Testicular LH Receptors Research Articles

The Effect of the Chronic Administration of a Potent Luteinizing Hormone Releasing Hormone Analog on the Rat Prostate

In order to assess the effect of the chronic administration of a potent luteinizing hormone releasing hormone analog, (D-SER(But)6) LHRH (1–9) nonapeptide-ethylamide (Buserelin, HOE 766) on the pituitary gonadal axis, and the prostate, adult male Wistar rats were administered either 0, 3, 10 or 50μg./kg. body weight Buserelin subcutaneously daily. At 7, 21, 35 and 42 days of treatment, groups of animals were sacrificed and certain serum endocrine and grava metric parameters determined. In addition, at 1, 21 and 42 days of treatment the 1-hour response of serum LH and serum testosterone to a single injection of 10μg./kg. body weight Buserelin was determined.All treatment doses had similar effects. Serum prolactin and the basal and “acute” response of serum LH to Buserelin (+Δ5,000 per cent) were unaltered throughout treatment. Testes weight, testicular LH receptor content, and basal and “acute” concentrations of serum testosterone were markedly decreased by 42 days of treatment (48, 89, 88 and 88 per cent, respectively). Although seminal vesicle weight declined 50 per cent at 42 days of treatment, prostate weight was not altered from initial weight, but was significantly lower than age matched control at 42 days of treatment.Buserelin remains a potent stimulator of pituitary LH release even during chronic administration. It markedly reduces serum testosterone through a predominant testicular site of action. Buserelin treatment inhibits the growth of the normal prostate, but does not cause its regression.

Gonadotropin binding and Leydig cell activation in the rat testis in vivo.

Testicular LH receptor occupancy and steroidogenic responses were measured in adult male rats after intracardiac injections of [125I]iodo-hCG (0.5--5 x 10(6) cpm) mixed with known amounts of nonradioactive hCG to yield doses ranging from 10 ng to 300 micrograms. Uptake of the hormone by the testis was measured in the whole tissue or the 20,000 x g homogenate, with correction for nonspecific binding in animals injected with a 100-fold excess of unlabeled hCG. The steroidogenic response to hCG was followed by measurements of serum and testicular testosterone. Maximum specific uptake of [125I]iodo-hCG by the testes was observed 4--6 h after hormone injection. Of the specific counts, 80% were recovered in the 20,000 x g pellet of the tissue homogenate. The testicular contents of hCG-binding sites were similar when measured by in vivo occupancy of the receptors and by the in vitro receptor assay, indicating the physiological validity of the receptor measurements in tissue homogenates. Serum and testicular testosterone levels reached a maximum at 1 h, independent of the hCG dose used. When receptor occupancy in vitro after injection of hCG was compared with stimulation of steroidogenesis, a significant (P less than 0.05) 3-fold elevation of serum testosterone was seen when only 0.05% of the receptors were occupied. The maximal testosterone response was reached with 0.8% receptor occupancy. It is concluded that the same number of testicular LH receptors can be occupied by the circulating hormone in vivo and in tissue homogenates in vitro. The spare receptor concept also applied to the in vivo situation, since stimulation of steroidogenesis in the intact animal requires occupancy of only a few receptors per Leydig cell. This may be a general feature of hormonal activation of endocrine target cells in vivo.

Effects of Experimentally Induced Hyperprolactinemia on the Hypothalamus, Pituitary, and Testes in the Golden Hamster

Hyperprolactinemia was induced in adult male golden hamsters by transplantation of four homologous pituitaries under the renal capsules. The resulting elevation of plasma prolactin (PRL) levels was accompanied by a dramatic increase in the concentration of FSH in peripheral plasma. In contrast, plasma LH levels were not affected. The content and concentration of testicular LH receptors, testicular weight, and plasma testosterone levels were significantly greater in hyperprolactinemic hamsters than in the sham‐operated control animals. Experimentally induced hyperprolactinemia was associated also with an increase in the hypothalamic content of LHRH and norepinephrine, while the dopamine level in the hypothalamus was not affected. Furthermore, hyperprolactinemia appeared to have increased the release of LH and FSH from the incubated hemipituitaries in the presence and in the absence of LHRH, but only release of LH under basal conditions was significantly affected. The authors conclude from these observations that experimentally induced hyperprolactinemia stimulates testicular function in the golden hamster by increasing the release of endogenous FSH and by increasing the number of LH receptors in the testes. Comparison of these findings with the results of similar experiments in rats and mice suggests that the number of testicular LH receptors can be profoundly influenced by chronic changes in plasma levels of both LH and PRL and that PRL can stimulate LH binding only when LH levels are reduced or unaltered.

Direct inhibition of testicular steroidogenesis and gonadotrophin receptor levels by [(imBzl)-D-His6, Pro9-NEt]GnRH and [D-Trp6, Pro9-NEt]GnRH, potent agonists of GnRH.

The effects of [(imBzl)-D-His6, Pro9-NEt]GnRH and [D-Trp6, Pro9-NEt]GnRH on testicular function in rats was evaluated. In adult rats the administration of 0.01, 0.1 or 10 micrograms of either agonist induced rapid increases in serum LH, FSH and testosterone (T) levels which started to decline within several hours. Both agonists caused a decrease in testicular LH and FSH receptor concentrations. The testicular FSH receptor concentration started to decline earlier than the LH receptor concentration but, both reached their lowest levels by day 2 after the administration of the agonists. The recovery of FSH receptor content was slower than that of LH. The extrapituitary effects of the 2 agonists were investigated in immature hypophysectomized animals. Administration of hCG (5 IU daily) to hypophysectomized rats for 5 days caused an increase in serum T levels. Concomitant administration of either of the agonists (10 micrograms) inhibited the steroidogenic action of hCG. Administration of the agonists alone caused a reduction in testicular LH receptor concentration in hypophysectomized rats. Treatment of the hypophysectomized rats for 0-4 days suggested that the direct antitesticular action of the agonists requires 1-2 days to become evident.

Human testicular LH receptors: correlations with circulating gonadotrophins and testicular steroid secretion.

Testicular androgen production is regulated by circulating LH, the effect of which is mediated by its testicular membrane receptors. In the present study, testicular [125I]hCG binding in man was investigated and found to be characterized by saturability and high affinity (mean KD in testes of 21 patients with prostatic carcinoma = 1.64 X 10(-10) M), and stimulated testosterone production in dispersed interstitial cells in vitro. The concentrations of the LH receptors correlated with serum FSH concentrations (r = 0.52, P less than 0.05) but not with circulating LH levels. Neither had they any correlation with intratesticular testosterone, 5 alpha-dihydrotestosterone, androstenedione, progesterone, 17-hydroxyprogesterone or pregnenolone concentrations, which may be due to the fact that LH receptors are confined to Leydig cells, whereas steroids may be unevenly distributed in the different cells of the testis. In contrast, the concentrations of the LH receptor displayed positive correlations with the concentrations of testosterone (r = 0.81, P less than 0.001), androstenedione (r = 0.54, P less than 0.05), 17-hydroxyprogesterone (r = 0.76, P less than 0.001) and progesterone (r = 0.60, P less than 0.05) in the spermatic vein serum of the patients. Our data suggest that the Leydig cells mainly responsible for steroid secretion into the blood are under gonadotrophic control exerted via their receptors, whereas Leydig cell function is not rapidly reflected in steroid concentrations within the testis itself.

Inhibition of Spermatogenesis and Steroidogenesis During Long‐Term Treatment with hCG in the Rat

The effects of chronic treatment with hCG (100 I.U. s.c. every second day) on testicular morphology, LH receptor levels and concentration of steroid intermediates of the δ4 and δ5 biosynthetic pathways were studied in adult rats for periods extending from one to 12 weeks. Treatment with hCG causes a decrease in testis weight, a maximal inhibitory effect being observed after two and four weeks of treatment. At these time intervals, the loss of testis weight is accompanied by degenerative changes in most seminiferous tubules and hypertrophy of Leydig cells. Administration of hCG for one week leads also to an almost complete loss of LH binding sites and to a marked stimulation of the levels of testicular steroids of the A and pathways, as well as to an increase in weights of secondary reproductive organs. The initial increment of testicular steroid levels is followed after two weeks of hCG administration by an apparent decrease of 17, 20‐desmolase activity suggested by a reduction in the levels of androst‐5‐ene‐3β, 17β‐diol, androstenedione, testosterone, and 5α‐dihydrotestosterone and an increase in the concentrations of pregnenolone, 17‐OH‐pregnenolone, progesterone and 17‐OH‐progesterone. Plasma and pituitary LH levels are maximally reduced at one and four weeks of treatment, respectively, while plasma and pituitary FSH levels are only slightly reduced after four weeks of hCG administration. The effects of hCG on all of the above‐mentioned parameters, except for testicular morphology and testis weight, are completely reversible at the eight and 12 week intervals. This transiency in the effects of hCG is accompanied by a gradual increase in plasma levels of hCG antibodies. The present data show that chronic treatment of adult rats with hCG induces a marked degeneration of the seminiferous tubules and an inhibition of spermatogenesis that accompanies the well‐known loss of testicular LH receptors and inhibition of the steroidogenic pathway.

Study of the direct action of luteinizing hormone-releasing hormone agonists at the testicular level in intact rats treated with an antiluteinizing hormone serum.

Is it well known that LHRH agonists can inhibit testicular functions by gonadal desensitization secondary to endogenous LH release and that a direct action at the gonadal level has also been demonstrated. Since an excess of anti-LH serum can be used, as an alternative to hypophysectomy, to neutralize the influence of endogenous LH release, the relative importance of the testis and pituitary gland in the inhibitory effect of a LHRH agonist, [D-Ser-(TBU)6,des-Gly-NH210]LHRH ethylamide (Buserelin), was studied on gonadal gonadotropin receptors in intact adult male rats treated with equine anti-LH or normal horse serum (NHS). A single administration of increasing doses (1-100 ng) of Buserelin leads to a progressive inhibition of testicular LH and PRL receptor levels by 70% and 40%, respectively, in animals injected with NHS. Treatment with the anti-LH serum completely prevents this inhibitory effect of a single dose of the LHRH agonist. After two successive injections of Buserelin in NHS-treated animals, testicular LH receptors are reduced by 70% and 80% with the 100- and 500-ng doses, respectively, while testicular PRL receptors are inhibited by 40-60%. In animals treated with the anti-LH serum, the inhibition of testicular LH receptors is reduced by only 18% (100 ng Buserelin) and 55% (500 ng Buserelin), while the inhibitory effect on PRL receptors is abolished. The present data show that endogenous LH release induced by a single injection of a LHRH agonist plays an essential role in the loss of testicular LH receptors measured 2 days later. Moreover, upon repeated injection of the LHRH agonist, neutralization of endogenous LH release by an anti-LH serum markedly reduces the inhibitory effect of the LHRH agonist on LH receptors, while it completely prevents the effect on PRL receptors, suggesting that the inhibitory effect of the LHRH agonist in the male rat is predominantly due to endogenous LH release rather than to a direct action of the peptide at the testicular level.

Genetic variation in testicular LH receptors in the mouse.

The concentration of testicular hCG (LH) binding sites was measured in five mutants with defective endocrine and/or reproductive functions, in a hypoandrogenic inbred strain and in six normal strains. A decrease in hCG binding was found in dw/dw mice, and an increase in Tfm/y and C57Bl10/J mice. Analysis of the results indicates the existence of a genetic factor(s) involved in the regulation of hCG (LH) binding.

Combined long-term treatment with an LHRH agonist and a pure antiandrogen blocks androgenic influence in the rat.

Daily administration for 5 months of the potent LHRH agonist (D-Ser(TBU)6, des-Gly-NH2(10)) LHRH ethylamide (250 ng) in combination with the pure antiandrogen RU23908 (5 mg) to adult male rats causes a marked inhibition of ventral prostate and seminal vesicle weight to 9% and 15% of control, respectively. At the doses used, owing to readjustments of the pituitary-testicular axis, neither treatment alone has an effect on prostate weight and exerts only minimal inhibitory effects on seminal vesicle weight. Whereas treatment with the LHRH agonist alone markedly inhibits testicular LH and PRL receptor levels, the antiandrogen alone stimulates the concentration of the two receptors and reverses the inhibitory effect of the LHRH agonist treatment on LH receptors. Treatment with the LHRH agonist decreases plasma PRL levels, whereas the antiandrogen increases the concentration of circulating LH and FSH by 250%. Treatment with the LHRH agonist decreases the concentration of testosterone and its precursors of the delta 4-pathway while stimulating 5 alpha-reductase activity in both the absence and presence of simultaneous treatment with the antiandrogen. The present data show that blockage of the delta 4-steroidogenic pathway induced by treatment with an LHRH agonist prevents the escape phenomenon observed during long-term treatment with a pure antiandrogen, and permits maximal inhibitory effects of the two treatments on secondary sex organ weight. Such combined treatment with an LHRH agonist (to block androgen formation) and an antiandrogen (to neutralize remaining androgens of testicular and adrenal origin) should be the hormonal therapy of choice in prostatic carcinoma.

Induction and Maintenance of Gonadotropin and Lactogen Receptors in Hypoprolactinemic Rats

The effect of 2-bromo-alpha-ergocriptine (BR)-induced hypoprolactinemia on the induction and maintenance of testicular gonadotropin and lactogen receptors was studied in 60-day-old rats after receptor regulation by a gonadotropin-releasing hormone agonist analog [[D-Ser-(tBu)6]des-Gly10-GnRH N-ethylamide (GnRH-A)] and in young animals during sexual maturation. In adult animals, BR treatment delayed the reappearance of LH binding in the testis after GnRH-A injection, but had no effect on the recovery of lactogen binding. BR treatment also inhibited the increase of LH binding that occurred in control animals during the experimental period, but did not affect lactogen binding. Furthermore, BR potentiated the effect of GnRH-A on the decrease of Leydig cell testosterone synthesis observed 2 days later in vitro. Eight days after GnRH-A injection, concomitant BR treatment significantly inhibited the recovery of Leydig cell cAMP production. In peripubertal (25- to 46-day-old) animals, BR diminished the normal rise in testicular LH receptors, but did not affect the increase in lactogen receptors. Serum testosterone levels and other features of pubertal development, such as balano-preputial separation and spermatogenesis, were unaffected by hypoprolactinemia. In neonatal female animals, significant lactogen binding was detected at 3 days of age, whereas hCG binding was not demonstrable until 9 days after birth. These findings indicate that the expression of lactogen receptors precedes that of LH receptors in the developing gonad, and that the increase of LH binding in the testis during pubertal development requires normal circulating PRL levels. In adult animals, hypoprolactinemia potentiates GnRH-A-induced desensitization of steroidogenesis and cAMP formation, as well as LH receptor down-regulation, delaying the recovery of these phenomena. Although decreased serum PRL levels were associated with a marked reduction in testicular LH receptors both during development and in adult life, the absence of changes in lactogen receptors indicates that the latter sites are largely independent of the circulating PRL concentration.

Regulation of luteinizing-hormone-receptors and steroidogenesis in the neonatal rat testis.

Testicular LH receptor regulation and steroidogenesis were studied in 5-day-old rats, in which Leydig cells are derived from the fetal population of interstitial cells. The affinity of hCG binding to the neonatal and adult testis was similar. The hCG-binding capacity per unit weight of tissue in the neonatal testis was 5–10% of the adult level but was very similar when expressed per Leydig cell. The sensitivity of the testosterone response to hCG was 4-fold higher in neonatal Leydig cells, whereas the basal and maximal rates of testosterone production were similar in neonatal and adult cells. No clear differences were seen in cAMP production. A low dose of hCG (20 IU/kg BW sc) induced in 1–3 days a 40–60% increase in LH receptors without affecting the Leydig cell number; such an effect was not observed in the adult. A higher dose of hCG (600 IU/kg BW sc) increased the number of Leydig cells per testis about 2-fold in 2 days, and a significant increase in testicular basal and hCGstimulated testosterone pro...

Direct inhibitory effect of glucocorticoids upon testicular luteinizing hormone receptor and steroidogenesis in vivo and in vitro.

The direct effects of glucocorticoids on testicular LH receptor content and steroidogenesis were studied in vivo and in vitro. Immature hypophysectomized rats were treated with varying doses of dexamethasone, corticosterone, or a synthetic progestin, 17,21-dimethyl-19-nor-pregna-4,9-diene-3,20-dione (R5020). Some animals were also treated concomitantly with FSH to prevent the hypophysectomy-induced decrease in testis functions. At the end of 5 days of treatment, testicular LH/hCG receptor content was measured by [125I]hCG binding assay while steroidogenic responsiveness was measured by in vitro incubation of testes. Dexamethasone decreased testicular LH receptor in control and FSH-treated hypophysectomized rats in doses as low as 10 microgram/day, whereas corticosterone (10 microgram/day) decreased testicular LH receptor in the FSH-treated rats but had no effect in rats not treated with FSH. In contrast, R5020 had no effect on testicular LH receptor content. In vivo treatment of hypophysectomized rats with FSH increased both basal and hCG-stimulated production of androstanediol in vitro. In contrast, concomitant treatment with dexamethasone, but not R5020, decreased both basal and hCG-stimulated testicular androstanediol production. The direct effect of glucocorticoids on testicular steroidogenic potentials was also studied in primary culture of testicular cells obtained from adult hypophysectomized rats. Treatment of cultured testicular cells wtih hCG increased testosterone production. The addition of various natural and synthetic glucocorticoids, but not R5020, to hCG-treated cells decreased testosterone production in a dose- and time-related manner (triamcinolone greater than or equal to dexamethasone greater than cortisol greater than or equal to corticosterone). A 40% decrease in testosterone production was apparent at 6 h after addition of 10(-7) M dexamethasone to hCG-treated cells. These results demonstrate the direct inhibitory effect of glucocorticoids on testicular LH receptor content and steroidogenesis, suggesting the adrenal glucocorticoids may regulate testis functions.

Hormonal regulation of testicular luteinizing hormone and prolactin receptors.

The effects of gonadotropic stimulation on testicular LH and PRL receptors were examined in the absence and presence of elevated serum PRL levels. In adult male rats, the administration of ovine LH (oLH) caused an increase in both PRL and LH receptors within 40–120 min. This initial rise in testicular receptors was followed by the transient loss and recovery of PRL sites and a more prolonged loss of LH receptors. In contrast, the administration of bovine PRL or the elevation of circulating PRL levels during metoclopramide infusion caused no change in the lactogenic sites. A significant positive effect of increased serum PRL levels on testicular LH receptors was observed during metoclopramide infusion, but this did not prevent the loss of LH receptors caused by the administration of oLH. Similarly, prior treatment with bovine PRL failed to prevent the LH-induced loss of testicular LH receptors. These findings demonstrate that PRL and LH receptors exhibit concomitant but distinct forms of regulation during ...

Relative importance of the adenohypophyseal and gonadal sites in inhibitory action of LHRH agonists.

Assessment of the role of endogenous LH release and direct gonadal action of LHRH agonists in the loss of gonadal LH receptors induced by treatment with gonadotropin-releasing peptides was first made by comparing the effect of single administration of 1 �g of ID-Ala’, des-Gly-NH2 ‘#{176} I LHRH ethylamide, ED-Ala’) LHRH-EA, 12 h before or immediately after hypophysectomy (HYPOX) on LH receptor levels measured 48 h after surgery in male and female adult rats. While administration of the LHRH agonist before HYPOX leads to a 50% loss of testicular LH receptors, only a 20% decrease is seen in HYPOX animals, thus suggesting a predominant role of endogenous LH release in the desensitization process in male animals. Treatment with increasing doses (1 to 100 ng/day) of another LHRH agonist, [D-Ser(TBU)’ I LHRH-EA, causes a 90% loss of testicular LH receptors in intact rats at the highest dose used (100 ng daily for 9 days) while the same treatment decreases LH receptors by only 35% in HYPOX animals. As measured by the steroidogenic response to 10 �g 0LH in vivo, while treatment of intact animals with [D-Ser(TBU)’ I LHRHEA leads to a marked blockage of the testicular steroidogenic pathway at the level of 17-hydroxylase and 17,20-desmolase activities with a corresponding increase of 5o-reductase activity, minimal or no signs of changes of enzymatic activity are seen after identical treatment in HYPOX animals. Chronic treatment (1 month) with the LHRH agonist in intact rats leads to degenerative changes of the seminiferous tubules, while no effect is observed in HYPOX animals, thus suggesting the essential role of the pituitary gland. Moreover, adrenalectomy does not influence the inhibitory effect of treatment with LHRH agonists on the loss of testicular LH receptors in HYPOX animals, thus eliminating the role of the adrenals in the action of LHRH agonists. Contrary to the results obtained in male animals, single administration of the LHRH agonist in female rats has similar inhibitory effects on ovarian gonadotropin receptors when administered either before or after hypophysectomy. Moreover, treatment with increasing doses of ED-Ser(TBU)’ I LHRH-EA leads to a similar inhibition of ovarian LH receptor levels in intact and HYPOX female animals. The present data show that LHRI-I agonist-induced endogenous LH release plays a predominant role in the desensitizing effect of treatment with LHRH agonists on testicular LH receptors and steroidogenesis as well as on inhibition of spermatogenesis; in the female, the direct gonadal effect, at least on LH and FSH receptors, appears to play a more important role than in male animals.

Prolactin Stimulation of Testicular Steroid Biosynthesis in the Male Rat

The effect of prolactin on testicular steroidogenesis was studied in intact adult male rats and in animals treated for 12 days with the LHRH‐agonist [D‐Ala6, des‐Gly‐NH210]LHRH ethylamide (LHRH‐agonist, 1 μg every third day). Testicular LH and prolactin receptors are decreased to 20 and 50% of control, respectively, 26 hours after the last injection of LHRH‐agonist. This loss of testicular receptors is accompanied by an increase in the concentration of testicular pregnenolone (500%) and progesterone (700%), whereas 17‐OH‐progesterone, androstenedione, testosterone, and dihydrotestosterone are decreased to 50, 25, 10, and 60% of control levels, respectively. The Injection of 2 mg of ovine prolactin in intact rats 2 hours before sacrifice leads to an increase in testicular 17‐OH‐progesterone (300%) and androgen (100%) levels. However, in animals treated with the LHRH‐agonist, prolactin injection leads to an increase in only progesterone and pregnenolone levels, whereas the concentration of the other steroids remains low. The present data indicate that the stimulatory effect of prolactin at an early stage(s) of the testicular steroidogenic pathway remains relatively intact In the desensitized testis and leads to an apparent accentuation of the LHRH‐agonist‐lnduced enzymatic blockage at the level of 17‐hydroxylase and 17, 20‐desmolase activities.

Hormone-dependent subpopulation of rat testicular luteinizing hormone (LH) receptors.

The effects of prolactin (PRL), growth hormone (GH), and luteinizing hormone (LU) treatment on testicular LI-I receptor content were studied in the hypophysectomized adult male rat. One and 2 weeks after hypophysectomy, testicular LH receptor binding capacity, as determined by 1125 1)hCG binding to whole testicular homogenates, was 14% and 16% of normal values, respectively. Seven days of s.c. LH treatment (5 pg/day), begun 1 week after hypophysectomy, caused a significant additional reduction of 1125 Il-hCG binding capacity to 10% of control values. Growth hormone administration (100 pg/day) for 7 days, begun 1 week after hypophysectomy, caused an increase in binding to 31% of control values, but simultaneous administration of LH (5 pg/day) prevented this GM-induced increase in binding capacity. Seven days of PRL treatment (100 pg/day) also caused a significant increase in [121 Il-hCG binding above the 2 week hypophysectomized control values (24%), but adding PRL to GH treatments did not result in an increase in 112511-hCG binding above that seen with GH or PRL alone. However, simultaneous administration of LH with PRL caused the induction of additional new receptors and returned total binding to 44% of normal intact control values. Combination of all three treatments (GH, PRL, and LH) showed an additive effect of the residual and PRL + LH treatment regimens and increased [12511 -hCG binding to 52% of that of normal controls. These experiments suggest 1) GM and PRL are capable of inducing new LII receptors in the regressed testis of the hypophysectomized rat, 2) PRL allows LH or LH allows PRL to induce a new LH receptor population resistant to LH down-regulation, 3) GIl causes the induction of a new population of LH receptors sensitive to LU down-regulation. These observations raise the possibility that subpopulations of Leydig cell LH receptors with different hormonal sensitivities reside within the testes.

Resensitization of testosterone production in men after human chorionic gonadotropin-induced desensitization.

In rats, monkeys, and man, 24−h exposure to hCG can lead to a loss of testicular LH receptors and/or a decrease in the ability of the testis to produce testosterone inresponse to LH (desensitization). Such desensitization seems to be related to a block in the conversion of 17−hydroxyprogesterone to testosterone. In man, this desensitization is reflected by a plateau in serum testosterone (after a small early rise) for 4−24 h after hCG administration despite high serum hCG and rising serum 17−hydroxyprogesterone. After 72−h exposure to hCG, serum testosterone in men doubles, suggesting that the testis has regained sensitivity to gonadotropin (resensitization) with regard to testosterone production. To explore themechanism of this resensitization, we measured the acute (2−4 h) changes in serum levels of testosterone or testosterone precursor steroids in young men after im injection of hCG in both the basal and the resensitized state (72−h exposure to hCG) as indicators of the ability of the testis to produce testosterone or testosterone precursor steroids at those times. The acutehCG-induced rises in serum testosterone were the samein the resensitized and basal states [649 ° 215 vs. 637 ° 131 (SE) area units], confirming the resensitization of testosterone production. The acute hCG−induced rises in the A′ precursors of testosterone, 17−hydroxyprogesterone, and androstenedione, which were significant in the basal state(427 ° 54 and 156 ° 27 area units, respectively; P < 0.001), were abolished in the resensitized state (59 ° 50 and 43 ° 42 area units; P x0030; NS). In contrast, the acute hCG−induced rise in the A5 steroid dehydroepiandrosterone, which was not significant in the basal state (182 ° 136 area units; P 0 NS), was significant in the resensitized state (333 °113 area units; P < 0.01). Ten days after a single injection of hCG,serum testosterone was reduced below basal (419 ° 52 vs. 575 ° 91 ng/dl;P < 0.05), while the acute hCG−stimulated rise in serum 17−hydroxyprogesterone was greater than basal (1302 ° 262 vs. 640 ° 123 area units; P < 0.02), suggesting prolonged persistence of the block in conversion of 17−hydroxyprogesterone to testosterone. FSH administration for 72 h did not affect the responses to hCG. We conclude that resensitization of testosterone production after 72 h of hCG exposure is associated with the loss of acutely hCGreleaseable 17−hydroxyprogesterone and androstenedione, while acutely releaseable dehydroepiandrosterone is not reduced. These findings are consistent with a shift in the pattern of testosterone biosynthesis from the A4 to the A5 pathway, depletion of acutely releaseableintracellular pools of A4 steroids without a shift to the A5 pathway, an enzymatic block in the conversion of A5 to A′ steroids, or some combination of these mechanisms.

Gonadotropin-releasing hormone and its agonist inhibit testicular luteinizing hormone receptor and steroidogenesis in immature and adult hypophysectomized rats.

The direct effect of gonadotropin-releasing hormone (GnRH) and its agonist on testicular LH receptor and steroidogenesis was studied in hypophysectomized immature and adult rats. Hypophysectomized rats were treated daily with varying doses of GnRH or [des-Gly10,D-Leu6(N alpha Me)Leu7, Pro9-NHEt]GnRH(a potent agonist). Some animals were also treated concomitantly with FSH, PRL, GH and/or LH to prevent the hypophysectomy-induced loss of testicular LH receptor and steroidogenic capacity. At the end of 5 days of treatment, testicular LH/hCG receptor concentration was measured by a [125I]-hCG-binding assay and steroidogenic responsiveness was determinded by in vitro incubations. GnRH and the GnRH agonist reduced testicular LH receptor in control and FSH-treated hypophysectomized immature rats. As little as 0.5 microgram agonist/day induced a greater than 40% decrease in the LH receptor content, whereas GnRH was less potent, with 50 micrograms/day inducing about a 50% decrease. The inhibitory effect of GnRH was shown to be the result of decreases in the concentration of LH receptor rather than changes in the receptor affinity (Kd = 1.1 X 10(-10)M). GnRH did not interfere with the [125I]hCG receptor assay. Treatment with PRL, GH, and FSH, alone or in various combinations, increased the testicular LH receptor content. The stimulatory effect of these pituitary hormones was depressed by concomitant treatment with the GnRH agonist. Similar inhibitory effects of GnRH and the agonist on testicular LH receptor were demonstrated in adult hypophysectomized rats. In vitro studies demonstrated that treatment with the GnRH agonist in vivo inhibited both basal and hCG-stimulated androgen production in FSH-primed immature hypophysectomized rats. Associated with decreases in androgens (testosterone and androstenedione) and reduced androgens (dihydrotestosterone, androstanediol, and androsterone), there was marked suppression of 17 alpha-hydroxylated precursors and C-21 steroid intermediates in animals treated with the GnRH agonist, thus suggesting that the inhibitory effect of the GnRH agonist was associated with possible defects in 17 alpha-hydroxylase and side-chain cleavage enzymes. Likewise, treatment with the GnRH agonist inhibited in vitro testicular steroidogenic responses in adult hypopysectomized rats. These results demonstrate the extrapituitary inhibitory effect of GnRH on testicular LH receptor content and Leydig cell steroidogenesis in immature and adult hypophysectomized rats.

Antifertility Effects of LHRH Agonists in the Male

Acute or chronic treatment of adult male rats with LHRH or its agonists leads to a loss of testicular LH and prolactin receptors accompanied by decreased testis, seminal vesicle, and ventral prostate weight. The inhibition of testosterone formation is due to a blockage of the steroidogenic pathway at the level of 17‐hydroxylase and 17,20‐desmolase activities. The testicular desensitization is accompanied by a decreased pituitary responsiveness to LHRH. Although the inhibitory effects at the testicular level could be explained by endogenous LH release induced by the LHRH agonist with secondary testicular desensitization, the LH‐releasing peptides also exert direct inhibitory effects on gonadotropin receptors at the testicular level. Moreover, LHRH and its agonists bind to a specific LHRH receptor in interstitial cells. Chronic treatment with LHRH agonists leads to marked degenerative changes in the seminiferous tubules, almost all tubules showing signs of histologic damage after four weeks of treatment. Single administration of an LHRH agonist by the intranasal route in normal adult men causes a transient inhibition of plasma testosterone levels with a temporary loss of diurnal cyclicity, whereas preliminary data obtained in patients with cancer of the prostate show inhibition of both testosterone and dihydrotestosterone plasma levels. Such data suggest the potential use of LHRH agonists in male contraception and for the treatment of cancer of the prostate.

Further Studies on the Inhibitory Effect of [D‐A1a6, des‐Gly‐NH210]LHRH Ethylamide on Spermatogenesis and Steroidogenesis in the Rat: Reversibility and Effect of Androgen Administration

Following the recent observation that chronic treatment with [D‐Ala6, des‐Gly‐NH210]LHRH ethylamide, a potent LHRH agonist, leads to an almost complete degeneration of seminiferous tubules in the rat, the time‐course of recovery of spermatogenesis after cessation of treatment was investigated. Adult rats injected with the LHRH agonist (100 ng, every second day) for four weeks were studied four, eight, and 16 weeks later. Although progressive improvement was noted from four to 16 weeks after cessation of treatment, about 25% of tubules still appeared completely degenerated at the end of the recuperation period. One month following cessation of treatment, testicular LH and prolactin receptor concentrations, testicular testosterone (T) and dihydrotestosterone (DHT) levels, and ventral prostate and seminal vesicle weights had returned to normal values. Testis weight, however, which was decreased by 40% one month after treatment with the LHRH analog, increased more slowly toward normal and was still at 85% of control value four months after treatment. Plasma LH and FSH levels, which were increased approximately 100% after treatment with the LHRH analogue, returned to within normal values between one and two months following cessation of treatment, respectively. Administration of androgens (T or DHT) concomitantly with the LHRH agonist only partially prevented the deleterious effects of the peptide on spermatogenesis. On the other hand, administration of the LHRH agonist had no effect on testicular morphology in hypophysectomized rats. These data show a rapid return to normal of the hypophyseal testicular elements responsible for androgen formation after desensitization by LHRH agonist treatment in adult rats. These results suggest that the inhibitory effect of chronic LHRH agonist administration on spermatogenesis is at least partially reversible, and that a decrease in androgen production is probably not the only factor involved in the inhibition of spermatogenesis by LHRH agonist treatment.