European hamsters (Cricetus cricetus) show a transient phase of insensitivity to long photoperiods after gonadal regression

Annual rhythms of body weight and reproduction in the European hamster (Cricetus cricetus) are the result of an interaction between seasonal changes in day length (photoperiod) and seasonal changes in the responsiveness of animals to these photoperiods. The present study demonstrates that under natural conditions European hamsters are not able to perceive long photoperiods (i.e., a 16L:8D cycle) before mid-November. This is an important difference to other hamster species, in which regrowth of the gonads can be stimulated by exposure to long photoperiods at any stage of gonadal regression. The experiments also demonstrate the existence of an annual phase of sensitivity to long photoperiods that starts around mid-November and extends until March/April. During this phase of sensitivity, exposure to a long photoperiod (16L:8D) induced gonadal regrowth within 3 wk. Additional experiments with an accelerated photoperiodic lighting regimen indicated that a photoperiod of approximately 13 h is necessary to stimulate gonadal regrowth. Under natural light conditions in Stuttgart (48.46 degrees N), a photoperiod of 13 h is reached by the beginning of April, which fits well with the finding that the majority of animals kept under a natural light:dark cycle had well-developed gonads by the end of April. Nevertheless, these animals showed a rather variable timing of gonadal regrowth, ranging from early January to late April. This is most likely the result of two processes: first, an endogenous mechanism (photorefractoriness) that induces gonadal recrudescence without any photoperiodic information while the animals are still in their hibernation burrows, and second, a direct stimulatory effect of long photoperiods.     

Synchronization of the circannual reproductive rhythm of the ewe by discrete photoperiodic signals

Although many species display endogenous circannual rhythms of biological activity that are synchronized by day length, the specific photoperiodic requirements for synchronizing such rhythms are not established for any species. We tested the hypothesis that the circannual reproductive rhythm of sheep can be synchronized by exposure to just one or two discrete blocks of photoperiodic information each year. Ewes were pinealectomized to prevent their ability to transduce photoperiodic information into altered reproductive neuroendocrine activity. During the 53/4 yr following pinealectomy, specific photoperiodic signals were restored for discrete periods of time via replacement of 24-h patterns of melatonin, the pineal hormone that transmits photic information to the reproductive neuroendocrine axis. The ewes were kept in a 12-mo photoycycle that alternated between short (8L:16D) and long (16L:8D) days every 6 mo and that was 6 mo out of phase with the geophysical year. Pineal-intact control ewes exhibited synchronous annual reproductive cycles. Noninfused pinealectomized control ewes did not exhibit synchronous cycles. Pinealectomized ewes infused with alternating 70-day blocks of short- and long-day patterns of melatonin every 6 mo for the first 21/2 yr of the experiment exhibited synchronous annual reproductive cycles that were 6 mo out of phase with those of ewes maintained outdoors. This synchrony persisted when the frequency of the melatonin treatment was reduced to just one 70-day block of a long-day pattern of melatonin each 365 days. Cycle period was 368 +/- 3 days; standard deviation of the date of onset of reproductive induction averaged only 3 days. Our study provides the first direct evidence that a single block of photoperiodic information a year can synchronize a circannual rhythm.     

Photoperiodic response of the hypothalamo-pituitary-gonad axis in male and female canaries,Serinus canaria

Although the gonadal photoperiodic response and its influence upon the song control system in canaries have been extensively studied, photoperiodic regulation of the GnRH system has not been investigated. To examine the relationship between photoperiod and the reproductive neuroendocrine system in male and female canaries, three groups of canaries were exposed to chronic short days (8L:16D; Phsens), acute long days (18L:6D; Phstim) and chronic long days (also 18L:6D; Phrefr) to induce the reproductive states of photosensitivity, photostimulation, and photorefractoriness, respectively. Brain sections were processed for GnRH immunocytochemistry. The canaries in this study did not demonstrate consistent or uniform responses to different photoperiodic treatments. In males, gonad size varied with photoperiod; Phstim males had larger gonads than either the Phsens or Phrefr males. In contrast, there was no difference between groups in female gonad size as a result of photoperiodic treatment. Brain GnRH cell number, cell size, and fiber density were similar in all groups. The results suggest that canaries are not as obligatory photoperiodic as previously thought (or at least not all varieties of domestic canaries are). This could be a result of many years of domestication, the natural history of the species, phylogenetic constraint, or a combination of these factors.     

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.     

Photoperiod and temperature affect reproductive and nonreproductive functions in male prairie voles (Microtus ochrogaster)

Adult male prairie voles (Microtus ochrogaster) were housed for 10 wk and exposed to long (16L:8D) or short (8L:16D) photoperiods at 21 degrees or 5 degrees C. Maintenance in short day lengths reduced testicular, epididymal, and seminal vesicle mass and also significantly depressed spermatogenic activity. Cold ambient temperature further suppressed gonadal size in voles exposed to short days. Several pelage characteristics were affected by photoperiod, but not by temperature. Increased fur density, fur depth, and length of guard hair and underhair were observed in voles exposed to short days. Intrascapular brown fat and gonadal fat pad mass as well as body mass were significantly less in voles housed in cold temperatures than in voles exposed to warm ambient temperatures; photoperiod did not affect these parameters. Approximately 30% of the male voles exposed to short days maintained their reproductive systems, yet they clearly processed photoperiodic information; all short-day males, regardless of reproductive condition, had comparable winter pelage development. Our results suggest that in prairie voles, photoperiod may be a predictive cue for reproductive function in nature; however, it appears that pelage development is a more obligatory response to photoperiod than is reproduction.     

Latitude of origin influences photoperiodic control of reproduction of deer mice (Peromyscus maniculatus)

Three subspecies of Peromyscus maniculatus originating from different latitudes were maintained from birth in light dark cycles that provided between 10 and 18 h of light per day. At 50 days of age, Chihuahua, Mexico mice (latitude of origin 27 degrees N) and South Dakota, U.S.A. mice (44 degrees N) kept in the 10L:14D photoperiod had reduced gonadal and seminal vesicle weights and a lower spermatogenic index than corresponding mice kept in a 14L:10D photoperiod. Some Chihuahua and South Dakota mice, apparently constituting nonphotoperiodic subpopulations, developed their gonads while kept in the short-day photoperiod. The critical day length for stimulation of sexual maturation was greater for mice from Manitoba, Canada (55 degrees N) than for mice from the lower latitudes. At 70 days of age, testes and seminal vesicle weights, and the spermatogenic index of Manitoba mice in the 14L:10D photoperiod, were lower than those of animals maintained in 16L:8D and 18L:6D photoperiods. Responsiveness to short day lengths was greater among adult South Dakota than adult Chihuahau mice and melatonin treatment significantly reduced testes weights of South Dakota but not of Chihuahua adult mice. Photoperiodic regulation of the reproductive system varies with latitude of origin. Differences in the critical day length necessary for stimulating development of functional reproductive activity and variations in the percent of photoperiodic animals within each subspecies, appear to contribute to latitudinal gradients in reproduction.     

Gonadal hormone-dependent and -independent regulation of immune function by photoperiod in Siberian hamsters

Siberian hamsters (Phodopus sungorus) exhibit changes in reproductive and immune function in response to seasonal variations in day length. Exposure to short days induces gonadal regression and inhibits testosterone secretion. In parallel, short days enhance immune function: increasing leukocyte numbers and attenuating cytokine and behavioral responses to infection. We examined whether photoperiodic changes in leukocyte phenotypes and sickness behaviors are dependent on concurrent photoperiodic changes in gonadal function. Male hamsters were gonadectomized or sham-gonadectomized and either exposed to short days (9 h light/day; SD) or kept in their natal long-day (15 h light/day; LD) photoperiod for 10-13 wk. Blood samples were obtained for leukocyte enumeration, and hamsters were challenged with bacterial LPS, which induced behavioral (anorexia, reductions in nest building) and somatic (weight loss) sickness responses. Among gonad-intact hamsters, exposure to SD increased total and CD62L+ lymphocytes and CD3+ T lymphocytes in blood and significantly attenuated LPS-induced sickness responses. Independent of photoperiod, castration alone increased total and CD62L+ lymphocyte and CD3+ T lymphocyte numbers and attenuated somatic and anorexic sickness responses. Among castrated hamsters, SD exposure increased lymphocyte numbers and suppressed sickness behaviors. In castrated hamsters, the magnitude of most immunological effects of SD were diminished relative to those evident in gonad-intact hamsters. The SD phenotype in several measures of immunity can be instated via elimination of gonadal hormones alone; however, photoperiodic effects on immune function persist even in castrated hamsters. Thus, photoperiod affects the immune system and neural-immune interactions underlying sickness behaviors via gonadal hormone-dependent and -independent mechanisms.     

Support for a rare pattern of temperature-dependent sex determination in archaic reptiles: evidence from two species of tuatara (Sphenodon)

Background

In many birds, day length (=photoperiod) regulates reproductive cycle. The photoperiodic environment varies between different seasons and latitudes. As a consequence, species at different latitudes may have evolved separate photoperiodic strategies or modified them as per their adaptive need. We studied this using house sparrow as a model since it is found worldwide and is widely investigated. In particular, we examined whether photoperiodism in house sparrows (Passer domesticus) at 27°N, 81°E shared features with those exhibited by its conspecifics at high latitudes.

Results

Initial experiment described in the wild and captive conditions the gonad development and molt (only in captives) cycles over a 12-month period. Both male and female sparrows had similar seasonal cycles, linked with annual variations in day length; this suggested that seasonal reproduction in house sparrows was under the photoperiodic control. However, a slower testis and attenuated follicular growth among captives indicated that other (supplementary) factors are also involved in controlling the reproductive cycle. Next experiment examined if sparrows underwent seasonal variations in their response to stimulatory effects of long day lengths. When birds were transferred every month over a period of 1 year to 16 hours light:8 hours darkness (16L:8D) for 17–26 weeks, there was indeed a time-of-year effect on the growth-regression cycle of gonads. The final experiment investigated response of house sparrows to a variety of light-dark (LD) cycles. In the first set, sparrows were exposed for 31 weeks to photoperiods that were close to what they receive in between the period from sunrise to sunset at this latitude: 9L:15D (close to shortest day length in December), 12L:12D (equinox, in March and September) 15L:9D (close to longest day length in June). They underwent testicular growth and regression and molt in 12L and 15L photoperiods, but not in 9L photoperiod. In the second set, sparrows were exposed for 17 weeks to photoperiods with light periods extending to different duration of the daily photosensitivity rhythm (e.g. 2L:22D, 6L:18D, 10L:14D, 14L:10D, 18L:6D and 22L:2D). Interestingly, a slow and small testicular response occurred under 2L and 10L photoperiods; 6L:18D was non-inductive. On the other hand, 14L, 18L and 22L photoperiods produced testicular growth and subsequent regression response as is typical of a long day photostimulation.

Conclusion

Subtropical house sparrows exhibit photoperiodic responses similar to that is reported for its population living at high latitudes. This may suggest the conservation of the photoperiodic control mechanisms in birds evolved over a long period of time, as a physiological strategy in a temporally changing environment ensuring reproduction at the best suited time of the year.     

Regulation of seasonality in the migratory male blackheaded bunting (Emberiza melanocephala)

The present study was carried out on a Palearctic-Indian migratory species, the blackheaded bunting (Emberiza melanocephala), to understand the importance of photoperiodism and circannual rhythms in determining seasonality in changes in body mass and testis size in birds. An initial experiment determined the effects of duration and intensity of light on photoperiodic induction. The birds were exposed to different photoperiods (hours of light:hours of darkness; 11.5L:12.5D, 12L:12D, 12.5L:11.5D and 13L:11D) at the same (approximately 450 lux) light intensity, and to 13L:11D at different light intensities (50-, 100-, 400-, 800- and 1000-lux). The induction and subsequent regression of photoperiodic responses were dependent upon duration and intensity of the light period until these reached threshold. A second experiment investigated if an endogenous seasonal rhythm underlies photoperiodism in buntings. Birds maintained since February on a 8L: 16D photoperiod (a non-inductive short day length invariably used to ensure photosensitivity in photoperiodic species) were subjected periodically to 16L:8D (a long day length), one group every month from mid-March to mid-August. The magnitude of long day response in body mass and testes decreased as the duration of the short days progressed, but testicular response was restored in birds that were exposed to long days in July and August. The birds exposed simultaneously to short, long, and natural day lengths for 32 weeks underwent an induction-regression cycle under long days and natural day lengths, but not under short days in which a decrease in body mass occurred after about 20 weeks. The last experiment examined the importance of latitudinal migration on photoperiodism, by comparing the response to long days of three groups which included birds from populations those were held in the outdoor aviary for 1 or 2 years at 27 degrees N and those immediately arrived from their breeding grounds (approximately 40 degrees N). There was no difference in the photoperiodic induction among the three groups, indicating that neither experience to changing photoperiods during a migratory journey, nor to long photoperiods at breeding grounds, were critical for a subsequent response (initiation-termination-reinitiation) cycle. Taken together, these findings suggest that (1) the blackheaded bunting has its own endogenous timing program, which is regulated by the photoperiod, and (2) the photoperiodic programs of bunting are flexible enough to accommodate variations in the amplitude of environmental cycles. Thus, it appears that photoperiodism has evolved independently of the evolution of migration in this species.     

Influence of photoperiod and gonadal steroids on hibernation in the European hamster

Torpor was monitored daily in adult male and female European hamsters (Cricetus cricetus) induced to hibernate by exposure to a cold environment (6 degrees C). The effect of photoperiodic manipulations or administration of exogenous gonadal steroids was examined in gonadectomized or intact hamsters. 1. Gonadal regression occurred in all short day, but only in some long day, cold-exposed hamsters. Entry into hibernation was not observed until reproductive regression had occurred. Thus, gonadal atrophy appears to be a necessary precondition for hibernation. 2. Castrated hamsters in the short day cold condition showed a significantly greater incidence of torpor than those in the long day cold condition. Hence, photoperiod affected torpor independently of its effect on the gonadal cycle. 3. Testosterone, when administered via silastic capsules at near physiological levels, completely inhibited torpor in gonadectomized male and female hamsters hibernating in the short day cold condition. 4. In ovariectomized females, torpor was unaffected by progesterone treatment, but partially inhibited by estradiol. A greater inhibition of torpor was observed when estradiol-primed females were administered both estradiol and progesterone simultaneously. Thus, the effect of both hormones may be functionally comparable to that of the single testicular hormone. 5. Estradiol inhibited torpor to a greater extent in intact and ovariectomized female hamsters hibernating in long days than those in short days, suggesting an effect of photoperiod on responsiveness to estradiol. These results indicate an inverse relationship between the gonadal and hibernation cycles, and a probable role for gonadal steroids to influence the timing of the hibernation season. However, non-gonadal factors must also be involved in controlling hibernation, since photoperiod affected the incidence of torpor in gonadectomized animals and because hamsters were able to terminate hibernation in the absence of gonadal hormones.     

Effects of gonadal steroid hormones on hibernation in the Turkish hamster (Mesocricetus brandti)

Summary The Turkish hamster (Mesocricetus brandti) is a photoperiodic species in which exposure to a short day photoperiod induces gonadal atrophy. When hibernating male Turkish hamsters were administered testosterone via subcutaneous silastic capsules hibernation was abruptly terminated. The threshold serum testosterone level required to prevent hibernation was intermediate between the levels observed in males with fully-regressed and fully-functional testes. Dihydrotestosterone was as effective as testosterone in blocking hibernation. Estradiol had a partial inhibitory effect. Progesterone and corticosterone implants had no effect on hibernation. Similar results were obtained in female hamsters. The total period of hibernation for untreated males under laboratory conditions was usually 5–6 months. In most animals testicular growth began to occur toward the end of that period. As gonadal growth continued bouts of torpor decreased in frequency and duration, and hibernation was terminated when the testes approached reproductive size. Castrated males continued to hibernate much longer than intact animals. These observations suggest that in nature decreasing day length in the fall initiates gonadal regression which is followed by cold-induced hibernation. In the spring spontaneous recrudescence of the testes may lead to termination of the dormancy period. This mechanism may enable the species to prepare for the reproductive season with maximum efficiency.Supported by NIH Research Grant HD-10478 and by a grant from the Connecticut Research FoundationThe data in this paper were presented in part at the Annual Meeting of the American Society of Zoologists, Toronto, December 26–30, 1977     

Photorefractoriness and energy availability interact to permit facultative timing of spring breeding

In seasonally breeding mammals, vernal reproductive development is not directly triggered by increases in day length, rather, an endogenous program of photorefractoriness to short winter days initiates spontaneous development in advance of spring. The transition to the reproductive phenotype is energetically demanding. How food availability in late winter and early spring impacts the onset and expression of photorefractoriness is not known. In this study, male Siberian hamsters were born into a simulated natural photoperiod, and at the winter solstice, they were subjected to a restricted feeding protocol in which a daily food ration was provided in an amount equal to ad libitum (AL) intake during the weeks preceding the solstice. Over the next several months, AL-fed control hamsters exhibited spontaneous recrudescence or spontaneous development. In contrast, vernal reproductive development was abolished in most food-rationed hamsters. In food-rationed hamsters that did exhibit recrudescence, conspicuous delays in the onset of gonadal development and decreases in the magnitude of growth were evident. In all hamsters, the termination of food rationing triggered rapid gonadal development. The data indicate that late winter/early spring increases in environmental food availability are required for the normal manifestation of photorefractoriness-induced reproductive development and suggest that a function of photorefractoriness may be merely to disinhibit the reproductive axis from photoperiodic suppression. Vernal gonadal development or recrudescence appears to be strongly affected by proximate energy availability.     

Photoperiodic response in the male laboratory rat

Normally photoperiodic laboratory rats can be induced to respond reproductively to a change in the length of the day by various experimental manipulations. One such paradigm that results in significant gonadal regression involves the treatment of rats with exogenous testosterone during exposure to short days. Studies were undertaken to assess various aspects of this model system including 1) the testicular response of testosterone-treated rats exposed to various photoperiods, 2) the time course for testicular regression under a short photoperiod, and 3) the role of the pineal gland as a mediator of the effects of day length on the neuroendocrine-gonadal axis. Photoperiods ranging in length from 2 to 22 h/24 h had no effect on testicular size in untreated rats. In contrast, while near normal testicular weights were maintained in laboratory rats treated with testosterone and exposed to 10 or more h of light per day, testicular regression occurred in rats implanted with testosterone-filled capsules and exposed to photoperiods of 8 or fewer h of light per day. Maximal testicular regression was reached in about 9 wk in testosterone-treated rats exposed to 6L:18D. Removal of the pineal gland totally blocked the inhibitory effects of exposure to short day lengths in testosterone-treated rats. These studies define some of the characteristics of an extant, but dormant, system for photoperiodic time measurement in the common laboratory rat and implicate a role for the pineal gland in this system. These experiments offer evidence that neuroendocrine factors that regulate continuous vs. seasonal reproductive patterns are malleable. Such flexibility in the photoperiodic response may also contribute to the evolution of seasonal to non-seasonal species and vice versa.     

Spontaneous "regression" of enhanced immune function in a photoperiodic rodent Peromyscus maniculatus.

Short days inhibit reproduction and enhance immune function in deer mice (Peromyscus maniculatus). Their reproductive inhibition is sustained by an endogenous timing mechanism: after ca. 20 weeks in short days, reproductive photorefractoriness develops, followed by spontaneous recrudescence of the reproductive system. It is unknown whether analogous seasonal timing mechanisms regulate their immune function or whether enhanced immune function is sustained indefinitely under short days. In order to test this hypothesis, we housed adult male deer mice under long (16 h light day(-1)) or short (8 h light day(-1)) day conditions for 32 weeks or under long day conditions for 20 weeks followed by 12 weeks of short days. Mice under the long day conditions remained photostimulated over the 32 weeks, whereas mice housed under the short day conditions exhibited gonadal regression followed by photorefractoriness and spontaneous recrudescence. Mice transferred to short days at week 20 were reproductively photoregressed at week 32. Total splenocytes, relative splenic mass and mitogen-activated splenocyte proliferation were greater in those mice transferred to short days at week 20 than in those mice housed under either long or short day conditions for 32 consecutive weeks, and immune function in mice exposed to short days for 32 weeks was comparable with that of long day animals. These data suggest that short day enhancement of immune function is not indefinite. With prolonged (< or = 32 weeks) exposure to short days, several measures of immune function exhibit "spontaneous" regression, restoring long day-like immunocompetence. The results suggest that formal similarities and, possibly, common substrates exist among the photoperiodic timekeeping mechanisms that regulate seasonal transitions in reproductive and immune function.     

Functional maturation of the gonads of Turkish hamsters under various photoperiods

The golden hamster, Mesocricetus auratus, is the only photoperiodic rodent to date that has been shown to fail to respond to inhibitory (i.e., short, less than 12.5 h/day) photoperiods until after pubertal onset. In other photoperiodic hamsters, mice, and voles, short photoperiods greatly retard gonadal maturation. The Turkish hamster, Mesocricetus brandti, is a photoperiodic rodent that as an adult is reproductively competent only on photoperiods of 15-17 h of light per day; photoperiods of less than 15 or greater than 17 h of light promote gonadal regression. In this report we addressed two questions: a) are prepubertal M. brandti photoperiodic, and b) if so, is gonadal maturation enhanced or suppressed by exposure to photoperiods of greater than 17 h of light per day? Turkish hamsters were raised on photoperiods of 12, 16, 20, or 24 (= LL) h of light per day. Testicular growth was retarded for 16 wk by 12L:12D. Very long days, 20L:4D, or LL did not retard testicular development. In females, pubertal onset, as indicated by first vaginal estrus, was delayed in young raised on 12L:12D and in 2 of 18 and 4 of 19 young raised on 20L:4D and LL, respectively. These results demonstrate that prepubertal Turkish hamsters are photoperiodic, but respond differently from adults to photoperiods greater than 17 h of light per day.     

What photoperiodic signal is provided by a continuous-release melatonin implant?

The purpose of this study was to evaluate whether the insertion of a continuous-release melatonin implant into ewes provides a short-day photoperiodic signal or acts as a functional pinealectomy (provides no specific photoperiodic signal but renders ewes incapable of responding to changes in photoperiod). Ewes primed with 60 long days (18L:6D) during the spring were moved to intermediate day length (13L:11D) for 66 days and then given one of five treatments: 1) short-day control, second drop in photoperiod to 8L:16D; 2) intermediate-photoperiod control, kept on 13L:11D; 3) pinealectomy and kept on 13L:11D; 4) melatonin implant and kept on 13L:11D; 5) melatonin implant and moved to 8L:16D. Mean number of estrous cycles per group and total duration of reproductive activity were determined. Ewes in all groups began to exhibit estrous cycles after the initial reduction in photoperiod. The number of estrous cycles and duration of reproductive activity differed among groups. The number of estrous cycles and duration of reproductive activity was extended in ewes receiving the second drop in photoperiod compared to that of the intermediate-photoperiod controls. Pinealectomized ewes had a number of estrous cycles and duration of reproductive activity similar to those of ewes maintained on the intermediate photoperiod. Melatonin implants increased the number of estrous cycles and prolonged reproductive activity in ewes maintained on the intermediate photoperiod; melatonin implants did not prevent the extension of reproductive activity in ewes receiving the second photoperiodic drop to the short daylength.(ABSTRACT TRUNCATED AT 250 WORDS)     

Termination of gonadal refractoriness in Turkish hamsters, Mesocricetus brandti

The Turkish hamster (Mesocricetus brandti) is a photoperiodic species. In this investigation, we characterized the photoperiodic requirements for termination of gonadal refractoriness, defined as the inability of the animal to respond to short-day treatment with gonadal regression. Paired testes weights were reduced to less than 20% of their original weight by 10 wk of 12L:12D treatment. This was followed by spontaneous testicular recrudescence (completed by Week 25 of 12L:12D treatment), the overt indication of refractoriness to short photoperiods. Next, the period of long-day exposure sufficient for termination of refractoriness was determined. Refractory males were exposed to 16L:8D for 5 to 20 wk. Ten weeks of 16L:8D treatment was enough for the animals to regain the sensitivity to a second challenge of 12L:12D treatment. Fifteen weeks of 20L:4D or 16L:8D terminated refractoriness in female Turkish hamsters; 20L:4D therefore was not interpreted as a short day by refractory hamsters. This was unexpected because in photosensitive animals this photoperiod acts like a short day, causing gonadal regression. These results suggest that Turkish hamsters are similar to Syrian hamsters in that both species require two or more months of long days in summer to recover sensitivity to the short days of the following fall.     

Involvement of threshold photoperiod in control of reproductive rhythmicity in migratory blackheaded bunting, Emberiza melanocephala

To explore the need for minimum threshold photoperiodicity in regulation of metabolic and reproductive activities of a migratory finch, various programmed light-dark (LD) schedules, such as P1 (3L/21D), P2 (6L/18D), P3 (9L/15D), P4 (12L/12D), P5 (15L/9D), P6 (18L/6D), P7 (21L/3D), and P8 (24L/0D), have been used on photosensitive female blackheaded buntings for 42 days. Results indicate that the photoperiodic thresholds of 3 h, 6 h, and 9 h completely failed to have any response on buntings, while threshold photoperiodicities of 12 h, 15 h, 18 h, 21 h, and 24 h had significant effect (P < .001) on body weight, ovarian weight, and circulating plasma estradiol concentration, suggesting the role of the photoperiod as a primary environmental source to regulate various metabolic and reproductive functions. Further, it has been suggested that the threshold photoperiod in this species appears to be of 12 h duration.     

First evidence for photoperiodic regulation of the life cycle in a millipede species, Polydesmus angustus (Diplopoda: Polydesmidae)

The hypothesis that the periods of dormancy previously described in the millipede Polydesmus angustus may be photoperiodically induced diapauses was tested experimentally. In this species, biennial individuals exhibit two successive periods of dormancy: aestivation in the penultimate stadium (stadium VII) and reproductive dormancy in the adults, which emerge in autumn. It was first established that the reproductive dormancy is not a thermally controlled state of quiescence. When adults emerging in autumn were kept at 16 °C under natural photoperiod, their reproduction was delayed for several months in comparison with adults emerging in spring at similar temperatures. This indicates that the reproductive dormancy begins with a period of diapause. Further experiments provided evidence of a photoperiodic induction of the adult diapause. When millipedes were reared under short day length (L:D 12:12 h) throughout their development, they required more time to reproduce than millipedes reared under long day length (L:D 16:8 h) at the same temperatures. Photoperiod influenced reproduction in females, but no significant effects were detected in adult males. On the other hand, stadium VII was markedly longer at L:D 16:8 h than at L:D 12:12 h in both sexes, which strongly suggests that aestivation is also induced by photoperiod. However, the effects on the duration of stadium VII varied among individuals, some of which showed no response to long days. This study is the first to document photoperiodic regulation of the life cycle in the class Diplopoda, a trait common in other classes of terrestrial arthropods.     

Molecular mechanism of the photoperiodic response of gonads in birds and mammals

Appropriate timing of various seasonal processes is crucial to the survival and reproductive success of animals living in temperate regions. When seasonally breeding animals are subjected to annual changes in day length, dramatic changes in neuroendocrine-gonadal activity take place. However, the molecular mechanism underlying the photoperiodic response of gonads remains unknown for all living organisms. It is well known that a circadian clock is somehow involved in the regulation of photoperiodism. Recently, rhythmic expression of circadian clock genes was observed in the mediobasal hypothalamus (MBH) of Japanese quail. The MBH is believed to be the center for photoperiodism. In addition, long-day-induced hormone conversion of the prohormone thyroxine (T(4)) to the bioactive triiodothyronine (T(3)) by deiodinase in the MBH has been proven to be important to the photoperiodic response of the gonads. Although the regulating mechanism for the photoperiodic response of gonads in birds and mammals has long been considered to be quite different, the long-day-induced expression of the deiodinase gene in the hamster hypothalamus suggests the existence of a conserved regulatory mechanism in avian and mammalian photoperiodism.