The effects of castration on the seasonal pattern of plasma LH concentrations in wild mallard drakes

Effect of photostimulation on concentrations of plasma prolactin in castrated bantams (Gallus domesticus)

The annual breeding cycle of 'unimproved' breeds of domestic chicken, including the bantam, at temperate latitudes, is terminated by decreasing daylength in autumn and is initiated in late winter, while daylengths are still short. Observations on photoperiodic birds that terminate seasonal breeding by the development of long day photorefractoriness suggest that the photoinduced pattern of prolactin secretion is associated with the pattern of gonadal growth and regression. It was predicted that, if there is a causal relationship between photoinduced changes in prolactin secretion and gonadal function in birds then, in the bantam, the pattern of prolactin secretion observed after photostimulation would not be the same as in birds terminating breeding by the development of long day photorefractoriness. Experiments were carried out on surgically castrated bantams to avoid confounding the effects of photostimulation and the stimulatory actions of testicular hormones on prolactin secretion. Transfer of photosensitive castrated bantams from 8 to 14, 16, 18 or 20 h light/day initially stimulated prolactin release and, subsequently, after 20-30 days, concentrations of plasma prolactin progressively decreased. After 148 days of photostimulation, concentrations of plasma prolactin approached but were still higher than short day controls. Transfer of photosensitive castrated bantam cockerels from 8 to 12 h light/day stimulated a slower increase in plasma prolactin that subsequently remained higher than in other photostimulated groups. A further 4 h increase in photoperiod in the birds exposed for 148 days to 12 or 16 h light/day resulted, respectively, in a transitory increase and no increase in prolactin secretion. Recovery of photosensitivity for prolactin release was observed in the birds transferred to 18 or 20 h light/day for 148 days after treatment with 8 h light/day for 35 days. Attempts to obtain an independent hormonal correlate of the prolactin responses to photostimulation by measurement of plasma luteinizing hormone (LH) were unsuccessful. The concentration of plasma LH in castrated bantams did not change in response to a change in photoperiod. These observations show that the photoinduced pattern of prolactin release in the bantam, a species which terminates seasonal breeding in response to decreasing daylength, is the same as that in birds which terminate seasonal breeding by the development of long day photorefractoriness. It is concluded that the photoinduced pattern of prolactin secretion in birds can be dissociated from the neuroendocrine mechanisms controlling the termination of seasonal breeding.

Photoperiodic control of sexual maturation in Muscovy drakes

The influence of photoperiodic control on reproductive development was investigated in Muscovy drakes by the use of night-interruption photoschedules consisting of a main photofraction of 6 hr and a secondary photofraction of 1 hr, interrupting the dark period 6 hr (Group A), 8 hr (Group B), 10 hr (Group C), and 12 hr (Group D) after dawn. Sexual maturity, assessed by the increase in plasma concentrations of luteinising hormone (LH) and testosterone, was achieved in all groups by Week 34. However, significant differences in the precocity of the rate of sexual maturation were evident on Week 27. At this time, plasma testosterone concentrations, relative to those at Week 15, had increased by 2.02, 0.29, 0.89, and 2.68 ng/ml in Groups A, B, C, and D respectively. Increases in LH concentrations followed a similar pattern. The more pronounced increases in LH and testosterone concentrations observed from Group B to Group D indicate that long days stimulate reproductive development in the Muscovy duck via a diurnal rhythm of photosensitivity. However, the nonphotostimulated increase in hormonal concentrations observed in Group A implies that a factor other than photostimulation may be involved. The dissipation of juvenile photorefractoriness by long-term short-day exposure and subsequent passive sexual maturation may explain these results. We hypothesize that the rate of sexual maturation results from a balance between the dissipation of juvenile photorefractoriness by short days and the relative strength of the photoperiodic drive as the result of long days.

Effect of low light intensity on testicular function in photorefractory domestic ganders

The seasonal variation of the plasma testosterone level of domestic ganders--living under natural photoperiod in Central Hungary--shows a bimodal rhythmicity. An experimental group of ganders within a photorefractory period was exposed to continuous low light intensity (1 lx/m2) from the middle of July until the beginning of August. They were then kept under natural daylight (14.5 L-9.5 D), and then had extra light (14 L-10 D) until the end of the experiment (end of November). The testis weight and the plasma concentration of testosterone were increased significantly as a result of low light intensity but failed to reach the seasonal (February) peak. Having readjusted the birds to natural photoperiod the testosterone plasma concentration did not continue to increase, but decreased slowly for 44 days (16 September). Between the 44th and 52nd day of the experiment (24 September) the testosterone concentration was reduced to the level of the control animals, which were maintained under natural photoperiod. It was established that the photorefractory period had not been terminated by 3 weeks' low light intensity, but a moderate gonadostimulation had occurred. Since photostimulation did not increase the plasma testosterone, but a superactive LH-RH analog increased it significantly in birds which had been in low light intensity, it is supposed that the cause of the refractoriness in ganders may also be decreased gonadotropin-releasing hormone secretion.

The development of photorefractoriness in castrated willow ptarmigan (Lagopus lagopus lagopus)

Changes in plasma LH levels were observed in castrated willow ptarmigan exposed to different lighting patterns to determine whether they become photorefractory. LH levels did not increase in long-term castrated birds exposed to a natural (70 degrees N) increase in daylength and did not fall abruptly in June when intact birds become photorefractory. Plasma LH levels in naturally photostimulated, long-term castrates fell gradually during the summer and fall. Plasma LH levels increased steeply when these birds were transferred in winter from short days to continuous light. Thereafter the levels fell indicating that the birds became photorefractory. In a further experiment, photosensitive male willow ptarmigan were castrated 3 or 6 weeks after transfer from short days to continuous light. The postcastration increase in plasma LH levels was significantly greater in birds castrated after 3 weeks than in those castrated after 6 weeks of photostimulation. In both groups of castrates, LH levels began to fall when LH levels decreased in intact controls, indicating the development of photorefractoriness. These findings confirm that the development of photorefractoriness is progressive rather than abrupt and that it may be accelerated in castrated birds under natural increases in daylength but not after acute transfer from short days to continuous light. These observations support the view that the development of photorefractoriness in willow ptarmigan involves an interaction between the gonads and the neuroendocrine or circadian system. They are not entirely consistent with the simple concept that photorefractoriness is caused by a decrease in the sensitivity of the hypothalamus to inhibitory steroid feedback.

The annual reproductive cycle in mallards

In wild mallard drakes plasma FSH and LH levels were elevated at the height of the breeding season in spring, decreased towards the end of the reproductive phase and were low during the time of photorefractoriness in summer, when the testes were regressed. In contrast plasma prolactin concentrations increased only towards the end of the breeding season and were at their annual height during the summer. Castration of photorefractory mallards caused a steep rise of plasma LH levels indicating that the testes were involved in the maintenance of the low LH concentrations during the refractory period. In these drakes LH titers remained high throughout the year and, thus, in the absence of the testes the birds did not become photorefractory. In photorefractory drakes removal of 90% of the testicular tissue not only increased plasma LH levels but induced complete spermatogenesis in the testicular remains within 6 weeks. Therefore it seems that photorefractoriness in mallards is due to an increased sensitivity of the hypothalamo-hypophyseal system to the negative feedback action of testicular androgens. After injection of tritiated testosterone the hypothalamus, anterior pituitary and testes, but not other organs studied, took up more radioactivity during the refractory phase than at the height of the breeding season. It is speculated that the increased sensitivity of the hypothalamo-hypophyseal unit to the negative androgenic feedback during photorefractoriness depends on an increased number of androgen receptors in these structures which, again, could be due to the elevated prolactin levels during this phase of the annual cycle.