Short-day enhancement of immune function is independent of steroid hormones in deer mice (Peromyscus maniculatus)

The effects of photoperiod and steroid hormones on immune function were assessed in male and female deer mice (Peromyscus maniculatus). In experiment 1, male deer mice were castrated, castrated and given testosterone replacement, or sham-operated. Half of each experimental group were subsequently housed in either long (LD 16:8) or short days (LD 8:16) for 10 weeks. Short-day deer mice underwent reproductive regression and displayed elevated lymphocyte proliferation in response to the T-cell mitogen concanavalin A, as compared to long-day mice. In experiment 2, female deer mice were ovariectomized, ovariectomized and given estrogen replacement, or sham-operated. Animals from each of these experimental groups were subsequently housed in either LD 16:8 or LD 8:16 for 10 weeks. Short-day deer mice underwent reproductive regression and displayed reduced serum estradiol concentrations and elevated lymphocyte proliferation in response to concanavalin A, as compared to long-day mice. Surgical manipulation had no effect on lymphocyte proliferation in either male or female deer mice. Neither photoperiod nor surgical manipulation affected serum corticosterone concentrations. These results confirm that both male and female deer mice housed in short days enhance immune function relative to long-day animals. Additionally, short-day elevation in splenocyte proliferation appears to be independent of the influence of steroid hormones in this species. Accepted: 17 April 1998     

Photoperiod alters central distribution of estrogen receptor alpha in brain regions that regulate aggression

Testosterone or its metabolite, estrogen, regulates aggression in males of many mammalian species. Because plasma testosterone levels are typically positively correlated with both aggression and reproduction, aggression is expected to be higher when males are in reproductive condition. However, in some photoperiodic species such as Siberian hamsters (Phodopus sungorus), males are significantly more aggressive in short day lengths when the testes are regressed and circulating testosterone concentrations are reduced. These results led to the formation of the hypothesis that aggression is modulated independently of circulating steroids in Siberian hamsters. Thus, recent studies have been designed to characterize the role of other neuroendocrine factors in modulating aggression. However, aggression may be mediated by testosterone or estrogen despite basal concentrations of these steroids by increasing sensitivity to steroids in specific brain regions. Consistent with this hypothesis, we found that males housed under short days have increased expression of estrogen receptor alpha in the bed nucleus of the stria terminalis, medial amygdala, and central amygdala. Neural activation in response to an aggressive encounter was also examined across photoperiod.     

Social environment modulates photoperiodic immune and reproductive responses in adult male white-footed mice (Peromyscus leucopus)

Social cues may interact with photoperiod to regulate seasonal adaptations in photoperiod-responsive rodents. Specifically, photoperiod-induced adjustments (e.g., reproduction and immune function) may differ among individuals in heterosexual pairs, same-sex pairs, or isolation. Heterosexual cues may be more influential, based on their potential fitness value, than same-sex cues or no social cues. The present study examined the effects of pair (with a male or female) or individual housing on reproductive and immune responses in male white-footed mice (Peromyscus leucopus) maintained in long or short photoperiods. Female pairing did not affect reproductive responses in short-day males. In long days, however, the presence of a female increased both testosterone concentrations and testes mass compared with individually housed and male-paired mice, respectively. Short-day, individually housed males enhanced delayed-type hypersensitivity (DTH) responses compared with single-housed mice in long days, but all paired groups decreased DTH responses regardless of photoperiod. The lack of enhanced DTH response in male mice paired with females coincided with reduced circulating corticosterone concentrations in both photoperiod treatments. Together, these results suggest that social environment may have important modulatory effects on photoperiod-regulated immune responses in male white-footed mice.     

Paternal aggression in a biparental mouse: parallels with maternal aggression

Environmental and social factors have important effects on aggressive behaviors. We examined the effect of reproductive experience on aggression in a biparental species of mouse, Peromyscus californicus. Estrogens are important in mediating aggressive behavior so we also examined estrogen receptor expression and c-fos for insights into possible mechanisms of regulation. Parental males were significantly more aggressive than virgin males, but no significant differences in estrogen receptor alpha or beta expression were detected. Patterns of c-fos following aggression tests suggested possible parallels with maternal aggression. Parental males had more c-fos positive cells in the medial amygdala, and medial preoptic area relative to virgin males. The medial preoptic area is generally considered to be relatively less important for male-male aggression in rodents, but is known to have increased activity in the context of maternal aggression. We also demonstrated through habituation-dishabituation tests that parental males show exaggerated investigation responses to chemical cues from a male intruder, suggesting that heightened sensory responses may contribute to increased parental aggression. These data suggest that, in biparental species, reproductive experience leads to the onset of paternal aggression that may be analogous to maternal aggression.     

Sex differences in hormonal responses to social conflict in the monogamous California mouse

Monogamous species are usually considered to be less likely to exhibit sex differences in behavior or brain structure. Most previous studies examining sex differences in stress hormone responses have used relatively sexually dimorphic species such as rats. We examined the stress hormone responses of monogamous California mice (Peromyscus californicus) to resident-intruder tests. We also tested males and females under different photoperiods, because photoperiod has been shown to affect both aggression and stress hormone responses. Females, but not males showed a significant increase in corticosterone levels immediately following a resident-intruder test. Males but not females showed elevated corticosterone levels under short days. Females tested in aggression tests also showed a significant increase in plasma oxytocin levels, but only when housed in long days. This was consistent with our observation that females but not males had more oxytocin positive cells in the paraventricular nucleus (PVN) when housed under long days. Our data show that sex differences in glucocorticoid responses identified in other rodents are present in a monogamous species.     

Opposing hormonal mechanisms of aggression revealed through short-lived testosterone manipulations and multiple winning experiences

Territorial aggression is influenced by many social and environmental factors. Since aggression is a costly behavior, individuals should account for multiple factors such as population density or reproductive status before engaging in aggression. Previous work has shown that male California mice (Peromyscus californicus) respond to winning aggressive encounters by initiating aggression more quickly in future encounters, and we investigated the physiological basis for this effect. We found that injections that produced a transient increase in testosterone (T) following an aggressive encounter caused males to behave more aggressively in an encounter the following day. Experience alone was not enough to change aggression, as males treated with saline injections showed no change in aggression. The effect of T injections on aggression was androgen-based, as the inhibition of aromatase did not block the T injections from increasing aggression. Aromatase inhibition did, however, increase aggression in the initial aggression tests (before application of T or saline injections), and aromatase activity in the bed nucleus of the stria terminalis (BNST) was negatively correlated with aggression. A previous study suggested that aromatase activity in the BNST decreases after males become fathers. Thus, distinct neuroendocrine mechanisms allow male California mice to adjust aggressive behavior in response to changes in social and reproductive status.     

Housing condition alters immunological and reproductive responses to day length in Siberian hamsters (Phodopus sungorus)

During winter, increased thermoregulatory demands coincide with limited food availability necessitating physiological tradeoffs among expensive physiological processes resulting in seasonal breeding among small mammals. In the laboratory, short winter-like day lengths induce regression of the reproductive tract, but also enhance many aspects of immune function. It remains unspecified the extent to which bolstered immune responses in short days represent enhanced immune function per se compared to long days or represents energetic disinhibition mediated by the regression of the reproductive tract. Cohabitation of male Siberian hamsters with intact female conspecifics can block short-day reproductive regression. We sought to determine whether female cohabitation could also block the enhanced immune function associated with short days. Adult male Siberian hamsters were housed in long or short day lengths in one of three housing conditions: (1) single-housed, (2) housed with a same sex littermate, or (3) housed with an ovariectomized female. Delayed-type hypersensitivity (DTH) responses were assessed after 8 weeks of photoperiod treatment. Housing with an ovariectomized female was not sufficient to block short-day reproductive regression, but prevented short-day enhancement of DTH responses. Housing with a male littermate did not alter reproductive or immune responses in either photoperiod. These data suggest that short day enhancement of immune function is independent of photoperiod-mediated changes in the reproductive system.     

Nongenomic effects of estradiol on aggression under short day photoperiods

In several vertebrate species, the effects of estrogens on male aggressive behavior can be modulated by environmental cues. In song sparrows and rodents, estrogens modulate aggression in the nonbreeding season or winter-like short days, respectively. The behavioral effects of estrogens are rapid, which generally is considered indicative of nongenomic processes. The current study further examined the hypothesis that estradiol acts nongenomically under short days by utilizing a protein synthesis inhibitor, cycloheximide (CX). Mice were housed in either short or long day photoperiods, and treated with an aromatase inhibitor. One hour before resident–intruder testing mice were injected with either CX or saline vehicle, and 30 min later were treated orally with either cyclodextrin conjugated estradiol or vehicle. Under short days, mice treated with estradiol showed a rapid decrease in aggressive behavior, independent of CX administration. CX alone had no effect on aggression. These results show that protein synthesis is not required for the rapid effects of estradiol on aggression, strongly suggesting that these effects are mediated by nongenomic processes. We also showed that estradiol suppressed c-fos immunoreactivity in the caudal bed nucleus of the stria terminalis under short days. No effects of estradiol on behavior or c-fos expression were observed in mice housed under long days. Previously we had also demonstrated that cage bedding influenced the directional effects of estrogens on aggression. Here, we show that the phenomenon of rapid action of estradiol on aggression under short days is a robust result that generalizes to different bedding conditions.     

Differential reproductive response to short photoperiod in deer mice: role of melatonin

Summary Inhibitory photoperiod differentially effects reproduction in deer mice (Peromyscus maniculatus nebrascensis). Pituitary-testicular function is arrested in about one-third of short-day exposed males (reproductively responsive mice), while an equal number remain fertile (reproductively nonresponsive mice). Both phenotypes are found in natural populations and their disparate reproductive responses have a genetic basis. To assess whether this difference is attributable to a prepineal/pineal or post-pineal mechanism, we compared spermatogenic responses of known and unknown phenotype to exogenous melatonin. Melatonin significantly reduced mean sperm number in long-day housed mice of unknown phenotype. But, individual responses ranged from azoospermia to normal spermatogenesis, and this range was not significantly different from that previously recorded for short-day exposed mice. Reproductively nonresponsive males were unaffected by melatonin administration when housed under long or short daylength. In contrast, melatonin significantly suppressed sperm production in reproductively responsive males housed under long photoperiod, but had no additional suppressive effect in short-day housed mice with regressed testes. These data demonstrate that melatonin is only effective in eliciting testicular regression in reproductively responsive males. Taken together, these results suggest that differential testicular response to photoperiod are caused by a post-pineal mechanism.Abbreviations LD long day - SD short day - 16L:8D 16 h light, 8 h dark - 8L:16D 8 h light, 16 h dark     

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.     

Photoperiodic regulation of adrenal hormone secretion and aggression in female Syrian hamsters

Seasonal changes in the length of the daily photoperiod induce significant changes in social behavior. Hamsters housed in winter-like short photoperiods (SP) can express significantly higher levels of aggression than hamsters housed in long photoperiods (LP) that mimic summer. The mechanisms responsible for increasing aggressiveness in SP-exposed female hamsters are not well understood but may involve seasonal changes in the endocrine system. In experiment 1, the effects of SP exposure on the circulating levels of three adrenal hormones were determined. Short photoperiod exposure was found to significantly depress the circulating levels of cortisol and the adrenal androgen dehydropiandrosterone (DHEA) but significantly increased the circulating levels of the sulfated form of DHEA, DHEAS. Experiment 2 examined the effects of gonadal hormones on several different measures of aggression including its intensity in females housed in both long and short photoperiod. Exposure to SP resulted in high levels of aggression regardless of the endocrine state of the animal or the measure used to quantify aggression. In contrast, administration of estradiol to hamsters housed in LP significantly reduced aggression. The data of the present study support the hypothesis that SP-housed females are more aggressive than LP-housed females because SP exposure renders females insensitive to the aggression-reducing effects of ovarian hormones.     

Geographic differences for delay of sexual maturation in Peromyscus leucopus: effects of photoperiod, pinealectomy, and melatonin

Effects of short-day photoperiod, pinealectomy, and melatonin on sexual maturation were tested in Peromyscus leucopus from either Connecticut (CT) or Georgia (GA). Laboratory reared-stocks from CT and GA were exposed to short daylength (photoperiod) from birth or 25 days of age. At 12 wk of age, delay in sexual maturation was indicated in most CT mice by decreased testis length, combined testes weight, and seminal vesicle weight. Conversely, GA animals did not delay sexual maturation when exposed to short-day photoperiod from either birth or 25 days of age. These results indicate that responses to short daylengths differ for juvenile CT and GA populations. In a second experiment, pinealectomized or sham-operated CT males were exposed to short-day (9L:15D) or long-day (16L:8D) photoperiod from birth. Pinealectomy blocked the effect of short daylength on reproduction. Therefore, the pineal must be involved in the delay of sexual maturation observed for short-day CT mice. The effects of melatonin, a pineal gland hormone, were tested with chronic s.c. implants or daily injections. In CT mice given either melatonin implants or afternoon injections, sexual maturation was delayed. GA mice were insensitive to all melatonin treatments. Further, no differences in circadian organization (phase angle, duration of activity, period under constant dark) between GA and CT animals were apparent. Collectively, these studies indicate that melatonin is involved in the mechanism responsible for delay of sexual maturation in CT mice. Short-day insensitivity of GA Peromyscus leucopus probably results from a deficiency in the melatonin effector pathway and is not due to a disruption of circadian organization.     

Short-day increases in aggression are independent of circulating gonadal steroids in female Siberian hamsters (Phodopus sungorus)

Among the suite of adaptations displayed by seasonally-breeding rodents, individuals of most species display reproductive regression and concomitant decreases in gonadal steroids during the winter. In addition, some species display increased aggression in short "winter-like" days compared with long "summer-like" day lengths. For example, male Syrian and Siberian hamsters held in short days express heightened levels of aggression that are independent of gonadal steroids. Virtually nothing is known, however, regarding seasonal aggression in female Siberian hamsters (Phodopus sungorus). Studies were undertaken to determine female levels of aggression in long and short days as well as the role of gonadal steroids in mediating this behavior. In Experiment 1, females were housed in long or short days for 10 weeks and resident-intruder aggression was assessed. Prior to testing, estrous cycle stages were determined by vaginal cytology and females were tested during both Diestrus I and Proestrus. In Experiment 2, hormone levels were experimentally manipulated; long-day females were ovariectomized (OVx) or given sham surgeries whereas short-day females were implanted with capsules containing 17beta-estradiol (E(2)) or Progesterone (P). In Experiment 3, both long- and short-day females were ovariectomized and implanted with either an exogenous E(2) or blank capsule, or given a sham surgery. Short-day hamsters displayed increased aggression relative to long-day females. Aggression was not affected by estrous stage. There was no difference in aggression between long-day OVx and sham animals. Furthermore, neither exogenous E(2) nor P had any significant effect on aggression. These results support previous findings of increased non-breeding aggression and suggest that short-day aggression is not likely mediated by circulating levels of gonadal steroids. These results also suggest that the endocrine regulation of seasonal aggression may be similar between the sexes.     

Photoperiodic stimulation of pubertal development in male deer mice: involvement of the circadian system

Pubertal development in prairie deer mice (Peromyscus maniculatus bairdii) is accelerated by exposure of juveniles to a long-day photoperiod, and, conversely, retarded by exposure to short days. The purpose of the present study was to evaluate the possible involvement of the circadian system in the photoperiodic regulation of puberty. Weanling males, previously housed on a short-day light cycle of 6L:18D, were subjected to a "resonance" protocol in which they received one of the following light cycles: 6L:18D, 6L:30D, 6L:42D, 6L:54D, or 16L:8D. Post-weaning exposure to cycles of 16L:8D, 6L:30D, and 6L:54D stimulated reproductive organ growth as measured at 6 weeks of age. Exposure to cycles of 6L:18D and 6L:42D failed to stimulate reproductive development. These data support the hypothesis that young male deer mice use a circadian rhythm of responsiveness to light to measure photoperiodic time and, consequently, regulate pubertal development.     

Short days and exogenous melatonin increase aggression of male Syrian hamsters (Mesocricetus auratus)

Many nontropical rodent species rely on photoperiod as a primary cue to coordinate seasonally appropriate changes in physiology and behavior. Among these changes, some species of rodents demonstrate increased aggression in short, "winter-like" compared with long "summer-like" day lengths. The precise neuroendocrine mechanisms mediating changes in aggression, however, remain largely unknown. The goal of the present study was to examine the effects of photoperiod and exogenous melatonin on resident-intruder aggression in male Syrian hamsters (Mesocricetus auratus). In Experiment 1, male Syrian hamsters were housed in long (LD 14:10) or short (LD 10:14) days for 10 weeks. In Experiment 2, hamsters were housed in long days and half of the animals were given daily subcutaneous melatonin injections (15 microg/day in 0.1 ml saline) 2 h before lights out for 10 consecutive days to simulate a short-day pattern of melatonin secretion, while the remaining animals received injections of the vehicle alone. Animals in both experiments were then tested using a resident-intruder model of aggression and the number of attacks, duration of attacks, and latency to initial attack were recorded. In Experiment 1, short-day hamsters underwent gonadal regression and displayed increased aggression compared with long-day animals. In Experiment 2, melatonin treatment also increased aggression compared with control hamsters without affecting circulating testosterone. Collectively, the results of the present study demonstrate that exposure to short days or short day-like patterns of melatonin increase aggression in male Syrian hamsters. In addition, these results suggest that photoperiodic changes in aggression provide an important, ecologically relevant model with which to study the neuroendocrine mechanisms underlying aggression in rodents.     

Phenotypic differences in the GnRH neuronal system of deer mice Peromyscus maniculatus under a natural short photoperiod

The neural mechanisms by which short photoperiod induces gonadal regression among seasonally breeding mammals are not well understood. One hypothesis suggests that the proximate cause of seasonal gonadal regression is a photoperiod-induced modification in GnRH secretion. This hypothesis is indirectly supported by our recent findings using immunocytochemistry which identified specific photoperiod-induced adjustments in the number and morphology of GnRH containing neurones between reproductively competent and reproductively regressed laboratory housed male deer mice. Herein, we report that the GnRH neuronal system is similarly affected in reproductively responsive and nonresponsive wild male deer mice Peromyscus maniculatus exposed to a natural short photoperiod. The distribution of immunoreactive (IR)-GnRH neurones was nearly identical in field caught animals and those housed under artificial photoperiod in the laboratory. Compared with reproductively nonresponsive males, reproductively responsive mice from the field population possessed a greater total number of IR-GnRH neurones, a greater number of IR-GnRH neurones within the lateral hypothalamus, and a greater proportion of bipolar IR-GnRH neurones. Each of these distributional and morphological characters was consistent with our findings in laboratory housed male deer mice exposed to an artificial short photoperiod. Taken together, these data underscore the validity of using an artificial photoperiod to evaluate seasonal adjustments in reproductive function in the laboratory.     

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.     

Effect of photoperiod on vasopressin-induced aggression in Syrian hamsters

Syrian hamsters are photoperiodic and become sexually quiescent when exposed to short "winter-like" photoperiods. In short photoperiods, male hamsters display significantly higher levels of aggression than males housed in long photoperiods. Arginine-vasopressin (AVP) within the anterior hypothalamus (AH) has been reported to modulate aggression in hamsters housed in long photoperiods. Previous studies have shown that AVP can facilitate aggression and its effects appear to be mediated by AVP V(1a) receptors (V(1a)R). In the present study, we investigated whether the increased levels of aggression observed after exposure to short photoperiod were the result of an increased responsiveness to AVP within the AH. Injections of AVP into the AH significantly increased aggression in hamsters housed in a long photoperiod, but had no effect in hamsters housed in a short photoperiod. In addition, injection of a V(1a)R antagonist into the AH significantly inhibited aggression in hamsters housed in long photoperiod, but had no effect in hamsters housed in a short photoperiod. These findings indicate that AVP within the AH increases aggression in hamsters housed in long photoperiods, but not in hamsters housed in short photoperiods.     

Photoperiod-dependent response to androgen in the medial amygdala of the Siberian hamster,Phodopus sungorus

The medial nucleus of the amygdala (MeA) is a steroid-sensitive region that has been implicated in the expression of behaviors such as mating and aggression. The male Siberian hamster (Phodopus sungorus) uses light cues to regulate its reproductive neuroendocrine system, reducing androgen synthesis in the autumn and increasing it in the spring. There is also evidence that short photoperiods reduce the sensitivity of the brain to the behavioral effects of androgen. The authors tested the hypothesis that MeA neurons are less responsive to androgen in short photoperiods by comparing the regional volume and average soma size of the four MeA subnuclei (anterodorsal [MeAD], anteroventral [MeAV], posterodorsal [MePD], and posteroventral) in adult male hamsters that had been castrated and then implanted with capsules containing either testosterone (T) or nothing. Animals from each group were housed in either long or short photoperiods for 15 weeks. MeAD and MeAV somata displayed photoperiod-dependent responses to androgen, increasing in size after T treatment only in long days. In contrast, the average soma size and the regional volume of the MePD subnucleus were significantly larger in T-treated males regardless of photoperiod. The authors conclude that photoperiod influences the sensitivity of the MeA to androgen.     

Importance of duration of nocturnal melatonin secretion in determining the reproductive response to inductive photoperiod in the ewe

The pineal gland, through its nocturnal melatonin secretion, mediates the effects of inhibitory (long) and stimulatory (short) photoperiod on reproduction in female sheep. Earlier studies revealed that duration of the nighttime melatonin rise is important in determining the inhibitory effect of day length on reproduction in the ewe. The present study tested whether the duration is also important in mediating the inductive response to short days. Pinealectomized ewes, housed under long days, received a short-day melatonin infusion (16-h duration) for 90 days. Reproductive status was monitored from the response to estradiol negative feedback of luteinizing hormone (LH) secretion. This short-day melatonin pattern led to unambiguous reproductive induction, despite the exposure to inhibitory long days. The increase in serum LH was comparable, in terms of latency and magnitude, to that in pinealectomized controls receiving the same short-day melatonin pattern under short days, and in pineal-intact controls transferred from long to short days. Since the reproductive status conformed to the length of time that melatonin was elevated each day rather than to photoperiod, these results support the conclusion that duration of the nighttime melatonin rise mediates the reproductive response of the ewe to an inductive photoperiod. In all, the melatonin rhythm is considered an integral component of the physiologic mechanism measuring day length; through duration of its nocturnal secretion, melatonin encodes both inhibitory and stimulatory photoperiods.