Assessment of Embryo-Induced Transcriptomic Changes in Hamster Uterus Using RNA-Seq.

BACKGROUND/AIMS: The mouse is widely used as an animal model for studying human embryo implantation. However, the mouse is unique in that both ovarian progesterone and estrogen are critical to implantation, whereas in the majority of species (e.g. human and hamster) implantation can occur in the presence of progesterone alone. METHODS: In this study, we analyzed embryo-induced transcriptomic changes in the hamster uterus during embryo implantation by using RNA-seq. Differentially expressed genes were characterized by bioinformatic analysis. RESULTS: We identified a total of 781 differentially expressed genes, of which 367 genes were up-regulated and 414 genes were down-regulated at the implantation site compared to the inter-implantation site. Functional clustering and gene network analysis highlighted the cell cycle process in uterus upon embryo implantation. By examining of the promoter regions of differentially expressed genes, we identified 7 causal transcription factors. Additionally, through connectivity map (CMap) analysis, multiple compounds were identified to have potential anti-implantation effects due to their ability to reverse embryo-induced transcriptomic changes. CONCLUSION: Our study provides a valuable resource for in-depth understanding of the mechanism underlying embryo implantation.

A circannual clock drives expression of genes central for seasonal reproduction.

Animals living in temperate zones anticipate seasonal environmental changes to adapt their biological functions, especially reproduction and metabolism. Two main physiological mechanisms have evolved for this adaptation: intrinsic long-term timing mechanisms with an oscillating period of approximately 1 year, driven by a circannual clock [1], and synchronization of biological rhythms to the sidereal year using day length (photoperiod) [2]. In mammals, the pineal hormone melatonin relays photoperiodic information to the hypothalamus to control seasonal physiology through well-defined mechanisms [3-6]. In contrast, little is known about how the circannual clock drives endogenous changes in seasonal functions. The aim of this study was to determine whether genes involved in photoperiodic time measurement (TSHß and Dio2) and central control of reproduction (Rfrp and Kiss1) display circannual rhythms in expression under constant conditions. Male European hamsters, deprived of seasonal time cues by pinealectomy and maintenance in constant photoperiod, were selected when expressing a subjective summer or subjective winter state in their circannual cycle of body weight, temperature, and testicular size. TSHß expression in the pars tuberalis (PT) displayed a robust circannual variation with highest level in the subjective summer state, which was positively correlated with hypothalamic Dio2 and Rfrp expression. The negative sex steroid feedback was found to act specifically on arcuate Kiss1 expression. Our findings reveal TSH as a circannual output of the PT, which in turn regulates hypothalamic neurons controlling reproductive activity. Therefore, both the circannual and the melatonin signals converge on PT TSHß expression to synchronize seasonal biological activity.

Adaptation to short photoperiods augments circadian food anticipatory activity in Siberian hamsters.

This article is part of a Special Issue "Energy Balance". Both the light-dark cycle and the timing of food intake can entrain circadian rhythms. Entrainment to food is mediated by a food entrainable circadian oscillator (FEO) that is formally and mechanistically separable from the hypothalamic light-entrainable oscillator. This experiment examined whether seasonal changes in day length affect the function of the FEO in male Siberian hamsters (Phodopus sungorus). Hamsters housed in long (LD; 15 h light/day) or short (SD; 9h light/day) photoperiods were subjected to a timed-feeding schedule for 10 days, during which food was available only during a 5h interval of the light phase. Running wheel activity occurring within a 3h window immediately prior to actual or anticipated food delivery was operationally-defined as food anticipatory activity (FAA). After the timed-feeding interval, hamsters were fed ad libitum, and FAA was assessed 2 and 7 days later via probe trials of total food deprivation. During timed-feeding, all hamsters exhibited increases FAA, but FAA emerged more rapidly in SD; in probe trials, FAA was greater in magnitude and persistence in SD. Gonadectomy in LD did not induce the SD-like FAA phenotype, indicating that withdrawal of gonadal hormones is not sufficient to mediate the effects of photoperiod on FAA. Entrainment of the circadian system to light markedly affects the functional output of the FEO via gonadal hormone-independent mechanisms. Rapid emergence and persistent expression of FAA in SD may reflect a seasonal adaptation that directs behavior toward sources of nutrition with high temporal precision at times of year when food is scarce.

Facilitation of male sexual behavior in Syrian hamsters by the combined action of dihydrotestosterone and testosterone.

BACKGROUND: Testosterone (T) controls male Syrian hamster sexual behavior, however, neither of T's primary metabolites, dihydrotestosterone (DHT) and estradiol (E(2)), even in highly supraphysiological doses, fully restores sexual behavior in castrated hamsters. DHT and T apparently interact with androgen receptors differentially to control male sexual behavior (MSB), but whether these two hormones act synergistically or antagonistically to control MSB has received scant experimental attention and is addressed in the present study. METHODOLOGY/PRINCIPAL FINDINGS: Sexually experienced male Syrian hamsters were gonadectomized and monitored 5 weeks later to confirm elimination of the ejaculatory reflex (week 0), at which time they received subcutaneous DHT-filled or empty capsules that remained in situ for the duration of the experiment. Daily injections of a physiological dose of 25 µg T or vehicle commenced two weeks after capsule implantation. MSB was tested 2, 4 and 5 weeks after T treatment began. DHT capsules were no more effective than control treatment for long-term restoration of ejaculation. Combined DHT + T treatment, however, restored the ejaculatory reflex more effectively than T alone, as evidenced by more rapid recovery of ejaculatory behavior, shorter ejaculation latencies, and a greater number of ejaculations in 30 minute tests. CONCLUSIONS/SIGNIFICANCE: DHT and T administered together restored sexual behavior to pre-castration levels more rapidly than did T alone, whereas DHT and vehicle were largely ineffective. The additive actions of DHT and T on MSB are discussed in relation to different effects of these androgens on androgen receptors in the male hamster brain mating circuit.

Hamster sperm capacitation: role of pyruvate dehydrogenase A and dihydrolipoamide dehydrogenase.

Recently, we demonstrated that pyruvate dehydrogenase A2 (PDHA2) is tyrosine phosphorylated in capacitated hamster spermatozoa. In this report, using bromopyruvate (BP), an inhibitor of PDHA, we demonstrated that hamster sperm hyperactivation was blocked regardless of whether PDHA was inhibited prior to or after the onset of hyperactivation, but the acrosome reaction was blocked only if PDHA was inhibited prior to the onset of the acrosome reaction. Further, inhibition of PDHA activity did not inhibit capacitation-associated protein tyrosine phosphorylation observed in hamster spermatozoa. It is demonstrated that the essentiality of PDHA for sperm capacitation is probably dependent on its ability to generate effectors of capacitation such as reactive oxygen species (ROS) and cAMP, which are significantly decreased in the presence of BP. MICA (5-methoxyindole-2-carboxylic acid, a specific inhibitor of dihydrolipoamide dehydrogenase [DLD]), another component of the pyruvate dehydrogenase complex (PDHc), also significantly inhibited ROS generation and cAMP levels thus implying that these enzymes of the PDHc are required for ROS and cAMP generation. Furthermore, dibutryl cyclic adenosine monophosphate could significantly reverse the inhibition of hyperactivation observed in the presence of BP and inhibition of acrosome reaction observed in the presence of BP or MICA. The calcium ionophore, A23187, could also significantly reverse the inhibitory effect of BP and MICA on sperm acrosome reaction. These results establish that PDHA is required for hamster sperm hyperactivation and acrosome reaction, and DLD is required for hamster acrosome reaction. This study also provides evidence that ROS, cAMP, and calcium are involved downstream to PDHA.

The hamster as a model for embryo implantation: insights into a multifaceted process.

Defects in preimplantation embryonic development, uterine receptivity, and implantation are the leading cause of infertility, pregnancy problems and birth defects. Significant progress has been made in our basic understanding of these processes using the mouse model, where implantation is ovarian estrogen-dependent in the presence of progesterone. However, an animal model where implantation is progesterone-dependent must also be studied to gain a full understanding of the embryo and uterine events that are required for implantation. In this regard, the hamster is a useful model and this review summarizes the information currently available regarding mechanisms involved in synchronous preimplantation embryo and uterine development for implantation in this species.

Estrogen and androgen repression of two female specific lacrimal lipocalins in hamster: Pituitary independent and sex hormone receptor mediated action.

Sexual dimorphism in lacrimal gland (LG) gene expression is believed to be due to direct inductive effects of androgens mediated by androgen receptors (AR) but hypophysectomy dramatically curtails these inductive effects. Since, functional estrogen receptors (ER) could not be detected in LG, estrogen effects on LG are believed to be indirectly mediated by changes in levels of pituitary hormones. We found that two lipocalins expressed in female hamster LG display an unusual and marked repression by both androgens and estrogens, which could be detected both at the level of transcripts and proteins. Here, we investigate whether these repressions, (i) require presence of pituitary and (ii) are mediated by androgen and estrogen receptors. Pituitary-ablation but not gonadectomy reduced LG weights in hamster. However, both pituitary-ablation and gonadectomy induced abundant expression of the LG lipocalins, which were markedly repressed by androgen or estrogen treatment. AR- and ER-antagonists prevented these repressions and only ER-alpha- but not ER-beta-specific agonist could mimic the estrogenic repression. AR transcript and protein and ER-alpha transcript were also detected in hamster LG. Thus, pituitary factors are neither essential for the expression of these LG lipocalins nor for their estrogenic or androgenic repressions and these repressions are very likely mediated by functional ER and AR present in LG.

Interactions of gonadotropin-releasing hormone (GnRH) and gonadotropin-inhibitory hormone (GnIH) in birds and mammals.

Gonadotropin-releasing hormone (GnRH) regulates secretion of both of the gonadotropins, luteinizing hormone (LH) and follicle-stimulating hormone. Thus, it is a key hormone for vertebrate reproduction. GnRH was considered to be unusual among hypothalamic neuropeptides in that it appeared to have no direct antagonist, although some neurochemicals and peripheral hormones (opiates, GABA, gonadal steroids, inhibin) can modulate gonadotropin release to a degree. Five years ago, a vertebrate hypothalamic neuropeptide that inhibited pituitary gonadotropin release in a dose-dependent manner was discovered in quail by Tsutsui et al. (2000. Biochem Biophys Res Commun 275:661-667). We now know that this inhibitory peptide, named gonadotropin-inhibitory hormone, or GnIH, is a regulator of gonadotropin release in vitro and in vivo. Its discovery has opened the door to an entirely new line of research within the realm of reproductive biology. In our collaborative studies, we have begun to elucidate the manner in which GnIH interacts with GnRH to time release of gonadotropins and thus time reproductive activity in birds and mammals. This paper reviews the distribution of GnIH in songbirds relative to GnRHs, and our findings on its modes of action in vitro and in vivo, based on laboratory and field studies. These data are simultaneously compared with our findings in mammals, highlighting how the use of different model species within different vertebrate classes can be a useful approach to identify the conserved actions of this novel neuropeptide, along with its potential importance to vertebrate reproduction.

Diurnal rhythms of common alpha-subunit mRNA expression in the pars tuberalis of hamsters and chickens.

To clarify whether the common alpha-subunit of glycoprotein hormones is involved in photic signal transduction, alpha-subunit mRNA levels in the pars tuberalis (PT) of both hamsters and chickens were estimated at different time points of the day/night cycle by laser capture microdissection (LCM) and real-time quantitative polymerase chain reaction (PCR). Distinct diurnal rhythms were found for alpha-subunit mRNA expression in both species. In the hamster PT, alpha-subunit mRNA levels gradually increased during the dark phase; the diurnal peak was found at time (ZT) 21. The lowest value was obtained at ZT 5 during the day. In the chicken PT, alpha-subunit mRNA levels were maintained at a low constant level at night between ZT 13 and 21. Thus, alpha-subunit mRNA expression in the PT depends on the light-dark cycle and may be controlled by the pineal hormone melatonin. The effect of various photoperiods on the hamster PT was examined by real-time PCR, immunohistochemistry, and electron microscopy. In hamsters kept under short photoperiod (L/D=8 h:16 h) or complete darkness, a dramatic decrease of alpha-subunit mRNA level was induced, and the PT-specific cells accumulated glycogen-like particles and enlarged secretory granules. Under long photoperiods (L/D=16 h:8 h), however, the alpha-subunit mRNA level was elevated and the PT-specific cells exhibited highly active features, i.e., piles of lamellar cisternae of rough endoplasmic reticulum and well-developed Golgi complexes. The alpha-subunit synthesized by the PT-specific cells may therefore participate in the circadian and seasonal regulation of endocrine activities.

Evidence for the disproportionate mapping of olfactory airspace onto the main olfactory bulb of the hamster.

Olfactory receptor neurons (ORNs) project to the rodent main olfactory bulb (MOB) from spatially distinct air channels in the olfactory recesses of the nose. The relatively smooth central channels of the dorsal meatus map onto the dorsal MOB, whereas the highly convoluted peripheral channels of the ethmoid turbinates project to the ventral MOB. Medial and lateral components of each projection stream innervate the medial and lateral MOB, respectively. To ascertain whether such topography entails the disproportionate representation seen in other sensory maps, we used disector-based stereological techniques in hamsters to estimate the number of ORNs associated with each channel in the nose and the number of their targets (glomeruli and mitral and tufted cells) in corresponding divisions of the MOB. Each circumferential half of the MOB (dorsal/ventral, medial/lateral) contained about 50% of the 3,100 glomeruli and about 50% of the 160,000 mitral and tufted cells per bulb. We found equivalent numbers of ORNs with dendritic knobs in the medial and lateral channels (4.5 million each). However, the central channels had only 2 million knobbed ORNs, whereas the peripheral channels had 7 million. Thus, there is a disproportionate mapping of the central-peripheral axis of olfactory airspace onto the dorsal-ventral axis of the MOB, encompassing a greater than threefold variation in the average convergence of ORNs onto MOB secondary neurons. We hypothesize that the disproportionate projections help to optimize chemospecific processing by compensating, with differing sensitivity, for significant variation in the distribution and concentration of odorant molecules along the olfactory air channels during sniffing.

Analysis of the golden Syrian hamster anterior pituitary gland proteome by ion trap mass spectrometry.

We utilized mass spectrometry (MS) and bioinformatics to investigate the proteome of the anterior pituitary gland (AP). Subcellular fractions of APs from 2-month-old male Golden Syrian hamsters were prepared for protein denaturation, treatment with trypsin and analyses utilizing micro liquid chromatography MS/MS and the database search software SEQUEST. In the nuclear, non-nuclear 100,000 x g and cytosolic fractions we identified 76, 52 and 52 different proteins, respectively. A total of 145 distinct proteins were detected. We identified growth hormone, prolactin, pro-opiomelanocortin, the alpha-subunit for the glycoprotein hormones, luteinizing hormone-beta and follicle-stimulating hormone-beta. Groups of other identified proteins included hormone processing, secretion granule associated, non-hormonal endoplasmic reticulum associated, calcium binding, protein kinase C associated histone and non-histone chromosomal material, other RNA-binding, splicing factors, heterogeneous nuclear ribonucleoproteins, helicases, lamins, microfilament associated, microtubule associated, adenosine triphosphate and guanosine diphosphate associated, keratins, lysosomal, ribosomal, enzymes in glycolysis and the tricarboxylic and pentose phosphate paths, glutathione associated, transmethylation, catabolic and unknown protein products as well as blood hemoglobins. Proteins previously not reported in the AP, such as fertility protein SP22, were identified. The proteins identified in the present study form a foundation for defining the proteome in normal adult male AP.

Insulin-induced repartitioning of metabolic fuels inhibits hamster estrous behavior: role of area postrema.

Excessive diversion of metabolic fuels away from oxidation and into adipose tissue storage depots, such as underfeeding or extraordinary levels of energy expenditure, can induce nutritional infertility. Treatment with pharmacological doses of insulin reduces metabolic fuel availability and suppresses both ovulatory cyclicity and pulsatile luteinizing hormone release in females of several mammalian species, but little attention has been paid to the effects of insulin treatments on reproductive behaviors. Ovariectomized Syrian hamsters were injected with long-acting insulin every 12 h for 72 h and were prevented from overeating by limiting their intake to approximately 110% of pretreatment levels. When given estradiol and progesterone, insulin-treated hamsters exhibited significantly reduced levels of sexual receptivity compared with saline-treated controls. This insulin-induced inhibition of estrous behavior was prevented by lesions of the area postrema. Insulin treatments also caused changes in the number of detectable estrogen receptor immunoreactive cells in the hypothalamus and preoptic area. Therefore, insulin-induced repartitioning of metabolic fuels induces changes in estrous behavior and neural estrogen receptors that are indistinguishable from those caused by food deprivation, cold exposure, or treatment with metabolic inhibitors.

Isolation of Paracoccidoides brasiliensis from armadillos (Dasypus novemcinctus) captured in an endemic area of paracoccidioidomycosis

Paracoccidioides brasiliensis, the causative agent of paracoccidioidomycosis (PCM), was first isolated from the Amazonian gerion where the mycosis is uncommon. In the present study, we report on the high incidence of PCM infection in armadillos from a hyperendemic region of the disease. Four nine-banded armadillos (Dasypus novemcinctus) were captured in the endemic area of Botucatu, Sao Paulo, Brazil, killed by manual cervical dislocation and autopsied under sterile conditions. Fragments of lung, spleen, liver and mesenteric lymph nodes were precessed for histology, cultured on Mycosel agar at 37ºC, and homogenized for inoculation into the testis and peritoneum of hamster. The animals were killed from week 6 to week 20 postinoculation and fragments of liver, lung, spleen, testis, and lymph nodes were cultured on brain heart infusion agar at 37ºC. Paracoccidioides brasiliensis was isolated from three armadillos both by direct organ culture and from the liver, spleen, lung, and mesenteric lymph node hamster. In addition, one positive armadillo presented histologically proven PCM disease in a mesenteric lymph node. The three aramdillos isolates (Pb-A1, Pb-A2, and Pb-A4) presented thermodependent dimorphism, urease activity, and casein assimilation, showed amplification of the gp43 gene, and were highly virulent in intratesticulary inoculation hamster. The isolates expressed the gp43 glycoprotein, the immunodominant antigen of the fungus, and reacted with a pool of sera from PCM patients. Taken together, the present data confirm that armadillos are a natural reservoir of P. brasiliensis and demonstrate that the animal is a sylvan host to the fungus

Beta-adrenergic signal transduction in the hypothalamus of the European hamster: relation with the seasonal hibernation cycle and the diurnal activity cycle.

Mammalian hibernation, an adaptation to survive harsh winter conditions, is one of the most prominent seasonal rhythmic processes exactly regulated on a low metabolic level. Diurnal variations in vegetative physiology are missing during hibernation; however, a precisely working diurnal system is mandatory for both the proper initiation and termination of the annual hibernation phase and the periodical arousal reactions. Biorhythms and the vegetative physiological processes connected with hibernation are, among others, controlled by hypothalamic noradrenaline systems. In this study, the density, binding capacity, and relative proportions of beta 1- and beta 2-adrenergic receptors (AR) within the hypothalamus of: 1) motorically inactive summer; 2) motorically active summer; 3) aroused, motorically active winter; and 4) deeply hibernating winter European hamsters (Cricetus cricetus) were studied. For further analysis of the beta-adrenergic signal transduction cascade, the activity of adenylyl cyclase (AC) was measured by formation of cAMP in controls, after stimulation of G proteins, or after forskolin stimulation without or in presence of manganese ions. While beta 1- and beta 2-AR subtypes were nearly equally abundant (50% beta 1:50% beta 2) in active summer, inactive summer, and hibernating hamsters, a significant redistribution in favor of beta 2-AR occurred after arousal (40% beta 1:60% beta 2). The activity of AC was much higher in active summer hamsters than in inactive summer, aroused winter, and hibernating winter hamsters. When AC was stimulated by guanylylimidophosphate [Gpp(NH)p], MnCl2, forskolin, or by forskolin in presence of MnCl2 instead of MgCl2, the potency to stimulate AC was found to show the following rank order: basal < Gpp(NH)p < MnCl2 < or = forskolin + MnCl2 < forskolin.

'Hamster-like' cycles in testicular size in the absence of gonadotrophin secretion in HPD rams exposed to long-term changes in photoperiod and treatment with melatonin.

Long term changes in the secretion of FSH, LH and testosterone, and the size of the testis were measured in groups of hypothalamo-pituitary disconnected Soay rams (HPD rams, n = 8) and control Soay rams (HPD-sham operated and unoperated, total n = 8) while exposed to an artificial lighting regimen of alternating 16-weekly periods of long days (16L: 8D) and short days (8L: 16D), and when treated with a constant-release implant of melatonin given under long days (total study: 136 weeks). Short term provocation tests using NMDA (glutamate receptor agonist), GnRH and LH were used to assess functionality of the hypothalamus, pituitary gland and testis, respectively. Control rams expressed normal photoperiod-induced cycles in the reproductive axis. Blood concentrations of FSH, LH and testosterone were significantly increased under short days, and decreased under long days with parallel changes in testicular diameter. Treatment with implants of melatonin under long days mimicked the effect of short days and induced rapid reactivation of the reproductive axis. In the HPD rams the blood concentrations of FSH, LH and testosterone declined immediately after the HPD surgery and values remained close to the lower limit of detection of the radioimmunoassays throughout the experiment. LH pulses were absent in the HPD rams and NMDA failed to induce LH secretion consistent with functional disconnection of the pituitary gland from the hypothalamus. The testes regressed to a significantly smaller size in the HPD rams compared with controls even at the nadir of the sexual cycle (testis diameter: 30.2 +/- 0.7 vs 41.3 +/- 0.8 mm, HPD vs control rams, respectively). A low amplitude cycle in testicular diameter (peak to nadir: 5.0 +/- 0.7 mm) persisted in the HPD rams with a temporal pattern opposite to the controls (growth under long days instead of short days; 'hamster like'). In the HPD rams, the treatment with melatonin blocked the long day-associated increase in testicular size, without effects on FSH, LH and testosterone secretion, pituitary responsiveness to GnRH (LH increment) or testicular responsiveness to LH (testosterone increment). This was in contrast to the cyclical changes in all parameters induced by melatonin in the control rams. At post-mortem, the reproductive tract in HPD rams was markedly regressed compared with the controls. The efficiency of spermatogenesis was reduced with few germ cells maturing beyond primary spermatocytes. Immunocytochemical staining, however, revealed the maintenance of androgen receptor expression in Sertoli cells, pertibular cells and Leydig cells, and steroid activity as measured by 17 alpha-hydroxylase expression in Leydig cells. Overall, the absence of photoperiod-induced changes in gonadotrophin secretion in the HPD rams illustrates the dependence on regulation by the hypothalamus, presumably through the secretion of GnRH. The residual cycle in the size of the testes in the HPD rams was closely correlated with the photoperiod-induced changes in prolactin secretion which persisted in these animals (summary of previous published data included). The combined results support the view that melatonin acts in the hypothalamus to mediate effects of photoperiod on gonadotrophin secretion and in the pituitary gland to mediate effects on prolactin secretion (dual site hypothesis), and that FSH, LH and prolactin act synergistically to regulate the long-term cycle in testicular activity in the ram.

Diurnal and seasonal changes in sympathetic signal transduction in cardiac ventricles of European hamsters.

This investigation of the relationship between cardiac beta-adrenoceptors and adenosine 3',5'-cyclic monophosphate (cAMP) formation in cardiac ventricles of the nocturnally active European hamster both during euthermia under a 12:12-h dark-light cycle and during hibernation under constant-darkness conditions showed that neither the densities, affinities, nor distribution of the beta 1- and beta 2-receptor subtypes differed between the dark phase, light phase, and hibernation. Basal formation of cAMP by the cardiac adenylyl cyclase of euthermic hamsters was higher in ventricles obtained at night [core temperature (Tcore) = 37.8 degrees C] than in ventricles obtained during the day (Tcore = 36.4 degrees C). Basal formation of cAMP was also significantly lower in hibernating hamsters (Tcore = 7.0 degrees C) than in euthermic hamsters. When adenylyl cyclase activity was stimulated by isoprenaline, guanylylimidodiphosphate [Gpp(NH)p], or forskolin, the rank order of potency was the same in euthermic hamsters and hibernating hamsters: isoprenaline < Gpp(NH)p < forskolin. Functional competition curves indicated that in the euthermic hamsters beta 1-receptors were responsible for 67% of the response to isoprenaline at night and 62% of the response during the day. In hibernating hamsters, in contrast, most of the response to isoprenaline (58%) was mediated via beta 2-receptors. This shift in the relative importance of the receptor subtypes may facilitate arousal from hibernation by making the heart more sensitive to circulating epinephrine.

An attempt to correlate brain areas containing melatonin-binding sites with rhythmic functions: a study in five hibernator species.

High affinity melatonin-binding sites have been described, by means of autoradiography with 2-125I-melatonin as the ligand, in more than 60 brain areas of about 20 mammalian species, with dramatic variations in the nature and number of labelled structures among the different species studied. As melatonin is involved in the synchronization of biological rhythms, we have tried to correlate the brain areas containing melatonin-binding sites with some rhythmic functions typical of given species. Therefore, we have studied the location of melatonin-binding sites in the complete brain of five long-day breeders with hibernation cycles, viz. one insectivore and four rodents. With the exception of the suprachiasmatic nuclei and the pars tuberalis of the pituitary, both of which contain binding sites in all five species, few reactive structures are common, even among species from the same family, e.g. the edible dormouse and the garden dormouse.

Seasonality and melatonin receptors in the pars tuberalis in some long day breeders.

The density of high-affinity melatonin receptors in the pars tuberalis of different long day breeders exhibits a seasonal variation, with high values in spring-summer and low values in autumn-winter. In all the studies, this decrease in receptor density was always associated with both a short photoperiod and sexual inactivity. Moreover, during the hibernation period, the reduction in receptor number was more marked in hypothermic than in normothermic animals. These seasonal changes in melatonin receptors were restricted to the pars tuberalis, which suggests that this structure represents an important target site for mediation of the photoperiodic effect of melatonin on seasonal functions. The mechanism of melatonin action at this level is discussed.