ABSTRACT

If starlings (Sturnus vulgaris) are transferred to long days soon after being thyroidectomized, they show gonadal maturation but no subsequent photorefractoriness, suggesting a specific role for thyroid hormones in the development of photorefractoriness. This study tests an alternative hypothesis: that following thyroidectomy all gonadal responses to photoperiodic changes are progressively inhibited. Photoperiodic responses of starlings were examined at two different times after thyroidectomy. Transfer of thyroidectomized birds to long days 4 weeks after thyroidectomy caused testicular growth but at a slower rate than in intact birds, whereas, in thyroidectomized birds kept on short days, testicular growth was greater than in corresponding intact birds. Thirteen weeks after thyroidectomy, there was little or no response to an increase or a decrease in daylength in thyroidectomized birds, whereas intact birds showed marked testicular growth or regression respectively. The results support the hypothesis that the reproductive system gradually becomes unresponsive to changes in daylength during the weeks following thyroidectomy. This long-term effect may mean that earlier conclusions regarding the effects of thyroidectomy need to be reassessed.

Journal of Endocrinology (1993) 139, 51–55

ABSTRACT

Is the extended prepubertal period which occurs in most birds due to a juvenile photorefractoriness analogous to photorefractoriness in adults? Starlings were taken from nest boxes 4 days after hatching and hand-reared on long days or short days. In females on long days, hypothalamic gonadotrophin-releasing hormone (GnRH) content and plasma LH concentration remained low. In intact males on long days, plasma LH remained low and the testes remained small, and in castrated birds, plasma LH also remained low. In females reared on short days, hypothalamic GnRH content began to increase dramatically from 4 weeks of age and plasma LH increased from 6 weeks of age. In intact males on short days, there was a slight increase in plasma LH and testicular size, and in castrated males, plasma LH increased markedly from six weeks of age. All birds on long days moulted into adult plumage, whereas those on short days retained juvenile plumage. The changes in birds reared on short days were similar to those found when photorefractory adult birds are transferred to short days. This adds to the evidence that juvenile photorefractoriness is analogous to photorefractoriness in adults, and therefore that the seasonal termination of photorefractoriness is an annual puberty. However, the responses to short days occur later in birds raised on short days than in older birds transferred to short days, which suggests either that short day-induced changes occur more slowly in young birds, or that birds only respond to short days after a certain age.

Journal of Endocrinology (1989) 123, 189–196

ABSTRACT

Intact and gonadectomized male and female starlings (Sturnus vulgaris) were held in outdoor aviaries for 1 year. Blood samples were taken frequently and assayed for LH and prolactin, and intact males were laparotomized to determine testis size.

Changes in testis size in intact males were similar to those in wild starlings with maximum size during April. Hormone levels in intact males and females also followed a similar pattern to those in wild birds although absolute levels were somewhat lower. Levels of LH increased during spring, were highest in early May and then declined rapidly as birds became photorefractory. Prolactin levels did not begin to increase until April, reached a peak in late May and then declined. In gonadectomized males and females, high levels of LH were maintained throughout the early part of the year when birds were photosensitive, but declined rapidly as they became photorefractory. This occurred at the same time as in intact birds. During photorefractoriness, LH levels in gonadectomized birds were similar to those in intact birds. Prolactin showed the same seasonal pattern in gonadectomized birds as that in intact birds. These results demonstrate that gonadal steroid feedback is unimportant in initiating and maintaining photorefractoriness and confirm that increased prolactin secretion coincides with the onset of photorefractoriness.

J. Endocr. (1984) 100, 213–218

Previous studies have shown that an increase in prolactin secretion coincides with the beginning of the photorefractory phase in starlings (Sturnus vulgaris). To determine how closely this increase is associated with the onset of refractoriness, we kept starlings on photoperiods which would induce refractoriness at different rates. Four groups of males and females were transferred from 8 h light: 16 h darkness (8L : 16D) to either 18L : 6D, 13L : 11D, 11L : 13D or 8L : 16D. Weekly blood samples were assayed for LH, FSH and prolactin and the males were frequently laparotomized to determine testis volume.

In males, both 18L and 13L induced marked increases in plasma gonadotrophin levels and rapid gonadal maturation. A photoperiod of 18L induced an earlier onset of refractoriness (2–4 weeks) than 13L (6 weeks). In both cases prolactin levels began to increase immediately before the onset of refractoriness. Males on 11L showed a slight increase in gonadotrophin levels and slow gonadal maturation. There was no increase in prolactin levels and these birds did not become refractory. Hormonal data from the females were similar to those from males. These results confirm that increased prolactin secretion is associated, in some way, with the onset of photorefractoriness in this species, although whether it is a cause or consequence of photorefractoriness, or simply of gonadal regression, remains unclear.

ABSTRACT

A comparison has been made between the glucose-induced changes in electrical activity, insulin release, lactate output and potassium permeability in normal and ob/ob obese (Norwich strain) mice. The electrical response of the islet membrane to high glucose (22·2 mmol/l) stimulation was different in the two types of mice, generating continuous spike activity in normal but producing bursts of activity in ob/ob mouse islets. The absolute amounts of insulin and lactate produced by ob/ob islets in response to both basal and high glucose concentrations were greater than the absolute amounts produced by normal islets, though the ratio of the amount produced in high glucose concentrations to the amount produced in basal glucose concentrations was not significantly different between normal and ob/ob islets for both parameters. Glucose-induced changes in potassium permeability were smaller in ob/ob than in normal mice. Cooling from 37 to 27 °C, during steady-state glucose stimulation, reduced both lactate output and insulin release, the temperature coefficients being similar in both types of mice. The effect of temperature reduction on electrical activity was more marked in the islets of ob/ob mice than in those of normal mice; spike frequency was unaffected in normal but reduced in ob/ob mice, whereas spike amplitude was decreased in both. Cooling-induced inhibition of potassium permeability was greater in the islets of ob/ob mice than in those of normal mice. These results show that this new colony of ob/ob mice (Norwich strain) is comparable to normal mice in terms of the dynamics of insulin release and lactate output in response to glucose, but that the response of the β-cell membrane electrical activity and potassium permeability to glucose is different.

J. Endocr. (1985) 107, 265–273

ABSTRACT

If adult starlings are kept on long days, they rapidly become refractory to long days (photorefractory). The recovery of the ability to respond to long days (photosensitivity) occurs after transfer to short days. Photosensitivity is associated with an increase in hypothalamic content of gonadotrophin-releasing hormone (GnRH), and, in castrated birds, with an increase in plasma LH. If photorefractory adult or juvenile starlings are transferred to short days, these events begin 3 and 4 weeks respectively after transfer. Starlings apparently hatch in a photorefractory state. However, if nestlings are hand-reared under short days, the increases in GnRH and LH begin at 6 and 7 weeks of age respectively. Is this 3-week delay, which corresponds to the time to reach adult body size, simply due to a slower response, or are nestlings incapable of responding to short days until fully grown? Castrated male nestling starlings were hand-reared under short days, under long days or under long days initially and transferred to short days at 2, 3, 6 or 9·5 weeks of age. The increase in plasma LH in birds transferred to short days at or before 3 weeks began at the same time as in birds kept on short days (7 weeks). In birds transferred at 3 weeks or later, the increase in LH began 4 weeks after transfer, irrespective of age. Intact female starlings showed similar responses. Birds kept on short days did not moult whereas birds transferred from long days to short days at 3 weeks did. These results suggest that nestlings are capable of perceiving daylength, but that the GnRH neurosecretory system does not respond to short days until birds are fully grown.

Journal of Endocrinology (1992) 132, 411–417

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.

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

We investigated the influence of maternal dietary restriction between days 28 and 80 of gestation followed by re-feeding to the intake of well-fed ewes up to 140 days of gestation (term is 147 days) in sheep, on expression of mRNA for insulin-like growth factor (IGF)-I, IGF-II and growth hormone receptor (GHR) in fetal liver and skeletal muscle. Singleton bearing ewes either consumed 3.2-3.8 MJ/day of metabolisable energy (ME) (i.e. nutrient restricted - approximately 60% of ME requirements, taking into account requirements for both ewe maintenance and growth of the conceptus) or 8.7-9.9 MJ/day (i.e. well fed - approximately 150% of ME requirements) between days 28 and 80 of gestation. All ewes were then well fed (150% of ME requirements) up to day 140 of gestation and consumed 8-10.9 MJ/day. At days 80 and 140 of gestation, five ewes were sampled from each group and fetal tissues taken. There was no difference in fetal body weight or liver weights between groups at either sampling date, or skeletal muscle (quadriceps) weight at 140 days. IGF-I mRNA abundance was lower in livers of nutrient-restricted fetuses at day 80 of gestation (nutrient restricted 2.35; well fed 3.70 arbitrary units), but was higher than well-fed fetuses at day 140 of gestation, after 60 days of re-feeding (restricted/re-fed 4.27; well fed 2.83;s.e.d. 0.98 arbitrary units, P=0.061 for dietxage interaction). IGF-II mRNA abundance was consistently higher in livers of nutrient-restricted fetuses (80 days: nutrient restricted 7.78; well fed 5.91; 140 days: restricted/re-fed 7.23; well fed 6.01;s.e.d. 1.09 arbitrary units, P=0.061 for diet). Nutrient restriction had no effect on hepatic GHR mRNA abundance, but re-feeding of previously nutrient-restricted fetuses increased GHR mRNA compared with continuously well-fed fetuses (80 days: nutrient restricted 70.6; well fed 75.1; 140 days: restricted/re-fed 115.7; well fed 89.4;s.e.d. 10.13 arbitrary units, P=0.047 for dietxage interaction). In fetal skeletal muscle, IGF-I mRNA abundance was not influenced by maternal nutrition and decreased with gestation age (P<0.01). IGF-II mRNA abundance was higher in skeletal muscle of nutrient-restricted fetuses compared with well-fed fetuses at day 80 of gestation (nutrient restricted 16.72; well fed 10.53 arbitrary units), but was lower than well-fed fetuses after 60 days of re-feeding (restricted/re-fed 7.77; well fed 13.72;s.e.d. 1.98 arbitrary units, P<0.001 for dietxage interaction). There was no effect of maternal nutrition or gestation age on fetal skeletal muscle GHR expression. In conclusion, maternal nutrient restriction in early to mid gestation with re-feeding thereafter results in alterations in hepatic and skeletal muscle expression of IGF-I, IGF-II and/or GHR in the fetus which may subsequently relate to altered organ and tissue function.