Photoperiodically-induced cycles in the secretion of prolactin in hypothalamo-pituitary disconnected rams: evidence for translation of the melatonin signal in the pituitary gland

Melatonin and Seasonal Synchrony in Mammals

In mammals, seasonal opportunities and challenges are anticipated through programmed changes in physiology and behavior. Appropriate anticipatory timing depends on synchronization to the external solar year, achieved through the use of day length (photoperiod) as a synchronizing signal. In mammals, nocturnal production of melatonin by the pineal gland is the key hormonal mediator of photoperiodic change, exerting its effects via the hypothalamopituitary axis. In this review/perspective, we consider the key developments during the history of research into the seasonal synchronizer effect of melatonin, highlighting the role that the pars tuberalis-tanycyte module plays in this process. We go on to consider downstream pathways, which include discrete hypothalamic neuronal populations. Neurons that express the neuropeptides kisspeptin and (Arg)(Phe)-related peptide-3 (RFRP-3) govern seasonal reproductive function while neurons that express somatostatin may be involved in seasonal metabolic adaptations. Finally, we identify several outstanding questions, which need to be addressed to provide a much thorough understanding of the deep impact of melatonin upon seasonal synchronization.

Thirty-seven years of MT1 and MT2 melatonin receptor localization in the brain: Past and future challenges

Identifying the target cells of a hormone is a key step in understanding its function. Once the molecular nature of the receptors for a hormone has been established, researchers can use several techniques to detect these receptors. Here I will review the different tools used over the years to localize melatonin receptors and the problems associated with each of these techniques. The radioligand 2-[125I] iodomelatonin was the first tool to allow localization of melatonin receptors on tissue sections. Once the MT1 and MT2 receptors were cloned, in situ hybridization could be used to detect the messenger RNA for these receptors. The deduced amino acid sequences for MT1 and MT2 receptors allowed the production of peptide immunogens to generate antibodies against the MT1 and MT2 receptors. Finally, transgenic reporters driven by the promoter elements of the MT1 and MT2 genes have been used to map the expression of MT1 and MT2 in the brain and the retina. Several issues have complicated the localization of melatonin receptors and the characterization of melatonin target cells over the last three decades. Melatonin receptors are expressed at low levels, leading to sensitivity issues for their detection. The second problem are specificity issues with antibodies directed against the MT1 and MT2 melatonin receptors. These receptors are G protein-coupled receptors and many antibodies directed against such receptors have been shown to present similar problems concerning their specificity. Despite these specificity problems which start to be seriously addressed by recent studies, antibodies will be important tools in the future to identify and phenotype melatonin target cells. However, we will have to be more stringent than previously when establishing their specificity. The results obtained by these antibodies will have to be confronted and be coherent with results obtained by other techniques.

Melatonin Administration Enhances Testicular Volume, Testicular Blood Flow, Semen Parameters and Antioxidant Status during the Non-Breeding Season in Bafra Rams

Our objectives were to investigate the effects of exogenous melatonin on testicular volume (TV), testicular blood flow (TBF), and semen quality in Bafra rams during the non-breeding season. One group of rams (MEL, n = 5) received a 36 mg melatonin implant twice, with 30 days in between, while the other group (CON, n = 5) served as the control. TBF, TV, and semen quality parameters were determined at three-week intervals starting three weeks before until twelve weeks after the first melatonin implant. Testicular blood flow was determined in the supratesticular (STA) and marginal testicular artery (MA) using color Doppler ultrasound. Semen was collected and evaluated, and the total oxidative status (TOS) and total antioxidative status (TAS) was determined using an ELISA. The MEL group had increased (p < 0.05) TV between the sixth and twelfth week after the start of treatment. Overall, the MEL group had lower resistance and pulsatility indexes (p < 0.05) between the third and ninth week, although there was no difference (p > 0.05) between the two groups in most semen quality parameters. However, TAS concentrations increased (p < 0.05) in the MEL group compared with the CON. The results of this study show that exogenous melatonin in the non-breeding season significantly increased both TBF and TV in Bafra rams. Therefore, giving rams implants with 36 mg melatonin twice at least one month prior to the non-breeding season is expected to improve testicular size and function and reproductive capacity.

Evaluation of the effect of melatonin implantation in rams and eCG dose in ewes synchronized by a CIDR-eCG protocol on reproductive performance of Lacaune sheep breed during non-breeding season

Sustainable production of milk is favorable in dairy sheep industry, which necessitates year-round reproduction of rams and ewes even during non-breeding season. Hence, protocols facilitating reproduction of rams and ewes during non-breeding season are of importance for this purpose. Therefore, the present study was conducted to investigate the effect of melatonin implantation in rams and administration of 400 IU eCG (E400) versus 300 IU eCG (E300) in ewes on reproductive performance of Lacaune sheep breed during non-breeding season. Rams were allocated to two groups including untreated (control, CON; n = 36) and melatonin-treated (MEL; n = 37). A subset of rams from CON (n = 7) and MEL (n = 7) groups were used for assessment of scrotal circumference, kinematic and functional characteristics of sperm, total antioxidant capacity (TAC) of semen and circulating testosterone on Days 0 (the beginning of study) and 60 (60 days after melatonin implantation). Further, the study had a 2 × 2 factorial design with four experimental groups including 1) ewes treated with E300 and introduced to CON rams (E300CON; n = 17 rams and 172 ewes), 2) ewes treated with E300 and introduced to MEL rams (E300MEL; n = 18 rams and 177 ewes), 3) ewes treated with E400 and introduced to CON rams (E400CON; n = 19 rams and 192 ewes), and ewes treated with E400 and introduced to MEL rams (E400MEL; n = 19 rams and 190 ewes). Melatonin implantation improved scrotal circumference, concentration, progressive motility, velocity, mitochondrial membrane potential and viability of sperm, TAC of semen, concentration of testosterone and fertility of rams (P < 0.05). Besides, E400 compared with E300 enhanced synchronization of estrus and fertility in ewes (P < 0.05). In conclusion, melatonin implantation promoted reproductive performance in Lacaune rams, and increase in dose of eCG improved reproductive performance in Lacaune ewes.

Biological timekeeping in polar environments: lessons from terrestrial vertebrates

The polar regions receive less solar energy than anywhere else on Earth, with the greatest year-round variation in daily light exposure; this produces highly seasonal environments, with short summers and long, cold winters. Polar environments are also characterised by a reduced daily amplitude of solar illumination. This is obvious around the solstices, when the Sun remains continuously above (polar 'day') or below (polar 'night') the horizon. Even at the solstices, however, light levels and spectral composition vary on a diel basis. These features raise interesting questions about polar biological timekeeping from the perspectives of function and causal mechanism. Functionally, to what extent are evolutionary drivers for circadian timekeeping maintained in polar environments, and how does this depend on physiology and life history? Mechanistically, how does polar solar illumination affect core daily or seasonal timekeeping and light entrainment? In birds and mammals, answers to these questions diverge widely between species, depending on physiology and bioenergetic constraints. In the high Arctic, photic cues can maintain circadian synchrony in some species, even in the polar summer. Under these conditions, timer systems may be refined to exploit polar cues. In other instances, temporal organisation may cease to be dominated by the circadian clock. Although the drive for seasonal synchronisation is strong in polar species, reliance on innate long-term (circannual) timer mechanisms varies. This variation reflects differing year-round access to photic cues. Polar chronobiology is a productive area for exploring the adaptive evolution of daily and seasonal timekeeping, with many outstanding areas for further investigation.

Seasonality of prolactin in birds and mammals

In most animals, annual rhythms in environmental cues and internal programs regulate seasonal physiology and behavior. Prolactin, an evolutionarily ancient hormone, serves as a molecular correlate of seasonal timing in most species. Prolactin is highly pleiotropic with a wide variety of well-documented physiological effects; in a seasonal context prolactin is known to regulate annual changes in pelage and molt. While short-term homeostatic variation of prolactin secretion is under the control of the hypothalamus, long-term seasonal rhythms of prolactin are programmed by endogenous timers that reside in the pituitary gland. The molecular basis of these rhythms is generally understood to be melatonin dependent in mammals. Prolactin rhythmicity persists for several years in many species, in the absence of hypothalamic signaling. Such evidence in mammals has supported the hypothesis that seasonal rhythms in prolactin derive from an endogenous timer within the pituitary gland that is entrained by external photoperiod. In this review, we describe the conserved nature of prolactin signaling in birds and mammals and highlight its role in regulating multiple diverse physiological systems. The review will cover the current understanding of the molecular control of prolactin seasonality and propose a mechanism by which long-term rhythms may be generated in amniotes.

Hypophysial angiogenesis decodes annual time and underlies physiological adaptation to seasonal changes in the environment

Adaptation to annual changes in the environment is controlled by hypophysial hormones. In temperate zones, photoperiod is the primary external cue that regulates annual biological cycles and is translated by the pattern of melatonin secretion acting primarily in the hypophysial pars tuberalis. Angiogenic mechanisms within this tissue contribute to decode the melatonin signal through alternative splicing of the vascular endothelial growth factor A (VEGF-A) gene in both the pars tuberalis and the capillary loops of the infundibulum. The resulting melatonin-evoked differential productions of VEGF-A isoforms will induce seasonal remodeling of the vascular connection between the hypothalamus and hypophysis, and act as paracrine messengers in the pars distalis to generate the required seasonal endocrine response. Specifically, the long melatonin signal in winter upregulates antiangiogenic VEGF-A isoforms, which will reduce the number of vascular loops and the density of VEGF receptors in endocrine and folliculo-stellate (FS) cells, inhibit prolactin secretion, and stimulate FSH. In contrast, the short melatonin signal in summer upregulates proangiogenic VEGF-A isoforms that will increase the number of vascular loops and the density of VEGF receptors in endocrine and FS cells, stimulate prolactin secretion, and suppress FSH. A similar system has been identified in long day seasonal breeders, revealing that this is a conserved mechanism of adaptation across species. Thus, an angiogenesis-based, intrahypophysial system for annual time measurement controls local microvascular plasticity and conveys the photoperiodic signal readout from the melatonin sensitive pars tuberalis to the endocrine cells of the pars distalis to regulate seasonal adaptation to the environment.

GnRH and the photoperiodic control of seasonal reproduction: Delegating the task to kisspeptin and RFRP-3

Synchronization of mammalian breeding activity to the annual change of photoperiod and environmental conditions is of the utmost importance for individual survival and species perpetuation. Subsequent to the early 1960s, when the central role of melatonin in this adaptive process was demonstrated, our comprehension of the mechanisms through which light regulates gonadal activity has increased considerably. The current model for the photoperiodic neuroendocrine system points to pivotal roles for the melatonin-sensitive pars tuberalis (PT) and its seasonally-regulated production of thyroid-stimulating hormone (TSH), as well as for TSH-sensitive hypothalamic tanycytes, radial glia-like cells located in the basal part of the third ventricle. Tanycytes respond to TSH through increased expression of thyroid hormone (TH) deiodinase 2 (Dio2), which leads to heightened production of intrahypothalamic triiodothyronine (T3) during longer days of spring and summer. There is strong evidence that this local, long-day driven, increase in T3 links melatonin input at the PT to gonadotropin-releasing hormone (GnRH) output, to align breeding with the seasons. The mechanism(s) through which T3 impinges upon GnRH remain(s) unclear. However, two distinct neuronal populations of the medio-basal hypothalamus, which express the (Arg)(Phe)-amide peptides kisspeptin and RFamide-related peptide-3, appear to be well-positioned to relay this seasonal T3 message towards GnRH neurons. Here, we summarize our current understanding of the cellular, molecular and neuroendocrine players, which keep track of photoperiod and ultimately govern GnRH output and seasonal breeding.

Cell Networks in Endocrine/Neuroendocrine Gland Function

Reproduction, growth, stress, and metabolism are determined by endocrine/neuroendocrine systems that regulate circulating hormone concentrations. All these systems generate rhythms and changes in hormone pulsatility observed in a variety of pathophysiological states. Thus, the output of endocrine/neuroendocrine systems must be regulated within a narrow window of effective hormone concentrations but must also maintain a capacity for plasticity to respond to changing physiological demands. Remarkably most endocrinologists still have a "textbook" view of endocrine gland organization which has emanated from 20^th century histological studies on thin 2D tissue sections. However, 21^st -century technological advances, including in-depth 3D imaging of specific cell types have vastly changed our knowledge. We now know that various levels of multicellular organization can be found across different glands, that organizational motifs can vary between species and can be modified to enhance or decrease hormonal release. This article focuses on how the organization of cells regulates hormone output using three endocrine/neuroendocrine glands that present different levels of organization and complexity: the adrenal medulla, with a single neuroendocrine cell type; the anterior pituitary, with multiple intermingled cell types; and the pancreas with multiple intermingled cell types organized into distinct functional units. We give an overview of recent methodologies that allow the study of the different components within endocrine systems, particularly their temporal and spatial relationships. We believe the emerging findings about network organization, and its impact on hormone secretion, are crucial to understanding how homeostatic regulation of endocrine axes is carried out within endocrine organs themselves. © 2022 American Physiological Society. Compr Physiol 12:3371-3415, 2022.

Responses to melatonin of 2 breeds of dairy ewes in early lactation under autumn photoperiod conditions

A total of 72 dairy ewes of 2 breeds (MN, Manchega, 72.4 ± 1.9 kg of body weight, n = 36; LC, Lacaune, 77.7 ± 2.3 kg of body weight; n = 36) were used to evaluate the lactational effects of melatonin implants in early lactation and under the short-day photoperiod conditions of autumn (experiment was centered on the winter solstice). Ewes lambed in autumn and were penned indoors in 12 balanced groups of 6 ewes by breed, body weight, age, and number of lambs, and randomly assigned to a 2 × 2 × 3 factorial design (treatment × breed × replicate). Ewes suckled their lambs for 28 d. Treatments were (1) melatonin (MEL), which received 1 subcutaneous implant of melatonin (18 mg/ewe) in the ear base at 35 ± 1 d (1 wk after lamb weaning), and (2) control, which did not receive any treatment. Ewes were fed ad libitum a total mixed ration (forage:concentrate, 60:40) and machine milked twice daily. Daily milk yield was automatically recorded from d 29 to 105 of lactation and sampled every 2 wk for composition. Jugular blood was sampled for plasma hormone analyses at 30, 50, 80, 110, and 124 d of lactation. Body reserves were assessed every 2 wk. Feed intake was measured by pen during 3 separated periods after the start of the treatments (wk 2 to 3, wk 6 to 7, and wk 10 to 11). Feed intake, and milk yield and composition varied by breed, but no MEL effects were detected on dry matter intake, milk yield, milk composition, or fat and protein standardized milk in either breed. As a result of the unique composition of the implants and the variable body weights of the ewes, the MEL treatment dose (on average, 0.24 mg/kg of body weight) was 6.8% greater in the MN (lighter) than in the LC (heavier) ewes. Plasmatic melatonin markedly increased in the MEL-treated ewes (on average, 111%), but despite the amount of MEL used, the MN responded greatly compared with the LC ewes (150 vs. 63%, respectively). No differences in basal plasmatic melatonin were detected between breeds (6.4 ± 1.1 pg/mL, on average), indicating the greater responsiveness to the implants of the lighter MN ewes. Plasmatic prolactin tended to decrease in the MEL-treated ewes (-35%, on average), but the effect was significant only in the MN ewes (-54%), in agreement with their greater response to MEL. No effects of MEL treatment were detected on plasmatic IGF-I in either breed. Moreover, body reserves did not vary by effect of MEL treatment or breed throughout the experiment. In conclusion, the use of exogenous melatonin as MEL implants, together with the endogenous melatonin naturally produced under short-day photoperiod conditions, had no effects on the early-lactation performances of dairy sheep, despite their breed and level of production.

Comparative Analysis and Identification of Differentially Expressed microRNAs in the Hypothalamus of Kazakh Sheep Exposed to Different Photoperiod Conditions

MicroRNAs (miRNA) plays an important role in several mammalian biological regulatory processes by post-transcriptionally regulating gene expression. However, there is little information on the miRNAs involved in the photoperiodism pathway that controls seasonal activity. To enhance our knowledge on the effect of different photoperiod conditions on miRNA, we divided Kazakh sheep into two groups: one exposed to a long photoperiod (LP, 16L:8D) and another with exposed to a short photoperiod (SP, 8L:16D) under supplemental feeding conditions. Further we compared the related miRNAs and target genes between the two groups. Fifteen differentially expressed miRNAs were identified, which were associated with 310 regulatory pathways covering photoperiodism, reproductive hormones, and nutrition. The miR-136-GNAQ pair was selected and validated as a differentially expressed, and a dual-luciferase reporter assay showed that the negative feedback loop existed between them. Examination of the expression profile revealed that the GNAQ expression was low in the estrous females both under LP and SP conditions, but high expression of GNAQ was observed in the anestrous females under LP conditions. Moreover, functional analysis revealed that KISS1 and GnRH expression was upregulated when GNAQ expression was downregulated in the hypothalamic cells, whereas DIO2 and TSHB expression was downregulated. Thus, miR-136-GNAQ might act as a switch in the regulation of seasonal estrus under different photoperiod conditions. These findings further enrich our understanding of the relationship between miRNAs and seasonal regulation of reproductive activity. Furthermore, our study provides novel insights into the miRNA-mediated regulatory mechanisms for overcoming photoinhibition in the seasonally breeding mammals, such as Kazakh sheep.

Photoperiod alters the choroid plexus response to LPS-induced acute inflammation in EWES

This study determined the influence of photoperiod on the expression of toll-like receptor 2 and 4 (TLR2 and TLR4), interleukin 1□ (IL1B), IL-1 receptor type I (IL1R1) and II (IL1R2), interleukin 6 (IL6), the IL-6 receptor (IL6R) and signal transducer (IL6ST), tumor necrosis factor α (TNF), and TNF□ receptor type I (TNFRSF1A) and II (TNFRSF1B) in the choroid plexus (ChP) of ewes with lipopolysaccharide (LPS)-induced acute inflammation. Under short-days (SD, n = 12, anestrous) and long-days (LD, n = 12, synchronized follicular phase), ewes were treated with saline or LPS. Compared to LD conditions, the ewes under SD were characterized by a greater (P<0.05) area under the curve (AUC) of cortisol in the LPS-treated group and by a lower (P<0.05) AUC of prolactin in the saline-treated group. Under both photoperiods, LPS increased (P<0.05) the expression of all examined genes except for TNFRSF1B (only under SD), TNF and TNFRSF1A (no stimulation), and IL6R (decreased (P<0.05) under SD). The LPS-induced increases in TLR2, TLR4, IL1B and its receptors, IL6 and TNFRSF1B were higher (P<0.05) under SD than LD. TLR4 was positively correlated with IL1B and IL6 in both saline- (r2 = 0.64, P<0.01 and r2 = 0.52, P<0.01) and LPS-treated (r2 = 0.81, P<0.0001 and r2 = 0.51, P<0.001) ewes. IL1B (r2 = 0.56, P<0.01 and r2 = 0.77, P<0.0001) and IL6 (r2 = 0.77, P<0.005 and r2 = 0.35, P<0.05) were positively correlated with TLR2 in saline- and LPS-treated ewes, respectively. This indicates that in ewes, the ChP response to acute systemic inflammation is dependent upon the photoperiod with stronger effects being observed under SD. Our results also suggest that gonadal hormones altering TLR4 signaling events are involved in the photoperiodic modulation of the ChP response to LPS. Further experiments are required to explain the mechanism involved in this phenomenon.

Gerald Lincoln: A man for all seasons

Gerald Anthony Lincoln died after a short illness on 15 July 2020 at the age of 75 years. Gerald was Emeritus Professor of Biological Timing at Edinburgh University and a Fellow of the Royal Society of Edinburgh. He was an outstanding scientist and naturalist who was a seminal figure in developing our understanding of the neuroendocrine mechanisms underlying seasonal rhythmicity. This review considers his life and some of his major scientific contributions to our understanding of seasonality, photoperiodism and circannual rhythmicity. It is based on a presentation at the online 2nd annual seasonality symposium (2 October 2020) that was supported financially by the Journal of Neuroendocrinology.

Thyroid hormone and hypothalamic stem cells in seasonal functions

Seasonal rhythms are a pervasive feature of most living organisms, which underlie yearly timeliness in breeding, migration, hibernation or weight gain and loss. To achieve this, organisms have developed inner timing devices (circannual clocks) that endow them with the ability to predict then anticipate changes to come, usually using daylength as the proximate cue. In Vertebrates, daylength interpretation involves photoperiodic control of TSH production by the pars tuberalis (PT) of the pituitary, which governs a seasonal switch in thyroid hormone (TH) availability in the neighboring hypothalamus. Tanycytes, specialized glial cells lining the third ventricle (3V), are responsible for this TH output through the opposite, PT-TSH-driven, seasonal control of deiodinases 2/3 (Dio 2/3). Tanycytes comprise a photoperiod-sensitive stem cell niche and TH is known to play major roles in cell proliferation and differentiation, which suggests that seasonal control of tanycyte proliferation may be involved in the photoperiodic synchronization of seasonal rhythms. Here we review our current knowledge of the molecular and neuroendocrine pathway linking photoperiodic information to seasonal changes in physiological functions and discuss the potential implication of tanycytes, TH and cell proliferation in seasonal timing.

Global Methylation and Protamine Deficiency in Ram Spermatozoa Correlate with Sperm Production and Quality but Are Not Influenced by Melatonin or Season

Simple Summary

Though environmental factors can alter the epigenome of mammalian spermatozoa, it is currently unclear whether these epigenetic changes are linked to sperm production, quality and fertility. This study aimed to identify whether the hormone melatonin, responsible for upregulating ram reproductive function, is able to alter broad epigenetic markers in spermatozoa, namely sperm global methylation and protamine deficiency. It was also investigated whether these parameters corresponded to ram endocrinology, semen production and quality. Though no effects of season or melatonin were found, both sperm global methylation and protamine deficiency correlated with several semen production and quality parameters. These moderate associations with sperm production and quality support that sperm protamine deficiency and global methylation are broadly indicative of testicular function.

Abstract

This study assessed whether the seasonal effects of melatonin that upregulate ram reproductive function alter sperm global methylation or protamine deficiency and whether these parameters corresponded to ram endocrinology, semen production and quality. Ejaculates were assessed from rams that received melatonin implants (n = 9) or no implants (n = 9) during the non-breeding season. Ejaculates (n = 2/ram/week) were collected prior to implantation (week 0), 1, 6 and 12 weeks post implantation and during the following breeding season (week 30). Flow cytometry was used to assess the sperm global methylation and protamine deficiency in each ejaculate, which had known values for sperm concentration, motility, morphology, DNA fragmentation, seminal plasma levels of melatonin, anti-Mullerian hormone and inhibin A. Serum levels of testosterone and melatonin were also evaluated. Though there was no effect of melatonin or season, sperm protamine deficiency was negatively correlated with sperm production and seminal plasma levels of anti-Mullerian hormone and positively correlated with sperm DNA fragmentation and morphology. Global methylation of spermatozoa was positively correlated with sperm DNA fragmentation, morphology and serum testosterone and negatively correlated with sperm motility. These moderate associations with sperm production and quality suggest that sperm protamine deficiency and global methylation are indicative of ram testicular function.

Overcoming neuroendocrine and metabolic barriers to puberty: the role of melatonin in advancing puberty in ewe lambs

Pubertal onset in the ewe is subject to a multitude of physiological and environmental constraints. As seasonal breeders, sheep rely on decreasing photoperiod to enter puberty and the subsequent breeding periods, hindering production. The initiation of puberty defines the reproductive yield of the ewe, and as such is a critical factor influencing production outcomes. Currently, the misconception that ovine puberty is reliant on age results in ewes being bred at over a year old, leading to a substantial unproductive period between birth and first conception. As such, transcending pubertal barriers to allow for earlier initiation of reproductive competency has significant commercial merit. The primary candidate to achieve this is the neurohormone melatonin, a key factor that naturally signals photoperiodic change that facilitates seasonal remodeling of the ovine hypothalamic-hypophyseal-gonadal axis. Despite being known to modulate reproductive seasonality in both the mature ewe and ram, the ability of melatonin to advance ewe puberty remains underutilized in industry. To optimize melatonin application and shape perceptions of breeding ewe lambs, a greater understanding of pubertal impediments and the natural role of melatonin is warranted. This review examines the physiological role and applications of melatonin to advance ewe puberty, and how this may act in conjunction with other physiological and metabolic cues.

Exogenous melatonin advances the ram breeding season and increases testicular function

Governed by melatonin, ovine reproductive seasonality limits production outcomes due to periods of decreased reproductive efficiency. Though it is established that slow-release melatonin implants improve out of season reproductive performance in the ewe, the comprehensive effects of exogenous melatonin in the ram remain inconclusive. This study aimed to ultimately clarify the ability of exogenous melatonin to alter ram reproductive function during the non-breeding season and the subsequent breeding season. Hence, we investigated the effect of exogenous melatonin on reproductive endocrinology, semen quality and production, testicular size and libido in Merino and Poll Dorset rams (n = 31, using a subset of 18 rams for analysis of semen production and quality). Melatonin treatment resulted in elevation of melatonin in seminal plasma from 1-8 weeks post-implantation and in blood plasma at 6 weeks post-implantation. The blood plasma testosterone of implanted rams was greater than controls at both 6 weeks post-implantation and during the following breeding season. Implanted rams exhibited increased testicular size and number of sperm per ejaculate from 3-12 weeks post-implantation but did not demonstrate any change in sperm motility or morphology in response to treatment. Compared to their control counterparts, melatonin-treated Poll Dorset rams exhibited a lower percentage of sperm DNA fragmentation during several weeks of the non-breeding season. Though melatonin increased the likelihood of ejaculate collection in Poll Dorset rams (P < 0.05), libido was otherwise unaffected by treatment. Melatonin did not alter seminal plasma concentrations of inhibin A or Anti-Mullerian hormone, however, for the first time in the ram we have shown Anti-Mullerian hormone to be positively correlated with the number of sperm per ejaculate and sperm motility (r = 0.464 and 0.3242 respectively, P < 0.001), and inhibin A to be correlated to the number of sperm per ejaculate (r = 0.1786, P = 0.0135). These results indicate that melatonin is able to both systemically upregulate reproduction and act directly upon testicular function in the ram.

Reproductive performance in three Slovenian sheep breeds with different alleles for the MTNR1A gene

The aim of the research was to describe the polymorphisms of the MTNR1A gene in three Slovenian sheep breeds, and determine whether these polymorphisms affect reproductive performance. A total of 100 Bovška, 110 Istrian Pramenka, and 108 Jezersko-Solčavska ewes, were used. The rams were placed with the ewes on 10 October (Bovška), and 20 September (Istrian Pramenka). At each farm, the lambing date and number of newborn lambs were recorded to evaluate the fertility rate, number of days from ram placement with ewes to lambing (DRIL), and litter size. The Jezersko-Solčavska rams were maintained with the ewes of the flock throughout the year and data related to lambing during the entire year were recorded. The DNA sequences indicated there were eight nucleotide variations and that in the position g.17355358 was always associated with that in the position g.17355452. For the Bovška breed, there was an association between the G/G genotype in the position g.17355452, and relatively greater fertility (P < 0.05) and a shorter DIRL (P < 0.05). For the Istrian Pramenka breed, the T/T genotype in the position g.17355458 was positively associated with a greater fertility (P < 0.05) and shorter DRIL (P < 0.05). For the Jezersko-Solčavska breed, ewes with the G/G genotype in the position g.17355452 lambed more when there was a decreasing photoperiod whereas the ewes with the A/A genotype lambed more (P < 0.05) during the period when there were increasing photoperiods. The different polymorphisms studied were associated with reproductive performance in the three Slovenian breeds.

The inhibitory effect of melatonin on mammary function of lactating dairy goats†

The direct role of melatonin in mammary glands of dairy goats has remained obscure. This study aimed to evaluate the expression of melatonin membrane receptors (MT1 and MT2) in the pituitary and mammary glands of dairy goats during lactation, and to investigate the role of melatonin in mammary function. Both MT1 and MT2 were consistently expressed in the pituitary and mammary eight glands throughout the lactation period, and their levels were lower in 9 March (group I), June (group III), and September (group V) than in May (group II) and August (group IV). The expression patterns of pituitary and mammary MT1 and MT2 were consistent with those of blood melatonin during lactation. Furthermore, the mammary prolactin (PRL), and pituitary growth hormone (GH) and PRL mRNA expression showed an inverse trend in relation to blood melatonin levels. In mammary tissues, MT1 and MT2 immunoreactivity was predominantly located in the mammary epithelial cells (MECs). In addition, a dose- and time-dependent inhibition on cell viability was observed in cultured MECs. At the dose of 10 and 100 pg/ml, melatonin decreased mammary β-casein and PRL expression. Furthermore, the inhibitory effects of melatonin were blocked by luzindole, a nonselective MT1 and MT2 receptor antagonist. In addition, melatonin promoted MT1 and MT2 expression in cultured MECs. In conclusion, the presence of MT1 and MT2 in the pituitary and mammary glands and the inhibitory effects of melatonin on cell viability, β-casein, and PRL expression in MECs suggest the potential regulation by melatonin in goat mammary function.

Patterns of prolactin secretion

Prolactin is pleotropic in nature affecting multiple tissues throughout the body. As a consequence of the broad range of functions, regulation of anterior pituitary prolactin secretion is complex and atypical as compared to other pituitary hormones. Many studies have provided insight into the complex hypothalamic-pituitary networks controlling prolactin secretion patterns in different species using a range of techniques. Here, we review prolactin secretion in both males and females; and consider the different patterns of prolactin secretion across the reproductive cycle in representative female mammals with short versus long luteal phases and in seasonal breeders. Additionally, we highlight changes in the pattern of secretion during pregnancy and lactation, and discuss the wide range of adaptive functions that prolactin may have in these important physiological states.

Adenosine stimulates neuromedin U mRNA expression in the rat pars tuberalis

Neuromedin U (NMU) shows circadian expression in the rat pars tuberalis (PT), and is known to be suppressed by melatonin. Here we examined the involvement of adenosine in the regulation of Nmu expression. We found that the rat PT expressed adenosine receptor A2b and that an adenosine receptor agonist, NECA, stimulated Nmu expression in brain slice cultures. In vitro promoter assays revealed that NECA stimulated Nmu promoter activity via a cAMP response element (CRE) in the presence of adenosine receptor A2b. NECA also increased the levels of phosphorylated CRE-binding protein. These findings suggest that adenosine stimulates Nmu expression by activating the cAMP signaling pathway through adenosine receptor A2b in the rat PT. This is the first report to demonstrate that Nmu expression in the PT is regulated by adenosine, which acts as an intravital central metabolic signal, in addition to melatonin, which acts as an external photoperiodic environmental signal.

Discontinuity in the molecular neuroendocrine response to increasing daylengths in Ile-de-France ewes: Is transient Dio2 induction a key feature of circannual timing?

In mammals, melatonin is responsible for the synchronisation of seasonal cycles to the solar year. Melatonin is secreted by the pineal gland with a profile reflecting the duration of the night and acts via the pituitary pars tuberalis (PT), which in turn modulates hypothalamic thyroid hormone status via seasonal changes in the production of locally-acting thyrotrophin. Recently, we demonstrated that, in the Soay sheep, photoperiodic induction of Tshb expression and consequent downstream hypothalamic changes occur over a narrow range of photoperiods between 12 and 14 hours in duration. In the present study, we aimed to extend our molecular characterisation of this pathway, based on transcriptomic analysis of photoperiodic changes in the pituitary and hypothalamus of ovariectomised, oestradiol-implanted Ile-de-France ewes. We demonstrate that photoperiodic treatments applied before the winter solstice elicit two distinctive modes of accelerated reproductive switch off compared to ewes held on a simulated natural photoperiod, with shut-down occurring markedly faster on photoperiods of 13 hours or more than on photoperiods of 12 hours and less. This pattern of response was reflected in gene expression profiles of photoperiodically sensitive markers, both in the PT (Tshb, Fam150b, Vmo1, Ezh2 and Suv39H2) and in tanycytes (Tmem252 and Dct). Unexpectedly, the expression of Dio2 in tanycytes did not show any noticeable increase in expression with lengthening photoperiods. Finally, the expression of Kiss1, the key activator of gonadotrophin-releasing hormone release, was proportionately decreased by lengthening photoperiods, in a pattern that correlated strongly with gonadotrophin suppression. These data show that stepwise increases in photoperiod lead to graded molecular responses at the level of the PT, a progressive suppression of Kiss1 in the hypothalamic arcuate nucleus and luteinising hormone/follicle-stimulating hormone release by the pituitary, despite apparently unchanged Dio2 expression in tanycytes. We hypothesise that this apparent discontinuity in the seasonal neuroendocrine response illustrates the transient nature of the thyroid hormone-mediated response to long days in the control of circannual timing.

An integrative view of mammalian seasonal neuroendocrinology

Seasonal neuroendocrine cycles that govern annual changes in reproductive activity, energy metabolism and hair growth are almost ubiquitous in mammals that have evolved at temperate and polar latitudes. Changes in nocturnal melatonin secretion regulating gene expression in the pars tuberalis (PT) of the pituitary stalk are a critical common feature in seasonal mammals. The PT sends signal(s) to the pars distalis of the pituitary to regulate prolactin secretion and thus the annual moult cycle. The PT also signals in a retrograde manner via thyroid-stimulating hormone to tanycytes, which line the ventral wall of the third ventricle in the hypothalamus. Tanycytes show seasonal plasticity in gene expression and play a pivotal role in regulating local thyroid hormone (TH) availability. Within the mediobasal hypothalamus, the cellular and molecular targets of TH remain elusive. However, two populations of hypothalamic neurones, which produce the RF-amide neuropeptides kisspeptin and RFRP3 (RF-amide related peptide 3), are plausible relays between TH and the gonadotrophin-releasing hormone-pituitary-gonadal axis. By contrast, the ways by which TH also impinges on hypothalamic systems regulating energy intake and expenditure remain unknown. Here, we review the neuroendocrine underpinnings of seasonality and identify several areas that warrant further research.

The effect of inflammation on the synthesis of luteinizing hormone and gonadotropin-releasing hormone receptor expression in the pars tuberalis of ewe during different photoperiodic conditions

The study was designed to determine the effect of endotoxin-induced inflammation on luteinizing hormone (LH) synthesis and gonadotropin-releasing hormone (GnRH) receptor expression in the pars tuberalis (PT) of ewes during anestrous season and follicular phase taking into account the time of the day. Moreover, the effect of inflammation on the release of melatonin and its type I receptor gene expression in the PT was also determined. Lipopolysaccharide administration reduced nocturnal release of melatonin only during anestrous season, but it did not influence the gene expression of melatonin type I receptor in the PT. Inflammation inhibited nocturnal increase in the gene and protein expression of LH in the PT during the follicular phase. Since in day-active species nocturnal accumulation of LH protein in the pituitary precedes the LH surge, this lowering of LH content may delay or disturb the surge occurrence. Suppression of LH secretion could have resulted from the decreased sensitivity of the PT on the action of GnRH because inflammation reduced GnRH receptor expression. The study suggests that the ability of endotoxin to suppress LH synthesis in the PT may be another mechanism by which inflammation disturbs reproductive neuroendocrine axis in seasonal breeders.

How can a binary switch within the pars tuberalis control seasonal timing of reproduction?

Life in seasonally changing environments is challenging. Biological systems have to not only respond directly to the environment, but also schedule life history events in anticipation of seasonal changes. The cellular and molecular basis of how these events are scheduled is unknown. Cellular decision-making processes in response to signals above certain thresholds regularly occur i.e. cellular fate determination, apoptosis and firing of action potentials. Binary switches, the result of cellular decision-making processes, are defined as a change in phenotype between two stable states. A recent study presents evidence of a binary switch operating in the pars tuberalis (PT) of the pituitary, seemingly timing seasonal reproduction in sheep. Though, how a binary switch would allow for anticipation of seasonal environmental changes, not just direct responsiveness, is unclear. The purpose of this review is to assess the evidence for a binary switching mechanism timing seasonal reproduction and to hypothesize how a binary switch would allow biological processes to be timed over weeks to years. I draw parallels with mechanisms used in development, cell fate determination and seasonal timing in plants. I propose that the adult PT is a plastic tissue, showing a seasonal cycle of cellular differentiation, and that the underlying processes are likely to be epigenetic. Therefore, considering the mechanisms behind adult cellular plasticity offers a framework to hypothesize how a long-term timer functions within the PT.

Female Reproductive Performance in the Mouse: Effect of Oral Melatonin

Although melatonin has some of the broadest ranges of actions on the physiology of vertebrates, especially on their reproductive processes, the mechanism by which melatonin regulates animal reproduction is still incompletely understood. This study was designed to determine the effect of oral melatonin on the reproductive performance of female mice. Female ICR mice (7 weeks old) were given melatonin-containing water (3, 30 and 300 μg/mL; melatonin) or water only (control) until 10 weeks of age. Then, some of the mice were successfully mated (confirmed by vaginal plugs), and the number of live births and their weights were recorded. Some mice were used for a histological analysis of the number of follicles in the ovaries. Others were used for oocyte collection after superovulation, and in vitro fertilization (IVF) was performed. The mRNA expression of the apopotosis-related genes (BAX, BCL2) in the IVF embryos were analyzed. After melatonin administration, the mice showed similar serum melatonin levels to that of the control. The number of antral follicles per mm² unit area in the 30 μg/mL melatonin-treated group (14.60) was significantly higher than that of the control (7.78), which was lower than that of the 3 μg/mL melatonin-treated group (12.29). The litter size was significantly higher in the 3 μg/mL melatonin-treated group (15.5) than in the control (14.3). After IVF, the hatched blastocyst formation rate in the 30 μg/mL melatonin-treated group (85.70%) was significantly higher than that of the control (72.10%), and it was the same for the BCL2/BAX expression ratio. Although oral melatonin did not appear to have an effect on the serum melatonin rhythm in the mouse, melatonin did increase litter size at the 3 μg/mL dose level, and improved the developmental competency of IVF embryos at the 30 μg/mL level.

Physiological Role of Vascular Endothelial Growth Factors as Homeostatic Regulators

The vascular endothelial growth factor (VEGF) family of proteins are key regulators of physiological systems. Originally linked with endothelial function, they have since become understood to be principal regulators of multiple tissues, both through their actions on vascular cells, but also through direct actions on other tissue types, including epithelial cells, neurons, and the immune system. The complexity of the five members of the gene family in terms of their different splice isoforms, differential translation, and specific localizations have enabled tissues to use these potent signaling molecules to control how they function to maintain their environment. This homeostatic function of VEGFs has been less intensely studied than their involvement in disease processes, development, and reproduction, but they still play a substantial and significant role in healthy control of blood volume and pressure, interstitial volume and drainage, renal and lung function, immunity, and signal processing in the peripheral and central nervous system. The widespread expression of VEGFs in healthy adult tissues, and the disturbances seen when VEGF signaling is inhibited support this view of the proteins as endogenous regulators of normal physiological function. This review summarizes the evidence and recent breakthroughs in understanding of the physiology that is regulated by VEGF, with emphasis on the role they play in maintaining homeostasis. © 2017 American Physiological Society. Compr Physiol 8:955-979, 2018.

Seasonal Reproduction in Vertebrates: Melatonin Synthesis, Binding, and Functionality Using Tinbergen's Four Questions

One of the many functions of melatonin in vertebrates is seasonal reproductive timing. Longer nights in winter correspond to an extended duration of melatonin secretion. The purpose of this review is to discuss melatonin synthesis, receptor subtypes, and function in the context of seasonality across vertebrates. We conclude with Tinbergen's Four Questions to create a comparative framework for future melatonin research in the context of seasonal reproduction.

The pars tuberalis: The site of the circannual clock in mammals?

Accurate timing and physiological adaptation to anticipate seasonal changes are an essential requirement for an organism's survival. In contrast to all other environmental cues, photoperiod offers a highly predictive signal that can be reliably used to activate a seasonal adaptive programme at the correct time of year. Coupled to photoperiod sensing, it is apparent that many organisms have evolved innate long-term timekeeping systems, allowing reliable anticipation of forthcoming environmental changes. The fundamental biological processes giving rise to innate long-term timing, with which the photoperiod-sensing pathway engages, are not known for any organism. There is growing evidence that the pars tuberalis (PT) of the pituitary, which acts as a primary transducer of photoperiodic input, may be the site of the innate long-term timer or "circannual clock". Current research has led to the proposition that the PT-specific thyrotroph may act as a seasonal calendar cell, driving both hypothalamic and pituitary endocrine circuits. Based on this research we propose that the mechanistic basis for the circannual rhythm appears to be deeply conserved, driven by a binary switching cell based accumulator, analogous to that proposed for development. We review the apparent conservation of function and pathways to suggest that these broad principles may apply across the vertebrate lineage and even share characteristics with processes driving seasonal adaptation in plants.

The effect of melatonin from slow-release implants on basic and TLR-4-mediated gene expression of inflammatory cytokines and their receptors in the choroid plexus in ewes

The present study concerns the effect of melatonin from slow-release implants on the expression of genes coding interleukin-1β (Il1B), inerleukin-6 (Il6), tumour necrosis factor α (Tnf) and their receptors: IL-1 receptor type I (Il1r1) and type II (Il1r2), IL-6 receptor (Il6r) and signal transducer (Il6st), TNFα receptor type I (Tnfrsf1a) and II (Tnfrsf1b) and retinoid-related orphan receptor α (RorA) and Rev.-erbα in the ovine choroid plexus (CP) under basal and lipopolysaccharide (LPS)-challenged conditions. Studies were performed on four groups: 1) sham-implanted and placebo-treated, 2) melatonin-implanted (Melovine, 18mg) and placebo-treated, 3) sham-implanted and LPS-treated (400ng/kg of body weight) and 4) melatonin-implanted and LPS-treated. Under basal conditions, we observed weak expression of Tnf, low expression of Il1B, Il6 and Il1r2 and intermediate expression of other cytokines receptors. LPS treatment induced (P≤0.05) expression in all cytokines and their receptors, except Il6r 3h after the administration. Melatonin attenuated (P≤0.05) LPS-induced up-regulation of Il6 but had no effect on other cytokines and their receptors and up-regulated (P≤0.05) Rev.-erbα expression under basal conditions. This indicates that melatonin from slow-release implants suppresses TLR4-mediated Il6 expression in the ovine CP via a mechanism likely involving clock genes.

Mechanisms regulating angiogenesis underlie seasonal control of pituitary function

Seasonal changes in mammalian physiology, such as those affecting reproduction, hibernation, and metabolism, are controlled by pituitary hormones released in response to annual environmental changes. In temperate zones, the primary environmental cue driving seasonal reproductive cycles is the change in day length (i.e., photoperiod), encoded by the pattern of melatonin secretion from the pineal gland. However, although reproduction relies on hypothalamic gonadotrophin-releasing hormone output, and most cells producing reproductive hormones are in the pars distalis (PD) of the pituitary, melatonin receptors are localized in the pars tuberalis (PT), a physically and functionally separate part of the gland. How melatonin in the PT controls the PD is not understood. Here we show that melatonin time-dependently acts on its receptors in the PT to alter splicing of vascular endothelial growth factor (VEGF). Outside the breeding season (BS), angiogenic VEGF-A stimulates vessel growth in the infundibulum, aiding vascular communication among the PT, PD, and brain. This also acts on VEGF receptor 2 (VEGFR2) expressed in PD prolactin-producing cells known to impair gonadotrophin secretion. In contrast, in the BS, melatonin releases antiangiogenic VEGF-A_xxxb from the PT, inhibiting infundibular angiogenesis and diminishing lactotroph (LT) VEGFR2 expression, lifting reproductive axis repression in response to shorter day lengths. The time-dependent, melatonin-induced differential expression of VEGF-A isoforms culminates in alterations in gonadotroph function opposite to those of LTs, with up-regulation and down-regulation of gonadotrophin gene expression during the breeding and nonbreeding seasons, respectively. These results provide a mechanism by which melatonin can control pituitary function in a seasonal manner.

Seasonal changes of metabolism and appetite in Soay rams

Relationships between photoperiod and cycles of voluntary food intake (VFI) and maintenance energy requirements (MER) were determined in Soay rams, subjected to a 6-month photoperiodic regime. Food was offered ad libitum (no. = 5) or at a predicted maintenance level (no. = 5). All rams demonstrated 6-month cycles of VFI, growth and reproductive status. Metabolizable energy intake (MEI) was greatest in rams given food ad libitum (666 (s.e. 21.7) kJ/kg metabolic live weight (M0·75) and food-restricted (528 (s.e. 12.2) kJ/kg M0·75) rams during sexual quiescence. Conversely, MEI was minimal (ad libitum, 289 (s.e. 8.4) kJ/kg M0·75; restricted, 428 (s.e. 8.1) kJ/kg M0·75) during the rut. Distinct cycles of heat production (HP) accompanied changes in MEL Changes in HP were similar (P > 0·05) for both groups (ad libitum, 520 (s.e. 22.1) to 394 (s.e. 9.2) kJ/kg M0·75; restricted 503 (s.e. 14.0) to 407 (s.e. 17.5) kJ/kg M0·75) and therefore energy retention varied more (P < 0·015) when rams were given food ad libitum (ad libitum, 131 (s.e. 43-1) to -106 (s.e. 38.2) kJ/kg M0·75; restricted, 78·0 (s.e. 27.1) to -53.0 (s.e. 38.2) kJ/ kg M0·75). Apparent digestibility of dietary energy varied inversely with MEI (P < 0·01). MERs ranged from 524 (s.e. 35.0) kJ/kg M0·75 to 401 (s.e. 27.3) kJ/kg M0·75, a proportional fluctuation of ±0·13. Changes in metabolic rate preceded those in appetite, suggesting a causal relationship.

Intrapituitary mechanisms underlying the control of fertility: key players in seasonal breeding

Recent studies have shown that, in conjunction with dynamic changes in the secretion of GnRH from the hypothalamus, paracrine interactions within the pituitary gland play an important role in the regulation of fertility during the annual reproductive cycle. Morphological studies have provided evidence for close associations between gonadotropes and lactotropes and gap junction coupling between these cells in a variety of species. The physiological significance of this cellular interaction was supported by subsequent studies revealing the expression of prolactin receptors in both the pars distalis and pars tuberalis regions of the pituitary. This cellular interaction is critical for adequate gonadotropin output because, in the presence of dopamine, prolactin can negatively regulate the LH response to GnRH. Receptor signaling studies showed that signal convergence at the level of protein kinase C and phospholipase C within the gonadotrope underlies the resulting inhibition of LH secretion. Although this is a conserved mechanism present in all species studied so far, in seasonal breeders such as the sheep and the horse, this mechanism is regulated by photoperiod, as it is only apparent during the long days of spring and summer. At this time of year, the nonbreeding season of the sheep coincides with the breeding season of the horse, indicating that this inhibitory system plays different roles in short- and long-day breeders. Although in the sheep, it contributes to the complete suppression of the reproductive axis, in the horse, it is likely to participate in the fine-tuning of gonadotropin output by preventing gonadotrope desensitization. The photoperiodic regulation of this inhibitory mechanism appears to rely on alterations in the folliculostellate cell population. Indeed, electron microscopic studies have recently shown increased folliculostellate cell area together with upregulation of their adherens junctions during the spring and summer. The association between gonadotropes and lactotropes could also underlie an interaction between the gonadotropic and prolactin axes in the opposite direction. In support of this alternative, a series of studies have demonstrated that GnRH stimulates prolactin secretion in sheep through a mechanism that does not involve the mediatory actions of LH or FSH and that this stimulatory effect of GnRH on the prolactin axis is seasonally regulated. Collectively, these findings highlight the importance of intercellular communications within the pituitary in the control of gonadotropin and prolactin secretion during the annual reproductive cycle in seasonal breeders.

Binary Switching of Calendar Cells in the Pituitary Defines the Phase of the Circannual Cycle in Mammals

Persistent free-running circannual (approximately year-long) rhythms have evolved in animals to regulate hormone cycles, drive metabolic rhythms (including hibernation), and time annual reproduction. Recent studies have defined the photoperiodic input to this rhythm, wherein melatonin acts on thyrotroph cells of the pituitary pars tuberalis (PT), leading to seasonal changes in the control of thyroid hormone metabolism in the hypothalamus. However, seasonal rhythms persist in constant conditions in many species in the absence of a changing photoperiod signal, leading to the generation of circannual cycles. It is not known which cells, tissues, and pathways generate these remarkable long-term rhythmic processes. We show that individual PT thyrotrophs can be in one of two binary states reflecting either a long (EYA3(+)) or short (CHGA(+)) photoperiod, with the relative proportion in each state defining the phase of the circannual cycle. We also show that a morphogenic cycle driven by the PT leads to extensive re-modeling of the PT and hypothalamus over the circannual cycle. We propose that the PT may employ a recapitulated developmental pathway to drive changes in morphology of tissues and cells. Our data are consistent with the hypothesis that the circannual timer may reside within the PT thyrotroph and is encoded by a binary switch timing mechanism, which may regulate the generation of circannual neuroendocrine rhythms, leading to dynamic re-modeling of the hypothalamic interface. In summary, the PT-ventral hypothalamus now appears to be a prime structure involved in long-term rhythm generation.

60 YEARS OF NEUROENDOCRINOLOGY: The hypothalamo-pituitary-gonadal axis

This review provides an outline of how our understanding of the neuroendocrine control of the hypothalamo-pituitary-gonadal axis has evolved since the publication of Geoffrey Harris' renowned monograph in 1955. Particular attention is directed to the neurobiology underlying pulsatile GnRH release from the hypothalamus, the neuroendocrine control of ovarian cycles, puberty and seasonality of gonadal function, and to ideas that have emerged as a result of examining the relationship between growth and the reproductive axis. The review closes with i) a brief discussion of how knowledge gained as a result of pursuing the early hypotheses of Harris has led to major clinical and therapeutic applications, and ii) a personal glimpse into the future of research in this fascinating area of biology.

60 YEARS OF NEUROENDOCRINOLOGY: Regulation of mammalian neuroendocrine physiology and rhythms by melatonin

The isolation of melatonin was first reported in 1958. Since the demonstration that pineal melatonin synthesis reflects both daily and seasonal time, melatonin has become a key element of chronobiology research. In mammals, pineal melatonin is essential for transducing day-length information into seasonal physiological responses. Due to its lipophilic nature, melatonin is able to cross the placenta and is believed to regulate multiple aspects of perinatal physiology. The endogenous daily melatonin rhythm is also likely to play a role in the maintenance of synchrony between circadian clocks throughout the adult body. Pharmacological doses of melatonin are effective in resetting circadian rhythms if taken at an appropriate time of day, and can acutely regulate factors such as body temperature and alertness, especially when taken during the day. Despite the extensive literature on melatonin physiology, some key questions remain unanswered. In particular, the amplitude of melatonin rhythms has been recently associated with diseases such as type 2 diabetes mellitus but understanding of the physiological significance of melatonin rhythm amplitude remains poorly understood.

Phenomenon of leptin resistance in seasonal animals: the failure of leptin action in the brain

The core of the leptin resistance hypothesis promulgated several years ago to explain obesity as a result of environmental causes consists of 2 tenets: the extinction of leptin-induced intracellular signaling downstream of leptin binding to the long form of the neuronal receptor LTRb in the hypothalamus and the impedance to leptin entry imposed at the blood-brain barrier (BBB). A recent comprehensive investigation concluded that a central leptin insufficiency associated with obesity can be attributed to a decreased efficiency of BBB leptin transport and not to leptin insensitivity within the hypothalamus. Interestingly, anorectic leptin's effects are counteracted in some individuals by a natural resistance associated with hyperleptinemia, which is related to changes in hypothalamic sensitivity to leptin (eg, due to malnutrition, obesity, or seasonal variations due to day-length-dependent reproduction changes). In sheep, it has been observed that the hypothalamus is resistant to leptin in some periods, which is related to the adaptation of these animals to annual changes in energy supply and demand. However, a broad range of ambiguities exists regarding the implications that the intracellular signaling of signal transducer and activator of transcription-2/suppressor of cytokine signaling 3 (STAT2/SOCS3) imparts central leptin resistance. Furthermore, several plausible alternative possibilities have been proposed, such as compensatory functional and anatomic reorganizations in the appetite regulating network, rearrangements in the afferent hormonal feedback signaling involved in weight homeostasis, and modifications in leptin transport to the hypothalamus across the BBB. Taken together, these observations suggest that the contention that impaired intracellular signaling downstream of leptin entry into the appetite regulating network expedites environmentally induced obesity remains unsubstantiated and requires further evidence. Furthermore, pregnancy decreases hypothalamic sensitivity to leptin (or other unknown mechanisms), and lactation can also alter the appetite-suppressing central activity of leptin. The objective of this review was to offer an approach to understanding (1) how information regarding nutritional status is transmitted to and interpreted within the hypothalamus in animals, with special attention on seasonally breeding animals and (2) whether central leptin resistance and/or leptin insufficiency in the hypothalamus favors the development of obesity.

Ultrastructural changes in lactotrophs and folliculo-stellate cells in the ovine pituitary during the annual reproductive cycle

In seasonal mammals living in temperate zones, photoperiod regulates prolactin secretion, such that prolactin plasma concentrations peak during the summer months and are lowest during the winter. In sheep, a short-day breeder, circulating prolactin has important modulatory effects on the reproductive system via inhibitory actions on pituitary gonadotrophs and hypothalamic gonadotrophin-releasing hormone release. The exact cellular mechanisms that account for the chronic hypersecretion of prolactin during the summer is not known, although evidence supports an intrapituitary mechanism regulated by melatonin. Folliculo-stellate (FS) cells are non-endocrine cells that play a crucial role in paracrine communication within the pituitary and produce factors controlling prolactin and gonadotrophin release. The present study examined the morphology of the FS and lactotroph cell populations and their distribution in the sheep pituitary during the annual reproductive cycle. Ovine pituitary glands were collected in the winter (breeding season; BS) and summer (nonbreeding season; NBS) and were prepared for quantitative electron microscopy to assess the effects of season on FS and lactotroph cell density, morphology and distribution, as well as on junctional contacts between cells. It was found that lactotrophs in the NBS are larger in size and contain more numerous PRL granules than lactotrophs in the BS. FS cells were also larger in the NBS compared to BS and showed altered morphology such that, in the BS, long cell processes surrounded clusters of adjacent secretory cells. Although no significant change in the number of junctions was observed between lactotrophs and FS cells, or lactotrophs and gonadotrophs, there was a significant increase in the number of adherens junctions between lactotrophs and between FS cells. These findings demonstrate seasonal plasticity in the morphology of lactotrophs and FS cells that reflect changes in PRL secretion.

Melatonin regulates somatotrope and lactotrope function through common and distinct signaling pathways in cultured primary pituitary cells from female primates

Melatonin (MT) is secreted by the pineal gland and exhibits a striking circadian rhythm in its release. Depending on the species studied, some pituitary hormones also display marked circadian/seasonal patterns and rhythms of secretion. However, the precise relationship between MT and pituitary function remains controversial, and studies focusing on the direct role of MT in normal pituitary cells are limited to nonprimate species. Here, adult normal primate (baboons) primary pituitary cell cultures were used to determine the direct impact of MT on the functioning of all pituitary cell types from the pars distalis. MT increased GH and prolactin (PRL) expression/release in a dose- and time-dependent fashion, a response that was blocked by somatostatin. However, MT did not significantly affect ACTH, FSH, LH, or TSH expression/release. MT did not alter GHRH- or ghrelin-induced GH and/or PRL secretions, suggesting that MT may activate similar signaling pathways as ghrelin/GHRH. The effects of MT on GH/PRL release, which are likely mediated through MT1 receptor, involve both common (adenylyl cyclase/protein kinase A/extracellular calcium-channels) and distinct (phospholipase C/intracellular calcium-channels) signaling pathways. Actions of MT on pituitary cells also included regulation of the expression of other key components for the control of somatotrope/lactotrope function (GHRH, ghrelin, and somatostatin receptors). These results show, for the first time in a primate model, that MT directly regulates somatotrope/lactotrope function, thereby lending support to the notion that the actions of MT on these cells might substantially contribute to the define daily patterns of GH and PRL observed in primates and perhaps in humans.

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.

On the value of seasonal mammals for identifying mechanisms underlying the control of food intake and body weight

This article is part of a Special Issue "Energy Balance". Seasonal cycles of adiposity and body weight reflecting changes in both food intake and energy expenditure are the norm in mammals that have evolved in temperate and polar habitats. Innate circannual rhythmicity and direct responses to the annual change in photoperiod combine to ensure that behavior and energy metabolism are regulated in anticipation of altered energetic demands such as the energetically costly processes of hibernation, migration, and lactation. In the last decade, major progress has been made into identifying the central mechanisms that underlie these profound long-term changes in behavior and physiology. Surprisingly they are distinct from the peptidergic and aminergic systems in the hypothalamus that have been identified in studies of the laboratory mouse and rat and implicated in timing meal intervals and in short-term responses to caloric restriction. Comparative studies across rodents, ungulates and birds reveal that tanycytes embedded in the ependymal layer of the third ventricle play a critical role in seasonal changes because they regulate the local availability of thyroid hormone. Understanding how this altered hormonal environment might regulate neurogenesis and plasticity in the hypothalamus should provide new insight into development of strategies to manage appetite and body weight.

Dim light at night disrupts the short-day response in Siberian hamsters

Photoperiodic regulation of physiology, morphology, and behavior is crucial for many animals to survive seasonally variable conditions unfavorable for reproduction and survival. The photoperiodic response in mammals is mediated by nocturnal secretion of melatonin under the control of a circadian clock. However, artificial light at night caused by recent urbanization may disrupt the circadian clock, as well as the photoperiodic response by blunting melatonin secretion. Here we examined the effect of dim light at night (dLAN) (5lux of light during the dark phase) on locomotor activity rhythms and short-day regulation of reproduction, body mass, pelage properties, and immune responses of male Siberian hamsters. Short-day animals reduced gonadal and body mass, decreased spermatid nuclei and sperm numbers, molted to a whiter pelage, and increased pelage density compared to long-day animals. However, animals that experienced short days with dLAN did not show these short-day responses. Moreover, short-day specific immune responses were altered in dLAN conditions. The nocturnal activity pattern was blunted in dLAN hamsters, consistent with the observation that dLAN changed expression of the circadian clock gene, Period1. In addition, we demonstrated that expression levels of genes implicated in the photoperiodic response, Mel-1a melatonin receptor, Eyes absent 3, thyroid stimulating hormone receptor, gonadotropin-releasing hormone, and gonadotropin-inhibitory hormone, were higher in dLAN animals than those in short-day animals. These results suggest that dLAN disturbs the circadian clock function and affects the molecular mechanisms of the photoperiodic response.

Negative regulation of neuromedin U mRNA expression in the rat pars tuberalis by melatonin

The pars tuberalis (PT) is part of the anterior pituitary gland surrounding the median eminence as a thin cell layer. The characteristics of PT differ from those of the pars distalis (PD), such as cell composition and gene expression, suggesting that the PT has a unique physiological function compared to the PD. Because the PT highly expresses melatonin receptor type 1, it is considered a mediator of seasonal and/or circadian signals of melatonin. Expression of neuromedin U (NMU) that is known to regulate energy balance has been previously reported in the rat PT; however, the regulatory mechanism of NMU mRNA expression and secretion in the PT are still obscure. In this study, we examined both the diurnal change of NMU mRNA expression in the rat PT and the effects of melatonin on NMU in vivo. In situ hybridization and quantitative PCR analysis of laser microdissected PT samples revealed that NMU mRNA expression in the PT has diurnal variation that is high during the light phase and low during the dark phase. Furthermore, melatonin administration significantly suppressed NMU mRNA expression in the PT in vivo. On the other hand, 48 h fasting did not have an effect on PT-NMU mRNA expression, and the diurnal change of NMU mRNA expression was maintained. We also found the highest expression of neuromedin U receptor type 2 (NMUR2) mRNA in the third ventricle ependymal cell layer, followed by the arcuate nucleus and the spinal cord. These results suggest that NMU mRNA expression in the PT is downregulated by melatonin during the dark phase and shows diurnal change. Considering that NMU mRNA in the PT showed the highest expression level in the brain, PT-NMU may act on NMUR2 in the brain, especially in the third ventricle ependymal cell layer, with a circadian rhythm.

Cells co-expressing luteinising hormone and thyroid-stimulating hormone are present in the ovine pituitary pars distalis but not the pars tuberalis: implications for the control of endogenous circannual rhythms of prolactin

BACKGROUND/AIMS

A mammalian circannual pacemaker responsible for regulating the seasonal pattern of prolactin has been recently described in sheep. This pacemaker resides within the pars tuberalis, an area of the pituitary gland that densely expresses melatonin receptors. However, the chemical identity of the cell type which acts as the pacemaker remains elusive. Mathematical-modelling approaches have established that this cell must be responsive to the static melatonin signal as well as prolactin negative feedback. Considering that in sheep the gonadotroph is the only cell in the pars tuberalis which expresses the prolactin receptor, and that in other photoperiodic species the thyrotroph is the only cell expressing the melatonin receptor in this tissue, a cell type which expresses both proteins would fulfil the theoretical criteria of a circannual pacemaker.

METHODS

Pituitary glands were obtained from female sheep under short days (breeding season) and long days (non-breeding season) and double immunofluorescent staining was conducted to determine the prevalence of bi-hormonal cells in the pars distalis and pars tuberalis using specific antibodies to luteinising hormone-β and thyroid-stimulating hormone-β.

RESULTS

The results reveal that whilst such a bihormonal cell is clearly present in the pars distalis and constitute 4% of the gonadotroph population in this region, the same cell type is completely absent from the pars tuberalis even though LH gonadotrophs are abundantly expressed.

CONCLUSIONS

Based on these findings, together with existing data, we are able to propose an alternative model where the gonadotroph itself is controlled indirectly by neighbouring melatonin responsive cells, allowing it to act as a pacemaker.

Circannual variation in thyroid hormone deiodinases in a short-day breeder

At temperate latitudes, many mammals and birds show internally timed, long-term changes in seasonal physiology, synchronised to the seasons by changing day length (photoperiod). Photoperiodic control of thyroid hormone levels in the hypothalamus dictates the timing. This is effected through reciprocal regulation of thyroid hormone deiodinase gene expression. The local synthesis of type 2 deiodinase (Dio2) promotes triiodothyronine (T3) production and summer biology, whereas type 3 deiodinase (Dio3) promotes T3 degradation and winter biology. In the present study, we investigated the extent to which the hypothalamic expression of Dio2 and Dio3 is circannually regulated in the Soay sheep, a short-day breeding mammal. Male sheep were exposed to a long photoperiod (LP; 16 : 24 h light/dark cycle) or a short photoperiod (SP; 8 : 24 h light/dark cycle), for up to 28 weeks to establish four different endocrine states: (i) LP animals in a spring/summer-like state of reproductive arrest; (ii) LP refractory (LPR) animals showing spontaneous reproductive reactivation; (iii) SP animals showing autumn/winter-like reproductive activation; and (iv) SP refractory (SPR) animals showing spontaneous reproductive arrest. A complex pattern of hypothalamic Dio2 and Dio3 expression was observed, revealing distinctive photoperiod-driven and internally timed effects for both genes. The patterns of expression differed both spatially and temporally, with phases of peak Dio2 expression in the median eminence and tuberoinfundibular sulcus, as well as in the paraventricular zone (PVZ) (maximal under LP), whereas Dio3 expression was always confined to the PVZ (maximal under SP). These effects likely reflect the distinct roles of these enzymes in the localised control of hypothalamic T3 levels. The spontaneous decline in Dio2 and spontaneous increase in Dio3 in LPR animals occurred with a corresponding decline in thyroid-stimulating hormone β expression in the neighbouring pars tuberalis (PT), although this relationship did not hold for the corresponding Dio2 increase/Dio3 decrease seen in SPR animals. We conclude that internally timed and spatially regulated changes in Dio2 and Dio3 expression may drive the cycling between breeding and nonbreeding states in long-lived seasonal species, and may be either PT-dependent or PT-independent at different phases of the circannual cycle.

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.