The influence of light wavelength on reproductive photorefractoriness in migratory blackheaded bunting (Emberiza melanocephala).

There are two effects of long day length on reproductive responses in birds, one is the photoinduction of gonadal growth and maturation and the other is the induction of gonadal regression and photorefractoriness. Although it is likely that the same photoreceptors are involved in the photoinduction of gonadal growth and the onset and maintenance of photorefractoriness. and so the influence of wavelength should be similar, this has not been investigated. Therefore, we investigated the influence of light wavelength on reproductive photorefractoriness in the migratory male blackheaded bunting held under long photoperiods. In mid May, when photoperiod was approximately 14L:10D (14 hours light:10 hours darkness), eight groups of sexually mature birds were moved indoors on an artificial photoperiod of 14L:10D (L - 450 lux. D - 0 lux). Then after 3 weeks, for six groups, a 4-h light period in the morning (zt 0-4; zt 0 [zeitgeber time 0] refers to the beginning of lights-on period) or in the evening (zt 10-14) was substituted with green (428 nm), red (654 nm) or white light at 16 +/- 2 lux intensity. Of the remaining two groups, one was maintained on 14L: 10D and the other transferred to 10L:14D: these served as controls. At the end of 4 weeks, all birds were found to have undergone testicular regression, irrespective of LD cycle they were exposed to. When these gonadally regressed birds were subjected to 16L:8D for another 4 weeks, to test their responsiveness to the stimulatory effects of long day lengths, only those exposed to 10L:14D and 14L:10D with a 4-h green light period showed testicular regrowth. On the other hand, birds exposed to 14L:10D with a 4-h white or red light period remained fully regressed, similar to 14L:10D controls. Except for some individual difference, there was no difference in response between the groups that received a 4-h light period in the morning and that received it in the evening. These results suggest that the wavelengths of light influence induction of buntings from the photosensitive state into the photorefractory state. Whereas the short light wavelengths facilitated recovery from the photorefractoriness, the long light wavelengths were more effective in maintaining the photorefractoriness.     

Thyroxine treatment prevents the development of photosensitivity in european starlings (Sturnus vulgaris)

Summary In starlings, the breeding season is terminated by a state of photorefractoriness. Birds remain completely reproductively inactive as long as long days are maintained, and only exposure to short days restores photosensitivity. Two experiments investigated the role of different doses of thyroxine in the development of photosensitivity in castrated starlings. First, photorefractory castrated male starlings were moved from long (18L:6D) to short (8L:16D) days, and received in the drinking water either 1 or 10 mg · 1^-1 thyroxine for the first 7 weeks of a 14-week observation period. Control birds regained photosensitivity after 5 weeks of short days, as signaled by a spontaneous increase in plasma LH, whereas the return to photosensitivity was delayed until weeks 7 and 9 in the 1- and 10-mg · 1^-1 thyroxine-treated birds, respectively. In the second experiment, the effect of different doses of thyroxine was explored at the level of the hypothalamic Gn-RH neurosecretory neurones. The acquisition of photosensitivity in control birds transferred from long to short days was characterized by a marked increase in hypothalamic Gn-RH content (while long-day controls maintained low Gn-RH content). Doses of 10 and 20 mg · 1^-1 of thyroxine completely prevented the return to photosensitivity, as seen through changes in either plasma LH concentrations or hypothalamic Gn-RH content, while a dose of 1 mg · 1^-1 allowed a partial recovery of photosensitivity, as hypothalamic Gn-RH content increased to an intermediate level and the spontaneous rise in plasma LH occurred slowly but steadily.Abbreviations Gn-RH gonadotrophin-releasing hormone - LH luteinizing hormone - LHRH-I luteinizing hormone releasing hormone     

Sexual maturation and moult in juvenile Starlings Sturnus vulgaris in response to different daylengths

Starlings, like most other species, show no gonadal development until spring of the year after they hatch, even though they hatch and attain full body size during long days. This could be because they develop in a physiological state analogous to that of photorefractory adults and so need to experience short days in order to activate the reproductive system. To test this possibility, young were hand-reared under different photoperiodic regimes. Young raised under constant long days showed no gonadal development, nor did birds initially raised under short days and then transferred to long days at 3 weeks of age. However, birds transferred from short to long days at 10 weeks of age did show gonadal development, followed by gonadal regression, while birds raised under constant short days showed slow continual gonadal development. This last group, unlike the other three groups, did not moult into adult plumage. Since 4 weeks of long days are required to terminate photorefractoriness in adult Starlings, these results demonstrate that the reproductive system of young birds is in a similar state to that of photorefractory adults, and hence that puberty is analogous to the termination of photofractoriness.     

Hypothalamic expression of thyroid hormone-activating and -inactivating enzyme genes in relation to photorefractoriness in birds and mammals

Photorefractoriness is the insensitivity of gonadal development to the stimulatory effects of long photoperiods in birds and to the inhibitory effects of short photoperiods in small mammals. Its molecular mechanism remains unknown. Recently, it has been shown that reciprocal expression of thyroid hormone-activating enzyme [type 2 deiodinase (Dio2)] and -inactivating enzyme [type 3 deiodinase (Dio3)] genes in the mediobasal hypothalamus is critical for photoperiodically induced gonadal growth. Since thyroid hormones are required not only for photoinduction, but also for the induction of photorefractoriness, we examined the expression of these genes in relation to photorefractoriness in birds and mammals. Transfer of birds to long photoperiods induced strong expression of Dio2. This was maintained in tree sparrow when they later became photorefractory, but decreased somewhat in quail. In hamsters, transfer to long photoperiods also induced strong expression of Dio2. High values were not maintained under long photoperiods, and, indeed, expression decreased at the same rate as in animals transferred to short photoperiods. There was no renewed expression of Dio2 associated with testicular growth as animals became refractory to short photoperiods. Expression of Dio3 was high under short photoperiods and low under long photoperiods in all the animals examined, except for the short photoperiod-refractory hamsters. Our present study revealed complex regulation of deiodinase genes in the photoinduction and photorefractory processes in birds and mammals. These gene changes may be involved in the regulation of photorefractoriness, as well as photoinduction.     

The thyroid and photoperiodic control of seasonal reproduction in American tree sparrows (Spizella arborea)

To explore the role of the thyroid gland in the control of seasonal reproduction in obligately photoperiodic American tree sparrows (Spizella arborea), the effects of (1) thyroxine administered in drinking water to thyroid-intact photosensitive or photorefractory birds, and (2) radiothyroidectomy before and after photostimulation and during photorefractoriness were examined. Chronic administration of pharmacological doses of thyroxine induced testicular growth and usually regression in initially photosensitive birds held on short or intermediate daylengths. Some thyroxine-treated birds with regressed testes were absolutely photorefractory, but most remained photosensitive. Exogenous thyroxine never induced testicular growth in photorefractory birds moved to short days, though it often impeded, and sometimes even blocked, the recovery of photosensitivity. Although circumstantial, these effects of exogenous thyroxine are consistent with an hypothesis that assigns to thyroid hormones two roles — one stimulatory and the other inhibitory — in the control of seasonal reproduction. Radiothyroidectomy before photostimulation inhibited (but did not prevent) photoinduced testicular growth, blocked spontaneous testicular regression, suppressed molt, and prevented photorefractoriness. Moreover, as demonstrated by testicular growth after thyroxine replacemnt therapy, radiothyroidectomy during photorefractoriness later restored photosensitivity despite continued photostimulation. Thus, euthyroidism is an essential condition for maximizing (but not for initiating) photoinduced testicular growth and for triggering and maintaining photorefractoriness in photostimulated tree sparrows. However, when performed early during photostimulation, radiothyroidectomy neither immediately induced nor later blocked spontaneous testicular regression. Thus, endogenous thyroid hormones and long days may interact during a critical period to program a sequence of physiological events that plays out as photorefractoriness in chronically photostimulated birds. Such an organizational event cannot be permanent, for seasonal reproduction is episodic and its control mechanism necessarily cyclic. Because thyroidectomy simulated the well-known restorative effect of short days (and exogenous thyroxine impeded it), short days may dissipate photorefractoriness by creating a milieu wherein thyroid hormones are deficient or inactive.Abbreviations ANOVA analysis of variance - bTSH bovine thyroid stimulating hormone - GnRH gonadotropin-releasing hormone - LH luteinizing hormone - nL: nD daily light: dark regime (n is duration in hours) - SEM standard error of the mean - SNK Student-Newman-Keuls test - T₄ thyroxine - TH thyroid hormone - TR thyroid hormone receptor     

Photoperiodic control of the termination of breeding and the induction ofmoult in House Sparrows Passer domesticus

Two photoperiodic mechanisms controlling gonadal regression in birds have been identified: absolute photorefractoriness, typical of species with short breeding seasons, where gonadal regression occurs spontaneously during long days, and relative photorefractoriness, where a decrease in daylength is required to induce regression. An experiment was designed to test whether these simply represent extremes of one underlying mechanism. Three groups of male House Sparrows Passer domesticus were transferred from a short photoperiod, 8 h of light: 16 h of darkness per day (8L:16D) to long photoperiods of either 18L:6D, 16L:8D or 13L:11D. Gonadal maturation rates were similar in all three groups; gonadal regression and moult began latest in the 13L:11D group. Four additional groups of sparrows were transferred from 8L:16D to 18L:6D and then transferred to either 13L: 11D or 16L:8D prior to, or shortly after, the onset of gonadal regression. The decrease in daylength prior to regression had no effect on the timing of regression but did advance the onset of moult. Decrease in daylength after the onset of regression increased the rate of regression and the rate of moult. Because a decrease in daylength did not affect the timing of regression, the data do not support the hypothesis that absolute and relative photorefractoriness represent extremes of a single underlying photoperiodic control mechanism. The adaptive significance of the effects of decreasing daylength on the rate of regression and moult 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.     

Effect of daylength on the rate of recovery of photosensitivity in male starlings (Sturnus vulgaris).

Four groups of castrated photorefractory starlings were transferred from a photoperiod of 18 h light/day (18 L) to photoperiods of 6 L, 8 L, 11 L or 12 L. A control group was kept on 18 L. Plasma concentrations of luteinizing hormone (LH) were low in all groups initially. The first significant increase in LH, which signals the recovery of photosensitivity, occurred after 4 weeks in the groups on 6 L and 8 L, after 8 weeks in the 11 L group and after 13 weeks in the 12 L group. There was no increase in the group on 18 L. The rate of recovery of photosensitivity is therefore inversely proportional to daylength, for daylengths between 8 L and 12 L.     

Extraocular control of photorefractoriness in American tree sparrows (Spizella arborea)

Photorefractoriness, a reversible state of unresponsiveness to daylengths of gonadostimulatory duration, terminates seasonal breeding in many photoperiodic species of birds. Whether the eyes are components of the mechanism that triggers photorefractoriness is an important, but heretofore unresolved, question. Although a role for extraocular photoreception in the mechanism of photoinduced gonadal growth is well documented, the eyes may be important in the mechanism of photorefractoriness if, as some evidence suggests, they are gonadoinhibitory. With American tree sparrows (Spizella arborea), I here confirm that the absence of eyes does not impede photoinduced testicular growth and establish that an extraocular mechanism mediates the transition from photosensitivity to photorefractoriness: Tree sparrows blinded by bilateral ocular enucleation, when photosensitivity to long days or by miniature self-powered lights implanted atop the skull, showed marked testicular growth and then, as evidenced by spontaneous testicular regression, became photorefractory, as did sighted controls.     

Response to experimental photoperiods by a migratory bunting, Emberiza melanocephala

Little is known about the effects of photoperiod on avian migrants that visit southeast Asia. In this paper, we report experiments performed on an emberizid finch, the Black-headed Bunting Emberiza melanocephala , to investigate its photoperiodic responses under artificial photoperiods, and continuous light and darkness.
Two series of experiments were performed with the photosensitive male birds. In the first series, different groups were exposed to seven different artificial photoperiods: 3L/21D, 6L/18D, 8L./16D, 11L/13D, 12L/12D, 15L/9D and 20L/4D, for 30 days. They were weighed and laparotomized at the beginning and end of the experiments. The birds responded to 12L/12D, 15L/9D and 20L/4D, but not to 3L/21D, 6L/18D, 8L/16D and 11L/13D. In the second series, photosensitive birds were placed under continuous light (LL) and dark (DD) conditions for 130 and 90 days. Periodic observations indicated that testicular growth and fattening followed by involution and fat-depletion had resulted in birds under LL, indicating the onset of photorefractoriness, while DD had no effect either on gonads or fattening in the buntings.
Our results demonstrate that light stimulation is a prerequisite to reproductive and metabolic activities (pre-migratory and migratory changes, fattening and weight gain) in the Black-headed Bunting, which has a photoperiodic threshold to these events at between 11 and 12 h daily photoperiods.     

Photoperiodic control of testicular regression and moult in male House Sparrows Passer domesticus

Testis size, bill colour and moult were monitored in male House Sparrows Passer domesticus kept under a natural daylength regime between February and November. On three occasions (at the summer solstice, 25 days later and 39 days later), groups of birds were transferred to a daylength of 18 h of light and 6 h of darkness per day (18L: 6D), the natural daylength at the solstice. In birds under natural daylengths, the testes had regressed significantly by 2 5 days after the solstice. In those transferred to 18L:6D at the solstice, the onset of regression was delayed by about 4 weeks. Transfer to 18L: 6D after the solstice did not cause recrudescence; the testes continued to regress. In birds transferred to 18L: 6D at the solstice, moult was delayed by 4 weeks and progressed more slowly. These results suggest that photoperiodically induced gonadal regression in this species contains elements characteristic of both absolute and relative photorefractoriness.     

Photoperiod and melatonin influence seasonal gonadal cycles in the grasshopper mouse (Onychomys leucogaster)

Development of the reproductive apparatus was delayed in grasshopper mice maintained from birth in short photoperiods (10 h light/day). The inhibitory effects of short photoperiods on sexual maturation eventually waned and mice in 10L:14D became reproductively active. Adult mice transferred from long (14 h light/day) to short photoperiods underwent testicular regression after 10 weeks and complete gonadal redevelopment after 30 weeks. A similar phenomenon was observed in adult female mice; oestrous cycles ceased within 3 weeks and resumed after 13 weeks in the short photoperiod. The regressive effects of short photoperiods on the male reproductive system were mimicked by daily injections of melatonin administered to mice housed in 14L:10D. Responsiveness of the female reproductive system to melatonin was reduced among photorefractory as compared to photosensitive mice. We suggest that the initial rate of sexual maturation and the timing of seasonal breeding in adult mice are regulated by photoperiod; effects of short daylengths on the neuroendocrine-reproductive axis appear to be mediated by the pineal gland.     

Lack of gonadotrophin-releasing hormone (GnRH) neuron response to decreasing photoperiod in thyroidectomized male starlings (Sturnus vulgaris)

Exposure of starlings to long days initially causes reproductive maturation, but eventually leads to photorefractoriness. During photorefractoriness, gonadotrophin-releasing hormone (GnRH) decreases in the GnRH cell bodies and fibers emanating from these to the median eminence, circulating gonadotrophin concentrations decrease to a minimum, and the gonads regress. Thyroidectomy profoundly affects these photoperiodic responses. In chronically thyroidectomized starlings, gonadal responses to changes in day length are attenuated. This investigation was conducted to determine whether, in the absence of gonadal responsiveness, the GnRH system of chronically thyroidectomized starlings responds to changes in day length. Two groups of thyroidectomized male starlings were transferred from short days (8L:16D) to long days (18L:6D) for four weeks, and testicular volume increased. One group was kept on long days (TxLD) and the other was returned to short days (TxSD). Testicular volume did not decrease in the TxSD group. The GnRH neurons of the two thyroidectomized groups were compared to those of two groups of intact starlings, one of them on long days and photorefractory (ILD), the other on short days and photosensitive (ISD). Group ILD had lower numbers of GnRH-stained cells than groups TxLD, TxSD and ISD, which did not differ in this respect. Similar differences were observed for GnRH cell size in the pre-optic area (POA) and for density of staining of GnRH fibers in the median eminence. The results confirm that thyroidectomy attenuates gonadal responses to change in day length and suggest that this results from an effect upon the central nervous system rather than a peripheral effect.     

Effect of intranasal irrigation with zinc sulphate on male-induced oestrus in the wild mouse, Mus musculus domesticus

Adult males of the subtropical house sparrow (Passer domesticus) were subjected to continuous darkness (DD) or short days (8L:16D) after photostimulation (15L:9D) for 30 days. One group was retained on the long day (15L:9D) schedule. Another two groups of photosensitive birds were placed under DD or 8L:16D. The photostimulated birds under DD or 8L:16D showed testicular regression and the rate of regression was similar in both the treatments. The photosensitive birds, on the other hand, did not show any initiation in testicular growth and development in either of the treatments. Long day birds (15L:9D) maintained enlarged testes without regression (the onset of photorefractoriness) during the treatment period. These results conform with the external coincidence model, but could also be explained by a modified internal coincidence model.     

Timing of photorefractoriness in the European starling: significance of photoperiod early and late in the reproductive cycle

In European starlings, as in many other birds inhabiting higher latitudes, gonads develop in response to the increasing daylengths in early spring. Later in the year, however, the hypothalamo-pituitary-gonadal axis becomes refractory to the previously stimulatory long photoperiods and the gonads regress in summer. The present study addresses the question of when during the gonadal growth phase photorefractoriness is determined. A 13-h photoperiod induces testicular development and subsequent testicular regression associated with refractoriness in male starlings. An 11-h photoperiod, in contrast, induces only testicular development, and photorefractoriness never develops. When starlings were transferred to an 11-h photoperiod, either 12 or 25 days following exposure to a 13-h photoperiod, their testes developed to full size, but remained large to the end of the experiment, i.e. refractoriness did not develop. The same was even true of most birds in a third group that were transferred to an 11-h photoperiod after 46 days of the 13-h photoperiod, when gonads had developed to near maximal size. These data show that, in contrast to some other species of passerine birds, the onset of photorefractoriness does not become fixed before the testes have undergone considerable development, and that the photoperiodic conditions experienced at the end of the testicular growth phase are still effective in determining the precise time of onset of photorefractoriness. It is suggested that this peculiarity of the starling is related to the fact that its gonadal development begins rather early in spring and, hence, under much shorter photoperiods than the other species studied.     

Short days induce premature reproductive maturation in juvenile starlings, Sturnus vulgaris

Castration of juvenile and photorefractory adult starlings caused no immediate increase in circulating concentrations of LH. In castrated juveniles and adults exposed to natural changes in daylength, plasma LH increased between mid-October and mid-November, although the increase was more rapid in adults. In castrated photorefractory adults, plasma LH increased 3-5 weeks after transfer to artificial short days (8L:16D). In castrated juvenile starlings plasma LH increased 4-6 weeks after transfer to 8L:16D, irrespective of the age of the birds. Birds as young as 17 weeks had high LH concentrations. These results suggest that the reproductive system of juvenile starlings is in the same state as that of photorefractory adults, and therefore the activation of the hypothalamo-pituitary axis for the first time in juveniles is analagous to the termination of photorefractoriness in adults.     

The LH-RH system of the male European starling: photoperiod induces changes to a possible multifunctional peptide system

In many birds reproduction is triggered by long daylengths but, paradoxically, continued exposure to long days leads to photorefractoriness and a complete shut down of the reproductive system. As these effects are thought to be mediated through the secretion of LH-RH, immunocytochemical techniques were used to investigate changes in the LH-RH system when European starlings were exposed to different photoperiods. Starlings exposed to 11L:13D and with mature testes show strong immunostaining both of LH-RH perikarya and fibers. Photosensitive short-day (8L:16D) starlings with undeveloped testes show an almost identical distribution of strongly immunoreactive perikarya but with less dense fibre staining. However, long-day (18L:6D) photorefractory starlings with fully regressed testes, show a profound reduction in LH-RH immunostaining. Perikarya have the same distribution but show a much reduced intensity of staining and fibers had almost entirely disappeared from all regions of the brain. Preliminary observations on the ultrastructure of immunocytochemically identified LH-RH neurones are also reported.     

Photorefractoriness in European starlings: associated hypothalamic changes and the involvement of thyroid hormones and prolactin

Responsivity to photostimulation in previously photorefractory European starlings is caused by subjection to short daylengths and is characterized by a marked activation of the hypothalamus in terms of synthesis of gonadotropin releasing hormone. This active hypothalamic state is amplified for a time by a subsequent exposure to long days but is soon completely reversed as the birds become photorefractory again. This latter effect of long photoperiods and the concurrent secretion of prolactin are dependent on the presence of thyroid hormones. Conceivably, prolactin causes photorefractoriness by inhibition at a hypothalamic level.     

Testosterone induction of song in photosensitive and photorefractory male sparrows

Song in male songbirds is activated by the sex steroid testosterone (T). Using male song sparrows (Melospiza melodia), we compared effects of T in the normal spring state of photosensitivity (i.e., when the pituitary-gonadal axis is sensitive to stimulation by increasing daylength) and in the late summer-early fall state of photorefractoriness (i.e., when they are insensitive to increasing daylength). Photosensitive males experienced short days for 8 weeks and then long days for another 22 weeks to induce photorefractoriness. T implants were given to the birds twice, first when on short days and photosensitive, and second when on long days and photorefractory. Song rates were compared among 5 conditions: (1) photosensitive, short days, low T titers; (2) photosensitive, short days, high T titers; (3) photosensitive, long days, high T titers; (4) photorefractory, long days, low T titers; and (5) photorefractory, long days, high T titers. Plasma levels of T were monitored throughout the experiment by radioimmunoassay. T was equally effective in inducing song in both the photosensitive and photorefractory conditions. Thus, no seasonal change was found in the sensitivity to hormone action of the neural target sites mediating this behavior in song sparrows. Photosensitive birds sang at a higher rate when on long days than when on short days, however, even though there was no concomitant increase in plasma levels of T. This finding suggests that environmental factors can alter the expression of song activated by similar levels of T.     

Photorefractoriness and Its Termination in the Subtropical House Sparrow, Passer Domesticus: Involvement of Circadian Rhythm

Groups of photorefractory female subtropical house sparrows, Passer domestkus, when treated with 6 weeks of a short photocycle (8L : 16D) showed significant ovarian growth on their return to a long photocycle (15L :9D). A 6-hr photophase coupled with scotophase of varying durations does not terminate the refractory period under photoperiod cycles of 12 (6L : 6D), 36 (6L :30D) and 60 (6L : S4D) hr but the refractory period is terminated by light-dark cycles of 24 (6L: 18D), 48 (6L :42D) and 72 (6L : 66D) hr. These results are consistent with the Biinning hypothesis of coincidence between endogenous photosensitive rhythmicity and environmental photoperiod timing that an endogenous circadian rhythm is involved in the maintenance and termination of photorefractoriness.