 expression). We also purport that the reproductive pattern binary should be reassessed given its lack of fit for many vertebrate species that demonstrate seasonal, mixed patterns of (a)synchrony between circulating sex hormones and reproductive behavior.

Acute stress, steroid plasma levels, and innate immunity in Brazilian toads

Stress from habitat fragmentation has been shown to impact amphibian declines. Studies from a variety of vertebrates indicate that stressed animals exhibit an acute increase in circulating plasma glucocorticoid (GC) levels and consequent immunomodulation. To further explore the relationship between GCs and immunity, we subjected three species of newly captured Brazilian toads, Rhinella ornata, R. icterica and R. schneideri to restraint with or without movement restriction (maintenance in a moistened cloth bag vs. maintenance in a bin) for 24 h. We compared various parameters from baseline (field conditions) with values after restraint, including those associated with stress (corticosterone [CORT] plasma levels), and the neutrophil/lymphocyte ratio [N:L ratio]), potential reproduction (testosterone [T] plasma levels), and innate immunity (bacterial killing ability [BKA]). General responses to the restraint challenge (baseline vs. restraint) included increased CORT levels and N:L ratio, and decreased T levels and BKA. Additionally, CORT levels and N:L ratio tended to increase more from restraint with movement restriction than to restraint without movement restriction, indicating toads showed increased stress response to the more intense stressor. All variables showed interspecific variation at baseline conditions: R. ornata had higher CORT levels when compared to the other two species, while R. icterica had the highest BKA values. After restraint (with or without movement restriction), R. ornata displayed higher values for T and N:L ratio, and showed higher CORT values after restraint without movement restriction; however, the CORT values were similar among species after restraint with movement restriction. In terms of immunity, in response to restraint, BKA was different among species only after restraint with movement restriction, with R. schneideri showing the lowest BKA values. Our results show that restraint increases common markers of the stress response, and could reduce potential reproduction and innate immune responses in toads from all studied species. Our results also showed variation at the interspecific level, with the amplitude of change in the studied variables being consistent and more pronounced following restraint with movement restriction for the three-studied species.

Estradiol and reproduction in the South American toad Rhinella arenarum (Amphibian, Anura)

Rhinella arenarum is a South American toad with wide geographic distribution. Testes of this toad produce high amount of androgens during the non reproductive season and shift steroid synthesis from androgens to 5α-pregnanedione during the breeding. In addition, plasma estradiol (E₂) in males of this species shows seasonal variations but, since testes of R. arenarum do not express aromatase, the source of plasma E₂ remained unknown for several years. However, the Bidder's organ (BO), a structure located at one pole of each testis, is proposed to be the main source of E₂ in male's toads since it expresses several steroidogenic enzymes and is able to produce E₂ from endogenous substrates throughout the year. In addition, there were significant correlations between plasma E₂ and total activity of BO aromatase, and between plasma E₂ and the amount of hormone produced by the BO in vitro. In the toad, apoptosis induced by in vitro treatment with E₂ was mostly detected in spermatocytes during the breeding and in spermatids during the post-reproductive season, suggesting that this steroid has an important role in controlling spermatogenesis. However, in vitro treatment with E₂ had no effect on proliferation. This evidence suggests that the mechanism of action of E₂ on amphibian spermatogenesis is complex and more studies are necessary to fully understand the role of estrogens regulating the balance between cellular proliferation and apoptosis. In addition, in R. arenarum in vitro studies suggested that E₂ has no effect on CypP450c17 protein levels or enzymatic activity, while it reduces 3β-hydroxysteroid dehydrogenase/isomerase (3β-HSD/I) activity during the post reproductive season. As well, E₂ regulates FSHβ mRNA expression all over the year suggesting a down regulation process carried out by this steroid. The effect on LHβ mRNA is dual, since during the reproductive season estradiol increases the expression of LHβ mRNA while in the non-reproductive season it has no effect. In conclusion, the effect of E₂ on gonadotropins and testicular function is complex, not clearly understood and probably varies depending on the species. The aim of the current article is to review evidence on reproductive endocrinology and on the role of estradiol regulating reproduction in amphibians, with emphasis on the South American species Rhinella arenarum.

Effect of glucocorticoids on androgen biosynthesis in the testes of the toad Rhinella arenarum (Amphibia, Anura)

In rat Leydig cells, glucocorticoids (GCs) inhibit testosterone production through the interaction with the glucocorticoid receptor (GR). However, the sensitivity of those cells to GCs is regulated by the enzyme 11β-hydroxysteroid dehydrogenase Type 1 (11β-HSD1). In the testes of the toad Rhinella arenarum, the presence of an 11β-HSD similar to type 2 and a cytosolic GR has also been described. However, there is a lack of information regarding the effects of GCs on amphibian testicular steroidogenesis. In this study, the effects of corticosterone on androgen production, and the activity of two steroidogenic enzymes in toad testes were reported. Corticosterone inhibits androgen production via the GR because the GR antagonist RU486 prevents corticosterone-induced inhibition of testosterone. Corticosterone also reduced the activity of the cytochrome P450 17-hydroxylase, C17,20-lyase (Cyp450 _c17 ) without affecting the 3β-hydroxysteroid dehydrogenase/isomerase activity. This effect on Cyp450 _c17 was likewise inhibited by RU486. On the other hand, corticosterone had no effect on the amount of steroidogenic acute regulator protein. These results suggest that GCs inhibit steroidogenesis in toad testes by reducing of Cyp450 _c17 activity via a GR-mediated mechanism.

Effect of estradiol on apoptosis, proliferation and steroidogenic enzymes in the testes of the toad Rhinella arenarum (Amphibia, Anura)

Estrogens inhibit androgen production and this negative action on amphibian steroidogenesis could be related to the regulation of steroidogenic enzymes. Estrogens are also involved in the regulation of amphibian spermatogenesis by controlling testicular apoptosis and spermatogonial proliferation. The Bidder's organ (BO) is a structure characteristic from the Bufonidae family and in adult males of Rhinella arenarum it is one of the main sources of plasma estradiol (E2). The purpose of this study is to analyze the effect of E2 on testicular steroidogenic enzymes, apoptosis and proliferation in the toad R. arenarum. For this purpose, testicular fragments were treated during 24h with or without 2 or 20nM of E2. After treatments, the activities of cytochrome P450 17α-hydroxylase-C17-20 lyase (CypP450c17) and 3β-hydroxysteroid dehydrogenase/isomerase (3β-HSD/I) were measured by the transformation of radioactive substrates into products, and CypP450c17 expression was determined by Western blot analysis. Apoptosis in testicular sections was detected with a commercial fluorescent kit based on TUNEL method, and proliferation was evaluated by BrdU incorporation. Results indicate that E2 has no effect on CypP450c17 protein levels or enzymatic activity, while it reduces 3β-HSD/I activity during the post reproductive season. Furthermore, although E2 has no effect on apoptosis during the pre and the post reproductive seasons, it stimulates testicular apoptosis during the reproductive season, mostly in spermatocytes. Finally, E2 has no effect on testicular proliferation all year long. Taken together, these results suggest that E2 is involved in the regulation of testicular steroidogenesis and spermatogenesis.

Characterization and seasonal changes in LHβ and FSHβ mRNA of Rhinella arenarum (Amphibia, Anura)

In anurans, two types of gonadotropins were described in several species of Ranidae and Pipidae families but only in one of the Bufonidae family. Rhinella arenarum is a bufonid that have the lowest concentration of plasma androgens during the breeding. The objective of this paper was to characterize the cDNA sequence of β subunit of LH and FSH from toad pituitary and study seasonal variation in gonadotropins mRNA using quantitative real-time RT-PCR. The LHβ cDNA is a 636 bp sequence containing an open reading frame (ORF), 45 bp of 5'-untranslated region (UTR) and 174 bp of 3'-UTR. The ORF encodes for a signal peptide of 26 amino acids and a mature protein of 113 amino acids with one N-glycosylation site at the 34th position. The FSHβ cDNA sequence is a 535 bp fragment containing an ORF, 8 bp of 5'-UTR and 152 bp of 3'-UTR. The ORF encodes for a signal peptide of 20 amino acids and a mature protein of 104 amino acids with two N-glycosylation sites at 25th and 42nd positions. Multiple alignments of aminoacid deduced sequences of LHβ and FSHβ (teleosts, amphibians, birds, mammals) showed that all the tetrapods studied conserve 12 cysteins and one (LH) or two (FSH) N-Glycosylation sites. LHβ is closer to teleosts than to mammals and birds while FSHβ is closer to mammals. The analysis of seasonal changes in LHβ and FSHβ mRNA indicates that transcript levels have seasonal variations and that the profile of androgens is opposite to that of the gonadotropins mRNA.

Non-invasive reproductive and stress endocrinology in amphibian conservation physiology

Non-invasive endocrinology utilizes non-invasive biological samples (such as faeces, urine, hair, aquatic media, and saliva) for the quantification of hormones in wildlife. Urinary-based enzyme immunoassay (EIA) and radio-immunoassay have enabled the rapid quantification of reproductive and stress hormones in amphibians (Anura: Amphibia). With minimal disturbance, these methods can be used to assess the ovarian and testicular endocrine functions as well as physiological stress in captive and free-living populations. Non-invasive endocrine monitoring has therefore greatly advanced our knowledge of the functioning of the stress endocrine system (the hypothalamo-pituitary-interrenal axis) and the reproductive endocrine system (the hypothalamo-pituitary-gonadal axis) in the amphibian physiological stress response, reproductive ecology, health and welfare, and survival. Biological (physiological) validation is necessary for obtaining the excretory lag time of hormone metabolites. Urinary-based EIA for the major reproductive hormones, estradiol and progesterone in females and testosterone in males, can be used to track the reproductive hormone profiles in relationship to reproductive behaviour and environmental data in free-living anurans. Urinary-based corticosterone metabolite EIA can be used to assess the sublethal impacts of biological stressors (such as invasive species and pathogenic diseases) as well as anthropogenic induced environmental stressors (e.g. extreme temperatures) on free-living populations. Non-invasive endocrine methods can also assist in the diagnosis of success or failure of captive breeding programmes by measuring the longitudinal patterns of changes in reproductive hormones and corticosterone within captive anurans and comparing the endocrine profiles with health records and reproductive behaviour. This review paper focuses on the reproductive and the stress endocrinology of anurans and demonstrates the uses of non-invasive endocrinology for advancing amphibian conservation physiology. It also provides key technical considerations for future research that will increase the accuracy and reliability of the data and the value of non-invasive endocrinology within the conceptual framework of conservation physiology.

5α-Reductase, an enzyme regulating glucocorticoid action in the testis of Rhinella arenarum (Amphibia: Anura)

The reduction of A-ring of glucocorticoids to produce 5α-dihydro-derivatives by 5α-reductases has been considered as a pathway of irreversible inactivation. However, 5α-reduced metabolites of corticosterone and testosterone have significant biological activity. In this paper, we investigated whether toad testicular 5α-reductase (5α-Red) is able to transform corticosterone into 5α-dihydrocorticosterone. Furthermore, we studied the role of 5α-reduced metabolite of corticosterone as a glucocorticoid receptor (GR) agonist. The activity of 5α-Red was assayed in subcellular fractions with [(3)H]corticosterone or [(3)H]testosterone as substrate. The enzyme localizes in microsomes and its optimal pH is between 7 and 8. The activity is not inhibited by finasteride. These results support the conclusion that toad 5α-Red resembles mammalian type 1 isoenzyme. Kinetic studies indicate that neither K(m) nor V(max) for both corticosterone and testosterone were significantly different among reproductive periods. The K(m) value for testosterone was significantly higher than that for corticosterone, indicating that the C-21 steroid is the preferred substrate for the enzyme. Studies of the binding capacity of 5α-dihydrocorticosterone (5α-DHB) to the testicular GR show that 5α-DHB is able to displace the binding of [(3)H]dexamethasone to testicular cytosol with a similar potency than corticosterone. The inhibition constant (Ki) values for corticosterone and 5α-DHB were similar, 31.33±2.9 nM and 35.24±2.3 nM, respectively. In vitro experiments suggest that 5α-DHB is an agonist of toad testicular GR, decreasing the activity of the key enzyme for androgen synthesis, the cytochrome P450 17-hydroxylase, C17,20-lyase.

Expression of steroidogenic acute regulatory protein (StAR) in male American bullfrog (Rana catesbeiana) and preliminary evaluation of the response to TNT

We examined the expression of steroidogenic acute regulatory (StAR) protein mRNA in the American bullfrog (Rana catesbeiana). Primers and probes were designed to obtain a partial sequence of bullfrog StAR cDNA consisting of 349 base pairs. Quantitative PCR analysis of StAR mRNA equivalents was performed in tissues of juvenile and adult bullfrogs. In this study 18S mRNA was used as an internal standard. There were no differences in the expression of 18S RNA among tissues or between age groups. In juvenile males, the rank order for the constitutive levels of StAR was testes>skin>brain>kidneys. In adult males, StAR mRNA equivalent was greatest in testes, followed by kidneys, brain, and skin. In addition, stimulation and induction of testicular StAR by human chorionic gonadotropin significantly increased expression of StAR at 2, 4, and 6h after injection. Preliminary evaluation of 2, 4, 6-trinitrotoluene (TNT) revealed that acute exposure is associated with reduction of StAR mRNA expression. The information provided in this study will be useful for future research on StAR gene expression in amphibian reproductive biology and the development of reproductive biomarkers.

Does testosterone influence activity budget in the male Greek tortoise (Testudo graeca graeca)?

The stimulatory effect of testosterone on male sexual activity is one of the clearest examples linking hormones and behaviors. However, this relationship is complex in Chelonians. We experimentally studied the influence of testosterone levels on the activity budget and space use in male Greek tortoises (Testudo graeca graeca) during the spring mating season. We first described the annual pattern of changes in plasma testosterone levels in free-ranging animals in Morocco. Two peaks, one in winter and one in summer, corresponded to periods of inactivity; whereas mating periods in spring and to a lesser extent in autumn were associated with low plasma testosterone levels. Second, we experimentally manipulated plasma testosterone levels in free-ranging males, and analyzed the behavioral consequences. The strong contrasts in plasma hormone levels induced by the experimental treatments did not result in changes in activity budget or space use, both in the short-term or more than one month after the beginning of the hormonal treatment. Our results suggest that testosterone levels did not influence directly behavioral activity in this species, either immediately or after a time delay of one month.

Cytosolic glucocorticoid receptor in the testis of Bufo arenarum: seasonal changes in its binding parameters

Glucocorticoids (GC) are the hormonal mediators of stress. In mammals, high levels of GC have negative effects on reproductive physiology. For instance, GC can inhibit testicular testosterone synthesis by acting via glucocorticoid receptors (GR), the extent of the inhibition being dependent on GC levels. However, the effect of GC on testicular function and even the presence of GR in amphibians are still unclear. The purpose of this work was to characterise testicular cytosolic GR in Bufo arenarum, determining the seasonal changes in its binding parameters as well as the intratesticular localisation. The binding assays were performed in testis cytosol with [3H]dexamethasone (DEX) and [3H]corticosterone (CORT). Binding kinetics of DEX and CORT fitted to a one-site model. Results were expressed as means +/- standard error. Apparent number of binding sites (Bapp) was similar for both steroids (Bapp DEX = 352.53 +/- 72.08 fmol/mg protein; Bapp CORT = 454.24 +/- 134.97 fmol/mg protein) suggesting that both hormones bind to the same site. Competition studies with different steroids showed that the order of displacement of [3H]DEX and [3H]CORT specific binding is: DEX approximately RU486 approximately deoxycorticosterone (DOC) > CORT > aldosterone > RU28362 > progesterone >>> 11-dehydroCORT. The affinity of GR for DEX (Kd = 11.2 +/- 1.5 nM) remained constant throughout the year while circulating CORT clearly increased during the reproductive season. Therefore, testis sensitivity to GC action would depend mainly on inactivating mechanisms (11beta-hydroxysteroid dehydrogenase type 2) and CORT plasma levels. Since total and free CORT are higher in the reproductive than in the non-reproductive period, the magnitude of GC actions could be higher during the breeding season. The intratesticular localisation of the GR was determined after separation of cells by a Percoll density gradient followed by binding assays in each fraction. DEX binds to two different fractions corresponding to Leydig and Sertoli cells. In conclusion, in the testis of B. arenarum GC could regulate the function of both cellular types particularly during breeding when CORT reaches the highest plasma concentration.

Relationship between steroidogenesis and spermiation in Rana catesbeiana and Leptodactylus ocellatus

The present study employs an in vitro system to analyse the role of steroid hormones in hCG-induced spermiation in two species of anuran amphibian: Rana catesbeiana and Leptodactylus ocellatus. In vitro spermiation was induced with 10 IU hCG and the effect of different steroid-biosynthesis inhibitors was analysed. Cyanoketone (10(-5)M), an inhibitor of 3-oxo-4-ene steroid biosynthesis, did not block hCG-inducing activity even when biosynthesis of androgen was significantly reduced. These results clearly showed that, in both species, spermiation-inducing action of hCG does not depend on the biosynthesis of 3-oxo-4-ene steroids. Moreover, when combined inhibitors, aminoglutethimide (10(-5)M) plus cyanoketone (10(-5)M), were employed, spermiation evoked by hCG was not modified while hCG-induced androgen secretion significantly decreased. Additionally, none of the steroids used, progesterone, 17, 20 alpha-dihydroxy-4-pregnen-3-one, testosterone and 5 alpha-dihydrotestosterone, were able to induce spermiation in the absence of hCG, confirming that steroids are not involved in that process. In conclusion, as previously described in Bufo arenarum, in L. ocellatus and R. catesbeiana hCG-induced spermiation does not depend on steroid biosynthesis.

11beta-hydroxysteroid dehydrogenase in the testis of Bufo arenarum: changes in its seasonal activity

In rat Leydig cells, glucocorticoids (GC) inhibit testosterone (T) synthesis via glucocorticoid receptor (GR). However, GC access to GR is regulated by the local expression of 11beta-hydroxysteroid dehydrogenase (11beta-HSD). Two isoforms were identified in mammals: type 1, a NADP+-preferring enzyme with K(m) in the muM range for GC and type 2, NAD+-dependent, with K(m) in the nM range for GC. In amphibians, a seasonal rhythm in baseline GC levels was described. However, a shift in the amount of deactivating 11beta-HSD activity could alter GC effects. The purpose of this work is to describe seasonal changes in testicular activity of 11beta-HSD in Bufo arenarum as well as the annual and seasonal patterns of plasma corticosterone (B) and T. The activity of 11beta-HSD was assayed in homogenate and subcellular fractions in pre-reproductive (Pre-R), reproductive (R) and post-reproductive (Post-R) periods, using [3H]B. Plasma B and T were determined by RIA. Testicular 11beta-HSD is a microsomal NAD+-dependent enzyme with a K(m) in the nM order, its activity being strongly reduced by glycyrrhetinic acid. These results indicate that toad testes express an 11beta-HSD similar to mammalian type 2. Although 11beta-HSD activity is higher in the Post-R than in the R and Pre-R seasons (V(max): Pre-R: 0.26+/-0.10, R: 0.14+/-0.01, Post-R: 1.37+/-0.45, pmol/minmg protein), K(m) value remains constant throughout the year. A seasonal rhythm in baseline GC concentrations inversely correlated with plasma T was also described. T concentration is lower in the R season than in the other periods (Pre-R: 90+/-6; R: 12+/-1; Post-R: 56+/-3, nM) while total B concentration is higher in the breeding than in the other seasons (Pre-R: 62+/-10; R: 145+/-18; Post-R: 96+/-10, nM). Furthermore, free B (Pre-R: 51+/-8; R: 94+/-12; Post-R: 70+/-7, nM) was always below K(m) values. In conclusion, this work shows that the activity of 11beta-HSD in toad testes could modulate GC action by transforming active hormones in the corresponding inactive steroid.

Sex and season influence gonadal steroid biosynthetic pathways, end-product production and steroid conjugation in blotched blue-tongued lizards (Tiliqua nigrolutea)

We examined differences in gonadal steroid production and biosynthetic pathway activity with changing reproductive condition and between sexes in the scincid lizard, Tiliqua nigrolutea. We observed clear seasonal and sexual variation in the production of androgens and steroid conjugates, but detected no 17beta-estradiol or 5alpha-dihydrotestosterone produced by the gonads. An alternative steroid, more polar than estradiol, was detected: an investigation of this steroid is reported separately [Gen. Comp. Endocrinol. 129 (2002) 114]. There were seasonal and sex-related differences in steroid biosynthetic pathway activity. The Delta5 pathway metabolite, dehydroepiandrosterone, was detected only in males, and only from incubations using regressed testicular tissue. There was also a seasonal difference between the sexes in rates of progesterone accumulation, although the absence of corresponding elevated plasma concentrations suggests that the role of progesterone switches from a directly acting hormone to a precursor for others during the reproductive cycle in females. These results suggest that within the traditional view that vertebrate biosynthetic pathway activity and end-products are phylogenetically conserved, there is likely to be considerably species- and/or genus-specific variation.

Steroid production in toads

In Bufo arenarum, androgen biosynthesis occurs through a complete 5-ene pathway, including 5-androstane-3beta,17beta-diol as the immediate precursor of testosterone. Besides, steroidogenesis changes during the breeding period, turning from androgens to C(21)-steroids such as 5alpha-pregnan-3alpha,20alpha-diol, 3alpha-hydroxy-5alpha-pregnan-20-one and 5alpha-pregnan-3,20-dione. In B. arenarum, steroid hormones are not involved in hCG-induced spermiation, suggesting that the steroidogenic shift to C(21)-steroids during the breeding be not related to spermiation. The activity of 17-hydroxylase-C(17-20) lyase (CypP450(c17)) decreases during the reproductive season, suggesting that this enzyme would represent a key enzyme in the regulation of seasonal changes. However, the increase in the affinity for pregnenolone of 3beta-hydroxysteroid dehydrogenase (3alphaHSD)/isomerase could also be involved. Moreover, the reduction in CypP450(c17) leading to a reduction in C(19)-steroids, among them dehydroepiandrosterone (DHE), would contribute to the conversion of pregnenolone into progesterone, avoiding the non-competitive inhibition exerted by DHE on this transformation. Additionally, CypP450(c17) possesses a higher affinity for pregnenolone than for progesterone, explaining the predominance of the 5-ene pathway for testosterone biosynthesis. Animals in reproductive condition showed a significant reduction in circulating androgens, enhancing the physiological relevance of all the in vitro results. The in vitro effects of mGnRH and hrFSH on testicular steroidogenesis revealed that both hormones inhibited CypP450(c17) activity. In summary, these results demonstrate that, in B. arenarum, the change in testicular steroidogenesis during the reproductive period could be partially due to an FSH and GnRH-induced decrease in CypP450(c17) activity.

Effects of mGnRH on testicular steroidogenesis in the toad Bufo arenarum

GnRH controls vertebrate reproduction in several ways. This hormone not only affects the secretion of gonadotropins from the pituitary gland but also has a direct influence on several gonadal functions such as steroidogenesis, spermatogenesis, and spermiation. In the present paper we have studied the in vitro effects of GnRH on the testicular steroidogenesis of Bufo arenarum to ascertain the role of this peptide in the control of the steroidogenic pathway previously described in this species. It was found that GnRH is able to reduce basal as well as hCG-stimulated testosterone release, having an inhibitory effect on P450(c17) activity. Thus, GnRH could be involved in the mechanism that regulates the metabolic change in the testicular steroidogenesis. Additionally, testicular GnRH binding site has been characterised, showing a K(d) of 34 nM and a maximum binding of 4.7 pmol/mg protein.

In vitro hCG and human recombinant FSH actions on testicular steroidogenesis in the toad Bufo arenarum

In order to study the regulation of testicular steroidogenesis in the toad Bufo arenarum, the effect of gonadotropins (hCG and hrFSH) on steroidogenic enzymes was determined using an in vitro system. 3beta-Hydroxysteroid dehydrogenase/isomerase activity was not affected by any of the gonadotropins, at any of the concentrations used. In contrast, 5alpha-reductase activity was strongly reduced by both hCG and hrFSH. Human chorionic gonadotropin inhibited the activity of cytochrome P450 17alpha-hydroxylase-C(17-20) lyase (P450(c17)), only at the highest concentration used, while hrFSH strongly reduced P450(c17) activity at all the doses assayed. In conclusion, these data suggest that LH (hCG) and FSH regulate steroidogenic enzymes such as 5alphaRed and P450(c17). The results also suggest that FSH could be involved in the regulation of the change in steroidogenesis undergone by the testis during the breeding season. In turn, the inhibition of P450(c17) activity could result in a reduction of androgen production and an increment of C21 steroids.