ABSTRACT

Male starlings which had fully developed testes under 11-h daily photoperiods were subjected for 10 weeks to daylengths either increasing or decreasing by 30 min per week. In both cases testicular regression occurred; in the former case this was complete due to photorefractoriness, and was accompanied by a postnuptial moult and a marked (P <0·01) rise in plasma prolactin concentration. In the latter case the rate of gonadal regression was slow and variable and no moulting was observed. Plasma prolactin remained very low. Eventually the birds which had experienced decreasing photoperiods were transferred to long (16 h) days; they were shown to be photosensitive since their testes redeveloped rapidly. There was also a rise in plasma prolactin, the birds eventually showed the full testicular regression characteristic of photorefractoriness, and moulting occurred. These results show that raised prolactin secretion is not associated with short-day-induced testicular regression but only with long-day-induced photorefractoriness.

J. Endocr. (1984) 102, 353–356

ABSTRACT

The breeding season in quail ends when they become relatively refractory to long photoperiods. The processes underlying the development of this refractoriness are dependent upon the thyroid gland.

Thyroidectomized male Japanese quail transferred from short (8 h light: 16 h darkness; 8L: 16D) to long (20L: 4D) daily photoperiods mature their gonads and develop the androgen-dependent cloacal gland at rates only marginally slower than controls. However, when the birds are retransferred to short days thyroidectomized individuals do not show the rapid testicular regression and moult which occurs in controls under such conditions. The testes remain large for a long period, eventually decreasing in size slowly and irregularly. Once such thyroidectomized birds are transferred back to short days the administration of thyroxine has little effect upon the slow rate of gonadal regression but if the hormone is administered during the preceding period of long days normal regression does occur under short days, even though treatment has ceased. This suggests that a process dependent upon thyroid hormones takes place under long daylengths that ensures the termination of reproduction when they are exposed either to short daylengths or to decreasing daylengths after the summer solstice. This view was substantiated by further experiments in which thyroidectomy was performed after the birds had been exposed to long daylengths. When transferred to 8L: 16D normal testicular regression and moult occurred rapidly. Presumably the thyroiddependent process had proceeded to an adequate extent in these quail before the thyroid glands were destroyed. Further support came from treating intact quail chronically with thyroxine. Its administration to quail on 20L: 4D had no observable effect; the testes grew rapidly to maximal size and remained large. However, if the birds were given thyroxine under 12L: 12D the gonads first developed to maximal size and then, after about 12 weeks of treatment, regressed spontaneously whether or not thyroxine was still being administered. It seems that thyroxine had made the quail so relatively refractory that 12L: 12D was no longer sufficiently stimulatory to overcome the developing inhibition on the photoperiodic system. The results in quail are very reminiscent of the finding that the absolute photorefractoriness and spontaneous testicular regression which develops in starlings under long daylengths is thyroid dependent. Quail do not become absolutely refractory on long daylengths and regress their gonads spontaneously but they do develop a 'relative' refractoriness so that only a small decrease in photoperiod is necessary to switch off the reproductive system. We suggest that quail-type refractoriness is homologous with that in starlings and that both involve progressive changes induced by exposure to long daylengths which profoundly alter the photoperiodic response: these changes will not occur without a functional thyroid gland.

J. Endocr. (1985) 107, 211–221

ABSTRACT

Nestling starlings (Sturnus vulgaris) were thyroidectomized, in the wild, when 8 days old. They were later brought into captivity and hand reared on a daylength of 16 h light: 8 h darkness per day.

Thyroidectomy dramatically affected somatic development so that even at 23 weeks of age these birds retained the physical characteristics of approximately 3-week-old euthyroid birds. Nevertheless, they appeared to be healthy and were active. Despite retaining juvenile features, these thyroidectomized birds became sexually mature. In males, testicular growth began at 8 weeks of age and by 23 weeks both males and females had well developed gonads.

Gonadectomized male Japanese quail maintained on short daylengths (8 h light: 16 h darkness) showed a reliable and repeatable response, in terms of increased gonadotrophin secretion, to a single long photoperiod (20 h light) followed by continuous darkness. Plasma levels of LH were significantly increased within 4 h of the end of the long day. Secretion continued to increase with time and LH and FSH concentrations reached a maximum some 24 h later, thereafter decreasing only slowly over the next 8–10 days. Quail could be used repeatedly at fortnightly intervals and gave a consistent response for at least 15 months. The size of the photoperiodic response and its considerable duration were much greater than found in intact quail. The birds showed a similar (though reduced) response if a short daylength was followed by a single 4-h light pulse interrupting the 16-h dark period. Using this system the time of maximum responsivity to the inductive effects of light lies 10–16 h from 'dawn'. The marked and rapid effects described provide an excellent experimental system in which to study how photoperiodic changes are recognized and how consequent neural activation is translated into enhanced secretion rates of hormones in the peripheral endocrine system.

ABSTRACT

Sexually immature male starlings were radiothyroidectomized while held under short daylengths. They were then subjected to long (18-h) photoperiods and the testes developed rapidly and apparently normally to full maturity. As expected, thyroidectomy prevented an onset of photorefractoriness and after 140 days the testes were still fully developed (testicular width 6·5 ± 0·8 mm). Half of the birds were then given thyroxine (0·011 mmol/l) in the drinking water for 14 days while the others were maintained as controls. At the end of this time the birds still had fully developed gonads (testicular width 7·8 ± 0·3 mm) but after a further 14 days testicular regression began in the thyroxine-treated birds. The testes of all individuals in this group were fully regressed (width 1·8 ± 0·1 mm) by 56 days after the end of thyroxine administration and moulting of the flight feathers had begun. No recrudescence of the gonads was subsequently noted in the next 2 months and the birds were apparently photorefractory. The short period of thyroxine treatment also caused a rapid and prolonged increase in plasma prolactin levels from 2·0 ± 0·3 to 16·8 ± 2·6 μg/l. No testicular regression or moulting was observed in the control birds and their plasma prolactin levels remained very low (below 4·0 μg/l) throughout the experiment.

In thyroidectomized and castrated starlings held on 18-h daylengths, 14 daily injections of thyroxine 100 μg/bird per day) caused a rapid and permanent decrease in circulating FSH to basal levels (reached about 36 days after thyroxine treatment began, at which time the birds moulted). The treatment also caused a marked increase in plasma prolactin lasting for about 50 days.

J. Endocr. (1985) 104, 99–103

SUMMARY

A comparison has been made between the effects of oestradiol-17β, oestrone, oestriol, progesterone, testosterone and cortisone on the plasma concentration of proteins and lipids in the South African clawed toad, Xenopus laevis Daudin. No major changes in the parameters studied were found with the non-oestrogenic steroids; only the three oestrogens induced the appearance in the plasma of a calcium-binding phospholipoprotein and esterified lipids associated with yolk formation. These results support the hypothesis that the oestrogens alone cause the vitellogenic response in oviparous vertebrates.

Dose—response curves for oestradiol-17β, oestrone and oestriol were established for the effects on plasma protein, calcium, protein-bound phosphorus, phospholipid, lipid and cholesterol. The curves obtained were log.-linear over the range of doses employed (oestradiol-17β, 1–50 μg./day; oestrone, 4–100 μg./day; oestriol, 20–500 μg./day) with the exception of the cholesterol response to oestradiol. The relative potencies of the three oestrogens were compared for each of the plasma components measured. Oestradiol-17β was the most potent, being 3·9 times more active than oestrone and 13·7 times more active than oestriol.

ABSTRACT

Changes in concentrations of hypothalamic gonadotrophin-releasing hormone (GnRH) and pituitary and plasma FSH and prolactin were measured in intact and thyroidectomized female starlings (Sturnus vulgaris) after transfer from short to long photoperiods.

In intact birds, hypothalamic GnRH did not increase significantly during the first 6 weeks of photostimulation, but by 12 weeks, as birds became photorefractory, it had decreased to levels significantly lower than those before photostimulation. In thyroidectomized birds, which did not become photorefractory, hypothalamic GnRH remained high after 12 weeks of photostimulation. Pituitary FSH increased in both intact and thyroidectomized birds; it then decreased to low levels in intact photorefractory birds, but remained high in thyroidectomized birds. Plasma FSH increased to a peak after 2 weeks, but by 6 weeks it had decreased to low levels in both groups. In intact birds there was a 70-fold increase in pituitary prolactin during the first 6 weeks, and levels were still high after 12 weeks of photostimulation. In thyroidectomized birds, pituitary prolactin remained low. The results suggest that while the initial effect of long daylengths is to cause gonadal maturation, the ultimate effect is to switch off the reproductive system.

J. Endocr. (1985) 105, 71–77

ABSTRACT

When starlings (Sturnus vulgaris) are transferred from short to long days, hypothalamic content of gonadotrophin-releasing hormone (GnRH) and pituitary gonadotrophin content eventually decrease, as birds become photorefractory, to values lower than they were on short days. This implies that both should increase as photorefractoriness is terminated some time after transfer from long to short days. Further, since thyroidectomy causes termination of photorefractoriness in birds held on long days, this should also result in an increase in hypothalamic GnRH and pituitary gonadotrophin contents. This study tests these hypotheses.

Male starlings were transferred from short to long days for 10 weeks, by which time they should have become photorefractory. One group of birds was then killed; blood was collected, hypothalami and pituitaries were excised, and the stage of moult and testicular weights recorded. The remaining birds were then kept on long days, transferred to short days or thyroidectomized and kept on long days. Groups of birds in each treatment group were killed and sampled 2, 8 and 14 weeks later. Hypothalamic content of GnRH, and pituitary and plasma FSH and prolactin concentrations, were measured by radioimmunoassay.

In birds sampled after 10 weeks of long days, hypothalamic content of GnRH was quite low, pituitary and plasma levels of FSH were very low, testes were small and pituitary and plasma prolactin levels were high. In intact birds kept on long days, hypothalamic GnRH content decreased further and remained low. Pituitary and plasma FSH levels remained low, testes remained small and pituitary and plasma prolactin levels decreased slowly. In intact birds transferred to short days, hypothalamic GnRH content increased to values significantly (P<0·01) higher than in birds kept on long days. Pituitary FSH content increased (P<0·01) but plasma FSH levels remained low and testes remained small in most birds. Pituitary and plasma prolactin levels decreased rapidly. In thyroidectomized starlings kept on long days, hypothalamic GnRH content increased to the same values as in birds on short days, pituitary FSH content increased markedly and testes grew although plasma FSH remained low. Pituitary and plasma prolactin levels decreased slowly.

These results demonstrate that short days activate the hypothalamus to a state of 'readiness to function' and confirm that the thyroid glands are involved in this response.

J. Endocr. (1986) 110, 73–79

SUMMARY

Female canaries were exposed to various lighting schedules in January—February to assess the photoperiod necessary for reproductive development. Nest building was not increased with lighting schedules of 8 h light: 16 h darkness per day (8L: 16D) and 10L: 14D but was increased after 4 weeks of 12L: 12D or 14L: 10D. Placing of nest material was greatest with a lighting schedule of 14L: 10D. Plasma and pituitary luteinizing hormone (LH) levels were low when 8L: 16D was used and the ovaries and oviducts remained undeveloped. There was a significant rise in the weights of the ovaries and oviducts with 10L: 14D per day. This change was not reflected in the plasma LH level but the amount of LH in the pituitary was increased. Exposure to 12L: 12D or 14L: 10D caused substantial development of ovaries and oviduct growth, and significantly increased levels of plasma and pituitary LH. These two groups were indistinguishable in all these parameters.

Oestrogen treatment of a group exposed to 8L: 16D decreased plasma and pituitary LH in four out of six birds. Behaviourally, some of these birds showed an increase in gathering and placing of nest material. In an oestrogen-treated group exposed to 14L: 10D no effect was seen on plasma LH concentration, but the group showed significantly more gathering and placing behaviour than untreated birds. Oviduct weight was increased in both oestrogen-treated groups.

ABSTRACT

The development of the reproductive system was studied in juvenile starlings during the acquisition of photosensitivity, the attainment of sexual maturation after photostimulation and the subsequent onset of photorefractoriness, using immunohistochemistry for LHRH and radioimmunoassay measurements of hypothalamic, pituitary and plasma hormone concentrations. The first stage of sexual development induced by exposure of photorefractory immature starlings to short days (8 h light:16 h darkness; 8L:16D) was characterized by a decrease in pituitary prolactin content within 1 week and an increase in hypothalamic LHRH content, in the size of the LHRH perikarya and in the intensity of immunostaining in the median eminence in 4–6 weeks. Sexual maturation occurring after exposure to long days (18L:6D) was associated with further increases in LHRH content and cell size, and increases in LH and prolactin concentrations. During testicular regression, LHRH perikarya were reduced in size and staining intensity but LHRH immunostaining in the median eminence and content in the hypothalamus remained high until gonadal regression was almost complete. Prolactin levels were maximal during testicular regression. These results suggest that gonadal regression is initiated by a reduction in LHRH synthesis and possibly, in addition, an external inhibitory influence on LHRH release. Hypothalamic LHRH content eventually declined and LHRH immunostaining in the median eminence was much reduced in fully photorefractory starlings maintained under long days.

Journal of Endocrinology (1989) 122, 255–268