Gonadal Regression Research Articles

Rapid Induction of Hypothalamic Iodothyronine Deiodinase Expression by Photoperiod and Melatonin in Juvenile Siberian Hamsters (Phodopus sungorus)

Production of T(3) in the mediobasal hypothalamus is critical for regulation of seasonal reproductive physiology. Type 2 iodothyronine deiodinase (DIO2) and DIO3 enzymes catalyze the prohormone T(4) into biologically-active T(3) and biologically-inactive rT(3), respectively. In several seasonally-breeding vertebrates, DIO2 and DIO3 expression is implicated in photoperiod signal transduction in adulthood. These experiments tested the hypothesis that juvenile Siberian hamsters, which are highly responsive to photoperiod at weaning (postnatal day [PND]18), exhibit rapid and sustained changes in hypothalamic dio3 mRNA expression during photoperiod-induced and photoperiod-inhibited puberty. Hypothalamic dio2 and dio3 expression was measured via quantitative PCR in hamsters born and reared in a long-day photoperiod (15L:9D) and weaned on PND18 into short-day photoperiods (9L:15D). In SD males, hypothalamic dio3 mRNA was elevated 2.5-fold within 3 days (PND21) and continued to increase (>20-fold) through PND32; changes in dio3 mRNA preceded inhibition of gonadotropin (FSH) secretion and gonadal regression in SD. Females exhibited comparable dio3 responses to SD. In LD males, dio3 remained low and invariant from PND18-PND32. In contrast, dio2 mRNA rose conspicuously on PND21, independent of photoperiod, returning to basal levels thereafter. In LD, a single afternoon melatonin (MEL) injection on PND18 or PND20 was sufficient to increase hypothalamic dio3 mRNA, and dio3 increased in proportion to the number of successive days of MEL treatment. SD photoperiods and MEL exert rapid, sustained, and additive effects on hypothalamic dio3 mRNA, which may play a central role in inhibiting maturation of the peripubertal hypothalamo-pituitary-gonadal axis.

Effects of Photoperiod on Epididymal and Sperm Morphology in a Wild Rodent, the Viscacha (Lagostomus maximus maximus).

The viscacha (Lagostomus maximus maximus) is a seasonal South American wild rodent. The adult males exhibit an annual reproductive cycle with periods of maximum and minimum gonadal activity. Four segments have been identified in the epididymis of this species: initial, caput, corpus, and cauda. The main objective of this work was to relate the seasonal morphological changes observed in the epididymal duct with the data from epididymal sperm during periods of activity and gonadal regression using light and scanning electron microscopy (SEM). Under light and electron microscopy, epididymal corpus and cauda showed marked seasonal variations in structural parameters and in the distribution of different cellular populations of epithelium. Initial and caput segments showed mild morphological variations between the two periods. Changes in epididymal sperm morphology were observed in the periods analyzed and an increased number of abnormal gametes were found during the regression period. During this period, anomalies were found mainly in the head, midpiece, and neck, while in the activity period, defects were found only in the head. Our results confirm that the morphological characteristics of the epididymal segments, as well as sperm morphology, undergo significant changes during the reproductive cycle of Lagostomus.

Photoperiodic Influences on Ultradian Rhythms of Male Siberian Hamsters

Seasonal changes in mammalian physiology and behavior are proximately controlled by the annual variation in day length. Long summer and short winter day lengths markedly alter the amplitude of endogenous circadian rhythms and may affect ultradian oscillations, but the threshold photoperiods for inducing these changes are not known. We assessed the effects of short and intermediate day lengths and changes in reproductive physiology on circadian and ultradian rhythms of locomotor activity in Siberian hamsters. Males were maintained in a long photoperiod from birth (15 h light/day; 15 L) and transferred in adulthood to 1 of 7 experimental photoperiods ranging from 14 L to 9 L. Decreases in circadian rhythm (CR) robustness, mesor and amplitude were evident in photoperiods ≤14 L, as were delays in the timing of CR acrophase and expansion of nocturnal activity duration. Nocturnal ultradian rhythms (URs) were comparably prevalent in all day lengths, but 15 L markedly inhibited the expression of light-phase URs. The period (τ’), amplitude and complexity of URs increased in day lengths ≤13 L. Among hamsters that failed to undergo gonadal regression in short day lengths (nonresponders), τ’ of the dark-phase UR was longer than in photoresponsive hamsters; in 13 L the incidence and amplitude of light-phase URs were greater in hamsters that did not undergo testicular regression. Day lengths as long as 14 L were sufficient to trigger changes in the waveform of CRs without affecting UR waveform. The transition from a long- to a short-day ultradian phenotype occurred for most UR components at day lengths of 12 L–13 L, thereby establishing different thresholds for CR and UR responses to day length. At the UR-threshold photoperiod of 13 L, differences in gonadal status were largely without effect on most UR parameters.

Winter Day Lengths Counteract Stimulatory Effects of Apomorphine and Yohimbine on Sexual Behavior of Male Syrian Hamsters

Yohimbine and apomorphine selectively act on noradrenergic and dopaminergic neural substrates to augment male sexual behavior (MSB) in several rodent species. The present study assessed whether these drugs can overcome the suppressive effects of short winter-like day lengths on MSB. Yohimbine treatments that markedly increase copulatory behavior of male hamsters in long days were completely ineffective in facilitating MSB when injected after gonadal regression induced by 16 wks of short day lengths and after complete gonadal recrudescence after 32 wks of short days; apomorphine was similarly ineffective. The brain circuit that mediates MSB either may be less responsive to yohimbine and apomorphine in short than long days, or these drugs may not produce equivalent neurotransmitter changes in the two day lengths. After 32 wks of short-day treatment, all males had undergone testicular recrudescence and successfully ejaculated on initial tests with sexually receptive females after a hiatus of at least 4 mo during which they were denied mating opportunities. This suggests that overwintering males in the field are in a state of reproductive readiness at the outset of spring conditions favorable for survival of offspring. (Author correspondence: piekarski@berkeley.edu)

Seasonal Variation in Sensitivity of the Photoperiodic Response System in the Subtropical Tree Sparrow (Passer montanus)

We investigated seasonal variations in response to photoperiods in subtropical tree sparrow (Passer montanus), by examining the effects of long and short days on stimulation of response and termination of refractoriness, respectively. We also assessed whether photoperiodic effects were sex dependent. In one study, wild sparrows were transferred in each month of the year to artificial long days (14L/10D) for 12 weeks. Birds transferred from November to March (female) or April (male) showed gradual increase in gonadal growth and darkening of bill color, while those transferred from April (female) or May (male) to July underwent gradual regression in the above responses. Moult in the wing primaries and body feathers progressed with gonadal regression and the birds transferred from April/May to October exhibited gradual increase and decrease in feathers moult. In another study, 6 weeks of short day (9L/15D) exposure could recover responsivity to long days in refractory birds. Male and female birds responded almost in similar fashion with some variations. Overall, our results show a gradual change over seasons in responsiveness of the endogenous response system to stimulatory effects of long day length. They suggest roles of both long and short day lengths in regulation of seasonal cycles in subtropical tree sparrows.

Effects of short periods of warm water fluctuations on reproductive endocrine axis of the pejerrey (Odontesthes bonariensis) spawning

The aim of this study was to assess fluctuations in daily water temperature in Chascomús Lagoon during one year, and to evaluate whether the highest temperature recorded during pejerrey spawning season can produce an endocrine disruption on brain–pituitary–gonads axis. Fish were subjected to daily temperature fluctuations: 17°C to 19°C (reproductive control), 19°C to 25°C, and 19°C to 27°C. After 8days, ten fish per treatment were sacrificed and gene expression of gonadotropin-releasing hormone (GnRH-I, GnRH-II, GnRH-III), gonadotropin subunits-β (FSH-β, LH-β), glycoprotein hormone-α (GPH-α), gonadotropin receptors (FSH-R, LH-R), and gonadal aromatase (cyp19a1a) was analyzed. Also, plasma levels of sexual steroids and gonadal reproductive status were studied. Fish exposed to high temperature fluctuations quit spawning, presenting clear signs of gonadal regression. Fish recovered its spawning activity 11weeks after heat treatment. At endocrine level, GnRH-I and FSH-β in both sexes, LH-β and GPH-α in males and FSH-R, LH-R and cyp19a1a in females decreased significantly in treated fish. Also, a strong reduction in plasma sex steroid levels was found for both sexes. This study demonstrated that pulses of warm water in natural environment during pejerrey spawning season can disrupt all levels of the reproductive axis, impairing reproduction.

Rapid loss of fat but not lean mass prior to chick provisioning supports the flight efficiency hypothesis in tree swallows

Summary1. Birds often lose body mass during nesting. Determining whether this mass loss represents an energetic cost of reproduction (energetic stress hypothesis), serves an adaptive function (flight efficiency hypothesis), or results from physiological processes that are neutral with respect to fitness (e.g. gonadal regression hypothesis) is important to interpreting variation in body mass and energy stores in the context of life‐history theory.2. New quantitative magnetic resonance technology enables precise, repeated measurements of body composition (fat, lean, and water masses) on the same individuals, and we used this method to test a series of predictions to distinguish among competing hypotheses explaining mass loss in female tree swallows (Tachycineta bicolor, Vieillot, 1808).3. Tree swallows lost mass abruptly prior to the peak foraging demands of feeding chicks. Lean mass and fat mass losses varied independently, with small and gradual losses in lean mass during incubation and dramatic losses of fat immediately prior to and following hatching. Females lost some body water early in incubation, but did not lose more water when brood patches would be expected to atrophy. The period of greatest parental foraging costs was not associated with any significant changes in total body mass, lean mass, fat mass or water. Net change in body mass from early incubation until mid‐way through chick rearing was associated strongly with initial body mass and to a lesser degree, brood size.4. These findings are consistent with the flight efficiency hypothesis. Females appear to facultatively modulate their endogenous energy stores to maximize insurance against bad weather and poor foraging opportunities during incubation, but then lower their body mass to maximize efficiency once chicks hatch, thereby reducing the costs of feeding nestlings.5. This study corroborates results of a growing number of studies of small, insectivorous birds (especially cavity‐nesting species), suggesting that increases in mass prior to reproduction (and the subsequent loss of that mass) are likely best viewed as part of an adaptive suite of interrelated reproductive decisions made by females each year.

Nitric oxide (NO) expression during annual reproductive activity in buffalo epididymis: A histochemical and immunocytochemical study

The buffalo is one of the few domestic animals that has a seasonal mating cycle, influenced by the photoperiod. It is known that the photoperiod regulates gonadal function probably via the pineal and/or hypothalamus-pituitary axis. Moreover, the hypothalamus (melatonin) and gonads influence the production of the signaling transmitter nitric oxide (NO), suggesting that the NO may have an important role in the regulation of gonadotropin-releasing hormone secretion. This further suggests the hypothesis that NO in the epididymis has an important role in the maturation of spermatozoa and their motility and posterior fertilization capacity. The aim of the present study is to investigate the seasonal variations in the morphology of the epididymis by means histochemical and immunocytochemical techniques. We used the NADPH-d, nitric oxide synthase (NOS) I and NOS III to clarify the relationship between epididymis function and NO signaling activity. The results of this work show that NO is present in the caput of epididymis during short photoperiods, i.e., periods of maximum gonadal activity (winter) and absent during long photoperiods, i.e., periods of gonadal regression according to the previously described role of NO in spermatozoa capacitation and motility in the caput epididymis.

Seasonal Morphological Variations and Age‐Related Changes of the Seminal Vesicle of Viscacha (Lagostomus maximus maximus): An Ultrastructural and Immunohistochemical Study

The viscacha is a seasonal rodent that exhibit an annual reproductive cycle with periods of maximum reproductive activity and gonadal regression. We studied seasonal variations in the morphology and cellular population of the seminal vesicles (SVs) during both periods and in impuber animals. Seminal vesicles were studied by light and electronic microscopy. Measurements of epithelial height, nuclear diameter, luminal diameter, and muscular layer were performed. Also, we studied the distribution of androgen receptors (AR) in this gland during the reproductive cycle and in impuber animal. During gonadal regression, principal and clear cells showed signs of reduced functional activity. These were characterized by an epithelium of smaller height, irregular nuclei, and cytoplasm with few organelles, dilated cisterns, and glycogen granules. In impuber animals, the principal cells showed large nuclei with chromatin lax and cytoplasm with small mitochondria, poorly developed Golgi apparatus, and granules of glycogen. On the other hand, the cells exhibited seasonal variations in the distribution and percentage of immunolabeled cells to AR throughout the annual reproductive cycle. During the gonadal regression period, glandular mucosa exhibited numerous epithelial cells with intense nuclear staining. However, fibromuscular stromal cells were weakly positive for AR in contrast to what was observed during the activity period. Considering that testosterone values are lower in adult animals during the period of gonadal regression and in impuber animals, our immunohistochemical results show a significant correlation with the percentage of AR-immunopositive cells. In conclusion, these results demonstrate that the structure of the SVs changes in the activity period of viscacha, probably because of elevated levels of testosterone leading to an increase in the secretory activity of epithelial cells.

RFRP neurons are critical gatekeepers for the photoperiodic control of reproduction

Seasonally breeding mammals rely on the photoperiodic signal to restrict their fertility to a certain time of the year. The photoperiodic information is translated in the brain via the pineal hormone melatonin, and it is now well-established that it is the variation in the duration of the nocturnal peak of melatonin which synchronizes reproduction with the seasons. The Syrian hamster is a long day breeder, and sexual activity is therefore promoted by exposure to a long day photoperiod and inhibited by exposure to a short day photoperiod. Interestingly, in this species electrolytic lesion of the mediobasal hypothalamus abolishes the short day-induced gonadal regression. We have shown that the expression of a recently discovered neuronal population, namely RFamide-related peptide (rfrp) neurons, present in the mediobasal hypothalamus, is strongly down-regulated by melatonin in short day conditions, but not altered by circulating levels of sex steroids. The role of rfrp and its product RFRP-3 in the regulation of reproductive activity has been extensively studied in mammals, and our recent findings indicate that this peptide is a potent stimulator of the reproductive axis in the Syrian hamster. It induces a marked increase in GnRH neuron activity and gonadotropin secretion, and it is able to rescue reproductive activity in short day sexually inactive hamsters. Little is known about the localization of the RFRP-3 receptor, GPR147, in the rodent brain. Accumulating evidence suggests that RFRP-3 could be acting via two intermediates, the GnRH neurons in the preoptic area and the Kiss1 neurons in the arcuate nucleus, but future studies should aim at describing the localization of Gpr147 in the Syrian hamster brain. Altogether our data indicate that the rfrp neuronal population within the mediobasal hypothalamus might be a serious candidate in mediating the photoperiodic effects of melatonin on the regulation of the reproductive axis.

Photoperiod-dependent effects of neuronal nitric oxide synthase inhibition on aggression in Siberian hamsters

Many nontropical species undergo physiological and behavioral adaptations in response to seasonal changes in photoperiod, or day length. In most rodent species, short winter photoperiods reduce testosterone concentrations, which provoke gonadal regression and reduce testosterone-dependent behaviors such as mating and aggression. Seasonally-breeding Siberian hamsters, however, are paradoxically more aggressive in short-days, despite much reduced reproductive activity and testosterone concentrations. Nitric oxide (NO) signaling has been proposed as part of an alternate mechanism underlying this phenomenon. A reduction in neuronal nitric oxide synthase (nNOS), the enzyme responsible for synthesizing NO in the brain, is associated with increased aggression in male short-day hamsters. In the present study, we hypothesized that pharmacological inhibition of nNOS would increase aggressive behavior in long days, but not in short days because nNOS is already reduced. Adult male Siberian hamsters were housed in either long (LD 16:8h) or short (LD 8:16h) photoperiods for 8weeks, then treated with either the selective nNOS inhibitor, 3-bromo-7-nitroindazole (3BrN) or oil vehicle, and subsequently tested for aggression in a resident-intruder test. Treatment with 3BrN increased attack frequency and duration in long days, but had no effect in short days. Short days also reduced testosterone concentrations, without any effect of treatment. These data provide further evidence linking reduced nNOS to elevated short-day aggression and support a role for NO signaling in this phenomenon.

Dim nighttime illumination alters photoperiodic responses of hamsters through the intergeniculate leaflet and other photic pathways

In mammals, light entrains the central pacemaker within the suprachiasmatic nucleus (SCN) through both a direct neuronal projection from the retina and an indirect projection from the intergeniculate leaflet (IGL) of the thalamus. Although light comparable in intensity to moonlight is minimally effective at resetting the phase of the circadian clock, dimly lit and completely dark nights are nevertheless perceived differentially by the circadian system, even when nighttime illumination is below putative thresholds for phase resetting. Under a variety of experimental paradigms, dim nighttime illumination exerts effects that may be characterized as enhancing the plasticity of circadian entrainment. For example, relative to completely dark nights, dimly lit nights accelerate development of photoperiodic responses of Siberian hamsters transferred from summer to winter day lengths. Here we assess the neural pathways underlying this response by testing whether IGL lesions eliminate the effects of dim nighttime illumination under short day lengths. Consistent with previous work, dimly lit nights facilitated the expansion of activity duration under short day lengths. Ablation of the IGL, moreover, did not influence photoperiodic responses in animals held under completely dark nights. However, among animals that were provided dimly lit nights, IGL lesions prevented the short-day typical expansion of activity duration as well as the seasonally appropriate gonadal regression and reduction in body weight. Thus, the present data indicate that the IGL plays a central role in mediating the facilitative effects of dim nighttime illumination under short day lengths, but in the absence of the IGL, dim light at night influences photoperiodic responses through residual photic pathways.

Histología del ovario y ciclo reproductivo de Columbina picui (Temminck, 1813) (Aves: Columbidae) en Córdoba, Argentina

The annual reproductive cycle of Columbina picui was studied and characterized by a structural and quantitative analysis of the ovary. Adult females were collected in Sierra de Macha (Totoral, Cordoba, Argentina) between the years 2005—2006. During the phase of recrudesence gonadal (June to March), three values maximum were determined, the first one was observed in June as a result of the first oviposition of the subadults (individual with external adult characteristics but vestigial Bursa of Fabricius presence), the remaining values were registered one in September and the other one in January, corresponding that of the mature females. The ovaric regression is initiated with the hatching, and the resting stage between April and May, in the most of the birds. Two types of atresia were identified: a) the non bursting type, with no rupture of the follicular wall, which includes lipoidal (primordial follicles) and lipoglandular atresia, (previtelogenic follicles and small vitelogenic follicle), of them were observed during the whole reproductive cycle ; and b) the bursting atresia, with rupture of the follicular wall (vitelogenic follicles > 800 µm) only was observed during the gonadal regression. The abundance of females with capacity of reproducing during most of the year and the extensive phase of gonadal recrudescence were characteristics of the C. picui reproductive cycle.

Peripheral kisspeptin reverses short photoperiod-induced gonadal regression in Syrian hamsters by promoting GNRH release

In seasonal breeders, reproduction is synchronised by day length via the pineal hormone melatonin. In short winter days (short day, SD), the Syrian hamster displays a complete gonadal atrophy together with a marked reduction in expression of kisspeptins (Kp), a family of potent hypothalamic stimulators of GNRH neurons. Both central and peripheral acute injections of Kp have been reported to activate the gonadotropic axis in mammals. The aim of this study was to determine if and how peripheral administration of Kp54 could restore gonadal function in photo-inhibited hamsters. Testicular activity of hamsters kept in SD was reactivated by two daily i.p. injections of Kp54 but not by chronic subcutaneous delivery of the same peptide via mini-pumps. Acute i.p. injection of Kp54-induced FOS (c-Fos) expression in a large number of GNRH neurons and pituitary gonadotrophs together with a strong increase in circulating testosterone. The activation of pituitary cells by Kp was inhibited by preadministration of the GNRH receptor antagonist acyline. Altogether, our results demonstrate that peripheral Kp54 activates the gonadotropic axis by stimulating GNRH release and indicate that an appropriate protocol of long-term systemic Kp administration can recrudesce a photo-inhibited reproductive axis.

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

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

An Intact Dorsomedial Hypothalamic Nucleus, but Not the Subzona Incerta or Reuniens Nucleus, Is Necessary for Short-Day Melatonin Signal-Induced Responses in Siberian Hamsters

Siberian hamsters provide a useful model to define mechanisms underlying obesity reversal as they naturally transition from their extreme seasonal obesity in long ‘summer-like’ days (LDs) to a leaner state in short ‘winter-like’ days (SDs). These day length changes are coded into durational melatonin (MEL) signals by the pineal gland resulting in stimulation of MEL receptors (MEL_1a-Rs). MEL_1a-R mRNA is colocalized centrally in sympathetic nervous system (SNS) outflow neurons comprising a chain of neurons that ultimately innervates white adipose tissue (WAT). Neural components in this circuit include the subzona incerta (subZI), dorsomedial hypothalamic nucleus (DMH) and thalamic reuniens nucleus (ReN). SD, long-duration MEL signals induce gonadal regression and increase WAT SNS drive triggering lipolysis and thereby reversing LD obesity. We attempted to block the reversal of SD MEL signal-induced obesity by making electrolytic or sham lesions of the subZI, ReN or DMH in LD-housed hamsters. To create SD-like, long-duration MEL signals, we injected MEL 3 h before lights out, thereby lengthening the naturally occurring nocturnal duration of circulating MEL. ReN and subZI lesions did not block SD-like MEL signal-induced decreases in body, WAT, testicular masses or food intake; by contrast, DMH lesions blocked decreases in WAT and testicular mass. This nonresponsiveness was not due to lesion-induced inappropriate nocturnal LD MEL secretion that would have altered our creation of SD-like signals. Therefore, the DMH appears to participate in the control of both SD energy and reproductive responses, and joins the suprachiasmatic nucleus as sites necessary for SD responses in this species.

Effect of climatic variables on seasonal morphological changes in the testis and epididymis in the wild rodent Microcavia australis from the andes mountains, Argentina

It has been shown that seasonal changes, especially in arid areas have a large influence on gonadal changes of the species that inhabit these areas. We studied a wild hystricomorph Microcavia australis in its natural habitat in the arid Andes Mountains. Sampling of adult males was carried out every 2 months. After autopsy, testes and epididymides were weighed and processed to obtain histological samples. Testes were analyzed with a microscope to measure seminiferous tubule area and diameter for each sampled month. Epididymides were used to determine spermatozoon storage in the cauda region. Results illustrate morphological changes in the testis and epididymis along the year. A high output of sperm cells was detected from middle winter to middle summer and a complete shutdown of spermatogenesis at the end of summer. The initiation of testicular activity coincided with month with the shortest day length, in dry season and very low temperature. On the other hand, gonadal regression started in the middle of summer with long day length, in the wet season, and high temperatures. Rainfall, temperature, and day length seem to be important for the testis cycle. We suggest that photoperiod could be a good predictor for an oncoming period suitable for breeding, and males may probably use it as a signal to regulate gonadal activity.

Seasonal regulation of reproduction: altered role of melatonin under naturalistic conditions in hamsters

The seasonal reproductive cycle of photoperiodic rodents is conceptualized as a series of discrete melatonin-dependent neuroendocrine transitions. Least understood is the springtime restoration of responsiveness to winter-like melatonin signals (breaking of refractoriness) that enables animals to once again respond appropriately to winter photoperiods the following year. This has been posited to require many weeks of long days based on studies employing static photoperiods instead of the annual pattern of continually changing photoperiods under which these mechanisms evolved. Maintaining Siberian hamsters under simulated natural photoperiods, we demonstrate that winter refractoriness is broken within six weeks after the spring equinox. We then test whether a history of natural photoperiod exposure can eliminate the requirement for long-day melatonin signalling. Hamsters pinealectomized at the spring equinox and challenged 10 weeks later with winter melatonin infusions exhibited gonadal regression, indicating that refractoriness was broken. A photostimulatory effect on body weight is first observed in the last four weeks of winter. Thus, the seasonal transition to the summer photosensitive phenotype is triggered prior to the equinox without exposure to long days and is thereafter melatonin-independent. Distinctions between photoperiodic and circannual seasonal organization erode with the incorporation in the laboratory of ecologically relevant day length conditions.

Differential DMRT1 Expression in the Gonads of Podarcis sicula (Reptilia: Lacertidae)

DMRT genes encode a large family of transcription factors which share an unusual cysteine-rich DNA-binding motif, the DM domain. DM family members have been studied in the context of sexual development; in particular, the DMRT1 gene appeared to be the one most directly involved in sex determination, but its activity is largely unexplored and possible downstream targets of this factor have yet to be identified. DMRT1 of the lacertid lizard Podarcis sicula (PsDMRT1) was isolated as a model to study differential gene expression during the seasonal reproductive cycle of an ectothermal species. The adult testis of P. sicula exhibits full activity in spring, complete regression in summer and a slow autumnal recrudescence without spermiation. We cloned a 591-bp partial ORF of the PsDMRT1 fragment, whose putative amino acid sequence contains the conserved DM domain. Northern blot analysis of mRNA from different tissues of P. sicula individuals captured in spring demonstrated DMRT1 transcripts only in testis. Semi-quantitative RT-PCR and in situ hybridization experiments showed peak PsDMRT1 expression in spring, lower expression in autumn and no expression during the period of gonad regression. A possible correlation between androgen level variations and PsDMRT1 transcripts is hypothesized and discussed. Finally, data showed that PsDMRT1 is expressed only in spermatogenic cells, before the second meiotic division, suggesting that its role is confined to the proliferation and maintenance of spermatogonia and spermatocytes.

Photoperiodic responses are modulated by voluntary exercise in Djungarian hamsters (Phodopus sungorus)

Introduction: In response to short day (SD) lengths, Phodopus sungorus reduces body mass, size and activity of the gonads and changes into a whitish winter pelage. In previous studies we reported a reversal of the seasonal reduction in body mass and an impairment of seasonal gonadal regression in Phodopus with access to a running wheel (RW) in SDs. However, the seasonal change to a whitish winter fur was not affected by RW activity, suggesting that the SD signal can be read, but is overridden by a physiological response to RW activity. To understand how RW activity influences the occurrence of SD physiology, we examined its effect on physiological parameters and genes regulated by photoperiod that are important for the seasonal adaptation of Phodopus sungorus.