Photoperiodic Gonadal Response Research Articles

Temperature alters the hypothalamic transcription of photoperiod responsive genes in induction of seasonal response in migratory redheaded buntings

We investigated the temperature effects on hypothalamic transcription of genes involved in the induction of photoperiodic response in redheaded buntings. Birds were exposed at 22 and 38 °C to 13-h long photoperiods (LP), with controls at 22 °C on 8-h short photoperiods (SP). At 22 °C, compared to SP, we found higher tshb, eya3 and dio2 and low dio3 and gnih mRNA expressions after a week of LP; concomitant with testis recrudescence this confirmed buntings’ responsiveness to LP-induced photostimulation. tshb, dio2 and gnrh mRNA levels were further increased by 2.5 weeks of LP at 38 °C. Temperature sensitive trpm8, but not trpv4, bdnf or adcyap1 also showed LP-induced expression at 22 °C. Concomitant changes in dnmt3b and tet2 mRNA expressions further suggested epigenetic modification of temperature influence on photoperiodic responses. These results demonstrate the role of temperature in hypothalamic molecular regulation of the photoperiodic gonadal response in seasonally breeding birds.

Daily, circadian and seasonal changes of rhodopsin-like encephalic photoreceptor and its involvement in mediating photoperiodic responses of Gambel’s white-crowned Sparrow, Zonotrichia leucophrys gambelii

Extra-retinal, non-pineal, encephalic photoreceptors (EP) play important roles in mediating development of the reproductive system by the annual change in day length (photoperiodic gonadal response – PGR) in birds. However, the distribution of rhodopsin-like EPs and their functional daily, circadian and seasonal changes are still unclear in the avian brain. This study identifies two novel groups of rhodopsin-immunoreactive cells in the nucleus paraventricularis magnocellularis (PVN) of the hypothalamus and in the medial basal hypothalamus (MBH) in a seasonally breeding species, Gambel’s white-crowned sparrow (Zonotrichia leucophrys gambelii). In the PVN, rhodopsin-ir cell number showed both daily and circadian changes with more labeled cells apparent in the night phase in photosensitive birds, while only circadian changes were observed involving fewer labeled cells in the night phase in photorefractory birds. Single long day photo-stimulation significantly decreased the rhodopsin-ir cell number only in photosensitive birds, coincident with a rise in plasma levels of luteinizing hormone (LH). In the MBH, rhodopsin-ir cell number did not show daily, circadian or single long day induced changes in either photoperiodic states. But, overall these rhodopsin expressing neurons significantly increased from photosensitive to photorefractory states. In the median eminence (ME), more intense rhodopsin-ir was detected in photorefractory birds compared to photosensitive birds. For expression of GnRH and vasoactive intestinal polypeptide (VIP), seasonal differences were found with opposite relationships, consistent with previous studies. Our results suggest different roles of the two groups of rhodopsin-like EPs in the regulation of PGR in white-crowned sparrows.

Interactive effect of light colours and temporal synergism of circadian neural oscillations in reproductive regulation of Japanese quail

Avian literature reports the modulation of ‘photoperiodic gonadal responses’ by the temporal phase relation of serotonergic and dopaminergic oscillations in Japanese quail. But, the modulation of ‘light colour responses’ by the temporal synergism of neural oscillations is not yet known. Hence the present study was designed to investigate the interaction of the light colour (blue, red) and the phase relation of neural oscillations in the reproductive regulation of Japanese quail. Three week old male Japanese quail were divided into two groups and maintained under a long day length condition (16L:8D) and were exposed to a 30 lux intensity of blue LED (light emitting diode) (B LED) and a red LED light (R LED). At the age of 15.5weeks, quail of one subgroup of B LED were injected with serotonin precursor (5-HTP) and dopamine precursor (l-DOPA) 12hrs apart (B LED+12-hr) and those of the R LED group were injected with the same drugs (5mg/100g body weight over a period of thirteen days) but 8hrs apart (R LED+8-hr). The remaining subgroups of both the light colour groups (B LED & R LED) received normal saline twice daily and served as controls. Cloacal gland volume was recorded weekly until 35.5weeks of age when the study was terminated and reproductive parameters (testicular volume, GSI, seminiferous tubule diameter and plasma testosterone) were assessed.Results indicate that the 8-hr temporal phase relation of neural oscillations suppresses reproductive activity even during the photosensitive phase of the red light exposed quail (R LED+8-hr) compare to the R LED controls. On the other hand, the 12-hr temporal phase relation stimulates the gonadal development of the B LED+12-hr quail compared to the B LED controls which after completing one cycle entered into a regressive phase and remained sexually quiescent. These experiments suggest that the temporal phase relations of circadian neural oscillations, in addition to modulating the classical photoperiodic responses, may also modulate the gonadal responses to blue (suppressive) and red (stimulatory) light. These studies led us to conclude that the temporal phase relation of serotonergic and dopaminergic oscillations is not only an important regulator of avian reproduction but may also override the classical effects of light colours in Japanese quail.

Deep-brain photoreceptors (DBPs) involved in the photoperiodic gonadal response in an avian species, Gallus gallus

Three primitive photoreceptors [melanopsin (Opn4), neuropsin/opsin5 (Opn5) and vertebrate ancient opsin (VAOpn)] were reported as possible avian deep-brain photoreceptors (DBPs) involved in the perception of photoperiodic information affecting the onset and development of reproduction. The objective of this study was to determine the effect of long-day photostimulation and/or sulfamethazine treatment (SMZ, a compound known to advance light-induced testes development) on gene expression of DBPs and key hypothalamic and pituitary genes involved in avian reproductive function. Two-week old chicks were randomly selected into four experimental groups: short-day control (SC, LD8:16), short-day+SMZ (SS, LD8:16, 0.2% diet SMZ), long-day control (LC, LD16:8), and long-day+SMZ (LS, LD16:8, 0.2% diet SMZ). Birds were sampled on days 3, 7, and 28 after initiation of a long-day photoperiod and/or SMZ dietary treatments. Three brain regions [septal–preoptic, anterior hypothalamic (SepPre/Ant-Hypo) region, mid-hypothalamic (Mid-Hypo) region, posterior-hypothalamic (Post-Hypo) region], and anterior pituitary gland were dissected. Using quantitative real-time RT-PCR, we determined changes of expression levels of genes in distinct brain regions; Opn4 and Opn5 in SepPre/Ant-Hypo and Post-Hypo regions and, VAOpn in the Mid-Hypo region. Long-day treatment resulted in a significantly elevated testes weight on days 7 and 28 compared to controls, and SMZ augmented testes weight in both short- and long-day treatment after day 7 (P<0.05). Long-day photoperiodic treatment on the third day unexpectedly induced a large 8.4-fold increase of VAOpn expression in the Mid-Hypo region, a 15.4-fold increase of Opn4 and a 97.8-fold increase of Opn5 gene expression in the Post-Hypo region compared to SC birds (P<0.01). In contrast, on days 7 and 28, gene expression of the three DBPs was barely detectable. LC group showed a significant increase in GnRH-1 and TRH mRNA in the Mid-Hypo compared to SC on day 3. Pituitary LHβ and FSHβ mRNA were significantly elevated in LC and LS groups compared to SC on days 3 and 7 (P<0.05). On days 3 and 7, TSHβ mRNA level was significantly elevated by long-day treatment compared to the SC groups (P<0.05). Results suggest that long-day photoperiodic activation of DBPs is robust, transient, and temporally related with neuroendocrine genes involved in reproductive function. Additionally, results indicate that two subsets of GnRH-1 neurons exist based upon significantly different gene expression from long-day photostimulation and long-day plus SMZ administration. Taken together, the data indicate that within 3days of a long-day photoperiod, an eminent activation of all three types of DBPs might be involved in priming the neuroendocrine system to activate reproductive function in birds.

Universality and diversity in the signal transduction pathway that regulates seasonal reproduction in vertebrates.

Most vertebrates living outside the tropical zone show robust physiological responses in response to seasonal changes in photoperiod, such as seasonal reproduction, molt, and migration. The highly sophisticated photoperiodic mechanism in Japanese quail has been used to uncover the mechanism of seasonal reproduction. Molecular analysis of quail mediobasal hypothalamus (MBH) revealed that local thyroid hormone activation within the MBH plays a critical role in the photoperiodic response of gonads. This activation is accomplished by two gene switches: thyroid hormone-activating (DIO2) and thyroid hormone-inactivating enzymes (DIO3). Functional genomics studies have shown that long-day induced thyroid-stimulating hormone (TSH) in the pars tuberalis (PT) of the pituitary gland regulates DIO2/3 switching. In birds, light information received directly by deep brain photoreceptors regulates PT TSH. Recent studies demonstrated that Opsin 5-positive cerebrospinal fluid (CSF)-contacting neurons are deep brain photoreceptors that regulate avian seasonal reproduction. Although the involvement of TSH and DIO2/3 in seasonal reproduction has been confirmed in various mammals, the light input pathway that regulates PT TSH in mammals differs from that of birds. In mammals, the eye is the only photoreceptor organ and light information received by the eye is transmitted to the pineal gland through the circadian pacemaker, the suprachiasmatic nucleus. Nocturnal melatonin secretion from the pineal gland indicates the length of night and regulates the PT TSH. In fish, the regulatory machinery for seasonal reproduction, from light input to neuroendocrine output, has been recently demonstrated in the coronet cells of the saccus vasculosus (SV). The SV is unique to fish and coronet cells are CSF-contacting neurons. Here, we discuss the universality and diversity of signal transduction pathways that regulate vertebrate seasonal reproduction.

Photoperiod and circadian rhythmicity in testicular response of the tree sparrow

Investigation on the mechanism of photoperiodic time measurement in regulation of reproduction was done on the male tree sparrows (Passer montanus). Photosensitive sparrows were divided into three different groups (one control and two experimental). They were photostimulated by subjecting them to artificial long days, i.e. 14L/10D for 30 days. The control group was continued under 14L/10D while the experimental groups were transferred to either short days (9L/15D) or continuous darkness (DD) for another 30 days. Observations at 10-day intervals revealed that the birds of control group continued testicular growth. Gonadal growth did not persist, and regression was observed upon transfer of photostimulated birds of experimental groups to 9L/15D or DD. Further, there was no significant difference in the testicular volume of the birds under DD and 9L/15D. These results clearly suggest the involvement of an endogenous circadian rhythm in photoperiodic gonadal responses of the tree sparrow and are consistent with the Bunning hypothesis. They are also in agreement with the avian external coincident model of photoperiodic time measurement.

Immunocytochemical study of rhodopsin-containing putative encephalic photoreceptors in house sparrow, Passer domesticus

In seasonally breeding birds, encephalic photoreceptors (EPs) play an important role in regulating photoperiodic gonadal responses. Multiple photopigments have been suggested as the putative EPs, including rhodopsin, melanopsin, VA opsin and the cryptochromes. As for rhodopsin, two potential brain sites, the lateral septum (SL) and the infundibulum (INF) have been reported to co-express rhodopsin immunoreactivity (rhodopsin-ir) with vasoactive intestinal polypeptide immunoreactivity (VIP-ir) in groups of cerebrospinal fluid-contacting (CSF) cells, hypothesized to be the EPs for gonadal responses. In order to confirm the presence of rhodopsin in seasonally breeding birds and examine whether these EPs show daily change as do the photopigments in the retina and pineal gland, the present study immunocytochemically investigated: (1) the presence of rhodopsin expression in the deep brain of the house sparrow, Passer domesticus maintained in short days, and (2) rhythmic expression of rhodopsin and VIP in both SL and INF at Zeitgeber time (ZT) 1 and ZT 17 in house sparrows. Rhodopsin-ir and VIP-ir were observed in both areas of sparrow brains as previously described in other avian species but with a novel rod-like rhodopsin-ir cell type in the INF and novel expression of rhodopsin-ir fiber close to the preoptic area. Daily changes of rhodopsin-ir and VIP-ir cell number were revealed in the INF, but not in the SL. More rhodopsin-ir and fewer VIP-ir cells were found at ZT 17 than at ZT 1. In the median eminence, rhodopsin-ir fibers were only observed at ZT 1, and the relative optic density (ROD) of VIP-ir fibers was higher at ZT 1 than ZT 17. The results indicate daily changes of EPs in the IN and ME, suggesting a role of EPs in the orchestration of photoperiodic gonadal recrudesence.

Comparative analysis of the molecular basis of photoperiodic signal transduction in vertebrates

In temperate zones, the reproductive physiology of most vertebrates is controlled by changes in photoperiod. Mechanisms for the regulation of photoperiodic gonadal responses are known to differ between mammals and birds: in mammals, melatonin is the photoperiodic signal messenger, whereas in birds, photoperiodic information is received by deep brain photoreceptors. Recently, the molecular mechanism of photoperiodism has been revealed by studies on Japanese quail, which exhibit a most remarkable responsiveness to photoperiod among vertebrates, and molecular cascades involved in photoperiodism have been elucidated. Long-day stimulus induces expression of the β-subunit of thyroid stimulating hormone (TSH-β) in the pars tuberalis (PT) of the pituitary gland, and TSH derived from the PT regulates reciprocal switching of genes encoding types 2 and 3 deiodinases (Dio2 and Dio3, respectively) in the mediobasal hypothalamus (MBH) by retrograde action. Dio2 locally converts prohormone thyroxine (T(4)) to bioactive triiodothyronine (T(3)) in the MBH, which subsequently stimulates the gonadal axis. These events have been confirmed to occur in mammals with seasonal breeding, such as hamsters and sheep, suggesting that similar mechanisms are involved among various vertebrates. In addition, nonphotoperiodic mice also appeared to possess the same molecular mechanisms at the hypothalamo-hypophysial level. It has been noted that melatonin regulates the above-mentioned key genes (Dio2, Dio3, and TSH-β) in mammals, while photoperiod directly regulates these genes in birds. Thus, the input pathway of photoperiod is different between mammals and birds (i.e., melatonin versus light); however, the essential mechanisms are conserved among these vertebrates.

Melatonin Transmits Photoperiodic Signals through the MT1 Melatonin Receptor

Melatonin transmits photoperiodic signals that regulate reproduction. Two melatonin receptors (MT1 and MT2) have been cloned in mammals and additional melatonin binding sites suggested, but the receptor that mediates the effects of melatonin on the photoperiodic gonadal response has not yet been identified. We therefore investigated in mice whether and how targeted disruption of MT1, MT2, or both receptor types affects the expression level of two key genes for the photoperiodic gonadal regulation, type 2 and 3 deiodinase (Dio2 and Dio3, respectively). These are expressed in the ependymal cell layer lining the infundibular recess of the third ventricle and regulated by thyrotropin produced in the pars tuberalis. In wild-type C3H mice, Dio2 expression was constantly low, and no photoperiodic changes were observed, whereas Dio3 expression was upregulated under short-day conditions. In C3H with targeted disruption of MT1 and MT1/MT2, Dio2 expression was constitutively upregulated, Dio3 expression was constitutively downregulated, and the photoperiodic effect on Dio3 expression was abolished. Under short-day conditions, C3H with targeted disruption of MT2 displayed similar expression levels of Dio2 and Dio3 as wild-type animals, but they responded to long-day condition with a stronger suppression of Dio3 than wild-type mice. Melatonin injections into wild-type C57BL mice suppressed Dio2 expression and induced Dio3 expression under long-day conditions. These effects were abolished in C57BL mice with targeted disruption of MT1. All data suggest that the melatonin signal that transmits photoperiodic information to the hypothalamo-hypophysial axis acts on the MT1 receptor.

Rhythmic expression of clock genes in the ependymal cell layer of the third ventricle of rodents is independent of melatonin signaling

Reproductive physiology is regulated by the photoperiod in many mammals. Decoding of the photoperiod involves circadian clock mechanisms, although the molecular basis remains unclear. Recent studies have shown that the ependymal cell layer lining the infundibular recess of the third ventricle (EC) is a key structure for the photoperiodic gonadal response. The EC exhibits daylength-dependent changes in the expression of photoperiodic output genes, including the type 2 deiodinase gene (Dio2 ). Here we investigated whether clock genes (Per1 and Bmal1) and the albumin D-binding protein gene (Dbp) are expressed in the EC of Syrian hamsters, and whether their expression differs under long-day and short-day conditions. Expression of all three genes followed a diurnal rhythm; expression of Per1 and Dbp in the EC peaked around lights-off, and expression of Bmal1 peaked in the early light phase. The amplitude of Per1 and Dbp expression was higher in hamsters kept under long-day conditions than in those kept under short-day conditions. Notably, the expression of these genes was not modified by exogenous melatonin within 25 h after injection, whereas Dio2 expression was inhibited 19 h after injection. Targeted melatonin receptor (MT1, MT2, and both MT1 and MT2) disruption in melatonin-proficient C3H mice did not affect the rhythmic expression of Per1 in the EC. These data show the existence of a molecular clock in the rodent EC. In the hamster, this clock responds to long-term changes in the photoperiod, but is independent of acute melatonin signals. In mice, the EC clock is not affected by deletion of melatonin receptors.

Non-visual photoreception by a variety of vertebrate opsins.

Extraretinal photoreceptors in animals are involved in a variety of physiological functions such as photo-entrainment of circadian rhythm, photoperiodicity and body colour change. We have identified pinopsin in the chicken pineal gland as a typical 'non-visual' photoreceptive molecule. Pinopsin with bound 11-cis-retinal shows a blue-light sensitivity (lambda max = 468 nm), and it may play a role in synchronizing the phase of the endogenous circadian oscillator with an environmental dark-light cycle. Pinopsin is not a unique pineal opsin in animals. In the zebrafish, we have detected expression of two rhodopsin genes, the nucleotide sequences of which are very similar but distinct from each other. One is canonical rhodopsin expressed in the retina, and the other is expressed in the pineal gland. The latter gene is widely distributed among teleosts, and we named it 'exo-rhodopsin' after extraocular rhodopsin. On the other hand, our effort to identify the 'deep brain opsin' responsible for the photoperiodic gonadal response resulted in the identification of two kinds of opsins; pinopsin in the toad anterior preoptic nucleus and rhodopsin in the pigeon lateral septum. Both of these opsins are localized in the cerebrospinal fluid-contacting neurons in the brain of the two animals. We also identified VAL opsin in zebrafish retinal horizontal cells, which have not been considered as photoreceptive cells. It has become evident that animals employ a wide variety of photoreceptive molecules for 'non-visual' purposes.

Temporal Dynamics of Type 2 Deiodinase Expression after Melatonin Injections in Syrian Hamsters

In many species living in temperate zones, reproduction is controlled by the photoperiod. Recent findings have clarified that type 2 iodothyronine deiodinase (Dio2) plays a significant role in the photoperiodic response of gonads in the mediobasal hypothalamus, converting the prohormone T(4) into bioactive T(3). In mammals, Dio2 expression is suppressed by long-term melatonin injections, although the signal transduction pathways that link the melatonin signal to Dio2 expression are unknown. As a first step to approach the problem, we have here investigated the temporal dynamics of the melatonin effect on Dio2 expression using male Syrian hamsters. Dio2 mRNA levels were found to show diurnal rhythms under long-day conditions in an area adjacent to the tuberoinfundibular sulcus and in the ependymal cell layer lining the ventrobasal walls of the third ventricle. Daily sc melatonin injections given in the late afternoon under long-day condition suppressed the Dio2 mRNA levels already at the first day after the onset of the treatment in the ependymal cell layer lining the ventrobasal walls of the third ventricle, and 1 d later in an area adjacent to the tuberoinfundibular sulcus. These suppressive effects were sustained for at least 2 d after a single injection. Furthermore, we examined the temporal changes of the Dio2 expression after the onset of the treatment, showing that the suppression did not occur until midday of the next day. These data suggest that melatonin is involved in the signal transduction mechanisms controlling the photoperiodic response of gonads by acting on Dio2 expression rather rapidly through indirect pathways.

Possible Involvement of Organic Anion Transporting Polypeptide 1c1 in the Photoperiodic Response of Gonads in Birds

The photoperiodic response of the gonads requires T3, which is generated photoperiodically from T4 by type 2 iodothyronine deiodinase in the hypothalamus. Although thyroid hormones were long thought to traverse the plasma membrane by passive diffusion due to their lipophilic nature, it is now known that several organic anion transporting polypeptides (Oatp) transport thyroid hormones into target cells. In this study, we have used database searches to isolate DNA sequences encoding members of the chicken Oatp family and constructed a molecular phylogenetic tree. Comprehensive expression analyses using in situ hybridization revealed strong expression of cOatp1c1 and weak expression of cOatp1b1 in the ventro-lateral walls of basal tuberal hypothalamus, whereas expression of four genes (cOatp1a1, cOatp1b1, cOatp1c1, and cOatp3a2) was observed in the choroid plexus. Expression levels of all these genes in both regions were not different between short-day and long-day conditions. Functional expression of cOatp1c1 in Chinese hamster ovary cells revealed that cOatp1c1 is a highly specific transporter for T4 with an apparent Km of 6.8 nm and a Vmax of 1.50 pmol per milligram of protein per minute. These results suggest that cOatp1c1 could be involved in the thyroxine transport necessary for the avian photoperiodic response of the gonads.

The Reciprocal Switching of Two Thyroid Hormone-Activating and -Inactivating Enzyme Genes Is Involved in the Photoperiodic Gonadal Response of Japanese Quail

The molecular mechanisms underlying photoperiodic time measurement are not well understood in any organism. Relatively recently, however, it has become clear that thyroid hormones play an important role in photoperiodism, and in a previous study we reported that long daylengths in Japanese quail increase hypothalamic levels of T(3) and of the thyroid hormone-activating enzyme, type 2 iodothyronine deiodinase. The present study extends these observations to measure gene levels of the thyroid hormone-inactivating enzyme, type 3 deiodinase. Levels decreased after exposure to long days, but increased under short days. Changes in the two genes were then analyzed during the precisely timed photoinduction that occurs in quail exposed to a single long day. The two gene switches are the earliest events yet recorded in the photoinduction process, and overall, these reciprocal changes offer the potential to regulate active brain thyroid hormone concentrations rather precisely at the site in the brain where photoinduction is triggered.

Oral thyroxine administration mimics photoperiodically induced gonadal growth in Japanese quail

ABSTRACTMost temperate‐zone animals are seasonal breeders. In a previous study, it was found that light‐induced hormone conversion of thyroxine (T4) prohormone to active 3,5,3′‐triiodothyronine (T3) in the mediobasal hypothalamus regulates photoperiodic response of gonads in Japanese quail. Here the effect of T4 or T3, administered in drinking water, on testicular growth in the Japanese quail kept under short days is shown. Testicular length was significantly increased in birds given T4 at doses of 4, 8 and 10 mg/L, while any dose of T3 had little effect on testicular growth. High doses (8 and 10 mg/L) of T4 and T3 resulted in high mortality and/or reduction of bodyweight. Among all of the treatment, 4 mg/L of T4 was the most effective on photoperiodic testicular growth, which caused little reduction in bodyweight. These data provide a new conventional method for promoting gonadal growth under short days.

Seasonal morphological changes in the neuro-glial interaction between gonadotropin-releasing hormone nerve terminals and glial endfeet in Japanese quail.

In a previous study we showed that photoperiodically generated T3 in the hypothalamus is critical for the photoperiodic response of gonads in Japanese quail. The expression of thyroid hormone receptors in the median eminence (ME) suggested that photoperiodically generated T3 acts on the ME. Because thyroid hormone is known to play a critical role in the development and plasticity of the central nervous system, in the present study we have examined ultrastructure of the ME in Japanese quail kept in short-day and long-day environments. Immunoelectron microscopy revealed that GnRH nerve terminals are in close proximity to the basal lamina under long-day conditions, and conventional transmission electron microscopy demonstrated the encasement of the terminals by the endfeet of glia under short-day conditions. These morphological changes may regulate photoperiodic GnRH secretion.

Photoperiodic Regulation of Type 2 Deiodinase Gene in Djungarian Hamster: Possible Homologies between Avian and Mammalian Photoperiodic Regulation of Reproduction

The molecular mechanisms responsible for seasonal time measurement have yet to be fully described. Recently, we used differential analysis to identify that the type 2 iodothyronine deiodinase (Dio2) gene is responsible for the photoperiodic response of gonads in Japanese quail. It was found that expression of Dio2 in the mediobasal hypothalamus is induced by light and that T(3) content in the mediobasal hypothalamus increased under long day conditions. In addition, we showed that intracerebroventricular infusion of T(3) mimics photoperiodically induced testicular growth. Because it is well known that thyroid hormone is also essential for the maintenance of the seasonal reproductive changes in a number of mammals, we examined expression of Dio2 in Djungarian hamsters and found expression in the ependymal cell layer lining the infralateral walls of the third ventricle and the cell-clear zone overlying the tuberoinfundibular sulcus. Signal intensity was high under long days and weak under short days. Although light pulse did not affect Dio2 expression, melatonin injections decreased Dio2 expression under long days. These results indicate that Dio2 may be involved in the regulation of seasonal reproduction in mammals in the same way as observed in birds.

Light-induced hormone conversion of T4 to T3 regulates photoperiodic response of gonads in birds.

Reproduction of many temperate zone birds is under photoperiodic control. The Japanese quail is an excellent model for studying the mechanism of photoperiodic time measurement because of its distinct and marked response to changing photoperiods. Studies on this animal have suggested that the mediobasal hypothalamus (MBH) is an important centre controlling photoperiodic time measurement. Here we report that expression in the MBH of the gene encoding type 2 iodothyronine deiodinase (Dio2), which catalyses the intracellular deiodination of thyroxine (T4) prohormone to the active 3,5,3'-triiodothyronine (T3), is induced by light in Japanese quail. Intracerebroventricular administration of T3 mimics the photoperiodic response, whereas the Dio2 inhibitor iopanoic acid prevents gonadal growth. These findings demonstrate that light-induced Dio2 expression in the MBH may be involved in the photoperiodic response of gonads in Japanese quail.

A test of the coincidence and duration models of melatonin action in Siberian hamsters: the effects of 1-hr melatonin infusions on testicular development in intact and pinealectomized prepubertal Phodopus sungorus.

The pineal hormone melatonin is known to play an important role in mediating photoperiodic messages to the reproductive system in seasonal breeding animals. Our goal was to test, in a single experimental paradigm, two hypotheses that have been forwarded to describe how the circadian rhythm of pineal melatonin transmits photoperiodic information to the reproductive system: 1) induction, i.e., a short-day effect, occurs when secreted melatonin and a circadian rhythm of sensitivity to melatonin coincide in time; 2) induction occurs following exposure to elevated circulating melatonin levels for a prescribed duration. In order to determine the relative validity of these hypotheses, we investigated the testicular maturation response to 1-hr daily infusions of 10, 25, and 50 ng of melatonin in pinealectomized intact and prepubertal Siberian hamsters (Phodopus sungorus). Animals received, beginning on day 15 of life, programmed subcutaneous infusions of melatonin or vehicle at one of five time points (19:00-20:00, 20:00-21:00, 21:00-22:00, 24:00-01:00, and 03:00-04:00 hr) for 15 days. In animals gestated and raised in a long photoperiod (LD16:8 = 16L, where L is the duration of light in hours, and D that of dark), melatonin infusion right after lights off (20:00-21:00 hr) significantly retarded gonadal maturation; this dose was ineffective at other times tested. Doses of 10 and 25 ng melatonin were ineffective at all time points. Identical results were obtained in prepubertal hamsters gestated in a short photoperiod (LD10:14 = 10L) and raised in 16L; these results were independent of the presence or absence of the pineal gland. In animals gestated and raised in 10L, melatonin infusions failed to suppress testicular development beyond that induced by the photoperiod; testicular development was maximally suppressed in all groups. The results of these investigations are best explained under the experimental conditions employed here: 1) the photoperiodic gonadal response in juvenile Siberian hamsters is regulated by the coincidence in time of exogenously administered melatonin with an intrinsic rhythm of sensitivity to melatonin, which, under the constraints imposed by our experimental design, occurred at 20:00-21:00 hr; and 2) the duration of the melatonin signal alone, equal in all groups, cannot explain the results.

A test of the coincidence and duration models of melatonin action in Siberian hamsters. II. The effects of 4- and 8-hr melatonin infusions on testicular development of pinealectomized juvenile Siberian hamsters (Phodopus sungorus).

In a previous paper we demonstrated that properly timed 1-hr infusions of 50 ng melatonin effectively suppressed testicular development in juvenile Siberian hamsters. Only melatonin infused between 20:00 and 21:00 hr was effective in animals exposed to 16L (lights off 20:00 hr). In this paper we further investigate the importance of the coincidence and duration hypotheses of daily exposure of melatonin. Prepubertal Siberian hamsters received either 4- or 8-hr melatonin infusions at various times either on long photoperiod (LD 16:8 = 16L) or on short photoperiod (LD 10:14 = 10L). Daily 8-hr melatonin infusions suppressed testicular development in both photoperiods. Daily 4-hr, 50 ng/hr, melatonin infusions at 17:00-21:00 hr inhibited testicular growth in 16L and daily 4-hr melatonin infusions (either 50 ng/h or 50 ng/day) inhibited testicular growth at 17:00-21:00 hr in 10L. We also tested the efficacy of an interrupted melatonin infusion of long duration (8 hr). Pinealectomized prepubertal male Siberian hamsters, born on 16L, were infused with two signals of 4 hr separated by an interval of 2 hr. Melatonin-infused groups had significantly inhibited testicular growth compared to vehicle-infused animals. Testicular development was maximally inhibited only in those groups in which the period of melatonin sensitivity identified in the previous paper (20:00-21:00 hr) overlapped or immediately followed a period of melatonin infusion. Considering the restrictions of the experimental design employed in these studies, the results are best explained by the hypothesis that the photoperiodic gonadal response in juvenile Siberian hamsters is regulated by the coincidence in time of exogenously administered melatonin with an intrinsic rhythm of sensitivity to melatonin, which occurred at 20:00-21:00 hr. The duration of the melatonin signal alone can not explain the results.