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Summary. Sexually mature rams were left intact, castrated (wethers), castrated and implanted with testosterone, or castrated, implanted with testosterone and pulse-infused every hour with LHRH. Serum concentrations of LH increased rapidly during the first week after castration and at 14 days had reached values of 13·1 ± 2·2 ng/ml (mean ± s.e.m.) and were characterized by a rhythmic, pulsatile pattern of secretion (1·6 ± 0·1 pulses/h). Testosterone prevented the post-castration rise in serum LH in wethers (1·0 ± 0·5 ng/ml; 0 pulses/h), but a castrate-type secretory pattern of LH was obtained when LHRH and testosterone were administered concurrently (10·7 ± 0·8 ng/ml; 1·0 pulse/h). We conclude that the hypothalamus (rather than the pituitary) is a principal site for the negative feedback of androgen in rams and that an increased frequency of LHRH discharge into the hypothalamo—hypophysial portal system contributes significantly to the post-castration rise in serum LH.

Summary. The cellular composition of CL from 6 cows on ∼Day 12 of the oestrous cycle, after synchronization with cloprostenol, was studied by ultrastructural morphometry. Point-count measurements of volume density (mean ± s.d.) showed that large luteal cells occupied 40·2 ± 7·0% of the luteal tissue, and small luteal cells 27·7 ± 6·3%. Of the total of 393·4 ± 52·0 × 10³ cells per mm³ of luteal tissue, large luteal cells made up only 3·5% and small luteal cells 26·7%, a ratio of 1:7·6. Endothelial cells/pericytes, at 52·3%, were the most numerous cell type. The mean volume per large luteal cell was 29·6 ± 6·3 × 10³ μm³, while that of small luteal cells was 2·7 ± 0·4 × 10³ μm³. In spherical form, these volumes would represent mean diameters of 38·4 μm and 17·2 μm respectively, and are consistent with published measurements on dispersed luteal cells. However, the values for cell numbers are much higher than published values based on luteal tissue dispersion, suggesting that dispersion may result in substantial and possibly selective losses of luteal cells.

Keywords: corpus luteum; oestrous cycle; cow; morphometry; luteal cells

Summary. Testosterone-filled Silastic capsules were implanted into mature rams at the time of castration (wethers). After 6 weeks, a tonic pattern of FSH secretion was observed in rams, wethers and wethers implanted with testosterone. Castration caused serum concentrations of FSH to increase 4–5-fold. Relatively low serum concentrations of testosterone (25–50% of intact ram values) did not significantly affect FSH secretion in wethers, but wethers exposed to concentrations of testosterone equivalent to those of intact rams had serum concentrations of FSH similar to those of intact rams. We suggest that testosterone feedback can account for the gross differences in FSH observed between intact and castrated male sheep.

Summary. Over a period of 8 weeks ram lambs (16 weeks old) were made hyperthyroidal (serum thyroxine ≅ 150 ng/ml, compared with control ≅ 48 ng/ml) by daily subcutaneous injections of thyroxine or maintained at a constant body weight by restriction of the feed intake. Hyperthyroidal and restricted-intake lambs remained at a constant body weight during the period of treatment whilst control rams gained body weight. Testicular growth was normal in restricted-intake lambs but was suppressed in hyperthyroidal animals. Hyperthyroidism, but not feed restriction, was also associated with decrease in LH pulse frequency (1·3 ±0·3/12 h compared with controls 4·8 ± 0·9/12 h. Hyperthyroidal lambs showed normal LH responses to exogenous LHRH. After cessation of treatment testicular growth continued to be suppressed for up to 16 weeks in previously hyperthyroidic rams; thereafter testes began to increase in size but at 30 weeks after treatment were still smaller than those of control rams. It is concluded that elevated thyroxine concentrations directly influence sexual maturation in ram lambs through actions at hypothalamic and/or higher brain centres which control LH secretion. Transient hyperthyroidism during sexual maturation may cause permanent impairment of sexual development.

Summary. Groups of heifer calves received a primary immunization against androstenedione (Group A; N = 11) or oestradiol-17β (Group E; N = 10) at 3 months of age and booster injections on 5 occasions at 2- to 3-month intervals. Controls (Group C, N = 11) were immunized against human serum albumin alone using the same protocol. Immunity was achieved against both steroids as judged by the secondary antisteroid antibody titres in Group A (1126 ± 261; reciprocal of titre) and Group E (10 357 ± 4067) heifers. In Groups A and E there was a general decline in the respective peak antibody titres after successive booster injections. From 3 to 9 months of age mean plasma concentrations of LH were higher (P < 0·05) in Group E heifers (0·89 ± 0·08 ng/ml) than in Group C (0·46 ± 0·03 ng/ml) and Group A (0·59 ± 0·05 ng/ml) heifers which did not differ from one another. There were no differences between groups in plasma FSH concentrations. At 10 months of age the LH response to exogenous LHRH was of higher (P < 0·05) amplitude for heifers in Group E (2·59 ± 0·56 ng/ml) than for those in Groups C (0·61 ± 0·07 ng/ml) and A (1·04 ± 0·22 ng/ml). Elevated plasma progesterone concentrations at 5 months of age were shown by 2 heifers in Group C, 10 in Group A, and 6 in Group E. From 8 to 14 months of age a consistently higher proportion of Group A heifers exhibited elevated progesterone compared with Group C and Group E heifers. After ovarian synchronization and booster injection at 15 months of age a corpus luteum was present in 2 heifers in Group C, 7 in Group A and none in Group E. The ovaries of Group A heifers were different from those of Groups C and E and were characterized by greater numbers of 2–4 mm follicles. It is concluded that active immunization against gonadal steroids influences both LH secretion and ovarian function in prepubertal heifers. Early increases in ovarian activity in androstenedione-immunized heifers are maintained after puberty and may therefore confer some lifetime reproductive advantages.

Keywords: heifer; gonadal steroid immunization; gonadotrophins; puberty; ovary

Summary. Post-partum acyclic beef cows received continuous long-term treatment with GnRH (200 or 400 ng/kg body wt/h) or the GnRH agonist buserelin (5·5 or 11 ng/kg body wt/h) using s.c. osmotic minipumps which were designed to remain active for 28 days. All treatments increased circulating LH concentrations whereas FSH remained unchanged. Ovulation and corpus luteum (CL) formation as judged by progesterone concentrations ≥ 1 ng/ml occurred in 0/5 control, 4/5 200 ng GnRH, 4/4 400 ng GnRH, 4/5 5·5 ng buserelin and 3/5 11 ng buserelin cows. The outstanding features of the progesterone profiles were the synchrony, both within and across groups, in values ≥ 1 ng/ml around Day 6, and the fact that most CL were short-lived (4–6 days). Only 3 cows, one each from the 400 ng GnRH, 5·5 ng buserelin and 11 ng buserelin groups, showed evidence of extended CL function. Cows failed to show a second ovulation which was anticipated around Day 10 and this could have been due to insufficient FSH to stimulate early follicular development, or the absence of an endogenously driven LH surge. The highest LH concentrations for the respective groups were observed on Days 2 and 6 and by Day 10 LH was declining, although concentrations did remain higher than in controls up to Day 20. The fact that LH remained elevated in treated cows, but CL were nevertheless short-lived, suggested that endogenous LH secretion in post-partum cows following a GnRH-induced ovulation is not limiting to CL function, and that other factors cause early demise of the first CL. The doses of GnRH used were not directly limiting to CL function since in a second trial extended CL were observed when GnRH was preceded by progesterone.

Keywords: cow; post partum; continuous GnRH; ovary

Summary. Normal female rats were mated to control males or males which were subjected to unilateral testicular heating (43°C for 30 min), irradiation (500 R), efferent duct ligation, arterial ligation or castration; in all males, the contralateral ductus deferens was ligated. All treatments caused reduced fertility and eventually infertility, as judged by the percentage of females becoming pregnant; the infertility was temporary after heating and irradiation. During the periods of reduced fertility, the numbers of fetuses per pregnant female and the fetus/corpus luteum ratios were reduced. In subsequent experiments, after heating of the testis, there was not only failure of fertilization despite the presence of normal numbers of spermatozoa in the uterus, but also an increased rate of embryonic degeneration in normal females.

These results provide evidence that the male, while still fertile, can affect the fecundity of the female and the rate of embryo mortality.

Keywords: embryonic mortality; subfertility induced by heat; rat

Summary. Groups of bull calves received a primary immunization against testosterone (Group T; N = 7) or oestradiol-17β (Group E; N = 9) at 3 months of age and booster injections on four occasions at ∼2 month intervals. Controls (Group C, N = 7) were immunized against human serum albumin alone using the same protocol. Immunity was achieved against both steroids as judged by the secondary antisteroid antibody titres in Group T (730 ± 231; reciprocal of titre) and Group E (12 205 ± 4366) bulls; however, peak antibody titres generally declined with successive booster injections. Mean plasma concentrations of LH, FSH and testosterone during the period from 3 to 10 months of age were higher (P < 0·05) in Group T bulls than in Groups C and E. Group T bulls had larger testes compared with controls from 6 months of age onwards. At castration at 14 months of age, testes of Group T bulls were heavier (P < 0·05) than those of Groups C and E (179 ± 13, 145 ± 8 and 147 ± 6 g, respectively). At 10 months of age, there were no differences among treatment groups in LH responses to LHRH, but the testosterone responses were greater (P < 0·05) in bulls in Group T (26·2 ± 4·9 ng/ml) and Group E (16·6 ± 1·8 ng/ml) compared with those in Group C (6·9 ± 0·6 ng/ml). Testosterone responses to hCG determined at 13 months of age were also greater (P < 0·05) in Groups T and E relative to controls. At 14 months of age daily sperm production rates per bull (× 10⁻⁹) were higher (P < 0·10) in Group T bulls (2·2 ± 0·1) than those in Groups C (1·6±0·2) and E (1·6±0·1). These results indicate that early immunity against testosterone is associated with increased gonadotrophin secretion and accelerated growth of the testes in prepubertal bulls. Also, chronic immunity against testosterone or oestradiol-17β enhances the steroidogenic response of bull testes to gonadotrophic stimulation. If the above responses observed in young bulls are shown to be sustained, then immunity against gonadal steroids early in life may confer some reproductive advantage in mature animals.