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T. J. Parkinson and B. K. Follett

Experiments were conducted to examine whether seasonal breeding patterns of male sheep are abrogated by thyroidectomy. In Expt 1, Welsh Mountain rams were thyroidectomized in early autumn (September) and then maintained on either 16 h light:8 h dark (long days; n = 6) or 8 h light:16 h dark (short days; n = 6) for 8 months. Intact rams (n = 6 per group) were also housed in long or short days, or in natural photoperiods. Results were similar in animals housed on long or short days. In thyroidectomized rams, plasma FSH concentrations and scrotal circumference were maintained at values typical of the breeding season throughout the investigation, whereas in intact animals both reached a nadir in December and January. In Expt 2, a further 11 rams were thyroidectomized in March and, together with 23 intact animals, were maintained thereafter in natural photoperiods. In control rams, scrotal circumference increased slowly between May and September, whereas in thyroidectomized animals the circumference increased rapidly in the first 4 weeks following thyroidectomy (3.7 ± 0.7 cm), with a further increase (5.9 ± 1.0 cm) in the next 4 weeks. The scrotal circumference of thyroidectomized rams was therefore significantly (P < 0.01) greater than that in intact animals between April and August. Plasma FSH concentrations were significantly (P< 0.01) higher in thyroidectomized than in control rams by two weeks after surgery. These results indicate that thyroidectomy overcomes the seasonal (or photorefractory) inhibition of reproductive activity in rams and supports a key role for thyroid hormones in the expression of seasonal patterns of breeding activity.

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T. J. Parkinson, J. A. Douthwaite, and B. K. Follett

Thyroidectomy of seasonally breeding birds and mammals prevents the return to a state of sexual quiescence at the end of the breeding season. In starlings, thyroidectomy also causes premature sexual maturity. In this study, the effect of thyroidectomy upon the time of sexual maturity of prepubertal (8 week-old) ram lambs was examined. Thyroidectomy of four prepubertal and six mature rams was performed early in the spring. These and sham-operated controls were maintained in ambient photoperiods (south-west England). Scrotal circumference and serum LH, FSH, prolactin and thyroxine were measured every 2 weeks. In both the prepubertal lambs and the mature rams, scrotal circumference increased significantly within 5 weeks of thyroidectomy. FSH concentrations increased significantly in the mature rams after thyroidectomy. The relatively high FSH concentrations of thyroidectomized animals at the start of the experiment were maintained, but the FSH concentrations of intact lambs decreased during the late spring. These results provide the first indication that the timing of puberty in seasonally breeding mammals is a thyroid-dependent phenomenon.

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Nest building was induced in ovariectomized photosensitive canaries with a standard dose of oestradiol (0·05 mg thrice weekly) at various times of year. On a range of daylengths between 6 and 18 hr light/day, building behaviour tended to be more intense if the birds were exposed to long photoperiods. A similar result was found in photorefractory birds, both intact and ovariectomized.

Differences in building were unlikely to have been due solely to the amount of time available to birds on different photoperiods as hourly building rates still revealed a difference between groups. A marked difference in the latency of the response to oestrogen was found in the refractory birds.

Ovariectomy was not complete in many birds but the evidence from such birds and from the few which had been totally ovariectomized, as well as from those in the refractory period when long days did not result in ovarian growth, showed that endogenous oestrogen secretion was negligible.

The effect of long photoperiods in augmenting the induction of nestbuilding by oestrogen could not be reproduced by injections of either ovine or avian LH. This and other evidence suggests that a direct effect of gonadotrophins is not involved.

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S. M. Simpson, B. K. Follett, and D. H. Ellis

Summary. Reproductive activity in the Djungarian hamster, Phodopus sungorus, is suppressed by short daylengths and induced and maintained by long daylengths. To determine the time course of changes in plasma and pituitary gonadotrophin concentrations during the photoperiodic response, sexually immature male hamsters were moved from short (9L:15D) to long days (14D:10D). This induced an increase in testicular weight in 5 days and full sexual maturity in about 1 month. Plasma and pituitary FSH concentrations were significantly elevated after 3 and 5 long days respectively, reaching peak levels after 7–14 days and then declining. In contrast, pituitary and plasma LH concentrations did not increase until 10 and 21 days of photostimulation respectively.

Castration of hamsters kept in short days led to a marked increase in plasma and pituitary FSH titres. Transfer to long days further increased plasma FSH although pituitary content did not change. Castration of hamsters kept in long days led to an elevation of plasma and pituitary FSH concentration to these same levels. Transfer to short days reduced plasma FSH to the levels seen in hamsters castrated and kept in short days. Pituitary content did not change. The results suggest that while there is substantial steroid negative feedback in intact animals on both long and short days, the photoperiodic modulation of gonadotrophin secretion does not necessarily involve changes in feedback sensitivity.

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R. F. T. Kinch, R. P. Craven, and B. K. Follett

Summary. Male voles were raised from birth to 100 days of age in photoperiods of 16L:8D or 6L:18D. In the long photoperiod testes increased in size between 15 and 80 days of age, and there was an increase in seminal vesicle weight from 60 days of age. Spermatozoa were present in the testes at 60 days of age. In the short photoperiod testicular growth did not begin until 50 days of age with the seminal vesicles beginning to increase at 80 days of age. Spermatozoa were present in the testes at 100 days of age. Pituitary secretion in vitro of LH and FSH in response to 1 pmol GnRH, as well as hypothalamic GnRH content, rose to peaks at 50 and 80 days of age respectively in animals exposed to long photoperiods. There was no change in pituitary secretion of FSH in response to GnRH stimulation in animals from the short photoperiod. However, pituitary release of LH in response to 1 pmol GnRH rose to a peak at 80 days of age. Hypothalamic GnRH content rose to a peak at 50 days of age and then declined. The relationship between the hypothalamic GnRH and the sensitivity of the pituitary to GnRH stimulation is compatible with the idea that GnRH can mediate its own receptor numbers.

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F. G. L. Hartley, B. K. Follett, S. Harris, D. Hirst, and A. S. McNeilly

The endocrine basis of reproductive failure in red fox vixens was examined over two breeding seasons in a total of 11 animals. Weekly blood samples were assayed for progesterone, prolactin, LH and cortisol. Vaginal smears taken every 2 days over the oestrous period indicated that all vixens had mated. Vixens that successfully gave birth to a litter of cubs demonstrated significantly higher plasma progesterone and prolactin concentrations but significantly lower cortisol concentrations than did females that had ovulated, but then failed to whelp. There were no significant differences in plasma LH concentrations. These data suggest that reproductive losses could result from lowered plasma progesterone concentrations, possibly resulting from inadequate luteotrophic support by prolactin. A stress-induced mechanism of reproductive failure is implicated and is discussed in relation to social suppression of reproduction.