Adult sheep which had been castrated either before or after puberty were treated with a variety of steroids. The administration of testosterone propionate, oestrone, oestradiol-17β or diethylstilboestrol to animals castrated before puberty caused them to mount oestrous ewes. Oestradiol-17α was less effective than these hormones in this regard, whilst oestriol, hexoestrol and 5α-dihydrotestosterone were ineffective. The response to oestradiol-17β was not altered by the concurrent administration of dexamethasone to block the pituitary–adrenal axis which suggests that oestradiol-17β was not exerting its effect indirectly by causing the release of adrenal steroids. When 5α-dihydrotestosterone was administered in conjunction with oestradiol-17β intromission and ejaculation were observed in addition to mounting behaviour.

When rams were castrated as adults their mating behaviour slowly declined over the course of 2 years. After this time, mounting behaviour was rapidly restored by the administration of oestradiol-17β but not by 5a-dihydrotestosterone. These results are consistent with the hypothesis that oestrogens are the ultimate agents responsible for promoting mating behaviour in male animals and hence aromatizable androgens, such as testosterone, are effective whereas non-aromatizable androgens, such as 5α-dihydrotestosterone, are not.

ABSTRACT

The capacity of the anterior pituitary gland and testes in mature bulls (705±9 (s.e.m.) kg body wt, n = 4) to respond to graded doses of LH-releasing hormone (LHRH) was assessed relative to endogenous profiles of LH and testosterone secretion. Endogenous hormone profiles were determined by bleeding bulls at 20-min intervals for 12 h. Responses to LHRH were assessed on successive days after single intravenous injections of 1, 5, 10, 50 or 100 ng LHRH/kg body wt. Blood samples were taken at −40, −20, 0, 10, 20, 30, 40, 60 and 120 min relative to LHRH injection. During a 12-h bleed bulls showed spontaneous pulses of LH and testosterone which had peak amplitudes of 2·6±0·5 μg/l and 44·5 ± 7·1 nmol/l respectively. Respective peak LH (μg/l) and testosterone (nmol/l) responses to LVRH were as follows: 1 ng LHRH (3·0±0·7: 47·3±4·1); 5 ng LHRH (8·0±1·2; 52·8 ± 6·2); 10 ng LHRH (11·1±2·3; 57·7 ± 9·1); 50 ng LHRH (19·2±2·8; 47·9±8·6); 100 ng LHRH (19·1±4·7; 43·9 ±6·4). A dose of 1 ng LHRH/kg produced LH and testosterone responses which were comparable in amplitude to spontaneous peaks in the respective hormone. There was a linear (y = 0·28x+5·72; r = 0·81) increase in the LH response to doses of LVRH between 1 and 50 ng/kg; corresponding testosterone responses showed no relationship with the dose of LHRH. The capacity of the anterior pituitary gland to release amounts of LH eight to ten times in excess of those secreted during spontaneous peaks suggests that (1) there exists a large releasable store of LH in the anterior pituitary gland and (2) hypothalamic LHRH is a limiting factor in gonadotrophin secretion. In contrast to LH release, the androgenic response of the testes to acute gonadotrophic stimulation is determined largely by prevailing steroidogenic activity.

J. Endocr. (1984) 103, 371–376

Testicular growth and secretory profiles of LH and testosterone were monitored in three bull calves implanted with oestradiol-17β and three bull calves implanted with oestradiol-17β and infused intravenously with LH releasing hormone (LH-RH; 500 ng/pulse per h, 30-s pulse) continuously between 34 and 42 weeks of age. Oestradiol-17β implants restricted testicular growth and spermatogenesis by interfering with the hypothalamo-pituitary-testicular endocrine axis. Initiation of pulsatile LH release by LH-RH pulse infusion was accompanied by a twofold increase in mean circulating levels of LH (3·4 v. 1·8 μg/l) and a marked increase in serum testosterone (13·0 v. 0·4 μg/l). Testicular diameter was enhanced significantly by week 4 of infusion and increased in a linear fashion up to and including week 8. Testicular weight (g) and total daily sperm production (× 10⁹) at 42 weeks of age were decreased in calves implanted with oestradiol-17β (105 ± 14 (s.e.m.); 0·0) when compared with calves implanted with oestradiol-17β and infused with LH-RH (254 ± 12; 1·2 ± 0·3). Differences in testicular size and sperm production rates between LH-RH-infused and control bulls without implants (352 ± 26; 3·3 ± 0·9) were attributed to the 2-month delay between oestradiol-17β treatment and the initiation of LH-RH treatment. These results (1) confirm our earlier conclusion that oestradiol-17β can interfere with normal pubertal development in beef bulls and (2) provide additional support that pulsatile LH secretion is important for the initiation of testicular growth and spermatogenesis in pubertal bulls.

ABSTRACT

Twenty acutely castrated bulls were used to investigate the role of androgenic and oestrogenic steroids in the feedback control of LH secretion. The effects of 5α-dihydrotestosterone (DHT) or the growth stimulants trenbolone acetate (TBA) or oestradiol-17β (OE₂) on serum LH secretory profiles were measured. In addition, pituitary LH responses to exogenous LH releasing hormone (LHRH) were determined to differentiate between hypothalamic and pituitary sites of steroid action. At the time of castration, two groups of animals were given implants of either 45 mg OE₂ or 200 mg TBA. Another group received equivalent to 30 mg daily injections of DHT. Control steers showed an increase in LH from 2·4 ± 0·5 (s.e.m.) μg/l to 7·0 ± 0·5 μg/l during the week after castration. Treatment with DHT and TBA prevented the post-castration rise in serum LH. In contrast, steers given implants of OE₂ showed a significantly greater increase in LH than controls 1 day after castration, but by day 5 LH declined in the OE₂-treated group to precastration values. Five weeks after castration control steers secreted LH in pulses at intervals of 40–50 min and with an amplitude of 4·2± 0·4 μg/l. Pulses were not detected in the LH profiles of the steroid-treated steers. Dihydrotestosterone and TBA significantly reduced pituitary LH responses to exogenous LHRH, whereas steers receiving OE₂ showed LH responses to LHRH which were similar to those observed in castrated controls.

These results support the hypothesis that androgenic and oestrogenic components participate separately in the feedback control of LH secretion in the bull. A similar LH response to exogenous LHRH in control and OE₂-treated animals suggests that the primary site of oestrogen feedback is at the level of the hypothalamus. Conversely, the small LH response to LHRH in androgen-treated animals suggests that androgen feedback is, in part, imposed at the level of the pituitary gland. Interestingly, LH secretion is regulated by dosages of androgenic and oestrogenic steroids which are available commercially as growth stimulants for cattle.

J. Endocr. (1984) 100, 107–112

ABSTRACT

This study aimed to obtain a better understanding of the relationship between circulating thyroxine (T₄) concentrations and reproductive endocrine function in the ram. Mature Merino rams were thyroidectomized and supplemented with 0, 30, 100 and 300% of normal T₄ for 10 weeks. Thyroidectomy had no apparent effect on spermatogenic function but interfered with sperm maturation, the latter being returned to normal by 30% T₄ replacement. Circulating testosterone levels were reduced by thyroidectomy and restored to control levels by 30% T₄; when T₄ levels were supranormal (300%), circulating testosterone levels were again reduced. The lowered circulating testosterone levels in thyroidectomized rams occurred as a result of suppressed testosterone secretion from the testis, observed under basal conditions and also following LH-releasing hormone (LHRH) and human chorionic gonadotrophin injection. In thyroidectomized rams, sex hormone binding globulin (SHBG) levels were depressed without changes in testosterone clearance rate (TCR), while in rams with supranormal T₄ levels, TCR was increased without changes in SHBG levels. Subnormal levels of T₄ also restored to normal the reduced LH pulse frequency in thyroidectomized rams. Reduced LH pulse frequency, together with diminished LH release following LHRH injection in thyroidectomized rams, suggested effects of T₄ at the hypothalamo-pituitary axis. The present study demonstrates that complete lack of thyroid hormones suppresses normal reproductive endocrine function in the ram, but that this can be restored to normal by 30% T₄ replacement. The results support the theory that T₄ plays a permissive rather than a regulatory role in reproductive function in males.

J. Endocr. (1986) 111, 245–253

ABSTRACT

Castrated sheep were used to study the effects of gonadectomy on sensitivity to testosterone of brain centres associated with gonadotrophin negative feedback and with mating behaviour. In the first experiment serum LH and FSH concentrations were determined in intact rams, recently castrated (2 days and 3 weeks) and long-term castrated animals (> 2 years, wethers) during intravenous testosterone infusion at physiological and supraphysiological levels. In intact rams, testosterone infusions effectively suppressed serum LH whilst FSH levels were suppressed only after prolonged infusion at the supraphysiological dose. Recently castrated sheep, which had higher gonadotrophin levels than intact rams, were less sensitive to testosterone feedback. Neither rate of testosterone infusion had any effect on the raised gonadotrophin levels in wethers. In a second experiment gonadotrophin concentrations and mating behaviour were determined in wethers bearing subdermal polydimethylsiloxane implants of testosterone, dihydrotestosterone and oestradiol. Testosterone implants stimulated mating behaviour in all wethers but suppressed gonadotrophins in only a proportion (three out of seven) of the animals. Both oestradiol and dihydrotestosterone suppressed LH and FSH in all wethers, whilst oestradiol, but not dihydrotestosterone, also stimulated mating behaviour. The present findings indicate that testosterone imposes continuing negative feedback on gonadotrophin secretion and that changes in the gonadotrophin regulatory system, which lead eventually to a loss in sensitivity to testosterone feedback, develop soon after gonadectomy. The results also provide the first direct evidence that long-term gonadectomy in male sheep has differential effects on sensitivity to testosterone of brain centres associated with gonadotrophin negative feedback and with mating behaviour. A loss in sensitivity to testosterone feedback in castrated animals may involve a lesion in 5α-reductase, the enzyme required for conversion of testosterone to dihydrotestosterone.

J. Endocr. (1985) 104, 69–75

ABSTRACT

To ascertain whether temporal changes in activity of the hypothalamo-pituitary axis in prepubertal bulls may occur independently of shifts in sensitivity to steroid feedback, the acute post-castration rise in serum gonadotrophins was monitored in bull calves castrated at monthly intervals from 4 to 9 months of age. Since a major feature of the gonadotrophin profiles of developing bulls is a change in LH pulse frequency early in life, pulsatile LH secretion after castration was used as an index of activity of the central LH-releasing hormone (LHRH) pulse generator. Relative to the day of castration (day 0) bull calves (n = 4) were bled at 20-min intervals for 8 h on day −3 and at 10-min intervals for 4 h on days 3, 5 and 7. During the first week after castration, 4-month-old bulls showed a higher (P<0·05) frequency of LH pulses compared with bulls at 8 and 9 months (1·13, 0·88 and 0·75 pulses/h respectively; pooled s.e.m.= 0·13). Mean LH levels before castration were higher (P<0·05) in 4-month-old bulls than in bulls at 7, 8 and 9 months (0·92, 0·37, 0·31, 0·38 μg/l respectively; pooled s.e.m. = 0·12). After castration mean LH levels did not differ with age. Mean FSH levels did not differ among age groups either before or after castration. Increased serum LH levels in 4-month-old bulls confirmed the transient rise in LH secretion that occurs at this time in developing bull calves. Age-related differences in LH pulse frequency observed after castration suggested that in prepubertal bulls changes in activity of the central LHRH pulse generator can occur independently of steroid feedback mechanisms.

J. Endocr. (1986) 111, 67–73

ABSTRACT

Mature Merino rams were made hypothyroid by daily oral drenching with methylthiouracil or hyperthyroid by daily subcutaneous injections of thyroxine for 8 weeks. Neither hypothyroidism nor hyperthyroidism had any apparent effect on either spermatogenesis or daily sperm production, but motility of ejaculated spermatozoa and circulating testosterone concentrations were reduced in both conditions. The ratio of testosterone concentrations in plasma from the internal spermatic vein to those in peripheral blood plasma was higher in hyperthyroid (21·2 ± 3·5) than in control (11·1 ±4·4 and hypothyroid (7·6±1·4) rams. The basal secretion rate for testosterone was slightly lower in hypothyroid rams and testosterone responses to human chorionic gonadotrophin and after LH-releasing hormone (LHRH) were very much reduced. Basal serum LH levels were low in both hypothyroid and hyperthyroid rams compared with controls whereas there were no differences in FSH levels. The LH response to exogenous LHRH was reduced in hypothyroid rams but not in hyperthyroid rams. Serum prolactin levels on the other hand were higher than control in both hypothyroid and hyperthyroid rams.

Reduced testosterone secretion in hypothyroid rams indicates that the normal function of Leydig cells depends on an adequate level of thyroid hormones. The decrease in circulating testosterone concentrations in hyperthyroid rams with normal secretion rates suggests an increased testosterone clearance rate in these animals. The decreased spermatozoal motility in hypo- and hyperthyroid rams suggests that the lowered testosterone level in these animals has altered the androgen-dependent maturation of spermatozoa in the epididymis. Changes in testis function together with decreased LH secretion in hypothyroid rams indicate that thyroid hormones can act at both the pituitary gland and testes to influence reproductive function in mature rams.

J. Endocr. (1985) 105, 39–46