Testicular LH Receptors Research Articles

Direct effects of hormones of the hypothalamic-pituitary-thyroid axis on testicular steroidogenesis in hamsters.

Hormones of the hypothalamic-pituitary-thyroid axis have been shown to influence testicular function in rodents directly or indirectly. Panke et al. (1978) reported that experimental induction of hypo- or hyperthyroidism in the rat led to reductioms in plasma testosterone and in testicular LH receptors without altering peripheral concentrations of LH, FSH or PRL. There is a well known relationship between hypo- and hyperthyroidism, and fertility disorders in both men and women (Ingbar, 1985). Vriend and Wasserman (1986) showed that the thiourea-induced increase in TSH can partially reverse the short-photoperiod related testicular regression in Syrian hamsters. Furthermore, in genetically hypothyroid mice in which thyroid hormones are absent and plasma TSH levels are extremely high, Leydig cell responsiveness to hCG in vivo is significantly increased by TSH (Amador et al., 1986). Moreover, specific receptors for both TSH and thyroid hormones have been detected in rodent testes (Oppenheimer et al., 1974; Davies et al., 1978). Therefore, the present study was designed to study the effects of TRH, TSH, calcitonin (CTH) and thyroid hormones in vitro on testicular steroidogenesis in Syrian and Siberian hamsters.KeywordsThyroid HormoneSyrian HamsterTesticular FunctionTestosterone SynthesisSiberian HamsterThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Inhibition of pituitary-gonadal function in male rats by a potent GnRH antagonist

The inhibitory effects of the potent GnRH antagonist, [Ac- d-pCl-Phe 1,2, d-Trp 3, d-Arg 6, dAla 10]GnRH (GnRHant) upon pituitary-gonadal function were investigated in normal and castrated male rats. The antagonist was given a single subcutaneous (s.c.) injections of 1–500 μg to 40–60 day old rats which were killed from 1 to 7 days later for assay of pituitary GnRH receptors, gonadal receptors for LH, FSH, and PRL, and plasma gonadotropins, PRL, and testosterone (T). In intact rats treated with low doses of the antagonist (1, 5 or 10 μg), available pituitary GnRH receptors were reduced to 40, 30 and 15% of the control values, respectively, with no change in serum gonadotropin, PRL, and T levels. Higher antagonist doses (50, 100 or 500 μg) caused more marked decreases in free GnRH receptors, to 8, 4 and 1 % of the control values, which were accompanied by dose-related reductions in serum LH and T concentrations. After the highest dose of GnRHant (500 μg), serum LH and T levels were completely suppressed at 24 h, and serum levels of the GnRH antagonist were detectable for up to 3 days by radioimmunoassay. The 500 μg dose of GnRHant also reduced testicular LH and PRL receptors by 30 and 50% respectively, at 24 h; by 72 h, PRL receptors and LH receptors were still slightly below control values. In castrate rats, treatment with GnRHant reduced pituitary GnRH receptors by 90% and suppressed serum LH and FSH to hypophysectomized levels. Such responses in castrate animals were observed following injection of relatively low doses of GnRHant (100 μg), after which the antagonist was detectable in serum for up to 24 h. These data suggest that extensive or complete occupancy of the pituitary receptor population by a GnRH antagonist is necessary to reduce plasma gonadotropin and testosterone levels in intact rats. In castrate animals, partial occupancy of the available GnRH receptor sites appears to be sufficient to inhibit the elevated rate of gonadotropin secretion.

STUDIES ON THE INFLUENCE OF GROWTH HORMONE AND GROWTH FACTORS ON TESTICULAR STEROIDOGENESIS

Growth hormone (GH) treatment can increase testicular LH receptor concentration and testicular responsiveness to LH in hypophy sectoral zed rats. To verify whether GH and growth factors (somatomedin C or IGF1 and IGF2) exert a direct control on Leydig cell function, their effect on cAMP and testosterone (T) production by highly purified immature porcine Leydig cells (L) in primary culture was studied. Porcine GH (pGH) (10−710−9 M) induced a significant increase in cAMP accumulation and T production and significantly enhanced the action of hCG (60 mU/ml) on T release. Human GH (hGH) had a similar effect at 10−7 M but not at 10−9 M indicating some species specificity. This effect was partially blocked by a specific anti-hGH antibody. hGH also increased IGFj production by L while IGF1 also enhanced basal and hCG stimulated T production by L. Finally, receptors to IGF1 and IGF2 have been detected and characterized in L. These data indicate that 1) GH has a stimulatory effect on steroidogenesis of L in culture. 2) Their effect is exerted directly on the Leydig cell and is not mediated by another cell-type, and 3) hCG and GH seem to enhance T production by separate mechanisms and to interact at different receptorial sites. The mechanisms by which this action is exerted and the possibility that growth factors may be involved in this action remain to be determined.

Kinetic and autoradiographic studies on binding of hCG to the testicular LH receptors of neonatal rats.

The properties of hCG binding to LH receptors of the neonatal (5-day-old) rat testis were analysed and compared with those of the adult testis. The equilibrium association constants (Ka) of hCG-binding were similar at both ages, 2-4 X 10(10) M-1. In contrast, kinetic binding studies revealed that the association and dissociation rate constants of hCG binding were more rapid in the neonatal testis. Likewise, it was observed that the progression from loose (easily dissociable) to tight (non-dissociable) binding was less complete in the young than in the adult testis. Autoradiography of 125I-labelled hCG binding to interstitial cell suspensions at the two ages showed that the gonadotrophin binding per Leydig cell was about 50% lower in the neonatal testis. Conversely, since the surface area of adult Leydig cells was about 4-fold larger, the receptor density appeared to be higher in the neonatal Leydig cells. The rapid recovery of LH receptors after hCG stimulation, typical of the neonatal cells, was due to rapid replenishment of binding in the cells initially occupied by the injected hormone, rather than to an hCG-induced increase of Leydig cell number. Finally, in-vivo experiments with cycloheximide revealed that the rapid recovery of LH receptors was dependent on protein synthesis. These differences in the kinetics of neonatal testicular LH receptor turnover may be involved in the unique functional features of the fetal-neonatal growth phase of rat testicular Leydig cells.

Stimulatory effect of prolactin on luteinizing hormone-induced testicular 5 alpha-reductase activity in hypophysectomized adult rats.

Two pituitaries from 7-week-old female rats were grafted under the capsule of the left kidney of 50-day-old male rat to induce hyperprolactinemia. All of the pituitary-grafted and sham-operated rats were hypophysectomized at 56 days of age. The hypophysectomized rats in groups of 4 were given daily sc injections of saline or 9 micrograms NIADDK-ovine-(o)LH-23 for 4 and 5 days starting from days 58 and 70, respectively (short and long term hypophysectomized groups). The metabolism of [3H]progesterone or [14C]androstenedione by testicular homogenates, concentrations of testosterone and 5 alpha-androgens (androsterone plus 5 alpha-androstane-3 alpha, 17 beta-diol) in the serum and testes, and testicular LH receptors were estimated. Hypophysectomy caused significant decreases in testicular enzyme activities per gram of tissue, androgen production, and testicular LH receptors. In the testes of hypophysectomized rats, LH treatment significantly stimulated 5 alpha-reductase and 17-hydroxylase activities. Although pituitary grafts alone showed little or no effect on these testicular enzyme activities, hyperprolactinemia induced by the grafts markedly enhanced the LH-stimulated 5 alpha-reductase activity in both groups, especially in the long term hypophysectomized group. Therefore, androsterone and 5 alpha-androstane-3 alpha,17 beta-diol were shown to be the major C19-steroid products (immature type of testicular androgen production) in the LH- and PRL-stimulated testes of long term hypophysectomized adult rats. On the other hand, hyperprolactinemia was associated with a significant inhibition and a slight increase of the LH-stimulated 17-hydroxylase activities in the short and long term hypophysectomized groups, respectively. This difference can be attributed to both a PRL-induced increase in testicular LH receptors and a PRL-induced inhibition of 17-hydroxylase via a postreceptor mechanism(s). The present findings demonstrate for the first time that PRL directly stimulates LH-induced 5 alpha-reductase activity in the testes. It appears that PRL may play a role in the increased production of 5 alpha-C19-steroids and the parallel decrease of testosterone production in immature rat testes.

Modulation of testicular receptors for LH, FSH and prolactin by the administration of ovine prolactin in mature rat.

To elucidate the role of prolactin on testicular function, we treated mature rats with ovine prolactin (oPRL) and investigated the dose and time-dependent changes in testicular LH, FSH and prolactin receptors as well as in serum gonadotropin and steroid levels. Twelve week-old rats were injected sc with a single dose of various amounts of oPRL (0.2, 1 and 5 IU) and killed on the first, second and third days after the treatment. Testicular LH receptor decreased to 59% of the control level as a function of time while prolactin receptor increased to 244% maximally of the control level on the second day. In contrast, FSH receptor changed in a different fashion. Smaller amounts of oPRL (0.2 and 1 IU) raised the receptor level to 193% of the control level on the first day whereas a larger amount (5 IU) did not change the receptor, which tended to remain in a low level throughout the experimental period. The serum FSH level significantly increased in every group on the second day, then returned to the control range by the third day. On the other hand, the serum testosterone level changed in a characteristic manner, decreased significantly in every group on the first day though not in a dose-dependent fashion, returned to normal on the second day and significantly increased in the 0.2 IU group on the third day (p less than 0.01). Similarly, the serum estradiol level decreased in the oPRL-treated groups on the first day and was restored on the second day.(ABSTRACT TRUNCATED AT 250 WORDS)

Photoperiodic modulation of testicular LH receptors in the bank vole (Clethrionomys glareolus).

The temporal changes in testicular binding of 125I-labelled hCG in juvenile bank voles (18 days of age, born and reared in a 18L:6D photoperiod) exposed to a long (18L:6D, Group L) or short (6L:18D, Group S) photoperiod for 0, 3, 7, 14 and 42-56 days were investigated. During testicular maturation, in Group L, there was a slight initial decrease in LH receptor numbers per testis followed by a marked prepubertal rise during the initial phase of rapid testicular growth after which a decrease took place. In Group S, during testicular regression, the temporal changes in LH receptor numbers per testis resembled those of Group L except that the corresponding increase in hCG binding during the initial week was considerably less marked and the receptor numbers remained thereafter at a significantly lower level than in Group L. Leydig cell count indicated that the observed changes in LH receptors per testis were due to changes in the number of Leydig cells as well as in LH receptors per Leydig cell. The present results indicate, that (1) photoperiod is an important modulator of testicular LH receptor numbers in this species, (2) photoperiod or age has no significant effect on the binding affinity of LH receptors, (3) short photoperiods arrest the induction of LH receptors as well as the increase in Leydig cell numbers associated with normal testicular maturation, and (4) changes in LH receptor numbers per testis correlate well with the photoperiod-induced changes in androgen biosynthesis, spermatogenesis and Leydig cell morphology observed in our previous studies.

Treatment of prostatic cancer with a gonadotropin-releasing hormone agonist analog: acute and long term effects on endocrine functions of testis tissue.

Six patients with advanced prostatic cancer were treated with a potent GnRH agonist analog (buserelin, Hoechst; 600 micrograms, intranasally, three times a day) for 6 months. The first 2-6 days of treatment were associated with a 50% elevation (P less than 0.01) in serum testosterone (T) and a simultaneous elevation of 20% (P less than 0.01) in serum prostate-specific acid phosphatase (PAP). Serum T fell to the castrate range (less than 1 nmol/liter) in all patients in 2-3 weeks, and PAP decreased concomitantly in five of the six patients. Serum LH progressively decreased by about 80% during the treatment, whereas a secondary rise of FSH levels occurred after the first month of treatment. The patients were orchiectomized after 6 months of treatment. No differences were found between the pre- and postsurgical levels of serum T or in comparison with those of six patients orchiectomized as the first therapeutic measure. Testicular endogenous T concentrations, LH and FSH receptors, and in vitro T production were measured in three testis samples and compared with those values in testis tissue obtained from five control patients. The endogenous levels and in vitro production of T were depressed by over 95% in testes from agonist-treated patients. The small residual T production responded to hCG stimulation as in control patients. Interestingly, no change was found in testicular LH receptor content, but FSH receptors decreased by 80%. The elevation in serum PAP at the beginning of the agonist treatment and the small residual testicular T production after 6 months may not be clinically important. However, they indicate the necessity of comparative long term studies between orchiectomy and GnRH agonists in the treatment of patients with prostatic cancer.

Pituitary and gonadal function in hypogonadotrophic hypogonadal (hpg) mice bearing hypothalamic implants

GnRH receptor values are 30-50% of normal in pituitaries of hpg male mice, and testicular LH receptors only 8% of normal (160.4 +/- 17.6 and 2013 +/- 208.1 fmol/testis respectively). In male hpg mice bearing fetal preoptic area (POA) hypothalamic implants for 10 days there was no change in pituitary GnRH receptors, pituitary gonadotrophin content, or seminal vesicle weight. However, testicular weights and LH receptors were doubled in 4/10 mice and 2 had increased serum FSH levels. Between 26 and 40 days after implantation pituitary GnRH receptors and pituitary LH increased to normal male levels, although at 40 days serum and pituitary FSH concentrations had reached only 50% of normal values. Testicular and seminal vesicle weights increased more than 10-fold by 40 days after implantation and LH receptors to 70% of normal. In hpg female mice bearing hypothalamic implants for 30-256 days pituitary gonadotrophin concentrations were normal, even though GnRH receptors reached only 60% of normal values (6.18 +/- 0.4 and 9.8 +/- 0.4 fmol/pituitary respectively). Serum FSH was substantially increased from values of less than 30 ng/ml in hpg mice to within the normal female range in hypothalamic implant recipients. Ovarian and uterine weights increased after hypothalamic grafting from only 4-5% to over 74% of normal values. LH receptors increased from 6.5 +/- 1.3 fmol/ovary for hpg mice to 566.9 +/- 39.2 fmol/ovary for implant recipients. Vaginal opening occurred about 23 days after implantation and these animals displayed prolonged periods of oestrus.(ABSTRACT TRUNCATED AT 250 WORDS)

Testicular LH receptors during aging in Fisher 344 rats.

Levels of serum LH, prolactin, testosterone, progesterone and 17-OH progesterone and the testicular concentration and total content of LH receptors were measured in 4-, 11-, 18-, and 27-month-old Fisher 344 rats. All 27-month-old rats had Leydig cell tumors. At first, testicular LH receptor levels decreased with age, but with the appearance of the testicular tumors, these levels increased dramatically. Serum prolactin levels fluctuated with age, but were significantly decreased in 27-month-old rats, as were serum LH levels. Serum testosterone levels decreased steadily with age, while the testosterone-LH receptor ratio remained constant until the appearance of the testicular tumors, after which the ratio decreased precipitously. Serum progesterone levels remained constant throughout the life of Fisher 344 rats until the appearance of testicular tumors, when they increased dramatically. Serum 17-OH progesterone levels were increased significantly at 11 and 27 months as compared to four months of age, but levels at 18 months were similar to those seen in the 4-month-old animals. Therefore, in aged Fisher 344 rats with spontaneous Leydig cell tumors, there is an alteration in the testicular testosterone synthesizing pathway at a step after progesterone.

Ontogeny of hypothalamo-adenohypophyseal-gonadal (HAG) interrelationships in the chick embryo.

In the male chick embryo the components of the hypothalamo-adenohypophyseal-testicular axis initially function independently of each other. It is not until days 12.5-13.5 that the adenohypophysis begins to regulate testosterone synthesis and secretion; on day 13.5 plasma testosterone reaches a maximum embryonic level. This feed forward regulation of the pituitary-testicular unit appears to involve a cause and effect relation between a statistically significant increase in the number of testicular interstitial cell LH receptors on days 12.5 and 13.0 and an increase in plasma LH levels on day 13.5 (up-regulation). Subsequently, events occur that are interpreted as indicative of the feedback phase of this endocrine axis. Plasma LH levels decrease after day 13.5. Also on day 13.5 and all subsequent embryonic days, there is a significant decline in the volume density of testicular LH receptor-positive interstitial cells (IC) associated with an internalization of the LH receptor complexes and a marked decline in plasma testosterone levels (down-regulation). It is strongly suggested that the decline in the number of LH receptor-positive ICs and the internalization of the LH receptor complexes is indicative of a "desensitization" of the ICs followed by a decrease in testosterone synthesis and secretion. Comparable events that occur in the female embryo with respect to the development of the hypothalamo-adenohypophyseal-ovarian axis are also discussed in this minireview.

Differences in the regulation of steroidogenesis and tropic hormone receptors between the scrotal and abdominal testes of unilaterally cryptorchid adult rats.

Rats were made unilaterally cryptorchid by cutting the gubernaculum testis at birth. At 100 days age, the rats were injected with 600 IU/kg hCG. A biphasic testosterone response was seen in the scrotal (Scr) testis in response to hCG, with maxima at 1 h and 3 days after injection. The acute peak of testosterone was of similar magnitude in the abdominal (Abd) testis, but the secondary peak was not present. The response of testicular progesterone concentration to hCG stimulation showed a maximum at 1 day in both gonads, but it was 10- to 20-fold higher (P less than 0.01) in the Abd testis. The content of LH, FSH, and PRL receptors per testis was decreased on the Abd side. After hCG injection, the loss of available LH receptors was faster in the Abd testis. Likewise, the recovery of binding was faster in the Abd testes; at day 10 of the experiment, it was 102 +/- 5% of the starting levels compared to 52 +/- 4% on the Scr side (P less than 0.01). hCG did not affect FSH binding of the Scr testes, but induced a transient drop of 25-35% on day 1 on the Abd side (P less than 0.05). Thereafter, on days 3-10, the FSH binding of the Abd testes was 20-40% higher than on the Scr side (P less than 0.05-0.01). In PRL binding, similar heterologous down-regulation of 50-80% was found in both testes between 12-24 h. Thereafter, the Abd testis PRL receptors showed a transient elevation of 25-70% (P less than 0.05-0.01) on day 3, which was not seen in the Scr testes. In conclusion, the Abd testis displays a dramatically enhanced blockade of C21 steroid side-chain cleavage upon gonadotropin stimulation. The kinetics of changes in testicular LH receptors after hCG stimulation is faster in the Abd testis. Only Abd testes displayed hCG-induced changes in FSH binding and transient up-regulation of PRL receptors. The altered tropic regulation of Leydig and Sertoli cells of the Abd testis are indicative of direct functional changes in these cells in the elevated intra-Abd temperature and/or of changes in the paracrine component of testicular regulation.

Regulation of testicular prolactin and luteinizing hormone receptors in golden hamsters.

Iodinated ovine PRL (oPRL) was used to detect the presence of PRL receptors in hamster testes. Binding to these receptors was specific for PRL and was saturable and incompletely reversible. Testicular PRL receptors possessed a single class of independent, high affinity binding sites (Ka = 2.4 X 10(10) M-1). The ability of PRL to regulate testicular PRL and LH receptors was measured in experiments wherein mature hamsters were injected daily for 10 days with 600 micrograms bromocriptine, 600 micrograms bromocriptine plus 250 micrograms oPRL, or injection vehicles. Bromocriptine injections, which caused significant reductions in serum PRL, produced commensurate decreases in concentrations (femtomoles per mg protein) of testicular LH and PRL receptors, without affecting binding of PRL to adrenal or liver tissue preparations. Decreases in testicular PRL binding were not always statistically significant, but were consistently measured. The administration of oPRL with bromocriptine significantly increased both concentration and total content (femtomoles per paired testes) of testicular LH receptors and PRL-binding sites above those measured in bromocriptine-injected and control hamsters. Liver PRL-binding sites were, likewise, increased above values measured in controls and bromocriptine-injected hamsters by this latter treatment, which had no significant effect on adrenal PRL-binding sites. Neither bromocriptine nor bromocriptine plus oPRL had any effect on the affinity (Ka) of testicular PRL receptors for labeled oPRL. Serum testosterone and FSH concentrations were unaltered by treatment, thereby suggesting that effects on receptors did not occur secondary to changes in these hormones, but, rather, represent direct effects of PRL. An additional experiment in prepubertal male hamsters (aged 14-30 days) demonstrated that bromocriptine injections for 17 days significantly reduced the total content of testicular PRL receptors. In vitro experiments eliminated the possibility of direct effects of bromocriptine on testicular receptors. The data demonstrate the possibility that PRL is required for maintenance of normal concentrations of testicular LH/hCG and PRL receptors in adult and immature golden hamsters. An increase in putative serum anti-oPRL antibodies was measured in oPRL-injected hamsters. Since such antibodies could contribute, to an unknown degree, to PRL binding measured in tissue preparations, the present data indicate only that increased testicular binding occurs subsequent to injection of heterologous PRL, without demonstrating that this binding is indeed due to increased PRL receptors.

Pituitary-testicular function in immature rats after treatment with GnRH antagonist, GnRH antiserum and bromocriptine

The effects of a GnRH antagonist analogue ( N-acetyl-Ala 1, d- p-Cl-Phe 2,D-Trp 3,6-GnRH, Ant.) and a GnRH antiserum (A/S) on the development of pituitary-testicular function were studied in immature (23/24–31/32-day-old) rats. In another experiment the Ant. treatment was combined with bromocriptine (BR)-induced hypoprolactinaemia. Ant. and A/S decreased serum and pituitary levels of LH and FSH, and BR those of Prl ( P < 0.01-0.05). Testicular testosterone (T) and progesterone (P) contents were significantly decreased only by Ant. ( P < 0.01). Ant. decreased the weights of the testes, ventral prostates and seminal vesicles, as well as testicular LH, FSH and Prl receptors (R) ( P < 0.01-0.05). BR decreased LH-R but had no effect on Prl-R. Both Ant. and A/S decreased available pituitary GnRH-R (P < 0.01), but free testicular GnRH-R were reduced only by Ant. BR increased GnRH receptors in the pituitaries. It is concluded that Ant.-induced low gonadotropin levels in immature animals inhibit the developmental increase of testicular weight, gonadotropin and Prl-R, steroidogenesis and androgen action on accessory sex glands. Hypoprolactinaemia had an additive inhibitory effect to the antigonadal effects of Ant. The testis tissue of immature ( 23 24 -day-old) animals already contains GnRH-R. In general, developing animals are clearly very sensitive to the antigonadal actions of Ant. and BR, whereas the effect of GnRH-A/S is less pronounced than in adults.

Effects of Alleles at the W Locus on Testicular Luteinizing Hormone Receptors in Adult Mice

The autoregulation of testicular LH receptors was studied in Wx/Wv mice with germ cell aplasia and in normal (+/+) mice. To assess the effects of each individual allele, Wx/+ and Wv/+ mice were also examined. Basal testicular LH receptor concentration was higher in Wx/Wv mice than in all other genotypes, and higher in Wx/+ than in +/+ mice. Twenty-four h after injection of 0.3 IU hCG/g bw, LH receptor concentration was decreased in +/+ and Wv/+ mice, but not in Wx/Wv or Wx/+ animals. Administration of hCG caused a significant increase in plasma testosterone levels in all genotypes. However, injection of the highest dose of hCG used (0.9 IU/g bw) caused a significantly greater elevation in plasma testosterone in Wx/Wv than in +/+ mice. Plasma gonadotropin levels were significantly higher in Wx/Wv mice than in all other genotypes. The present results indicate that the Wx allele is responsible for the changes in testicular function observed in Wx/Wv mice, and suggest that this allele may be involved in the genetic regulation of testicular LH receptors in the mouse.

Comparison of oestrogen and GnRH agonist analogue-induced inhibition of the pituitary-testicular function in rat.

The purpose of the present study was to compare the inhibitory effects of oestrogen and a gonadotrophin releasing hormone agonist analogue (GnRH-A) on the pituitary-testicular function of adult rats. Animals were treated with sc injections of oestrogen (diethylstilboestrol, DES, 5 or 50 micrograms/kg body weight/day) or a gonadotrophin releasing hormone agonist analogue. [(D-Ser-(tBu)6)des-Gly10-GnRH N-ethylamide, GnRH-A, 0.4 or 4 micrograms/kg/day] up to 12 days. Serum LH (24 h after the last hormone injection) decreased by 83% in 3 days with DES, but was unchanged during 12 days of GnRH-A treatment. Serum testosterone (T) decreased by 98% during DES treatment, and also clearly but less profoundly, by 89% with GnRH-A. Maximal decrease in the weights of the ventral prostate and seminal vesicles were 73-78% with DES-treatment, but clearly slower, and to a lesser extent, with GnRH-A (33-41%). Testicular weights decreased consistently (up to 41%) with GnRH-A treatment only. DES decreased the content of testicular LH receptors by 40% in 12 days whereas GnRH-A caused a loss of 97% in LH binding. Testicular lactogen receptors decreased to similar extents (by 68-78%) with both treatments. A clear increase in serum progesterone/T ratio was observed with both types of treatment, suggesting blockade of steroidogenesis at the C21 steroid level. These findings suggest that the antigonadal actions of oestrogen in the intact animal are largely due to a decrease of circulating gonadotrophin levels, and that those of GnRH-A are predominately due to Leydig cell LH-receptor down-regulation and steroidogenic lesions, induced by transiently elevated gonadotrophin levels. The inhibitory effects of oestrogen on testicular function appeared to be faster and more complete that those of GnRH-A in the present short-term experiments.

Changes in testicular gonadotropin receptors and steroid content through postnatal development until puberty in the lamb.

Changes in testicular LH and FSH receptor numbers were studied in lambs from 10–120 days of age and in 2-yr-old rams. Testicular testosterone and 17β-estradiol were also measured, and a histological examination of testicular parameters was performed in 20-day-old and 2-yr-old animals. The binding affinities of testicular LH receptors (1.4 ± 0.1 × 1010 M-1) and FSH receptors (3.7 ± 0.2 109 M-1) did not differ with age. The concentration of LH receptors (expressed per milligram of membrane protein) decreased from 31 to 6 fmol between 10– 120 days of age, but that of FSH receptors increased from 65 to 95 fmol. The total number of LH receptors per testis increased from 0.06 to 0.6 pmol between 10–80 days of age, then to 2.0 pmol by 120 days of age. The total number of LH receptors per testis did not differ between 120-day-old and 2-yr-old rams, but this was the result of a concomitant increase in the number of Leydig cells and a decrease in the number of LH receptors per Leydig cell. The change in the total n...

Seasonal changes in testicular gonadotropin receptors and steroid content in the ram.

To determine if seasonal changes in plasma gonadotropin levels and testicular weight are accompanied by changes in the number of testicular gonadotropin receptors, testicular LH and FSH receptor numbers were measured every 3 months in the testes of 2-yr-old rams during 2 successive yr. The intratesticular contents of testosterone and 17β-estradiol were also measured. The binding affinity of testicular receptors for LH (1.9 ± 0.2 1010 M-1) and FSH (4.1 ± 0.2 109 M-1) did not vary with season. When expressed per milligram of membrane proteins or per gram of wet tissue, the concentrations of LH and FSH receptors were maximum in June. When expressed per testis, however, the number of LH receptors increased linearly from December to September (from approximately 2 to 5 pmol/ testis), then decreased rapidly from September to December. The number of FSH receptors per testis was minimum between December and March (∼4 pmol/testis), increased by J ne, was maximum between June and September (∼12 pmol/testis), and th...

Mice are insensitive to the antitesticular effects of luteinizing hormone-releasing hormone agonists.

Treatment of male rats with [(imBzl)-D-His6, Pro9-NEt]LHRH or [D-Trp6,Pro9-NEt]LHRH, potent agonists of LHRH, led to a marked decrease in serum testosterone levels and a reduction in testicular LH receptor concentration. Similar treatment of mice showed that they were resistant to the antitesticular effects of the LHRH agonists. To further explore the differences between rats and mice, the direct antitesticular effects of these peptides were investigated in hypophysectomized animals. Hypophysectomized rats and mice were given ovine FSH (50 micrograms), with or without a LHRH agonist (10 micrograms), daily for 5 days. On day 6, the testicular steroidogenic response to hCG was studied. In these studies the in vivo as well as the in vitro steroidogenic response of rat testes to hCG were inhibited by the LHRH analogs. In contrast, pretreatment of mice with the LHRH analogs did not affect their testicular steroidogenic response. Binding studies with the [125I]LHRH analog demonstrated receptors for this peptide on Leydig cells from adult rats. Receptors for LHRH were not, however, detectable on murine Leydig cells. These results suggest that one of the reasons for the lack of an antitesticular effect of LHRH agonists in mice may be due to the inability of these peptides to have a direct effect on testes and may relate to a lack of LHRH receptors.

Acquisition of regulatory mechanisms for gonadotropin receptors and steroidogenesis in the maturing rat testis.

Testicular LH, lactogen, and estrogen receptors and in vitro steroidogenic and cAMP responses were measured in rats between the ages of 1 and 60 days. Testicular LH and lactogen receptors increased gradually with advancing age, although there was no direct correlation between the concentrations of the two receptors. Cytosolic estrogen receptors were present in the testis at 6 days of age, but α-fetoprotein represented the major estrogen-binding component until 20 days of age. The basal and maximally stimulated in vitro production rates of testosterone, pregnenolone, and cAMP per unit weight were more than 10-fold higher in neonatal rats than in 60-dayold animals. These production rates decreased rapidly to a nadir at 15 days and increased again between 15 and 60 days of age. Treatment of 1-day-old rats with a low dose of hCG (10 IU/kg daily) for 3 days caused an increase of 91% (P < 0.01) -in LH receptors per testis. This effect of hCG diminished gradually with advancing age and became insignificant in animals older than 23 days. A single high dose of hCG (600 IU/kg) caused rapid loss of free testicular LH receptors within 12–24 h in animals of all ages. The recovery of LH-binding sites occurred within 2–3 days in 5-day-old animals, but became progressively slower with advancing age and reached 14 days in 60-day-old rats. The in vitro testosterone responses of testes from 5-dayold rats were increased by 2- to 3-fold when measured 3 days after the high dose of hCG. This positive effect of hCG treatment also decreased with age and became insignificant in animals older than 20 days. In adult animals, the same hCG treatment caused complete loss of testosterone responses to gonadotropic stimulation, consistent with the development of steroidogenic lesions in the Leydig cell. These results demonstrate that the production of testosterone, pregnenolone, and cAMP per unit tissue weight is higher in the neonatal testis. Also, hCG has a rapid stimulatory effect upon LH receptors and steroidogenesis in the neonatal testis, but does not cause LH receptor downregulation and steroidogenic lesions at this age. These stimulatory responses to hCG gradually diminish during development and are replaced by the adult-type responses, with sterodogenic lesions and LH receptor down-regulation. The transition from neonatal to adult-type responses which occurs around 15 days of age emphasizes the functional differences between fetal and adult populations of Leydig cells. The development of estrogenmediated inhibitory actions in the testis may contribute to the differences observed between neonatal and adult rat Leydig cells.