Development Of Photorefractoriness Research Articles

Reproductive axis gene regulation during photostimulation and photorefractoriness in Yangzhou goose ganders

BackgroundThe Yangzhou goose is a long-day breeding bird that has been increasingly produced in China. Artificial lighting programs are used for controlling its reproductive activities. This study investigated the regulations of photostimulation and photorefractoriness that govern the onset and cessation of the breeding period.ResultsIncreasing the daily photoperiod from 8 to 12 h rapidly stimulated testis development and increased plasma testosterone concentrations, with peak levels being reached 2 months after the photoperiod increase. Subsequently, testicular activities, testicular weight, spermatogenesis, and plasma testosterone concentrations declined steadily and reached to the nadir at 5 months after the 12-hour photoperiod. Throughout the experiment, plasma concentrations of triiodothyronine and thyroxine changed in reciprocal fashions to that of testosterone. The stimulation of reproductive activities caused by the increasing photoperiod was associated with increases in gonadotropin-releasing hormone (GnRH), but decreases in gonadotropin-inhibitory hormone (GnIH) and vasoactive intestinal peptide (VIP) gene messenger RNA (mRNA) levels in the hypothalamus. In the pituitary gland, the levels of follicle-stimulating hormone (FSH) and luteinizing hormone (LH) mRNA abruptly increased during the longer 12-hour photoperiod. The occurrence of photorefractoriness was associated with increased GnIH gene transcription by over 250-fold, together with increased VIP mRNA levels in the hypothalamus, and then prolactin and thyroid-stimulating hormone in the pituitary gland. FSH receptor, LH receptor, and StAR mRNA levels in the testis changed in ways paralleling those of testicular weight and testosterone concentrations.ConclusionsThe seasonal reproductive activities in Yangzhou geese were directly stimulated by a long photoperiod via upregulation of GnRH gene transcription, downregulation of GnIH, VIP gene transcription, and stimulation of gonadotrophin. Development of photorefractoriness was characterized by hyper-regulation of GnIH gene transcription in the hypothalamus, in addition of upregulation of VIP and TRH gene transcription, and that of their receptors, in the pituitary gland.

Photoperiodic regulation of seasonal reproduction, molt and body weight in the migratory male yellow-breasted bunting (Emberiza aureola)

Photoperiod has been shown to be a major source of temporal information regulating reproduction and associated functions in a number of avian species. We studied seasonal cycles of testicular volume, molt and body weight in natural and temperature-controlled conditions and under different artificial photoperiods in the yellow-breasted buntings. Buntings posses seasonal cycles of testicular volume, molt, body weight and fattening with no major difference between natural and temperature-controlled conditions. These cycles follow an annual solar cycle suggesting the possibility of their photoperiodic control. To confirm this, photosensitive birds were studied under 9L/15D (close to shortest day length), 12L/12D (equinox day length) and 14L/10D (close to longest day length) for 18 months. Buntings showed testicular growth followed by regression and development of photorefractoriness; molt and body weight change only under 12L/12D and 14L/10D but not under 9L/15D. Reinitiation of above responses did not occur following initial cycles under stimulatory photoperiods precluding the possibility of circannual rhythm involvement. Birds exhibited an incomplete prenuptial molt of body feathers during gonadal stimulation under long days followed by complete postnuptial molt of body and primary feathers that progressed with gonadal regression. Exposure of photosensitive birds to light–dark cycles constituting 9–16h of light/day suggested that daily photoperiod of about 12h or more is essential in inducing testicular growth and function. These results clearly indicate that buntings are capable of fine discrimination of photoperiodic information and use annual changes in day length as an environmental factor to time their seasonal responses.

Photoperiod as a proximate factor in control of seasonality in the subtropical male Tree Sparrow, Passer montanus

BackgroundMost species of birds exhibit well-defined seasonality in their various physiological and behavioral functions like reproduction, molt, bill color etc. such that they occur at the most appropriate time of the year. Day length has been shown to be a major source of temporal information regulating seasonal reproduction and associated events in a number of avian species. The present study aims to investigate the role of photoperiod in control of seasonal cycles in the subtropical male tree sparrow (Passer montanus) and to compare its responses at Shillong (Latitude 25°34'N, Longitude 91°53'E) with those exhibited by its conspecifics and related species at other latitudes.ResultsInitial experiment involving study of seasonal cycles revealed that the wild tree sparrows posses definite seasonal cycles of testicular volume, molt and bill color. These cycles were found remarkably linked to annual solar cycle suggesting the possibility of their photoperiodic control. To confirm this possibility in the next experiment, the photosensitive birds were exposed to three different light-dark regimes that were close to what they experience at this latitude: 9L/15D (close to shortest day length), 12L/12D (equinox day length) and 14L/10D (close to longest day length) for 18 months. Tree sparrows showed testicular growth followed by regression and development of photorefractoriness, molting and bill color changes only under long daily photoperiods (12 L and 14 L) but not under short daily photoperiod (9 L). Birds, under stimulatory photoperiods, did not show reinitiation of the above responses after the completion of initiation regression cycle even after their exposure to these photoperiods for 18 months. This precludes the possibility of circannual rhythm generation and suggests the involvement of photoperiodic mechanism in control of their seasonal cycles. Further, replacement of body and primary feathers progressed with gonadal regression only under long days suggesting that the two high energy demanding events of reproduction and molt are phased at two different times in the annual cycle of the bird and are photoperiodically regulated. Results of the final experiment involving exposure of photosensitive birds to a variety of photoperiodic treatments (9L/15D, 10L/14D, 11L/13D, 12L/12D, 14L/10D and 16L/8D) for 30 days suggested that the light falling for 11 h or more is important in inducing testicular growth and function in this species.ConclusionThese results clearly indicate that despite of small photofluctuation, subtropical tree sparrows are capable of fine discrimination of photoperiodic information and use day length as a proximate environmental factor to time their seasonal responses similar to their conspecifics and related species at other latitudes suggesting the conservation of photoperiodic control mechanism in them.

Reproductive Physiology: Songbird Study Removes Long-Standing Neuroendocrinology Research Roadblock

Songbirds have long been favored as models to investigate the neuroendocrine mechanisms that regulate seasonal reproduction. Progress in this area has for many years been hindered by our inability to identify the songbird gene that codes for GnRH1, the hypothalamic decapeptide that stimulates the secretion of gonadotropins by the pituitary gland and, consequently, gonadal development (1–3). A new study removes this roadblock (4), thus creating new opportunities to address several long-standing questions. In this study, Stevenson et al. (4) cloned the GNRH1 cDNAs of two songbirds, the European starling and zebra finch, that are extensively used for research in reproductive endocrinology. The nucleotide sequence that is predicted to code for the GnRH1 decapeptidewas found topresentanumberofdifferencesbetween thezebra finch and starling and also between these species and nonsongbird avian species studied to date. Despite these differences, the amino acid structure of GnRH1 was found to be conserved across species examined. Immunoreactive GnRH1-containing neurons in songbirds are located primarily in the preoptic region (5–7). Confirming this observation, Stevenson et al. found GNRH1 mRNA expression, as determined by RT-PCR and in situ hybridization, to likewise be confined to this area of the brain. The activity of the reproductive system in many songbirds is regulated primarily by day length (1, 2, 8). In these birds, exposure to sufficiently long days stimulates reproductive development by increasing the production and secretion of GnRH1. Prolonged exposure to these long days, however, desensitizes the GnRH1 system to this stimulus. Birds in this condition are said to be photorefractory and undergo gonadal regression (8–10). The development of photorefractoriness is a key event in the annual reproductive cycle of seasonally breeding birds, but even though the reproductive consequences of photorefractoriness are well described, the process that initiates is speculative. Recently photorefractory birds generally retain a large pool of releasable GnRH1 whereas brain stores of GnRH1 in deeply photorefractory birds are low (6, 11–13). This observation led to hypothesize that photorefractoriness develops as a two-step process consisting in decreased secretion and then decreased production of GnRH1 (1). To test this hypothesis, Stevenson et al. (4) measured the gonad size and preoptic region expression of the GNRH1 gene in male starlings that they transferred from short days to chronically long days until birds became photorefractory. In this experiment, photoinduced changes in GNRH1 gene expression and gonad size followed parallel time courses as starlings became photorefractory. This finding is significant because it suggests that contrary to the current model, an early event associated with photorefractoriness in starlings consists in decreased expression of the GNRH1 gene and therefore presumably also of GnRH1 peptide production. The identification of the songbird GNRH1 gene will facilitate the use of these organisms to more directly address a number of unresolved issues in avian neuroendocrinology: Species such as the Japanese quail, Coturnix coturnix japonica, and desert sparrows of the genus Aimophila terminate a reproductive cycle not by becoming absolutely photorefractory, as do starlings but by developing relative photorefractoriness (14–16). By contrast with absolutely refractory birds, relatively photorefractory birds are thought to retain large hypothalamic storesofGnRH1thatcanbereleased inresponse tosufficiently long photoperiod or appropriate nonphotic or pharmacological stimulation (17).Does significantproductionofGnRH1continueas long asbirds remainrelativelyphotorefractory? Is thecontrolofGnRH1 production in relatively photorefractory species fundamentally different from that in absolutely photorefractory species? Studies comparing GNRH1 gene expression in absolutely and relatively photorefractory species should help answer these questions. As in other photoperiodic vertebrates, the activity of the reproductive system in songbirds is influenced by nonphotoperiodic factors such as water and food availability (18–20), tem-

Ambient temperature effects on photo induced gonadal cycles and hormonal secretion patterns in Great Tits from three different breeding latitudes

The present study determines how populations of Great Tits ( Parus major) breeding in southern, mid and northern European latitudes have adjusted their reproductive endocrinology to differences in the ambient temperature during the gonadal cycle. A study based on long-term breeding data, using the Colwell predictability model, showed that the start of the breeding season has a high predictability (≈ 0.8–0.9) at all latitudes, and that the environmental information factor ( I e) progressively decreased from mid Italy ( I e > 4) to northern Finland ( I e < 1). The results indicate that integration of supplementary information, such as ambient temperature, with photoperiodic initial predictive information (day length), becomes progressively more important in maintaining the predictability of the breeding season with decreasing latitude. This hypothesis was verified by exposing photosensitive Great Tits from northern Norway, southern Sweden and northern Italy to sub-maximal photo-stimulatory day lengths (13L:11D) under two different ambient temperature regimes (+ 4 °C and + 20 °C). Changes in testicular size, plasma levels of LH and testosterone were measured. The main results were: (1) Initial testicular growth rate, as well as LH secretion, was affected by temperature in the Italian, but not in birds from the two Scandinavian populations. (2) Maximum testicular size, maximum LH and testosterone levels were maintained for a progressively shorter period of time with increasing latitude, regardless of whether the birds were kept on a low or a high ambient temperature. (3) In birds from all latitudes, the development of photorefractoriness, as indicated by testicular regression and a decrease in plasma levels of LH and testosterone, started much earlier (with the exception for LH Great Tits from northern Scandinavia) when kept on + 20 °C than when kept on + 4 °C. The prolonging effects of a low temperature was more pronounced in Mediterranean birds, than in birds from Scandinavia, and more pronounced in Great Tits from southern Scandinavia than in Great Tits from northern Scandinavia. Ecological implications of the results are discussed, as well as possible impact of global warming on the breeding success of European Great Tits from different breeding latitudes.

Refractoriness to Short Day Lengths Augments Tonic and Gonadotrophin‐Releasing Hormone‐Stimulated Lutenising Hormone Secretion

Siberian hamsters (Phodopus sungorus) undergo reproductive involution following exposure to short winter day lengths. Following approximately 20 weeks of exposure to short day (SD) lengths, hamsters become refractory to the inhibitory effects of SD, and reproductive competence is restored in anticipation of spring. The extent to which changes in gonadal steroid-dependent and -independent regulation of gonadotrophin secretion participate in this vernal reactivation of the gonads is not known. This experiment tested whether tonic and gonadotrophin-releasing hormone (GnRH)-stimulated regulation of lutenising hormone (LH) secretion differs between photoresponsive and photorefractory Siberian hamsters. Male hamsters born into long day (LD) lengths were castrated or subjected to a sham-castration surgery at 17 days of age, implanted s.c. with blank or testosterone-filled capsules, and housed in LD or SD thereafter. Baseline LH and LH responses to GnRH (200 ng/kg, s.c) were measured at 14 (photoresponsive) and 40 (photorefractory) weeks of age. Despite lower circulating testosterone concentrations in gonadally regressed SD hamsters on week 14, tonic LH concentrations were comparable among all groups of gonad-intact hamsters on weeks 14 and 40; however, week 14 SD hamsters exhibited significantly higher GnRH-stimulated LH responses. Tonic LH concentrations were indistinguishable among all groups of castrated hamsters bearing empty implants on week 14, but prolonged exposure to LD led to a decrease in resting LH, whereas prolonged exposure to SD resulted in an increase in LH. In castrated hamsters bearing testosterone implants, baseline LH concentrations were comparable in all groups, but GnRH treatment resulted in significantly higher LH concentrations in photorefractory (week 40, SD) hamsters relative to all other groups. The data suggest that the development of photorefractoriness in Siberian hamsters is characterised by enhanced gonadal hormone-independent stimulation of LH secretion, and diminished sensitivity to inhibitory negative-feedback effects of testosterone on LH secretion. Decreases in responsiveness of gonadotrophin secretion to gonadal hormone negative feedback may contribute to the process of photorefractoriness and assist in maintaining the growth of reproductive organs during the process of gonadal recrudescence.

Hypothalamic Pro‐GnRH‐GAP, GnRH‐I and GnRH‐II During the Onset of Photorefractoriness in the White‐Crowned Sparrow (Zonotrichia leucophrys gambelii)

Gambel's white-crowned sparrow is a long distance migrant that undergoes spontaneous gonadal regression as a result of long day exposure. This termination of breeding is caused by the development of photorefractoriness and the birds become insensitive to long days, including continuous light. The present study investigated its possible mechanisms by examining the activity of the gonadotrophin-releasing hormone (GnRH) system under different photoperiodic regimes. We investigated the localisation and distribution of GnRH-I, its precursor pro-GnRH-GAP and GnRH-II in Gambel's white-crowned sparrow brain using immunocytochemistry with specific antibodies during photostimulation and the development of photorefractoriness. The study revealed that photoperiodic treatment, including the onset of photorefractoriness, had no significant effect on the size or number of GnRH-I, pro-GnRH-GAP or GnRH II immunoreactive cells, or the density of the GnRH-I, pro-GnRH-GAP immunoreactive fibres at the median eminence. GnRH-II was not found in the median eminence, suggesting that it does not regulate pituitary gonadotrophin secretion. GnRH-I measurement in hypothalamic extracts by radioimmunoassay did not reveal any significant difference between birds that were photostimulated or in the early stages of photorefractoriness. Furthermore, the action of the excitatory amino acid glutamate agonist N-methyl-D-aspartate on GnRH neurones in photorefractory birds was demonstrated by the significant blockade of luteinising hormone release with a specific GnRH antagonist. Taken together, these results suggest that, in Gambel's white-crowned sparrow, a decrease in GnRH-I secretion is the initial step for the onset of photorefractoriness and not a decrease in GnRH-I biosynthesis.

Flexible reproduction in wild canaries is independent of photoperiod

In temperate zone species, timing of the breeding season is determined by the minimum photoperiod or “critical day length” and the development of photorefractoriness. The integration of environmental cues like food, temperature, rainfall, and behavioural interactions determines the breeding window. However, there are a number of examples for breeding activities outside this time window. Here we investigated the possible mechanisms for early seasonal breeding activities in a population of wild canaries ( Serinus canaria) that inhabits an isolated island within the Madeiran archipelago. In December 1999, breeding activities were observed six weeks before the usual onset of the breeding season on the island, which can be related to unusual heavy rainfall during the two previous months. Moreover, testosterone (T) levels of the birds were significantly higher as during the same time in previous years and showed no difference to the T levels found in the breeding season. Thus, birds do not rely on the cues given by the photoperiod alone, but react independently of day-length to favourable environmental conditions, like water availability. Therefore, the individuals of this wild canary population can be considered as seasonal opportunistic breeders.

Breeding Birds as a Commodity

Understanding of the physiology, molecular genetics and developmental biology of avian reproduction is essential for advances in the efficient breeding of birds for commercial gain. 2. Reproduction is controlled at the central nervous level by gonadotrophin releasing hormone-I (GnRH-I) and prolactin. 3. The activity of GnRH-I neurones can be manipulated photoperiodically, through understanding of the physiology of photorefractoriness, to prevent premature onset, or to enhance persistency of reproductive function. 4. Active immunisation against prolactin releasing hormone, vasoactive intestinal peptide, delays the development of photorefractoriness and enhances persistency of reproductive function. 5. The creation of molecular maps of the chicken genome makes it possible to identify quantitative trait loci for repro- ductive traits, which will be used to identify DNA sequences for marker assisted selection. Polymorphisms in alleles of physiological candidate genes for reproductive efficiency might also be useful for marker assisted selection. 6. New understanding of the developmental biology of the chicken opens up the prospect of developing methods to control sex ratios, and to make transgenic birds. Transgenic technology will be used to introduce genes for disease resis- tance, and to make proteins for biomedical applications. 7. Nuclear transfer technology might be applied to save endangered species from extinction.

Testosterone accelerates the onset of photorefractoriness in baya weaver Ploceus philippinus.

The baya weaver Ploceus philippinus, despite its purely tropical origin and tropical/subtropical distribution, exhibits all features of a typically photoperiodic species. The onset of reproduction in this species is triggered by increasing daylength and breeding is terminated by the development of distinct photorefractory phase. In order to ascertain the involvement of androgens in the development of photorefractoriness the long day response of birds was tested after a prior exposure to varied doses of testosterone. Testosterone treatment accelerated the onset of photorefractoriness as judged from the gonadal status, LH-dependent yellow plumage and testosterone-dependent beak pigmentation. While gonadal development and yellow plumage occurred in the control birds due to the stimulatory long photoperiod, the testosterone administered birds failed to show such developments. These findings indicate that testosterone might impair the higher photoresponding mechanisms resulting in the development of photorefractoriness.

Role of the pituitary gland in the development of photorefractoriness and generation of long-term changes in prolactin secretion in rams.

Hypothalamo-pituitary disconnected Soay rams were exposed to two photoperiodic treatments: 1) constant long days (16L:8D) for 48 wk after pretreatment under short days (LD group), and 2) constant short days (8L:16D) for 48 wk after pretreatment under long days (SD group). In the LD group, plasma prolactin (PRL) concentrations increased from 0 to 8 wk (maximum: 143.3 +/- 8.4 microg/l; 8.8 +/- 1. 2 wk), decreased from 9 to 34 wk (minimum: 15.6 +/- 1.6 microg/l; 34. 5 +/- 1.5 wk), and finally increased again under the constant conditions, with a similar cyclical pattern for all individuals. In the SD group, PRL concentrations showed an inverse pattern (minimum: 8.6 +/- 2.6 microg/l; 17.1 +/- 2.0 wk; maximum: 46.4 +/- 5.5 microg/l; 30.2 +/- 3.2 wk), with more variability. Plasma concentrations of FSH were basal in both groups. The duration of the daily nocturnal melatonin peak (measured at 10, 24, and 44 wk) remained close to 8 h under long days (high-fidelity melatonin signal) but decreased significantly (13.8 h to 9.3 h) under short days (low-fidelity melatonin signal). The results support the conclusion that the melatonin signal encoding photoperiod acts within the pituitary gland to induce both acute (inductive) and chronic (refractory) effects photoperiod on PRL secretion.

Changes in Brain Gonadotropin-Releasing Hormone- and Vasoactive Intestinal Polypeptide-like Immunoreactivity Accompanying Reestablishment of Photosensitivity in Male Dark-Eyed Juncos (Junco hyemalis)

In seasonally breeding, photoperiodic birds, the development of photorefractoriness is associated with decreased brain expression of gonadotropin-releasing hormone-like immunoreactivity (GnRH-li ir) and increased expression of vasoactive intestinal polypeptide-like immunoreactivity (VIP-li ir). Dissipation of photorefractoriness and reestablishment of photosensitivity are associated with increased GnRH-li ir brain production, but concurrent changes in VIP-li ir expression have not been investigated. To address this question, we compared the expression of VIP-li ir in the infundibulum (INF) of adult male dark-eyed juncos (Junco hyemalis) that were made photorefractory (PR) by prolonged exposure to long days with that of birds that were not photostimulated (PS), but had regained photosensitivity by exposure to short days for 5 (short-term-PS, ST-PS) or 13 (long-term-PS, LT-PS) consecutive months. Photosensitive males had smaller INF VIP-li ir cell bodies than PR males, but the numbers of INF VIP-li ir cells were independent of photoperiodic condition. Changes in infundibular VIP-li ir were correlated with changes in preoptic area (POA) GnRH-li expression. Specifically, photosensitive males had more and larger POA GnRH-li ir cells and more GnRH-li ir fibers in this region than PR males. Further, LT-PS males had more GnRH-li ir POA fibers and larger testes than ST-PS juncos. Thus, induction of photorefractoriness is associated with increased VIP and decreased GnRH brain expression whereas dissipation of photorefractoriness concurs with decreased VIP and increased GnRH brain expression. These results suggest a physiological role for VIP in the control of changes in GnRH expression as a function of the photosensitive condition.

Thyroidectomy of House Sparrows (Passer domesticus) Prevents Photo-Induced Testicular Growth but Not the Increased Hypothalamic Gonadotrophin-Releasing Hormone

Thyroidectomy of starlings (Sturnus vulgaris) prevents the decrease in hypothalamic gonadotrophin-releasing hormone (GnRH) which normally occurs at the onset of photorefractoriness. To extend this observation to another species, changes in testicular mass, bill colour, moult, and hypothalamic GnRH content were monitored in photostimulated and nonphotostimulated intact and thyroidectomized house sparrows (Passer domesticus). Photostimulated intact birds rapidly increased testicular mass and GnRH. Later, testicular mass and GnRH decreased, and birds moulted, as they became photorefractory. Nonphotostimulated intact birds showed an increase in testicular mass and GnRH. Neither photostimulated nor nonphotostimulated thyroidectomized birds showed a marked increase in testicular mass, but both showed an increase in GnRH. Photostimulated thyroidectomized birds showed no subsequent decrease in GnRH and they did not moult. It is suggested that in this species, thyroidectomy inhibits the release of GnRH as well as preventing the downregulation of GnRH synthesis normally associated with the development of photorefractoriness.

The role of prolactin in the development of reproductive photorefractoriness and postnuptial molt in the European starling (Sturnus vulgaris).

Seasonal breeding in many birds, including the European starling, is terminated by the development of absolute reproductive photorefractoriness, followed by a postnuptial molt, when photo-induced PRL secretion is at its seasonal maximum. To determine whether this photo-induced increase in PRL secretion has a causal role in the development of photorefractoriness or molt, European starlings were actively immunized against vasoactive intestinal polypeptide (VIP), the PRL releasing hormone in birds, or against PRL, during a photo-induced breeding cycle. In half of the VIP-immunized birds, the photo-induced increase in PRL was completely suppressed. Although these birds became photorefractory, the rate of gonadal regression was markedly slowed. These birds did not molt. In the remaining VIP-immunized birds, the photo-induced increase in PRL was inhibited but not completely suppressed. In these birds, and in those immunized against PRL, gonadal regression was also slowed, but molt progressed as normal. There were no significant differences in concentrations of plasma thyroxine between treatment and control groups, indicating that the effects of immunization on gonadal regression were not mediated by the induction of hypothyroidism. These results are consistent with the view that in the European starling the seasonal photo-induced increase in PRL accelerates gonadal regression during the onset of photorefractoriness but does not itself cause photorefractoriness. Further, the seasonal increase in PRL is required for the induction of the postnuptial molt.

A test for photorefractoriness in high‐producing stocks of laying pullets

1. Pullets of 2 high-producing commercial stocks (both brown-egg layers) were exposed to 5 different lighting patterns between 18 and 72 weeks to test the hypothesis that photoperiods used in commercial lighting programmes early in the laying year may be unnecessarily long and, by accelerating the development of photorefractoriness, may contribute to the decline in egg production observed after the initial peak. Two rooms of 288 pullets were allocated to each treatment. 2. The rate of lay observed with a Step-Up treatment which gave increases in photoperiod from 8L:16D at 18 weeks to 15L:9D at 27 weeks of age was not significantly different from that of treatments which held the birds on 11L:13D during peak egg production but gave increments up to 15L:9D later in the laying year. 3. A control group maintained on 11L:13D from 20 to 72 weeks laid 295 eggs per bird housed and a further group held on 8L:16D from 0 to 72 weeks laid 284 eggs per bird. These yields were lower than the Step-Up treatment (299 eggs) but show the potential of modern hybrid stocks to lay prolifically even without light stimulation. 4. It is concluded that the stocks tested in this experiment showed no advantage when given lighting programmes in the first laying year which were designed to minimise the adverse effects of photorefractoriness.

Thyroidectomy progressively renders the reproductive system of starlings (Sturnus vulgaris) unresponsive to changes in daylength.

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.

Egg-Laying and Photorefractoriness in the European Kestrel Falco tinnunculus

Breeding and moult cycles were investigated in pairs of European Kestrels Falco tinnunculus exposed to four different photoperiods (LD regimes of 17.5:6.5, 15:9, 13:11, and 8:16). Laying, incubation and feeding of the young occurred in all four photoperiods, with all birds moulting after the breeding season. The time until the first egg, the duration of the period during which first clutches were laid, and the time until birds began to moult, were all negatively correlated with daylength. Two groups of birds (LD 13:11 and 8:16) were held under constant conditions for 4.5 and 2 yr, respectively. After the first breeding/moult cycle, the LD 13:11 group did not begin a second cycle for the next 19 months, and only started laying again, and subsequently moulting, after exposure to an 8 h day for 6 weeks. The LD 8:16 group went through two breeding/moult cycles during the two years. It is concluded that the development and expression of photorefractoriness is related to daylength, and that photorefractory Kestrels need short days to become photosensitive again. The possibility that the development of photorefractoriness is involved in the seasonal decrease in clutch size, found in both free-living and captive breeding Kestrels, is discussed.

The development of photorefractoriness in termination of the breeding season in the tropical brahminy myna: role of photoperiod

Gonads of brahminy myna (Sturnus pagodarum) spontaneously regress in July/August when the daylength is still stimulatory. Experiments were conducted to investigate if photoperiod was involved in the timing of gonadal regression and if photorefractoriness terminated the breeding season in this species. The observations obtained in the present study clearly show that: i) increasing photoperiods of spring/summer programmed for eventual gonadal regression in the late summer; ii) the birds developed photorefractoriness to all stimulatory daylengths and consequently the breeding season could not be extended by providing more stimulatory photoperiods; and iii) exposure to short daylength treatment failed to overcome the onset of refractoriness in birds after they had attained full gonadal growth and development. These results suggest that refractoriness is a process used by the brahminy myna to terminate the breeding season, and that this species becomes totally photorefractory.

Extraocular control of seasonal reproduction in female tree sparrows ( Spizella arborea)

The work reported here explored extraocular control of seasonal reproduction in a photoperiodic species. Photosensitive female tree sparrows ( Spizella arborea) were subjected to bilateral or unilateral ocular enucleation or to sham operation and exposed to a photoregimen designed to stimulate luteinizing hormone (LH) secretion, to induce photorefractoriness, and to restore photosensitivity. As revealed by plasma LH profiles constructed from measurements taken over 181 days, neither bilateral nor unilateral ocular enucleation had a substantive effect on photoinduced LH release, on the development of photorefractoriness, or on the recovery of photosensitivity. In a related experiment, photorefractory sighted females were implanted epicranially with miniature self-powered lights (SPLs) or with nonemitting helium blanks (HBs) and transferred to short days. After 8 weeks, SPLs and HBs were removed, and birds were challenged with long days. As indicated by plasma LH and ovarian responses to the challenge, previously HB-implanted birds had regained photosensitivity, but previously SPL-implanted birds remained photorefractory. When interpreted within the context of the effects of blinding, these findings suggest that an extraocular encephalic mechanism maintains photorefractoriness. Collectively, the data support the hypothesis that photoinduced LH release and the biannual transitions between photosensitivity and photorefractoriness are controlled by an extraocular mechanism(s).

Turkey Plasma Thyroid Hormone and Prolactin Concentrations Throughout an Egg Laying Cycle and in Relation to Photorefractoriness ,

Plasma concentrations of thyroxine (T4), triiodothyronine (T3), and prolactin were determined weekly through an egg laying cycle induced with 30 wk of 14 h light: 10 h dark. Egg production, primary remige molting, and occurrence of photorefractoriness were monitored. Hens were assigned to one of two groups; those that exhibited photorefractoriness (PR) and those that remained photosensitive (PS).Weekly plasma T4 concentrations relative to the time of initiation of photostimulation did not differ between PR and PS groups; concentrations peaked prior to the onset of egg production, were lowest during peak egg production, and increased between Weeks 6 and 30 of photostimulation. Plasma T4 levels of PR hens peaked 3 wk prior to the onset of photorefractoriness. Plasma T3 concentrations of PS and PR hens were similar until increases occurred in PR hens at the onset of photorefractoriness. Plasma prolactin levels in PS and PR hens increased after photostimulation, declined sooner in PR hens, and were generally greater in PS than PR hens during the last 20 wk of the study.Mean time of onset of photorefractoriness in PR hens was 23.0 wk (range = 18 to 28 wk) after initiation of photostimulation. Egg production of PS hens was greater than that of PR hens after 20 wk of photostimulation, and PR hens molted a greater number of primary remiges than PS hens. Results indicated that 1) elevated plasma T4 may be involved in development of photorefractoriness in turkey hens, 2) increases in plasma T3 may be involved in gonadal regression and molting, and 3) elevated plasma prolactin was associated with persistence of photosensitivity.