Decoding photoperiodic time and melatonin in mammals: what can we learn from the pars tuberalis?

Seasonal changes in activity of hypothalamic thyroid hormone system in different winter phenotypes of Djungarian hamster (Phodopus sungorus)

Although the Djungarian hamster (Phodopus sungorus) is a seasonality model, it presents substantial variability in winter acclimation. In response to short photoperiod, some individuals express a suite of winter traits such as low body mass, regressed gonads, white fur, and daily torpor, while others develop only some adjustments or maintain a summer phenotype. Despite comprehensive research, the mechanisms underlying polymorphism of winter phenotype are still unknown. We compared key elements of the hypothalamic thyroid hormone system, as well as the tanycyte architecture in hamsters of both sexes. Individuals presented different responses to short photoperiod characterized either as phenotypes (non-responder, partial-responder and full-responder) or photoresponsive index. We measured the expression of genes coding iodothyronine deiodinase 2 and 3, monocarboxylate transporter 8, thyrotropin-releasing hormone, and somatostatin in 40 individuals and counted the number of immunolabeled tanycyte processes in standardized regions of interest around the third ventricle in 30 individuals. Animals acclimated to short photoperiod presented a downregulation of diodinase 2 and somatostatin and an upregulation of deiodinase 3, as well as a decreased number of tanycyte processes, compared to long photoperiod-exposed individuals. Although phenotypes did not differ in gene expression, the higher the photoresponsive index, the lower was the deiodinase 2 expression and the higher the deiodinase 3 expression. Partial-responders and full-responders had less tanycyte processes than non-responders, and the number of tanycyte processes correlated with the photoresponsive index. Sexes differed neither in their seasonal response, nor hypothalamic gene expression, but females had more tanycyte processes. Our results are in accordance with studies emphasizing the pivotal role of thyroid hormones in seasonal response. We suggest that the whole spectrum of winter phenotypes exists within the population of Djungarian hamsters and that it is reflected also at the level of neuroendocrine regulation. However, the neuroendocrine underpinnings of winter phenotype polymorphism require further investigation.

Modulation of bone morphogenetic protein activity by melatonin in ovarian steroidogenesis

BACKGROUND

Melatonin regulates circadian and seasonal rhythms and the activities of hormones and cytokines that are expressed in various tissues, including the ovary, in which melatonin receptors are expressed. In the ovary, follicular growth occurs as a result of complex interactions between pituitary gonadotropins and autocrine and paracrine factors, including bone morphogenetic proteins (BMPs) that are expressed in the ovary.

METHODS

The effects of melatonin and BMPs on steroidogenesis were examined by using the primary cultures of rat granulosa cells.

MAIN FINDINGS RESULTS

It was shown that melatonin has antagonistic effects on BMP-6 actions in the granulosa cells, suggesting that melatonin is likely to contribute to balancing the biological activity of endogenous BMPs that maintain progesterone production and luteinization in the growing follicles. Similar interactions between melatonin and BMP-smad signaling also were shown in the mechanism of controlling ovarian steroidogenesis by other ligands.

CONCLUSION

A new role of melatonin in the regulation of endocrine homeostasis in relation to BMP activity is introduced in this review.

Commentary: miR-132/212 Modulates Seasonal Adaptation and Dendritic Morphology of the Central Circadian Clock

Daily rhythms in behavior and physiology are coordinated by an endogenous clock located in the suprachiasmatic nucleus (SCN) of the hypothalamus. This central pacemaker also relays day length information to allow for seasonal adaptation, a process for which melatonin signaling is essential. How the SCN encodes day length is not fully understood. MicroRNAs (miRNAs) are small, non-coding RNAs that regulate gene expression by directing target mRNAs for degradation or translational repression. The miR-132/212 cluster plays a key role in facilitating neuronal plasticity, and miR-132 has been shown previously to modulate resetting of the central clock. A recent study from our group showed that miR-132/212 in mice is required for optimal adaptation to seasons and non-24-hour light/dark cycles through regulation of its target gene, methyl CpG-binding protein (MeCP2), in the SCN and dendritic spine density of SCN neurons. Furthermore, in the seasonal rodent Mesocricetus auratus (Syrian hamster), adaptation to short photoperiods is accompanied by structural plasticity in the SCN independently of melatonin signaling, thus further supporting a key role for SCN structural and, in turn, functional plasticity in the coding of day length. In this commentary, we discuss our recent findings in context of what is known about day length encoding by the SCN, and propose future directions.

Interaction of Melatonin and BMP-6 in Ovarian Steroidogenesis

The bone morphogenetic protein (BMP) system in the ovary plays a physiological role as a luteinization inhibitor in growing follicles. BMP-6 secreted from oocytes and granulosa cells can exert an inhibitory effect on follicle-stimulating hormone (FSH) actions by suppressing adenylate cyclase activity downstream of the FSH receptor. The inhibition of FSH-induced progesterone production by BMP-6 is impaired by melatonin treatment in granulosa cells. Intracellular Smad signaling induced by BMP-6 is suppressed by melatonin, suggesting that melatonin has a regulatory role in BMP receptor signaling in granulosa cells. Since the expression of BMP-6 in granulosa cells is increased in patients with polycystic ovary syndrome, melatonin may play an important role in the maintenance of progesterone production by suppressing BMP-6 signaling, leading to the preservation of ovarian function.

miR-132/212 Modulates Seasonal Adaptation and Dendritic Morphology of the Central Circadian Clock

The central circadian pacemaker, the suprachiasmatic nucleus (SCN), encodes day length information by mechanisms that are not well understood. Here, we report that genetic ablation of miR-132/212 alters entrainment to different day lengths and non-24 hr day-night cycles, as well as photoperiodic regulation of Period2 expression in the SCN. SCN neurons from miR-132/212-deficient mice have significantly reduced dendritic spine density, along with altered methyl CpG-binding protein (MeCP2) rhythms. In Syrian hamsters, a model seasonal rodent, day length regulates spine density on SCN neurons in a melatonin-independent manner, as well as expression of miR-132, miR-212, and their direct target, MeCP2. Genetic disruption of Mecp2 fully restores the level of dendritic spines of miR-132/212-deficient SCN neurons. Our results reveal that, by regulating the dendritic structure of SCN neurons through a MeCP2-dependent mechanism, miR-132/212 affects the capacity of the SCN to encode seasonal time.

Melatonin counteracts BMP-6 regulation of steroidogenesis by rat granulosa cells

The ovarian bone morphogenetic protein (BMP) system is a physiological inhibitor of luteinization in growing ovarian follicles. BMP-6, which is expressed in oocytes and granulosa cells of healthy follicles, specifically inhibits FSH actions by suppressing adenylate cyclase activity. In the present study, we studied the role of melatonin in ovarian steroidogenesis using rat primary granulosa cells of immature female rat ovaries by focusing on the interaction with BMP-6 activity. Treatment with melatonin had no direct effect on FSH-induced progesterone or estradiol production by granulosa cells, and the results were not affected by the presence of co-cultured oocytes. In addition, synthesis of cAMP by granulosa cells was not significantly altered by melatonin treatment. To elucidate the interaction between activities of melatonin and BMPs, the effect of melatonin treatment on suppression of progesterone synthesis by BMP-6 was investigated. Interestingly, the inhibitory effect of BMP-6 on FSH-induced progesterone production was impaired by co-treatment with melatonin. Granulosa cells express higher levels of MT1 than MT2, and BMP-6 had no significant effect on MT1 expression in granulosa cells. However, BMP-6-induced Smad1/5/8 phosphorylation and Id-1 transcription were suppressed by melatonin, suggesting that melatonin has an inhibitory effect on BMP receptor signaling in granulosa cells. Although the expression levels of ALK-2, -6, ActRII and BMPRII were not affected by melatonin, inhibitory Smad6, but not Smad7, expression was upregulated by melatonin. Thus, melatonin plays a role in the regulation of BMP-6 signal intensity for controlling progesterone production in the ovary. These findings suggest that the effect of melatonin on maintenance of ovarian function is, at least in part, due to the regulation of endogenous BMP activity in granulosa cells.

Clocks for all seasons: unwinding the roles and mechanisms of circadian and interval timers in the hypothalamus and pituitary

Adaptation to the environment is essential for survival, in all wild animal species seasonal variation in temperature and food availability needs to be anticipated. This has led to the evolution of deep-rooted physiological cycles, driven by internal clocks, which can track seasonal time with remarkable precision. Evidence has now accumulated that a seasonal change in thyroid hormone (TH) availability within the brain is a crucial element. This is mediated by local control of TH-metabolising enzymes within specialised ependymal cells lining the third ventricle of the hypothalamus. Within these cells, deiodinase type 2 enzyme is activated in response to summer day lengths, converting metabolically inactive thyroxine (T4) to tri-iodothyronine (T3). The availability of TH in the hypothalamus appears to be an important factor in driving the physiological changes that occur with season. Remarkably, in both birds and mammals, the pars tuberalis (PT) of the pituitary gland plays an essential role. A specialised endocrine thyrotroph cell (TSH-expressing) is regulated by the changing day-length signal, leading to activation of TSH by long days. This acts on adjacent TSH-receptors expressed in the hypothalamic ependymal cells, causing local regulation of deiodinase enzymes and conversion of TH to the metabolically active T3. In mammals, the PT is regulated by the nocturnal melatonin signal. Summer-like melatonin signals activate a PT-expressed clock-regulated transcription regulator (EYA3), which in turn drives the expression of the TSHβ sub-unit, leading to a sustained increase in TSH expression. In this manner, a local pituitary timer, driven by melatonin, initiates a cascade of molecular events, led by EYA3, which translates to seasonal changes of neuroendocrine activity in the hypothalamus. There are remarkable parallels between this PT circuit and the photoperiodic timing system used in plants, and while plants use different molecular signals (constans vs EYA3) it appears that widely divergent organisms probably obey a common set of design principles.

Melatonin receptor activation suppresses adrenocorticotropin production via BMP-4 action by pituitary AtT20 cells

The role of melatonin, a regulator of circadian rhythm, in adrenocorticotropin (ACTH) production by corticotrope cells has not been elucidated. In this study, we investigated the effect of melatonin on ACTH production in relation to the biological activity of bone morphogenetic protein (BMP)-4 using mouse corticotrope AtT20 cells that express melatonin type-1 (MT1R) but not type-2 (MT2R) receptors. We previously reported that BMP-4 inhibits corticotropin-releasing hormone (CRH)-induced ACTH production and proopiomelanocortin (POMC) transcription by inhibiting MAPK signaling. Both melatonin and an MT1R/MT2R agonist, ramelteon, suppressed CRH-induced ACTH production, POMC transcription and cAMP synthesis. The inhibitory effects of ramelteon on basal and CRH-induced POMC mRNA and ACTH levels were more potent than those of melatonin. Treatment with melatonin or ramelteon in combination with BMP-4 additively suppressed CRH-induced ACTH production. Of note, the level of MT1R expression was upregulated by BMP-4 stimulation. The suppressive effects of melatonin and ramelteon on POMC transcription and cAMP synthesis induced by CRH were not affected by an MT2R antagonist, luzindole. On the other hand, BMP-4-induced Smad1/5/8 phosphorylation and the expression of a BMP target gene, Id-1, were augmented in the presence of melatonin and ramelteon. Considering that the expression levels of BMP receptors, ALK-3/BMPRII, were increased by ramelteon, MT1R action may play an enhancing role in BMP-receptor signaling. Among the MT1R signaling pathways including AKT, ERK and JNK pathways, inhibition of AKT signaling functionally reversed the MT1R effects on both CRH-induced POMC transcription and BMP-4-induced Id-1 transcription. Collectively, MT1R signaling and BMP-4 actions were mutually augmented, leading to fine-tuning of ACTH production by corticotrope cells.

Melatonin

Melatonin (MEL) is a hormone synthesized and secreted by the pineal gland deep within the brain in response to photoperiodic cues relayed from the retina via an endogenous circadian oscillator within the suprachiasmatic nucleus in the hypothalamus. The circadian rhythm of melatonin production and release, characterized by nocturnal activity and daytime quiescence, is an important temporal signal to the body structures that can read it. Melatonin acts through high-affinity receptors located centrally and in numerous peripheral organs. Different receptor subtypes have been cloned and characterized: MT(1) and MT(2) (transmembrane G-protein-coupled receptors), and MT(3). However, their physiological role remains unelucidated, although livestock management applications already include the control of seasonal breeding and milk production. As for potential therapeutic applications, exogenous melatonin or a melatonin agonist and selective 5-hydroxytrypiamine receptor (5-HT(2c)) antagonist, eg, S 20098, can be used to manipulate circadian processes such as the sleep-vake cycle, which are frequently disrupted in many conditions, most notably seasonal affective disorder.

Melatonin in animal models

Melatonin is a hormone synthesized and secreted during the night by the pineal gland. Its production is mainly driven by the Orcadian clock, which, in mammals, is situated in the suprachiasmatic nucleus of the hypothalamus. The melatonin production and release displays characteristic daily (nocturnal) and seasonal patterns (changes in duration proportional to the length of the night) of secretion. These rhythms in circulating melatonin are strong synchronizers for the expression of numerous physiological processes. In mammals, the role of melatonin in the control of seasonality is well documented, and the sites and mechanisms of action involved are beginning to be identified. The exact role of the hormone in the diurnal (Orcadian) timing system remains to be determined. However, exogenous melatonin has been shown to affect the circadian clock. The molecular and cellular mechanisms involved in this well-characterized “chronobiotic” effect have also begun to be characterized. The circadian clock itself appears to be an important site for the entrapment effect of melatonin and the presence of melatonin receptors appears to be a prerequisite. A better understanding of such “chronobiotic” effects of melatonin will allow clarification of the role of endogenous melatonin in circadian organization.

Reproductive seasonality in the mare: neuroendocrine basis and pharmacologic control

Reproductive seasonality in the mare is characterized by a marked decline in adenohypophyseal synthesis and secretion of LH beginning near the autumnal equinox. Thus, ovarian cycles have ceased in most mares by the time of the winter solstice. Endogenous reproductive rhythms in seasonal species are entrained or synchronized as a result of periodic environmental cues. In the horse, this cue is primarily day length. Hence, supplemental lighting schemes have been used managerially for decades to modify the annual timing of reproduction in the mare. Although a full characterization of the cellular and molecular bases of seasonal rhythms has not been realized in any species, many of their synaptic and humoral signaling pathways have been defined. In the mare, neuroendocrine-related studies have focused primarily on the roles of GnRH and interneuronal signaling pathways that subserve the GnRH system in the regulatory cascade. Recent studies have considered the role of a newly discovered neuropeptide, RF-related peptide 3 that could function to inhibit GnRH secretion or gonadotrope responsiveness. Although results that used native peptide sequences have been negative in the mare and mixed in all mammalian females, new studies that used an RFRP3 antagonist (RF9) in sheep are encouraging. Importantly, despite continuing deficits in some fundamental areas, the knowledge required to control seasonal anovulation pharmacologically has been available for >20 yr. Specifically, the continuous infusion of native GnRH is both reliable and efficient for accelerating reproductive transition and is uniquely applicable to the horse. However, its practical exploitation continues to await the development of a commercially acceptable delivery vehicle.

Circadian regulation of pineal gland rhythmicity

The pineal gland is a neuroendocrine organ of the brain. Its main task is to synthesize and secrete melatonin, a nocturnal hormone with diverse physiological functions. This review will focus on the central and pineal mechanisms in generation of mammalian pineal rhythmicity including melatonin production. In particular, this review covers the following topics: (1) local control of serotonin and melatonin rhythms; (2) neurotransmitters involved in central control of melatonin; (3) plasticity of the neural circuit controlling melatonin production; (4) role of clock genes in melatonin formation; (5) phase control of pineal rhythmicity; (6) impact of light at night on pineal rhythms; and (7) physiological function of the pineal rhythmicity.

First evidence of a cDNA encoding for a melatonin receptor (mel 1b) in brain, retina, and testis of Pelophylax esculentus

Melatonin, nocturnally secreted by the pineal gland, regulates a variety of physiological functions, including reproduction. Here, we investigated the evidence of melatonin binding sites in frog tissue (brain, retina, and testis) through saturation and competition binding experiments. In the frog, Pelophylax esculentus, our results confirm the presence of a single class of melatonin-specific binding sites in the brain and retina, but not in the testis. Further experiments have been done using biomolecular approaches (PCR analysis). Here, we report the isolation of a cDNA encoding for a melatonin receptor type (mel 1b) from brain, retina, and testis of the P. esculentus. PCR analysis revealed that melatonin expression is higher in the brain and retina, whereas it is lower in the testis. The presence of a melatonin receptor transcript in the frog testis corroborates our previous results obtained in in vitro experiments that suggest that melatonin might act directly in male vertebrate gonads, and indicates that the frog testis may be a suitable model to verify the role of indolamine in testicular activity.

Encoding and decoding photoperiod in the mammalian pars tuberalis

In mammals, the nocturnal melatonin signal is well established as a key hormonal indicator of seasonal changes in day-length, providing the brain with an internal representation of the external photoperiod. The pars tuberalis (PT) of the pituitary gland is the major site of expression of the G-coupled receptor MT1 in the brain and is considered as the main site of integration of the photoperiodic melatonin signal. Recent studies have revealed how the photoperiodic melatonin signal is encoded and conveyed by the PT to the brain and the pituitary, but much remains to be resolved. The development of new animal models and techniques such as cDNA arrays or high throughput sequencing has recently shed the light onto the regulatory networks that might be involved. This review considers the current understanding of the mechanisms driving photoperiodism in the mammalian PT with a particular focus on the seasonal prolactin secretion.

Expression of melatonin (MT1, MT2) and melatonin-related receptors in the adult rat testes and during development

It is well known that melatonin provokes reproductive alterations in response to changes in hours of daylight in seasonally breeding mammals, exerting a regulatory role at different levels of the hypothalamic–pituitary–gonadal axis. Although it has also been demonstrated that melatonin may affect testicular activity in vertebrates, until now, very few data support the hypothesis of a local action of melatonin in the male gonads. The aim of this study was to investigate whether MT1, MT2 melatonin receptors and the H9 melatonin-related receptor, are expressed in the adult rat testes and during development. A semi-quantitative RT-PCR method was used to analyse the expression of MT1, MT2 and H9 receptors mRNAs in several rat tissues, mainly focusing on testes during development and adult life. Our results provide molecular evidences of the presence of both MT1 and, for the first time, MT2 melatonin receptors as well as of the H9 melatonin-related receptor in the examined tissues, including adult testes. During development MT1 and MT2 transcripts are expressed at lower levels in testes of rats from 1 day to 1 week of age, lightly increased at 2 weeks of age and remained permanently expressed throughout development until 6 months. These data strongly support the hypothesis that melatonin acts directly in male vertebrate gonads suggesting that rat testes may be a suitable model to verify the role of indolamine in vertebrate testicular activity.

Melatonin Entrainment of Circannual Rhythms

A melatonin-based photoperiod timing mechanism and a circannual rhythm-generating system interact to govern seasonal cycles in physiology and behavior in many vertebrates. This paper focuses on the pars tuberalis (PT) of the mammalian pituitary gland as a model melatonin-responsive tissue to investigate the molecular basis of these two basic long-term timing processes.

Melatonin in the multi‐oscillatory mammalian circadian world

In mammals, the complex interaction of neural, hormonal, and behavioral outputs from the suprachiasmatic nucleus (SCN) drives circadian expression of events, either directly or through coordination of the timing of peripheral oscillators. Melatonin, one of the endocrine output signals of the clock, provides the organism with circadian information and can be considered as an endogenous synchronizer, able to stabilize and reinforce circadian rhythms and to maintain their mutual phase-relationship at the different levels of the circadian network. Moreover, exogenous melatonin, through an action on the circadian clock, affects all levels of the circadian network. The molecular mechanisms underlying this chronobiotic effect have also been investigated in rats. REV-ERB alpha seems to be the initial molecular target.

Endogenous melatonin provides an effective circadian message to both the suprachiasmatic nuclei and the pars tuberalis of the rat

The suprachiasmatic nuclei (SCN) distribute the circadian neural message to the pineal gland which transforms it into a humoral circadian message, the nocturnal melatonin synthesis, which in turn modulates tissues expressing melatonin receptors such as the SCN or the pars tuberalis (PT). Nuclear orphan receptors (NOR), including rorbeta and rev-erbalpha, have been presented as functional links between the positive and negative loops of the molecular clock. Recent findings suggest that these NOR could be the initial targets of melatonin's chronobiotic message within the SCN. We investigated the role of these NOR in the physiological effect of endogenous melatonin on these tissues. We monitored rorbeta and rev-erbalpha mRNA expression levels by quantitative in situ hybridization after pinealectomy. Pinealectomy had no effect on NOR circadian expression rhythms in the SCN in 8-day pinealectomized (PX) animals. However in animals PX for 3 months, significant desynchronization between per1 and per2 transcription patterns appeared. These results suggest that endogenous melatonin could sustain the circadian rhythmicity and the phase relationship between the molecular partners of the SCN circadian system on a long-term basis. On the other hand, pinealectomy decreased the level and abolished the rhythmicity of NOR mRNA expression in the PT. These effects were partially prevented by daily melatonin administration in the drinking water. These results show that NOR can be regulated by the melatonin circadian rhythm in the PT and could be the link between the physiological action of melatonin and the core of the molecular circadian clock in this tissue.

A physiological model of a circannual oscillator

Recent evidence based on studies in hypothalamo-pituitary disconnected Soay sheep suggests that the generation of circannual rhythms may be local to specific tissues or physiological systems. Now, the authors present a physiological model of a circannual rhythm generator centered in the pituitary gland based on the interaction between melatonin-responsive cells in the pars tuberalis that act to decode photoperiod, and lactotroph cells of the adjacent pars distalis that secrete prolactin. The model produces a self-sustained, circannual rhythm in endocrine output that the authors explore by mathematical modeling. The circannual oscillation requires a delayed negative feedback mechanism. The authors highlight specific features of the pituitary dynamics as a guide to future research on circannual rhythms.

Redefining the limits of day length responsiveness in a seasonal mammal

At temperate latitudes, increases in day length in the spring promote the summer phenotype. In mammals, this long-day response is mediated by decreasing nightly duration of melatonin secretion by the pineal gland. This affects adenylate cyclase signal transduction and clock gene expression in melatonin-responsive cells in the pars tuberalis of the pituitary, which control seasonal prolactin secretion. To define the photoperiodic limits of the mammalian long day response, we transferred short day (8 h light per 24 h) acclimated Soay sheep to various longer photoperiods, simulating those occurring from spring to summer in their northerly habitat (57 degrees N). Locomotor activity and plasma melatonin rhythms remained synchronized to the light-dark cycle in all photoperiods. Surprisingly, transfer to 16-h light/day had a greater effect on prolactin secretion and oestrus activity than shorter (12 h) or longer (20 and 22 h) photoperiods. The 16-h photoperiod also had the largest effect on expression of circadian (per1) and neuroendocrine output (betaTSH) genes in the pars tuberalis and on kisspeptin gene expression in the arcuate nucleus of the hypothalamus, which modulates reproductive activity. This critical photoperiodic window of responsiveness to long days in mammals is predicted by a model wherein adenylate cyclase sensitization and clock gene phasing effects of melatonin combine to control neuroendocrine output. This adaptive mechanism may be related to the latitude of origin and the timing of the seasonal transitions.

Seasonal variations of clock gene expression in the suprachiasmatic nuclei and pars tuberalis of the European hamster (Cricetus cricetus)

In mammals, day length (photoperiod) is read and encoded in the main circadian clock, the suprachiasmatic nuclei (SCN). In turn, the SCN control the seasonal rhythmicity of various physiological processes, in particular the secretion pattern of the pineal hormone melatonin. This hormone then operates as an essential mediator for the control of seasonal physiological functions on some tissues, especially the pars tuberalis (PT). In the European hamster, both hormonal (melatonin) and behavioral (locomotor activity) rhythms are strongly affected by season, making this species an interesting model to investigate the impact of the seasonal variations of the environment. The direct (on SCN) and indirect (via melatonin on PT) effect of natural short and long photoperiod was investigated on the daily expression of clock genes, these being expressed in both tissues. In the SCN, photoperiod altered the expression of all clock genes studied. In short photoperiod, whereas Clock mRNA levels were reduced, Bmal1 expression became arrhythmic, probably resulting in the observed dramatic reduction in the rhythm of Avp expression. In the PT, Per1 and Rev-erbalpha expressions were anchored to dawn in both photoperiods. The daily profiles of Cry1 mRNA were not concordant with the daily variations in plasma melatonin although we confirmed that Cry1 expression is regulated by an acute melatonin injection in the hamster PT. The putative role of such seasonal-dependent changes in clock gene expression on the control of seasonal functions is discussed.

Seasonal variation and the immune response: a fish perspective

The environment in which an animal lives affects the physiology and psychology of that animal. The greater the distance from the equator the more profound this influence becomes, as the environment becomes more variable over the years. Temperature, photoperiod, precipitation and other environmental conditions, which are directly or indirectly controlled by the season, can affect an animal. It is becoming apparent that these conditions may impact on the immune system, and this can affect animal health. This review looks at the known mechanisms for transducing environmental cues and how these can affect immune parameters and function. The main focus is fish, especially in relation to aquaculture and the associated disease risks. Work on other animal classes is included for comparison.

Regulation of the ovine MT1 melatonin receptor promoter: interaction between multiple pituitary transcription factors at different phases of development

Pineal secretion of melatonin provides a neuroendocrine representation of the light-dark cycle, which is used to synchronise daily and annual rhythms of physiology and behaviour. In mammals, melatonin primarily acts through MT(1) melatonin receptors that exhibit a highly restricted tissue distribution. Expression of MT(1) receptors is subject to developmental and circadian control, which likely modulates the physiological actions of melatonin. To investigate the mechanisms controlling MT(1) expression we cloned the proximal 1.5kb region of the ovine MT(1) promoter. Sequence analysis revealed putative cis-elements for transcription factors involved in pituitary development, namely Pitx-1 and Egr-1, and multiple putative E-boxes, which are involved in both circadian and developmental gene regulation. Nuclear protein from ovine pars tuberalis (PT) cells, a site of high endogenous MT(1) expression, stimulated gene expression from a MT(1) expression construct, indicating the presence of a functional promoter. Pitx-1 was strongly expressed in the ovine PT and stimulated MT(1) promoter activity in transfection assays. Co-transfection with Egr-1 induced promoter-specific effects: Pitx-1-stimulated MT(1) activity was inhibited, whereas betaLH promoter activity was enhanced. In addition to Pitx-1 the circadian clock genes Clock and Bmal1 were also expressed in the PT. However, despite multiple putative E-boxes in the MT(1) promoter, transfected Clock and Bmal1 were unable to regulate either basal or Pitx-1-stimulated MT(1) promoter activity. The current data, in conjunction with our previous study of the rat MT(1) promoter, suggests a general model in which melatonin receptor expression in the mammalian pituitary is determined by the developmentally changing balance between stimulatory and inhibitory transcription factors. Furthermore, our data suggest that circadian variation in MT(1) gene expression does not depend upon the direct action of circadian clock genes on E-box cis-elements.

Melatonin induces gene-specific effects on rhythmic mRNA expression in the pars tuberalis of the Siberian hamster (Phodopus sungorus)

In mammals, circadian and photoperiodic information is encoded in the pineal melatonin signal. The pars tuberalis (PT) of the pituitary is a melatonin target tissue, which transduces photoperiodic changes and drives seasonal changes in prolactin secretion from distal lactotroph cells. Measurement of photoperiodic time in the PT is believed to occur through melatonin dependent changes in clock gene expression, although it is unclear whether the PT should be considered a melatonin sensitive peripheral oscillator. We tested this hypothesis in the Siberian hamster (Phodopus sungorus) firstly by investigating the effects of melatonin injection, and secondly by determining whether temporal variation in gene expression within the PT persists in the absence of a rhythmic melatonin signal. Hamsters preconditioned to long days were treated with melatonin during the late light phase, to advance the timing of the nocturnal melatonin peak, or placed in constant light for one 24 h cycle, thereby suppressing endogenous melatonin secretion. Gene expression in the PT was measured by in situ hybridization. We show that melatonin rapidly induces cry1 mRNA expression without the need for a prolonged melatonin-free interval, acutely inhibits mt1 expression, advances the timing of peak rev-erb alpha expression and modulates per1 expression. With the exception of cry1, these genes continue to show temporal changes in expression over a first cycle in the absence of a melatonin signal. Our data are consistent with the hypothesis that the hamster PT contains a damped endogenous circadian oscillator, which requires a rhythmic melatonin signal for long-term synchronization.

Melatonin regulates type 2 deiodinase gene expression in the Syrian hamster

In seasonal species, photoperiod organizes various physiological processes, including reproduction. Recent data indicate that the expression of type 2 iodothyronine deiodinase (Dio2) is modulated by photoperiod in the mediobasal hypothalamus of some seasonal species. Dio2 is believed to control the local synthesis of bioactive T(3) to regulate gonadal response. Here we used in situ hybridization to study Dio2 expression in the hypothalamus of a photoperiodic rodent, the Syrian hamster. Dio2 was highly expressed in reproductively active hamsters in long day, whereas it was dramatically reduced in sexually inhibited hamsters maintained in short day. This contrasted with the laboratory rat, a nonphotoperiodic species, in which no evidence for Dio2 photoperiodic modulation was seen. We also demonstrate that photoperiodic variations of Dio2 expression in hamsters are independent from secondary changes in gonadal steroids. Studies in pinealectomized hamsters showed that the photoperiodic variation of Dio2 expression is melatonin dependent, and injections of long day hamsters with melatonin for only 7 d were sufficient to inhibit Dio2 expression to that of short day levels. Finally, because in some seasonal species thyroid hormones are involved in photorefractoriness, we examined Dio2 expression in short day-refractory hamsters and found that Dio2 mRNA levels remained low despite full reproductive recrudescence. Altogether, these results demonstrate that in the Syrian hamster Dio2 is photoperiodically modulated via a melatonin-dependent process. Furthermore, refractoriness to photoperiod in hamsters appears to occur independently of Dio2. These results raise new perspectives for understanding how thyroid hormones are involved in the control of photoperiodic neuroendocrine processes.

Decoding the nightly melatonin signal through circadian clockwork

Photoperiod regulates the timing of seasonal cycles in reproduction, energy metabolism, moult and other seasonal characteristics, and the effects are transduced through changes in the duration of nocturnal melatonin secretion from the pineal gland. Short daily melatonin signals (4-8 h/day) activate a summer physiology, while long signals (> 10 h/day) produce a winter phenotype. Decoding signal duration occurs in specific target cells in the brain and pituitary gland, each governing a different component of the seasonal adaptation. The pars tuberalis (PT) of the pituitary regulates prolactin release and provides a tractable model system to investigate the molecular decoding mechanism. In the PT, melatonin onset at dusk activates cryptochrone (Cry1) gene expression and melatonin offset at dawn activates period (Per1) gene expression, thus the Cry/Per interval varies directly with nightlength, and inverse to daylength. It is proposed that photoperiod-induced changes in this phase-relationship dictates the level of CRY/PER protein heterodimer formation, and in turn, the level of transcriptional drive to the genes that control PT output--up-regulated under long days stimulating prolactin secretion and a summer physiology, and--down-regulated by short days in winter. The melatonin signal is thus decoded through circadian clock genes.

Multiple effects of melatonin on rhythmic clock gene expression in the mammalian pars tuberalis

In mammals, changing day length modulates endocrine rhythms via nocturnal melatonin secretion. Studies of the pituitary pars tuberalis (PT) suggest that melatonin-regulated clock gene expression is critical to this process. Here, we considered whether clock gene rhythms continue in the PT in the absence of melatonin and whether the effects of melatonin on the expression of these genes are temporally gated. Soay sheep acclimated to long photoperiod (LP) were transferred to constant light for 24 h, suppressing endogenous melatonin secretion. Animals were infused with melatonin at 4-h intervals across the final 24 h, and killed 3 h after infusion. The expression of five clock genes (Per1, Per2, Cry1, Rev-erbalpha, and Bmal1) was measured by in situ hybridization. In sham-treated animals, PT expression of Per1, Per2, and Rev-erbalpha showed pronounced temporal variation despite the absence of melatonin, with peak times occurring earlier than predicted under LP. The time of peak Bmal1 expression remained LP-like, whereas Cry1 expression was continually low. Melatonin infusion induced Cry1 expression at all times and suppressed other genes, but only when they showed high expression in sham-treated animals. Hence, 3 h after melatonin treatment, clock gene profiles were driven to a similar state, irrespective of infusion time. In contrast to the PT, melatonin infusions had no clear effect on clock gene expression in the suprachiasmatic nuclei. Our results provide the first example of acute sensitivity of multiple clock genes to one endocrine stimulus and suggest that rising melatonin levels may reset circadian rhythms in the PT, independently of previous phase.

Photorefractoriness in mammals: dissociating a seasonal timer from the circadian-based photoperiod response

In seasonal animals, prolonged exposure to constant photoperiod induces photorefractoriness, causing spontaneous reversion in physiology to that of the previous photoperiodic state. This study tested the hypothesis that the onset of photorefractoriness is correlated with a change in circadian expression of clock genes in the suprachiasmatic nucleus (circadian pacemaker) and the pars tuberalis (PT, a melatonin target tissue). Soay sheep were exposed to summer photoperiod (16-h light) for either 6 or 30 wk to produce a photostimulated and photorefractory physiology, and seasonal changes were tracked by measuring the long-term prolactin cycles. Animals were killed at 4-h intervals throughout 24 h. Contrary to the hypothesis, the 24-h rhythmic expression of clock genes (Rev-erbalpha, Per1, Per2, Bmal1, Cry1) in the suprachiasmatic nucleus and PT reflected the ambient photoperiod/melatonin signal and not the changing physiology. Contrastingly, the PT expression of alpha-glycoprotein hormone subunit (alphaGSU) and betaTSH declined in photorefractory animals toward a short day-like endocrinology. We conclude that the generation of long-term endocrine cycles depends on the interaction between a circadian-based, melatonin-dependent timer that drives the initial photoperiodic response and a non-circadian-based timer that drives circannual rhythmicity in long-lived species. Under constant photoperiod the two timers can dissociate, leading to the apparent refractory state.

Photoperiod regulates multiple gene expression in the suprachiasmatic nuclei and pars tuberalis of the Siberian hamster (Phodopus sungorus)

Photoperiod regulates the seasonal physiology of many mammals living in temperate latitudes. Photoperiodic information is decoded by the master circadian clock in the suprachiasmatic nuclei (SCN) of the hypothalamus and then transduced via pineal melatonin secretion. This neurochemical signal is interpreted by tissues expressing melatonin receptors (e.g. the pituitary pars tuberalis, PT) to drive physiological changes. In this study we analysed the photoperiodic regulation of the circadian clockwork in the SCN and PT of the Siberian hamster. Female hamsters were exposed to either long or short photoperiod for 8 weeks and sampled at 2-h intervals across the 24-h cycle. In the SCN, rhythmic expression of the clock genes Per1, Per2, Cry1, Rev-erbalpha, and the clock-controlled genes arginine vasopressin (AVP) and d-element binding protein (DBP) was modulated by photoperiod. All of these E-box-containing genes tracked dawn, with earlier peak mRNA expression in long, compared to short, photoperiod. This response occurred irrespective of the presence of additional regulatory cis-elements, suggesting photoperiodic regulation of SCN gene expression through a common E-box-related mechanism. In long photoperiod, expression of Cry1 and Per1 in the PT tracked the onset and offset of melatonin secretion, respectively. However, whereas Cry1 tracked melatonin onset in short period, Per1 expression was not detectably rhythmic. We therefore propose that, in the SCN, photoperiodic regulation of clock gene expression primarily occurs via E-boxes, whereas melatonin-driven signal transduction drives the phasing of a subset of clock genes in the PT, independently of the E-box.

Genetic aspects of melatonin biology

For decades, the important physiological roles of the pineal hormone have inspired scientific investigations. Research efforts have generated a broad amount of information relevant to various genetic aspects of melatonin biology. Nevertheless, our understanding of the effect of genetic factors upon melatonin biosynthesis and the mechanisms of gene expression regulation by melatonin in target tissues is far from complete. The present review makes an effort to summarize and systematize the existing information on the subject, sequentially discussing (i) the effect of genetic factors upon melatonin biosynthesis, (ii) melatonin receptor expression profiles, and (iii) the effect of melatonin upon expression of genes in target tissues.

The brain's calendar: neural mechanisms of seasonal timing

The suprachiasmatic nucleus (SCN) of the hypothalamus is the principal component of the mammalian biological clock, the neural timing system that generates and coordinates a broad spectrum of physiological, endocrine and behavioural circadian rhythms. The pacemaker of the SCN oscillates with a near 24 h period and is entrained to the diurnal light-dark cycle. Consistent with its role in circadian timing, investigations in rodents and non-human primates furthermore suggest that the SCN is the locus of the brain's endogenous calendar, enabling organisms to anticipate seasonal environmental changes. The present review focuses on the neuronal organization and dynamic properties of the biological clock and the means by which it is synchronized with the environmental lighting conditions. It is shown that the functional activity of the biological clock is entrained to the seasonal photic cycle and that photoperiod (day length) may act as an effective zeitgeber. Furthermore, new insights are presented, based on electrophysiological and molecular studies, that the mammalian circadian timing system consists of coupled oscillators and that the clock genes of these oscillators may also function as calendar genes. In summary, there are now strong indications that the neuronal changes and adaptations in mammals that occur in response to a seasonally changing environment are driven by an endogenous circadian clock located in the SCN, and that this neural calendar is reset by the seasonal fluctuations in photoperiod.

Prolactin cycles in sheep under constant photoperiod: evidence that photorefractoriness develops within the pituitary gland independently of the prolactin output signal

The present study investigated photorefractoriness in the prolactin (PRL) axis in hypothalamopituitary-disconnected (HPD) sheep exposed to prolonged long days. In experiment 1, HPD Soay rams transferred from short (8L:16D) to long (16L:8D) days for 48 wk to induce a cycle of activation, decline (photorefractoriness), and reactivation in PRL secretion were treated chronically with bromocriptine (dopamine-receptor agonist) or vehicle from the onset of photorefractoriness. Bromocriptine (0.01-0.04 mg kg-1 day-1; 12-24 wk of long days) blocked PRL release and caused a rebound response after the treatment, but it had no effect on the long-term PRL cycle (posttreatment PRL minimum, mean +/- SEM, 35.3 +/- 0.6 and 37.0 +/- 0.4 wk for bromocriptine and control groups, respectively; not significant). In experiment 2, HPD rams were treated with sulpiride (dopamine-receptor antagonist) during photorefractoriness. Sulpiride (0.6 mg/kg twice daily; 22-30 wk of long days) induced a marginal increase in blood PRL concentrations, but again, it had no effect on the long-term PRL cycle (PRL minimum, 37.9 +/- 0.4 and 37.6 +/- 0.9 wk for sulpiride and control groups, respectively; not significant). The 24-h blood melatonin profile consistently reflected the long-day photoperiod throughout, and blood FSH concentrations were minimal, confirming the effectiveness of the HPD surgery. The results support the conclusion that photorefractoriness is regulated at the level of the pituitary gland independently of the PRL output signal.

MT1 melatonin receptor mRNA expressing cells in the pars tuberalis of the European hamster: effect of photoperiod

Melatonin, secreted only during the night by the pineal gland, transduces the photoperiodic message to the organism. One important target for the hormone is the pars tuberalis (PT) of the adenohypophysis which displays a very high number of melatonin binding sites in mammals and is implicated in the seasonal regulation of prolactin secretion. To gain insight into the mechanism by which the melatonin signal is decoded in the PT, we studied the effect of photoperiod on the PT cells expressing the MT1 melatonin receptor in a highly photoperiodic species, the European hamster. Recently, we showed that, in the rat, the MT1 receptor mRNA is expressed in PT-specific cells characterized by their expression of beta-thyroid stimulating hormone (beta-TSH) along with the alpha-glycoprotein subunit (alpha-GSU). As the cellular composition of the PT shows variability among species, we first identified the cell type expressing the MT1 receptor in the European hamster by combining immunocytochemistry and nonradioactive in situ hybridization for the MT1 receptor mRNA. Our results show that, in the European hamster, as in the rat, the MT1 receptor is only expressed by the PT-specific-cells, beta-TSH and alpha-GSU positive. In a second step, we analysed the effects of photoperiod on the MT1 mRNA, and on beta-TSH and alpha-GSU both at the mRNA and protein levels. Our data show that, compared to long photoperiod, short photoperiod induces a dramatic decrease of MT1, beta-TSH and alpha-GSU expression. Protein levels of beta-TSH and alpha-GSU were also dramatically reduced in short photoperiod. Together, our data suggest that melatonin exerts its seasonal effects in the PT by signalling to PT specific-cells through the MT1 receptor subtype.

Generation of the melatonin endocrine message in mammals: a review of the complex regulation of melatonin synthesis by norepinephrine, peptides, and other pineal transmitters

Melatonin, the major hormone produced by the pineal gland, displays characteristic daily and seasonal patterns of secretion. These robust and predictable rhythms in circulating melatonin are strong synchronizers for the expression of numerous physiological processes in photoperiodic species. In mammals, the nighttime production of melatonin is mainly driven by the circadian clock, situated in the suprachiasmatic nucleus of the hypothalamus, which controls the release of norepinephrine from the dense pineal sympathetic afferents. The pivotal role of norepinephrine in the nocturnal stimulation of melatonin synthesis has been extensively dissected at the cellular and molecular levels. Besides the noradrenergic input, the presence of numerous other transmitters originating from various sources has been reported in the pineal gland. Many of these are neuropeptides and appear to contribute to the regulation of melatonin synthesis by modulating the effects of norepinephrine on pineal biochemistry. The aim of this review is firstly to update our knowledge of the cellular and molecular events underlying the noradrenergic control of melatonin synthesis; and secondly to gather together early and recent data on the effects of the nonadrenergic transmitters on modulation of melatonin synthesis. This information reveals the variety of inputs that can be integrated by the pineal gland; what elements are crucial to deliver the very precise timing information to the organism. This also clarifies the role of these various inputs in the seasonal variation of melatonin synthesis and their subsequent physiological function.

Evidence for an endogenous per1- and ICER-independent seasonal timer in the hamster pituitary gland

Most mammals use changing annual day-length cycles to regulate pineal melatonin secretion and thereby drive many physiological rhythms including reproduction, metabolism, immune function, and pelage. Prolonged exposure to short winter day lengths results in refractoriness, a spontaneous reversion to long-day physiological status. Despite its critical role in the timing of seasonal rhythms, refractoriness remains poorly understood. The aim of this study was therefore to describe cellular and molecular mechanisms driving the seasonal secretion of a key hormone, prolactin, in refractory Syrian hamsters. We used recently developed single cell hybridization and reporter assays to show that this process is initiated by timed reactivation of endocrine signaling from the pars tuberalis (PT) region of the pituitary gland, a well-defined melatonin target site, causing renewed activation of prolactin gene expression. This timed signaling is independent of per1 clock gene expression in the suprachiasmatic nuclei and PT and of melatonin secretion, which continue to track day length. Within the PT, there is also a continued short day-like profile of ICER expression, suggesting that the change in hormone secretion is independent of cAMP signaling. Our data thus identify the PT as a key anatomical structure involved in endogenous seasonal timing mechanisms, which breaks from prevailing day length-induced gene expression.

Melatonin: from seasonal to circadian signal

In mammals, the role of melatonin in the control of seasonality is well documented, and the sites and mechanisms of action involved are beginning to be identified. The exact role of the hormone in the circadian timing system remains to be determined. However, exogenous melatonin has been shown to affect the circadian clock. Identification of the molecular and cellular mechanisms involved in this well characterized chronobiotic effect will allow clarification of the role of endogenous melatonin in circadian organization.

Clock genes and the long-term regulation of prolactin secretion: evidence for a photoperiod/circannual timer in the pars tuberalis

Prolactin secretion is regulated by photoperiod through changes in the 24-h melatonin profile and displays circannual rhythmicity under constant photoperiod. These two processes appear to occur principally within the pituitary gland, controlled by the pars tuberalis. This is evident because: (i) hypothalamic-pituitary disconnected (HPD) sheep show marked changes in prolactin secretion in response to switches in photoperiod and manipulations of melatonin, similar to brain-intact controls; (ii) HPD sheep also show photoperiod-specific, long-term cycles in prolactin secretion under constant long or short days, with the timing maintained even when prolactin secretion is blocked for 2-3 months; and (iii) pars tuberalis cells, but not lactotrophs, express high concentrations of melatonin (MT1) receptor, and exhibit a duration-sensitive, cAMP-dependant, inhibitory response to physiological concentrations of melatonin. This suggests the existence of an intrinsic, reversible photoperiod-circannual timer in pars tuberalis cells. A full complement of clock genes (Bmal1, Clock, Per1, Per2, Cry1 and Cry2) are expressed in the ovine pars tuberalis, and undergo 24-h cyclical expression as observed in a cell autonomous, circadian clock. Activation of Per genes occurs in the early day (melatonin off-set), while activation of Cry genes occurs in the early night (melatonin on-set). This temporal association is evident under both long and short days, thus the Per-Cry interval varies directly with photoperiod. Because, PER : CRY, protein : protein interactions affect stability, nuclear entry and gene transcription based on rodent data, the change in phasing of Per/Cry expression provides a potential mechanism for decoding the long day/short day melatonin signal. A speculative, but testable, extension of this hypothesis is that intrinsically regulated changes in the phase of Per/Cry rhythms, regulates both photorefractoriness and the generation of circannual rhythms in prolactin secretion.

[Chrono-endocrinology--quo vadis?]

The present review deals with important new chronobiological results especially in the field of chronoendocrinology, shedding new light on the circadian organisation of mammals including man. In vitro studies have shown that the concept of the existence of a single circadian oscillator located in the suprachiasmatic nucleus has to be extended. Circadian oscillators have also been found to exist in the retina, islets of Langerhans, liver, lung, and fibroblasts. Another major result is the detection of a new photopigment, melanopsin, present in a subpopulation of retinal ganglion cells which are lightsensitive and project to the suprachiasmatic nucleus, acting as zeitgeber for the photic entrainment of the circadian rhythm. We are only beginning to understand how the circadian oscillator transmits the circadian message to the endocrine system. The generation of circadian and seasonal rhythms of hormone synthesis is best understood in the pineal gland and its hormone melatonin. Seasonal changes of melatonin synthesis are transduced in the pars tuberalis of the adenohypophysis which is now entering the limelight of chronoendocrinological research. Currently, the elucidation of the genetic basis and the molecular organisation of the circadian oscillator within individual cells is a major thrust in chronobiological research.

Temporal expression of seven clock genes in the suprachiasmatic nucleus and the pars tuberalis of the sheep: evidence for an internal coincidence timer

The 24-h expression of seven clock genes (Bmal1, Clock, Per1, Per2, Cry1, Cry2, and CK1 epsilon ) was assayed by in situ hybridization in the suprachiasmatic nucleus (SCN) and the pars tuberalis (PT) of the pituitary gland, collected every 4 h throughout 24 h, from female Soay sheep kept under long (16-h light/8-h dark) or short (8-h light/16-h dark) photoperiods. Locomotor activity was diurnal, inversely related to melatonin secretion, and prolactin levels were increased under long days. All clock genes were expressed in the ovine SCN and PT. In the SCN, there was a 24-h rhythm in Clock expression, in parallel with Bmal1, in antiphase with cycles in Per1 and Per2; there was low-amplitude oscillation of Cry1 and Cry2. The waveform of only Per1 and Per2 expression was affected by photoperiod, with extended elevated expression under long days. In the PT, the high-amplitude 24-h cycles in the expression of Bmal1, Clock, Per1, Per2, Cry1, and Cry2, but not CK1 epsilon, were influenced by photoperiod. Per1 and Per2 peaked during the day, whereas Cry1 and Cry2 peaked early in the night. Hence, photoperiod via melatonin had a marked effect on the phase relationship between Per/Cry genes in the PT. This supports the conclusion that an "external coincidence model" best explains the way photoperiod affects the waveform of clock gene expression in the SCN, the central pacemaker, whereas an "internal coincidence model" best explains the way melatonin affects the phasing of clock gene expression in the PT to mediate the photoperiodic control of a summer or winter physiology.

Insect photoperiodism and circadian clocks: models and mechanisms

Photoperiodic clocks allow organisms to predict the coming season. In insects, the seasonal adaptive response mainly takes the form of diapause. The extensively studied photoperiodic clock in insects was primarily characterized by a "black-box" approach, resulting in numerous cybernetic models. This is in contrast with the circadian clock, which has been dissected pragmatically at the molecular level, particularly in Drosophila. Unfortunately, Drosophila melanogaster, the favorite model organism for circadian studies, does not demonstrate a pronounced seasonal response, and consequently molecular analysis has not progressed in this area. In the current article, the authors explore different ways in which identified molecular components of the circadian pacemaker may play a role in photoperiodism. Future progress in understanding the Drosophila circadian pacemaker, particularly as further output components are identified, may provide a direct link between the clock and photoperiodism. In addition, with improved molecular tools, it is now possible to turn to other insects that have a more dramatic photoperiodic response.

Mammalian photoperiodic system: formal properties and neuroendocrine mechanisms of photoperiodic time measurement

Photoperiodism is a process whereby organisms are able to use both absolute measures of day length and the direction of day length change as a basis for regulating seasonal changes in physiology and behavior. The use of day length cues allows organisms to essentially track time-of-year and to "anticipate" relatively predictable annual variations in important environmental parameters. Thus, adaptive types of seasonal biological changes can be molded through evolution to fit annual environmental cycles. Studies of the formal properties of photoperiodic mechanisms have revealed that most organisms use circadian oscillators to measure day length. Two types of paradigms, designated as the external and internal coincidence models, have been proposed to account for photoperiodic time measurement by a circadian mechanism. Both models postulate that the timing of light exposure, rather than the total amount of light, is critical to the organism's perception of day length. In mammals, a circadian oscillator(s) in the suprachiasmatic nucleus of the hypothalamus receives photic stimuli via the retinohypothalamic tract. The circadian system regulates the rhythmic secretion of the pineal hormone, melatonin. Melatonin is secreted at night, and the duration of secretion varies in inverse relation to day length; thus, photoperiod information is "encoded" in the melatonin signal. The melatonin signal is presumably "decoded" in melatonin target tissues that are involved in the regulation of a variety of seasonal responses. Variations in photoperiodic response are seen not only between species but also between breeding populations within a species and between individuals within single breeding populations. Sometimes these variations appear to be the result of differences in responsiveness to melatonin; in other cases, variations in photoperiod responsiveness may depend on differences in patterns of melatonin secretion related to circadian variation. Sites of action for melatonin in mammals are not yet well characterized, but potential targets of particular interest include the pars tuberalis of the pituitary gland and the suprachiasmatic nuclei. Both these sites exhibit uptake of radiolabeled melatonin in various species, and there is some evidence for direct action of melatonin at these sites. However, it appears that there are species differences with respect to the importance and specific functions of various melatonin target sites.

Of rodents and ungulates and melatonin: creating a uniform code for darkness by different signaling mechanisms

Melatonin synthesis in the mammalian pineal gland is one of the best investigated output pathways of the circadian clock because it can be readily measured and is tightly regulated by a clearly defined input, the neurotransmitter norepinephrine. In this system, a regulatory scenario was deciphered that is centered around the cyclic AMP pathway but shows peculiar species-specific differences. In rodents, the cyclic AMP-mediated, temporally sequential up-regulation of two transcription factors, the activator CREB (cyclic AMP-responsive element-binding protein) and the inhibitor ICER (inducible cyclic AMP-dependent early repressor), is the core mechanism to determine rhythmic accumulation of the mRNA encoding for the rate-limiting enzyme in melatonin synthesis, the arylalkylamine N-acetyltransferase (AA-NAT). Thus, in rodents, the regulation of melatonin synthesis bears an essential transcriptional component, which, however, is flanked by posttranscriptional mechanisms. In contrast, in ungulates, and possibly also in primates, AA-NAT appears to be regulated exclusively on the posttranscriptional level. Here, increasing cyclic AMP levels inhibit the breakdown of constitutively synthesized AA-NAT protein by proteasomal proteolysis, leading to an elevated enzyme activity. Thus, self-restriction of cellular responses, as a reaction to external cues, is accomplished by different mechanisms in pinealocytes of different mammalian species. In such a temporally gated cellular adaptation, transcriptionally active products of clock genes may play a supplementary role. Their recent detection in the endogenously oscillating nonmammalian pineal organ and, notably, also in the slave oscillator of the mammalian pineal gland underlines that the mammalian pineal gland will continue to serve as an excellent model system to understand mechanisms of biological timing.

Biology of mammalian photoperiodism and the critical role of the pineal gland and melatonin

In mammals, photoperiodic information is transformed into a melatonin secretory rhythm in the pineal gland (high levels at night, low levels during the day). Melatonin exerts its effects in discrete hypothalamic areas, most likely through MT1 melatonin receptors. Whether melatonin is brought to the hypothalamus from the cerebrospinal fluid or the blood is still unclear. The final action of this indoleamine at the level of the central nervous system is a modulation of GnRH secretion but it does not act directly on GnRH neurones; rather, its action involves a complex neural circuit of interneurones that includes at least dopaminergic, serotoninergic and aminoacidergic neurones. In addition, this network appears to undergo morphological changes between seasons.

Photoperiodism in humans and other primates: evidence and implications

Most of the anatomical and molecular substrates of the system that encodes changes in photoperiod in the duration of melatonin secretion, and the receptor molecules that read this signal, have been shown to be conserved in monkeys and humans, and the functions of this system appear to be intact from the level of the retina to the level of the melatonin-duration signal of change of season. While photoperiodic seasonal breeding has been shown to occur in monkeys, it remains unclear whether photoperiod and mediation of photoperiod's effects by melatonin influence human reproduction. Epidemiological evidence suggests that inhibition of fertility by heat in men in summer contributes to seasonal variation in human reproduction at lower latitudes and that stimulation of fertility by lengthening of the photoperiod in spring contributes to the variation at higher latitudes. Parallels between the seasonality of human reproduction and seasonal affective disorder suggest that they may be governed by common biological processes. Historical and experimental evidence indicates that human responses to seasonal changes in the natural photoperiod may have been more robust prior to the Industrial Revolution and that subsequently they have been increasingly suppressed by alterations of the physical environment.