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

Adrenergic Agonists Activate Transcriptional Activity in Immortalized Neuronal Cells From the Mouse Suprachiasmatic Nucleus

The suprachiasmatic nucleus of the hypothalamus (SCN) houses the central circadian oscillator of mammals. The main neurotransmitters produced in the SCN are γ-amino-butyric acid, arginine-vasopressin (AVP), vasoactive intestinal peptide (VIP), pituitary-derived adenylate cyclase-activating peptide (PACAP), prokineticin 2, neuromedin S, and gastrin-releasing peptide (GRP). Apart from these, catecholamines and their receptors were detected in the SCN as well. In this study, we confirmed the presence of β-adrenergic receptors in SCN and a mouse SCN-derived immortalized cell line by immunohistochemical, immuno-cytochemical, and pharmacological techniques. We then characterized the effects of β-adrenergic agonists and antagonists on cAMP-regulated element (CRE) signaling. Moreover, we investigated the interaction of β-adrenergic signaling with substances influencing parallel signaling pathways. Our findings have potential implications on the role of stress (elevated adrenaline) on the biological clock and may explain some of the side effects of β-blockers applied as anti-hypertensive drugs.

The ISL LIM-homeobox 2 transcription factor is negatively regulated by circadian adrenergic signaling to repress the expression of Aanat in pinealocytes of the rat pineal gland

Melatonin is synthesized in the pineal gland during nighttime in response to nocturnal increase in the activity of the enzyme aralkylamine N-acetyltransferase (AANAT), the transcription of which is modulated by several homeodomain transcription factors. Recent work suggests that the homeodomain transcription factor ISL LIM homeobox 2 (ISL2) is expressed in the pineal gland, but its role is currently unknown. With the purpose of identifying the mechanisms that control pineal expression of Isl2 and the possible function of Isl2 in circadian pineal biology, we report that Isl2 is specifically expressed in the pinealocytes of the rat pineal gland. Its expression exhibits a 24 h rhythm with high transcript and protein levels during the day and a trough in the second half of the night. This rhythm persists in darkness, and lesion studies reveal that it requires intact function of the suprachiasmatic nuclei, suggesting intrinsic circadian regulation. In vivo and in vitro experiments show that pineal Isl2 expression is repressed by adrenergic signaling acting via cyclic AMP; further, Isl2 is negatively regulated by the nocturnal transcription factor cone-rod homeobox. During development, pineal Isl2 expression is detectable from embryonic day 19, preceding Aanat by several days. In vitro knockdown of Isl2 is accompanied by an increase in Aanat transcript levels suggesting that ISL2 represses its daytime expression. Thus, rhythmic expression of ISL2 in pinealocytes is under the control of the suprachiasmatic nucleus acting via adrenergic signaling in the gland to repress nocturnal expression, while ISL2 itself negatively regulates daytime pineal expression of Aanat and thereby suggestively enhances the circadian rhythm in melatonin synthesis.

The effect of light regime and time of slaughter in broiler on broiler performance, liver antioxidant status, and expression of genes related to peptide absorption in the jejunum and melatonin synthesis in the brain

This study aimed to assess the effects of light regime and time of slaughter on primal cut and organ weights, peptide transporter 1 (PEPT1) gene expression in the jejunum, arylalkylamine N-acetyltransferase (AANAT) gene expression in the brain, and liver oxidant/antioxidant status in broilers aged 37 days. The experiment was conducted in a factorial completely randomized design, with two light regimes (intermittent light varying according to bird age and continuous light under an 18 h light/6 h dark photoperiod) and four times of slaughter (2:00, 8:00, 14:00 and 20:00 h). There was an interaction effect on PEPT1 and AANAT expression, lipid and protein oxidation and superoxide dismutase (SOD) activity. In both light regimes, PEPT1 expression responded cubically to slaughter time. In the continuous light group, PEPT1 expression was highest in birds slaughtered at 2:00 and 14:00 h, whereas, in the intermittent light treatment, expression was highest at 8:00 h. In the continuous light regime, AANAT expression had a cubic relationship with time of slaughter, with the greatest values recorded at 20:00 h. In the intermittent light regime, slaughter time showed a cubic effect on lipid oxidation, which was highest at 8:00 h. In the continuous light group, there was a cubic effect on nitrite concentration, lipid oxidation, protein oxidation, and SOD activity; nitrite levels, lipid oxidation, and protein oxidation were highest and SOD activity was lowest in birds slaughtered at 14:00 h. Time of slaughter influenced catalase activity, which responded cubically; catalase activity was lowest at 8:00 and 14:00 h. This study is the first to demonstrate that PEPT1 expression in the jejunum of broilers follows a diurnal rhythm and varies according to light regime. The results also suggest that mainly continuous lighting and slaughter at 14:00 h when the animals are possibly more active may be more stressful to broilers.

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.

Melatonin and Pathological Cell Interactions: Mitochondrial Glucose Processing in Cancer Cells

Melatonin is synthesized in the pineal gland at night. Since melatonin is produced in the mitochondria of all other cells in a non-circadian manner, the amount synthesized by the pineal gland is less than 5% of the total. Melatonin produced in mitochondria influences glucose metabolism in all cells. Many pathological cells adopt aerobic glycolysis (Warburg effect) in which pyruvate is excluded from the mitochondria and remains in the cytosol where it is metabolized to lactate. The entrance of pyruvate into the mitochondria of healthy cells allows it to be irreversibly decarboxylated by pyruvate dehydrogenase (PDH) to acetyl coenzyme A (acetyl-CoA). The exclusion of pyruvate from the mitochondria in pathological cells prevents the generation of acetyl-CoA from pyruvate. This is relevant to mitochondrial melatonin production, as acetyl-CoA is a required co-substrate/co-factor for melatonin synthesis. When PDH is inhibited during aerobic glycolysis or during intracellular hypoxia, the deficiency of acetyl-CoA likely prevents mitochondrial melatonin synthesis. When cells experiencing aerobic glycolysis or hypoxia with a diminished level of acetyl-CoA are supplemented with melatonin or receive it from another endogenous source (pineal-derived), pathological cells convert to a more normal phenotype and support the transport of pyruvate into the mitochondria, thereby re-establishing a healthier mitochondrial metabolic physiology.

Melatonin is involved in the modulation of the hypothalamic and pituitary activity in the South American plains vizcacha, Lagostomus maximus

Melatonin, the key messenger of photoperiodic information, is synthesized in the pineal gland by arylalkylamine N-acetyltransferase enzyme (AANAT). It binds to specific receptors MT₁ and MT₂ located in the hypothalamus and pituitary gland. Melatonin can modulate the reproductive axis affecting the secretion of gonadotropin-releasing hormone (GnRH) and luteinizing hormone (LH). The South American plains vizcacha, Lagostomus maximus, shows natural poliovulation of up to 800 oocytes per estrous cycle, a 154-day long pregnancy, and reactivation of the reproductive axis at mid-gestation with pre-ovulatory follicular recruitment, presence of active corpora lutea, and variations of the endocrine status. Here we analyzed the involvement of melatonin in the modulation of the hypothalamic and pituitary gland physiology of vizcacha thorough several approaches, including histological localization of melatoninergic system components, assessment of melatoninergic components expression throughout the reproductive cycle, and evaluation of the effect of melatonin on hypothalamic and pituitary activities during the follicular and luteal phases of the estrous cycle. AANAT and melatonin receptors were localized in the pineal gland and preoptic area of the hypothalamus. Increase in pineal AANAT and serum melatonin expression was observed as pregnancy progressed, with the lowest hypothalamic MT₁ and MT₂ levels at mid-pregnancy. Pulsatility assays demonstrated that melatonin induces GnRH and LH secretion at luteal phase. The melatoninergic system effects on hypothalamic and pituitary gland hormones secretion during pregnancy pinpoint to melatonin as a potential key factor underlying the reactivation of the reproductive axis activity at mid-gestation.

Evaluation of potential reproductive seasonality in brown brocket deer (Mazama gouazoubira) bucks

Photoperiod is the main cue that controls seasonal reproduction in deer from temperate regions, like secretion of testosterone by the testis, neck enlargement, and testis development. However, little is known about its effect on the reproduction of tropical deer species. This research aimed to assess the potential seasonality of the brown brocket deer (Mazama gouazoubira) by melatonin treatment. Reproductive parameters such as testicular volume, neck circumference, and semen quality were evaluated from day 0 to 90, and fecal androgen metabolites (FAM) from day -28 to 91. Melatonin treatment was administered from day 0 to 60. In total, seven individuals were used in the study. Four animals were allocated to the treated group (TG) and three to the control group (CG). The neck circumference and some seminal parameters did not show a significant difference between CG and TG and along the experimental period. High FAM levels were observed for the TG on days 7, 14, 21, 35, 84, and 91 (p < 0.05). The mean testicular volume was reduced from day 0 to 30 (p < 0.05) and from day 0 to 90 (p < 0.05), with no increase in the mean testicular volume from day 30 to 60 (p > 0.05). Sperm motility was the highest on day 60 compared with day 0 (p < 0.05). Although not statistically different, membrane integrity and major defects tended to be higher and lower, respectively, in the TG compared with the CG. For these parameters, the incremental change (%) per individual showed that males from the TG tended to have a higher percentage during the melatonin treatment, hence melatonin might affect these seminal characteristics. These findings suggest that exogenous melatonin might enhance FAM levels and sperm motility.

Reproductive seasonality in primates: patterns, concepts and unsolved questions

Primates, like other mammals, exhibit an annual reproductive pattern that ranges from strictly seasonal breeding to giving birth in all months of the year, but factors mediating this variation are not fully understood. We applied both a categorical description and quantitative measures of the birth peak breadth based on daily observations in zoos to characterise reproductive seasonality in 141 primate species with an average of 941 birth events per species. Absolute day length at the beginning of the mating season in seasonally reproducing species was not correlated between populations from natural habitats and zoos. The mid-point of latitudinal range was a major factor associated with reproductive seasonality, indicating a correlation with photoperiod. Gestation length, annual mean temperature, natural diet and Malagasy origin were other important factors associated with reproductive seasonality. Birth seasons were shorter with increasing latitude of geographical origin, corresponding to the decreasing length of the favourable season. Species with longer gestation periods were less seasonal than species with shorter ones, possibly because shorter gestation periods more easily facilitate the synchronisation of reproductive activity with annual cycles. Habitat conditions with higher mean annual temperature were also linked to less-seasonal reproduction, independently of the latitude effect. Species with a high percentage of leaves in their natural diet were generally non-seasonal, potentially because the availability of mature leaves is comparatively independent of seasons. Malagasy primates were more seasonal in their births than species from other regions. This might be due to the low resting metabolism of Malagasy primates, the comparatively high degree of temporal predictability of Malagasy ecosystems, or historical constraints peculiar to Malagasy primates. Latitudinal range showed a weaker but also significant association with reproductive seasonality. Amongst species with seasonal reproduction in their natural habitats, smaller primate species were more likely than larger species to shift to non-seasonal breeding in captivity. The percentage of species that changed their breeding pattern in zoos was higher in primates (30%) than in previous studies on Carnivora and Ruminantia (13 and 10%, respectively), reflecting a higher concentration of primate species in the tropics. When comparing only species that showed seasonal reproduction in natural habitats at absolute latitudes ≤11.75°, primates did not differ significantly from these two other taxa in the proportion of species that changed to a less-seasonal pattern in zoos. However, in this latitude range, natural populations of primates and Carnivora had a significantly higher proportion of seasonally reproducing species than Ruminantia, suggesting that in spite of their generally more flexible diets, both primates and Carnivora are more exposed to resource fluctuation than ruminants.

Stimulation role of epinephrine in transcription of the melatonin synthesis key enzyme AANAT in the pineal gland of broilers

Melatonin is a crucial neurohormone synthesized in the pineal gland that influences the physiology of animals. The molecular mechanism of norepinephrine control of the synthesis of melatonin is well documented; however, few reports have described the effects of epinephrine on the synthesis of melatonin. In this study, the effect of epinephrine on melatonin synthesis was investigated by adding different concentrations of epinephrine or norepinephrine to broiler pineal glands cultured in vitro. In addition, we investigated how epinephrine regulates the synthesis of melatonin and the transcription of the key melatonin synthesis enzyme AANAT. We determined the abundance of melatonin, norepinephrine, and epinephrine in broiler serum and the mRNA expression levels of key enzymes under different light conditions. The minimum concentrations of epinephrine and norepinephrine required to recover the melatonin synthesis rhythm in pineal cells were 10^-13 and 10^-11 mol/L, respectively. Under various light durations, epinephrine reached maximum levels two hours earlier than melatonin. These results demonstrate for the first time that epinephrine can increase the synthesis of melatonin by increasing the transcription of AANAT.

Overexpression of ovine AANAT and HIOMT genes in switchgrass leads to improved growth performance and salt-tolerance

Melatonin is a well-known bioactive molecule with an array of health-promoting properties. Here, we detected the physiological function of melatonin in transgenic switchgrass overexpressing the homologous sheep arylalkylamine N-acetyltransferase and hydroxyindole O-methyltransferase genes, which catalyze the last two steps of melatonin synthesis. Compared to the wild-type (WT) and transgenic control (EV, expressing the empty vector only) plants, the transgenic switchgrass showed higher melatonin levels. Melatonin was detected in almost all switchgrass tissues, and relatively higher levels were detected in the roots and stems. Besides, melatonin showed diurnal or circadian rhythms in switchgrass similar to that in other species. Furthermore, we also found that melatonin positively affected switchgrass growth, flowering and salt tolerance. The genes related to flowering (APL3, SL1, FT1, FLP3, MADS6 and MADS15) and salt stress resistance (PvNHX1) in transgenic switchgrass exhibited a different expression profiles when compared to the control plants. Our study provided valuable findings that melatonin functions as a promoter in the regulation of switchgrass growth, flowering and salt tolerance.

Adrenergic regulation of cytoplasmic structures related to secretory processes in pig pinealocytes-an ultrastructural, quantitative study

Two structures, considered as secretory in nature, are present in the pinealocytes in of the domestic pig show the presence of two structures, which are considered as secretory in nature - the dense core vesicles (DCV) and the membrane bounded (dense) bodies (MBB). The latter are extremely numerous in pig pinealocytes (they occupy 6-20% of the cytoplasm), and the number of MBB changes under different physiological and experimental conditions. Norepinephrine is the main neurotransmitter that regulates the secretion of pineal melatonin. The present study was carried out to 1) clarify whether the DCV and their source - the Golgi apparatus (GA) - as well as the MBB are controlled by norepinephrine, 2) determine the effect of adrenergic stimulation on these structures, and 3) identify the receptors involved in the regulation of these structures. The studies were performed using a static organ culture of pig pineal explants. The explants were incubated in a control medium between 08:00 and 20:00 and in a medium with 10μM norepinephrine or alpha- or beta-adrenoceptor agonists between 20:00 and 08:00 on five consecutive days. The tissues were subsequently prepared for ultrastructural analysis. The results distinctly showed that the DCV, GA and MBB in pig pinealocytes are under adrenergic control. The stimulation of the beta-adrenoceptors resulted in an increase in the numerical density of the DCV and a decrease in the relative volume of the GA in the perikarya, while the incubation with agonists of the alpha1-adrenoceptors was ineffective. The relative volume of the MBB in the perikarya significantly decreased after treatment with both beta-agonists and alpha1-agonists, which suggested the involvement of two types of adrenoceptors in the regulation of these structures.

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.

Diurnal profiles of melatonin synthesis-related indoles, catecholamines and their metabolites in the duck pineal organ

This study characterizes the diurnal profiles of ten melatonin synthesis-related indoles, the quantitative relations between these compounds, and daily variations in the contents of catecholamines and their metabolites in the domestic duck pineal organ. Fourteen-week-old birds, which were reared under a 12L:12D cycle, were killed at two-hour intervals. The indole contents were measured using HPLC with fluorescence detection, whereas the levels of catecholamines and their metabolites were measured using HPLC with electrochemical detection. All indole contents, except for tryptophan, showed significant diurnal variations. The 5-hydroxytryptophan level was approximately two-fold higher during the scotophase than during the photophase. The serotonin content increased during the first half of the photophase, remained elevated for approximately 10 h and then rapidly decreased in the middle of the scotophase. N-acetylserotonin showed the most prominent changes, with a more than 15-fold increase at night. The melatonin cycle demonstrated only an approximately 5-fold difference between the peak and nadir. The 5-methoxytryptamine content was markedly elevated during the scotophase. The 5-hydroxyindole acetic acid, 5-hydroxytryptophol, 5-methoxyindole acetic acid and 5-methoxytryptophol profiles were analogous to the serotonin rhythm. The norepinephrine and dopamine contents showed no significant changes. The DOPA, DOPAC and homovanillic acid levels were higher during the scotophase than during the photophase. Vanillylmandelic acid showed the opposite rhythm, with an elevated level during the daytime.

Adrenergic activation of melatonin secretion in ovine pineal explants in short-term superfusion culture occurs via protein synthesis independent and dependent phenomena

The ovine pineal is generally considered as an interesting model for the study on adrenergic regulation of melatonin secretion due to some functional similarities with this gland in the human. The present investigations, performed in the superfusion culture of pineal explants, demonstrated that the norepinephrine-induced elevation of melatonin secretion in ovine pinealocytes comprised of two subsequent periods: a rapid increase phase and a slow increase phase. The first one included the quick rise in release of N-acetylserotonin and melatonin, occurring parallel to elevation of NE concentration in the medium surrounding explants. This rapid increase phase was not affected by inhibition of translation. The second, slow increase phase began after NE level had reached the maximum concentration in the culture medium and lasted about two hours. It was completely abolished by the treatment with translation inhibitors. The obtained results showed for the first time that the regulation of N-acetylserotonin synthesis in pinealocytes of some species like the sheep involves the on/off mechanism, which is completely independent of protein synthesis and works very fast. They provided strong evidence pointing to the need of revision of the current opinion that arylalkylamines N-acetyltransferase activity in pinealocytes is controlled exclusively by changes in enzyme abundance.

Endocrine rhythms in the brown bear (Ursus arctos): Evidence supporting selection for decreased pineal gland size

Many temperate zone animals adapt to seasonal changes by altering their physiology. This is mediated in large part by endocrine signals that encode day length and regulate energy balance and metabolism. The objectives of this study were to determine if the daily patterns of two important hormones, melatonin and cortisol, varied with day length in captive brown bears (Ursus arctos) under anesthetized and nonanesthetized conditions during the active (March-October) and hibernation periods. Melatonin concentrations varied with time of day and season in nonanesthetized female bears despite exceedingly low nocturnal concentrations (1-4 pg/mL) in the active season. In contrast, melatonin concentrations during hibernation were 7.5-fold greater than those during the summer in anesthetized male bears. Functional assessment of the pineal gland revealed a slight but significant reduction in melatonin following nocturnal light application during hibernation, but no response to beta-adrenergic stimulation was detected in either season. Examination of pineal size in two bear species bears combined with a phylogenetically corrected analysis of pineal glands in 47 other species revealed a strong relationship to brain size. However, pineal gland size of both bear species deviated significantly from the expected pattern. Robust daily plasma cortisol rhythms were observed during the active season but not during hibernation. Cortisol was potently suppressed following injection with a synthetic glucocorticoid. The results suggest that melatonin and cortisol both retain their ability to reflect seasonal changes in day length in brown bears. The exceptionally small pineal gland in bears may be the result of direct or indirect selection.

Twice daily melatonin peaks in Siberian but not Syrian hamsters under 24 h light:dark:light:dark cycles

The daily pattern of blood-borne melatonin varies seasonally under the control of a multi-oscillator circadian pacemaker. Here we examine patterns of melatonin secretion and locomotor activity in Siberian and Syrian hamsters entrained to bimodal LDLD8:4:8:4 and LD20:4 lighting schedules that facilitate novel temporal arrangements of component circadian oscillators. Under LDLD, both species robustly bifurcated wheel-running activity in distinct day scotophase (DS) and night scotophase (NS) bouts. Siberian hamsters displayed significant melatonin increases during each scotophase in LDLD, and in the single daily scotophase of LD20:4. The bimodal melatonin secretion pattern persisted in acutely extended 16 h scotophases. Syrian hamsters, in contrast, showed no significant increases in plasma melatonin during either scotophase of LDLD8:4:8:4 or in LD20:4. In this species, detectable levels were observed only when the DS of LDLD was acutely extended to yield 16 h of darkness. Established species differences in the phase lag of nocturnal melatonin secretion relative to activity onset may underlie the above contrast: In non-bifurcated entrainment to 24 h LD cycles, Siberian hamsters show increased melatonin secretion within ≈ 2 h after activity onset, whereas in Syrian hamsters, detectable melatonin secretion phase lags activity onset and the L/D transition by at least 4 h. The present results provide new evidence indicating multi-oscillator regulation of the waveform of melatonin secretion, specifically, the circadian control of the onset, offset and duration of nocturnal secretion.

Dynamics in enzymatic protein complexes offer a novel principle for the regulation of melatonin synthesis in the human pineal gland

Time of day is communicated to the body through rhythmic cues, including pineal gland melatonin synthesis, which is restricted to nighttime. Whereas in most rodents transcriptional regulation of the arylalkylamine N-acetyltransferase (Aanat) gene is essential for rhythmic melatonin synthesis, investigations into nonrodent mammalian species have shown post-transcriptional regulation to be of central importance, with molecular mechanisms still elusive. Therefore, human pineal tissues, taken from routine autopsies were allocated to four time-of-death groups (night/dawn/day/dusk) and analyzed for daytime-dependent changes in phosphorylated AANAT (p31T-AANAT) and in acetyl-serotonin-methyltransferase (ASMT) expression and activity. Protein content, intracellular localization, and colocalization of p31T-AANAT and ASMT were assessed, using immunoblotting, immunofluorescence, and immunoprecipitation techniques. Fresh sheep pineal gland preparations were used for comparative purposes. The amount of p31T-AANAT and ASMT proteins as well as their intracellular localization showed no diurnal variation in autoptic human and fresh sheep pineal glands. Moreover, in human and sheep pineal extracts, AANAT could not be dephosphorylated, which was at variance to data derived from rat pineal extracts. P31T-AANAT and ASMT were often found to colocalize in cellular rod-like structures that were also partly immunoreactive for the pinealocyte process-specific marker S-antigen (arrestin) in both, human and sheep pinealocytes. Protein-protein interaction studies with p31T-AANAT, ASMT, and S-antigen demonstrated a direct association and formation of robust complexes, involving also 14-3-3. This work provides evidence for a regulation principle for AANAT activity in the human pineal gland, which may not be based on a p31T-AANAT phosphorylation/dephosphorylation switch, as described for other mammalian species.

Evolution of time-keeping mechanisms: early emergence and adaptation to photoperiod

Virtually all species have developed cellular oscillations and mechanisms that synchronize these cellular oscillations to environmental cycles. Such environmental cycles in biotic (e.g. food availability and predation risk) or abiotic (e.g. temperature and light) factors may occur on a daily, annual or tidal time scale. Internal timing mechanisms may facilitate behavioural or physiological adaptation to such changes in environmental conditions. These timing mechanisms commonly involve an internal molecular oscillator (a 'clock') that is synchronized ('entrained') to the environmental cycle by receptor mechanisms responding to relevant environmental signals ('Zeitgeber', i.e. German for time-giver). To understand the evolution of such timing mechanisms, we have to understand the mechanisms leading to selective advantage. Although major advances have been made in our understanding of the physiological and molecular mechanisms driving internal cycles (proximate questions), studies identifying mechanisms of natural selection on clock systems (ultimate questions) are rather limited. Here, we discuss the selective advantage of a circadian system and how its adaptation to day length variation may have a functional role in optimizing seasonal timing. We discuss various cases where selective advantages of circadian timing mechanisms have been shown and cases where temporarily loss of circadian timing may cause selective advantage. We suggest an explanation for why a circadian timing system has emerged in primitive life forms like cyanobacteria and we evaluate a possible molecular mechanism that enabled these bacteria to adapt to seasonal variation in day length. We further discuss how the role of the circadian system in photoperiodic time measurement may explain differential selection pressures on circadian period when species are exposed to changing climatic conditions (e.g. global warming) or when they expand their geographical range to different latitudes or altitudes.

Absence of a serum melatonin rhythm under acutely extended darkness in the horse

BACKGROUND

In contrast to studies showing gradual adaptation of melatonin (MT) rhythms to an advanced photoperiod in humans and rodents, we previously demonstrated that equine MT rhythms complete a 6-h light/dark (LD) phase advance on the first post-shift day. This suggested the possibility that melatonin secretion in the horse may be more strongly light-driven as opposed to endogenously rhythmic and light entrained. The present study investigates whether equine melatonin is endogenously rhythmic in extended darkness (DD).

METHODS

Six healthy, young mares were maintained in a lightproof barn under an LD cycle that mimicked the ambient natural photoperiod outside. Blood samples were collected at 2-h intervals for 48 consecutive h: 24-h in LD, followed by 24-h in extended dark (DD). Serum was harvested and stored at -20°C until melatonin and cortisol were measured by commercial RIA kits.

RESULTS

Two-way repeated measures ANOVA (n = 6/time point) revealed a significant circadian time (CT) x lighting condition interaction (p < .0001) for melatonin with levels non-rhythmic and consistently high during DD (CT 0-24). In contrast, cortisol displayed significant clock-time variation throughout LD and DD (p = .0009) with no CT x light treatment interaction (p = .4018). Cosinor analysis confirmed a significant 24-h temporal variation for melatonin in LD (p = .0002) that was absent in DD (p = .51), while there was an apparent circadian component in cortisol, which approached significance in LD (p = .076), and was highly significant in DD (p = .0059).

CONCLUSIONS

The present finding of no 24 h oscillation in melatonin in DD is the first evidence indicating that melatonin is not gated by a self-sustained circadian process in the horse. Melatonin is therefore not a suitable marker of circadian phase in this species. In conjunction with recent similar findings in reindeer, it appears that biosynthesis of melatonin in the pineal glands of some ungulates is strongly driven by the environmental light cycle with little input from the circadian oscillator known to reside in the SCN of the mammalian hypothalamus.

A circadian clock is not required in an arctic mammal

Seasonally breeding mammals use the annual change in the photoperiod cycle to drive rhythmic nocturnal melatonin signals from the pineal gland, providing a critical cue to time seasonal reproduction. Paradoxically, species resident at high latitudes achieve tight regulation of the temporal pattern of growth and reproduction despite the absence of photoperiodic information for most of the year. In this study, we show that the melatonin rhythm of reindeer (Rangifer tarandus) is acutely responsive to the light/dark cycle but not to circadian phase, and also that two key clock genes monitored in reindeer fibroblast cells display little, if any, circadian rhythmicity. The molecular clockwork that normally drives cellular circadian rhythms is evidently weak or even absent in this species, and instead, melatonin-mediated seasonal timing may be driven directly by photic information received at a limited time of year specific to the equinoxes.

Pineal melatonin synthesis is decreased in type 2 diabetic Goto-Kakizaki rats

AIMS

It is not well understood why the amplitude of melatonin rhythms is reduced in diabetic animals and humans. This paper addresses the differences in the pineal melatonin synthesis of type 2 diabetic Goto-Kakizaki (GK) rats compared to non-diabetic Wistar rats (8 and 50 weeks old).

MAIN METHODS

Plasma melatonin concentrations and the pineal content of melatonin and its precursors (tryptophan, 5-hydroxytryptophan, serotonin, and N-acetylserotonin) were quantified at the middle of the day and night. Additionally, the expression of melatonin synthesizing enzymes, pineal noradrenaline content, and pineal protein content were considered, and the melatonin secreting capacity of pineal glands was studied in vitro.

KEY FINDINGS

The pineal glands of diabetic GK rats have a different expression pattern of melatonin synthesizing enzymes. The amount of all precursors of melatonin is reduced in pineal glands of diabetic GK rats. The pineal glands of diabetic GK rats contain less noradrenaline, indicating a reduced stimulation of nighttime melatonin synthesis. The pineal glands of diabetic GK rats produce less melatonin in reaction to noradrenaline in vivo and in vitro. The pineal glands of diabetic GK rats contain less protein, probably a consequence of diabetic neuropathy.

SIGNIFICANCE

This is the first time that melatonin synthesis is examined in a type 2 diabetic rat model. The present data unveiled several reasons for a reduced melatonin secretion in diabetic animals and presents an important link in the interaction between melatonin and insulin.

Differential ovarian expression of KiSS-1 and GPR-54 during the estrous cycle and photoperiod induced recrudescence in Siberian hamsters (Phodopus sungorus)

Kisspeptins, coded by the KiSS-1 gene, regulate aspects of the reproductive axis by stimulating GnRH release via the G protein coupled receptor, GPR54. Recent reports show that KiSS/GPR54 may be key mediators in photoperiod-controlled reproduction in seasonal breeders, and that KiSS-1/GPR54 are expressed in the hypothalamus, ovaries, placenta, and pancreas. This study examined the expression of KiSS-1/GPR54 mRNA and protein in ovaries of Siberian hamsters (Phodopus sungorus). Ovaries from cycling hamsters were collected during proestrus (P), estrus (E), diestrus I (DI), and diestrus II (DII). To examine KiSS-1/GPR54 during stimulated recrudescence, additional hamsters were maintained either in long day (LD 16L:8D, control) or short day (SD 8L:16D) for 14 weeks and then transferred to LD for 0-8 weeks. Staining of KiSS-1/GPR54 protein was detected by immunohistochemistry in steroidogenic cells of pre-antral and antral follicles, and corpora lutea. Immunostaining peaked in P and E, but decreased in the diestrus stages (P < 0.05). In recrudescing ovaries, KiSS-1/GPR54 immunostaining was low after 14 weeks of SD exposure (post-transfer [PT] week 0), and increased during the early weeks of recrudescence. Expression of KiSS-1/GPR54 mRNA was low with short day exposure, but increased during recrudescence and was higher at PT week 8 as compared to PT weeks 0 and 2 (P < 0.05). The elevated KiSS-1/GPR54 expression during P and E suggests a potential role in ovulation in Siberian hamsters. Transient increases in KiSS-1/GPR54 expression following LD stimulation are also suggestive of possible involvement in ovulation and/or restoration of ovarian function.

KiSS‐1: A Likely Candidate for the Photoperiodic Control of Reproduction in Seasonal Breeders

In seasonal species, photoperiod exerts tight regulation of reproduction to ensure that birth occurs at the most favorable time of yr. A distinct photoneuroendocrine circuit composed of the retina, suprachiasmatic nucleus (SCN) of the hypothalamus, and pineal gland transduces daylength into a rhythmic secretion of melatonin. The duration of the night-time rise of this hormone conveys daylength information to the organism. Melatonin is known to mediate the control of seasonal reproduction, but how it modulates sexual activity is far from understood. Recent data indicate that the product of the KiSS-1 gene is a potent stimulator of the hypothalamic-pituitary-gonadal axis and may play, together with its receptor GPR54, a central role in the neuroendocrine regulation of gonadotropin secretion. This article briefly reviews these findings and presents arguments that KiSS-1 could take part in the seasonal control of reproduction.

Daily Aa-nat gene expression in the camel (Camelus dromedarius) pineal gland

Arylalkylamine N-acetyltransferase (AA-NAT) is the rhythm-generating enzyme for the synthesis of pineal melatonin. Molecular investigations have revealed two biological models for the activation of AA-NAT. In rodent species, Aa-nat gene transcription is turned off during the daytime and markedly activated at night. In primates, sheep, and cows, the Aa-nat gene is constitutively transcripted with no visible daily variations. This inter-species difference in Aa-nat gene regulation leads to different daily profiles in melatonin synthesis and release. Thus, the nighttime onset of the rise in circulating melatonin is delayed and slow in rodents, whereas it is fast and sharp in sheep. In the camel (Camelus dromedarius), we have observed that circulating melatonin rises immediately after sunset, suggesting AA-NAT activity is regulated at the post-transcriptional level. In agreement with this hypothesis, we report herein the amount of Aa-nat mRNA in the camel pineal gland is high, during both the day and night with no daily variations, while melatonin concentration in the same pineal tissue is five times higher during the night than daytime.

Redefining the limits of day length responsiveness in a seasonal mammal

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

The effect of α- and β-adrenergic blockade on daily rhythms of body temperature, urine production, and urinary 6-sulfatoxymelatonin of social voles Microtus socialis

To examine the effect of adrenergic blockade on daily rhythms of rectal body temperature (T(b)), urine production rate, and melatonin (MEL; measured as urinary 6-sulfatoxymelatonin; 6-SMT), social voles Microtus socialis received a single intra-peritoneal injection of either prazosin (PRAZ, 1 mg/kg) or propranolol (PROP, 4.5 mg/kg); alpha- and beta-adrenergic blocking agents respectively, 1 h prior to scotophase onset (light/dark, 12L:12D; lights on 07:00 h). Both blockers caused significant decrease in T(b) values mainly during scotophase. Nocturnal urine production rates were higher for M. socialis treated with the drugs compared with controls. Overall, urine production rates were systematically higher in PROP-voles over the 24 h period when compared with PRAZ-voles; however these differences were not statistically significant. Interestingly, PROP caused significant elevation in urinary 6-SMT at the second half of the dark phase, whereas PRAZ had no effects. These data suggest that the mechanisms regulating MEL synthesis and thermoregulatory responses in M. socialis are different from those described in other rodents' species. Importantly, the data also suggest that the beta-blockade-induced elevation in MEL levels may be directly associated with increased urination in M. socialis.

Adaptations for life in the Arctic: evidence that melatonin rhythms in reindeer are not driven by a circadian oscillator but remain acutely sensitive to environmental photoperiod

In reindeer Rangifer tarandus, a high latitude species, the rhythmic production of melatonin periodically dissipates under natural photoperiods when, in mid-winter, there is near permanent darkness and again, in summer, when there is permanent light. In spring and autumn, as expected, melatonin production reflects the ambient light:dark (LD) cycle. We investigated the expression of circadian mechanisms on blood levels of melatonin in reindeer. Two experiments were conducted in which animals were transferred from natural photic conditions into continuous darkness for 3 days: (i) in February, when they had been exposed to an LD cycle (11L:13D) and (ii) in July, when they had been exposed to permanent light. In July, plasma levels of melatonin rose abruptly on exposure to darkness but then declined over 24 hr before displaying a second rise and decline over the following 36 hr. In contrast, in February, levels of melatonin rose abruptly but then remained elevated for more than 60 hr in darkness. Melatonin secretion upon exposure to darkness did not conform to a circadian pattern and did not, therefore, support the hypothesis that pineal activity in reindeer is tightly regulated by circadian mechanisms. Instead the secretion of melatonin appeared to be acutely and directly sensitive to ambient lighting. The results are consistent with a model in which Arctic resident animals have adapted to extreme photic conditions by disconnecting the generation of the pineal melatonin signal from their circadian machinery and relying, instead, on its being driven by the LD cycle for just a few weeks annually in spring and autumn.

Day-night expression patterns of clock genes in the human pineal gland

Rhythm generation within the mammalian circadian system is achieved by clock genes and their protein products. As an integral part of this system, the pineal gland serves the need to tune the body to the temporal environment by the rhythmic synthesis and release of melatonin. A number of human disorders and syndromes are associated with alterations in circadian rhythms of clock genes and their protein products and/or a dysfunction in melatonin synthesis. In the human, little is known about the molecular signature of time management. Pineal tissue from regular autopsies was allocated to asserted time-of-death groups (dawn, day, dusk, night), and analyzed by RT-PCR, immunoblotting, immunohistochemistry, and confocal laser scanning microscopy for expression of clock genes. Despite the observed diurnal rhythms in activity of the arylalkylamine N-acetyltransferase and in melatonin content, mRNA levels for the clock genes Period1, Cryptochrome1, Clock, and Bmal1, and also amounts of corresponding clock gene proteins showed no differences between time- of-death groups. In contrast, a time-of-day-dependent nucleocytoplasmic shuttling of clock gene proteins was detected. These data confirm the minor importance of a transcriptional regulation for dynamics in the human pineal gland, and offer a novel twist in the molecular competence of clock gene proteins.

Transcription factor dynamics in pineal gland and liver of the Syrian hamster (Mesocricetus auratus) adapts to prevailing photoperiod

The anticipation of day length and duration of darkness is necessary and advantageous for animals to survive and requires a photoperiodic memory. In the Syrian hamster this adaptation to photoperiod is mirrored by seasonal changes in the animal's reproductive state and its liver metabolism. Both events are linked to season-dependent alterations of the nocturnally elevated synthesis of the pineal hormone melatonin. To decipher molecules that are involved in this temporal gating, hamsters were exposed to long photoperiod (16 hr light:8 hr darkness; LP), or short photoperiod (8 hr light:16 hr darkness; SP). Dynamics in gene expression was investigated in the pineal gland [inducible cAMP early repressor (ICER)], and in the liver (ICER; C/EBPdelta; clock genes) using immunochemistry and reverse transcriptase PCR. While in the pineal, ICER rhythms tightly follow the prior duration of light and dark with decreasing levels at the beginning of the dark period in both LP and SP, ICER is not rhythmic in liver. In the liver, clock genes and their protein products reflect differences in photoperiodic history, with enhanced rhythm amplitudes of PER, CRY, CLOCK, and BMAL1 under SP conditions. Thus, in the Syrian hamster transcription factor expression patterns lock onto the prevailing photoperiod in two peripheral oscillators, the pineal gland and the liver, to function as mediators of suprachiasmatic nucleus-derived information on environmental light and dark. This tissue-specific gating in gene transcription represents a strategy to ameliorate consequences of altering environmental lighting conditions on endocrine and metabolic parameters that endow a strong circadian bias.

Rhythmic control of AANAT translation by hnRNP Q in circadian melatonin production

The circadian rhythm of pineal melatonin requires the nocturnal increment of serotonin N-acetyltransferase (arylalkylamine N-acetyltransferase [AANAT]) protein. To date, only limited information is available in the critical issue of how AANAT protein expression is up-regulated exclusively at night regardless of its species-specific mRNA profiles. Here we show that the circadian timing of AANAT protein expression is regulated by rhythmic translation of AANAT mRNA. This rhythmic control is mediated by both a highly conserved IRES (internal ribosome entry site) element within the AANAT 5' untranslated region and its partner hnRNP Q (heterogeneous nuclear ribonucleoprotein Q) with a peak in the middle of the night. Consistent with the enhancing role of hnRNP Q in AANAT IRES activities, knockdown of the hnRNP Q level elicited a dramatic decrease of peak amplitude in the AANAT protein profile parallel to reduced melatonin production in pinealocytes. This translational regulation of AANAT mRNA provides a novel aspect for achieving the circadian rhythmicity of vertebrate melatonin.

The mammalian pineal gland: known facts, unknown facets

In the mammalian pineal gland, information on environmental lighting conditions that is neuronally encoded by the retina is converted into nocturnally elevated synthesis of the hormone melatonin. Evolutionary pressure has changed the morphology of vertebrate pinealocytes, eliminating direct photoreception and the endogenous clock function. Despite these changes, nocturnally elevated melatonin synthesis has remained a reliable indicator of time throughout evolution. In the photo-insensitive mammalian pineal gland this message of darkness depends on the master circadian pacemaker in the hypothalamic suprachiasmatic nuclei. The dramatic change in vertebrate pinealocytes has received little attention; here, we therefore link the known evolutionary morphodynamics and well-investigated biochemical details responsible for rhythmic synthesis of melatonin with recently characterized patterns of gene expression in the pineal gland. We also address the enigmatic function of clockwork molecules in mammalian pinealocytes.

Melatonin synthesis in the human pineal gland: advantages, implications, and difficulties

Rhythms in the mammalian pineal organ depend on afferent information that is derived from the endogenous clock residing in the hypothalamic suprachiasmatic nucleus (SCN). The best characterized function of the pineal gland is the nocturnally elevated synthesis of the hormone melatonin, which provides the body with the signal of the duration of the night period. The rate-limiting enzyme for melatonin synthesis is arylalkylamine N-acetyltransferase (AANAT). In contrast to the transcriptional regulation of the Aanat gene in rodents, a post-translational shaping of the melatonin pattern is indicated in the human pineal gland. Despite the fact that melatonin levels can be determined easily in various body fluids, the molecular elements involved in shaping the rhythmic hormone synthesis cannot be analyzed experimentally in the living organism. However, the use of post-mortem pineal material seems to constitute a valid approach to decipher the regulation of human melatonin synthesis.

UV-A light regulation of arylalkylamine N-acetyltransferase activity in the chick pineal gland: role of cAMP and proteasomal proteolysis

Acute exposure of dark-adapted, cultured chick pineal glands to UV-A light significantly decreased the tissue cAMP concentration and the activity of arylalkylamine N-acetyltransferase (AANAT), the penultimate and key regulatory enzyme in the melatonin biosynthetic pathway. The magnitude of these changes was dependent on the duration of UV-A exposure. The UV-A light-evoked decline in pineal AANAT activity was blocked by cAMP protagonists (forskolin and dibutyryl-cAMP) and by inhibitors of the proteasomal degradation pathway (MG-132, proteasome inhibitor I, and lactacystin). These results indicate that the chick pineal gland is directly sensitive to UV-A light. By analogy to white light, the suppressive action of UV-A radiation on AANAT activity in the chick pineal gland involves changes in the tissue cAMP level and enhanced proteasomal proteolysis.

Seasonal variations in the nycthemeral rhythm of plasma melatonin in the camel (Camelus dromedarius)

Seasonal changes in the pattern of plasma melatonin were investigated in two groups of camels (Camelus dromedarius): 11 adult and six young camels. Animals were subjected to the outdoor conditions of a desert environment. Blood samples were taken at regular intervals of about 3 hr (added to particular samples at 1 hr before then 30 min and 1 hr after sunset, and 1 hr before and 1 hr after sunrise) for 24 hr at both solstices and equinoxes of the year. The plasma melatonin levels steeply increased soon after sunset and remained elevated throughout all the night. Then, melatonin concentrations progressively declined shortly before sunrise and returned to daytime basal levels 1 hr later. There was no seasonal variation in the amplitude or in the offset of the melatonin peak or in the daytime basal levels. The onset of the nocturnal peak was delayed by 2 hr in June at the summer solstice (P < 0.05), which can be related to the changes in night length between the two solstices. A significant effect of age was observed in all seasons. Melatonin levels were higher in the young camel group (fall equinox: P < 0.001; spring equinox: P < 0.01; winter solstice: P < 0.01; summer solstice: P < 0.05). The pattern of melatonin secretion in the camel showed a significant seasonal variation parallel to the photoperiodic changes of the year. The observed decline of melatonin levels during an extra-light pulse in the middle of the night indicates the light control of melatonin synthesis. It is not yet known if, in this low latitude desert region, the seasonal breeding period of the camel is cued by the photoperiod. The data obtained, however, clearly demonstrate that the camel has the capacity to follow and to integrate photoperiodic changes through melatonin changes.

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

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

N-acetyltransferase is not the rate-limiting enzyme of melatonin synthesis at night

Circadian melatonin production in the pineal gland and retina is under the control of serotonin N-acetyltransferase (NAT) and hydroxyindole-O-methyltransferase. Because NAT activity varies diurnally, it has been considered both the melatonin rhythm-generating enzyme and the rate-limiting enzyme of melatonin synthesis. In rats with dramatically reduced NAT activity due to a H28Y mutation in NAT, melatonin levels remained the same as in wildtype controls, suggesting that NAT does not determine the rate of melatonin production at night. Using a combination of molecular approaches with a sensitive in vivo measurement of pineal diurnal melatonin production, we demonstrate that (i) N-acetylserotonin (NAS), the enzymatic product of NAT, is present in vast excess in the night pineals compared with melatonin; (ii) the continuous increase in NAT protein levels at late night does not produce a proportional increase in melatonin; and (iii) an increase in NAS in the same animal over several circadian cycles do not result in corresponding increase in melatonin output. These results strongly suggest that NAT is not the rate-limiting enzyme of melatonin formation at night.

Hippocampal melatonin receptors modulate seizure threshold

PURPOSE

The pineal hormone melatonin has been shown to enhance hippocampal excitability. We therefore investigated whether inactivation of hippocampal melatonin receptors affects behavioral seizures.

METHODS

Intrahippocampal infusions were performed in rats to study the effect of different melatonin receptor antagonists on behavioral activity, EEG, and seizure susceptibility. Experiments were conducted at 2 times of the day that coincided with the peak and trough of the daily melatonin rhythm.

RESULTS

Local infusion of the Mel(1b) receptor antagonist 4-phenyl-2-propionamidotetralin (4-P-PDOT) into the hippocampus, but not the overlying neocortex, significantly increased seizure latency and in some cases provided complete protection against seizure development. In addition, 4-P-PDOT suppressed open field activity and hippocampal EEG amplitude. The mixed Mel(1a)/Mel(1b) receptor antagonist luzindole also increased seizure latency but to a lesser degree than 4-P-PDOT. The behavioral effects of Mel(1b) receptor inhibition were comparable to those of the gamma-aminobutyric acid (GABA)(A) receptor agonist muscimol and were observed during the dark phase (2400-0200 h) but not the light phase (1200-1400 h) of the daily photocycle. The anticonvulsant effect of intrahippocampal infusion of 4P-P-DOT was blocked by coadministration of the GABA(A) antagonist bicuculline.

CONCLUSIONS

Our results suggest that nocturnal activation of hippocampal Mel(1b) receptors depresses GABA(A) receptor function in the hippocampus and enhances seizure susceptibility.

Dopamine receptor activation in bovine pinealocyte via a cAMP-dependent transcription pathway

D1- and D2-dopamine receptors in the bovine pineal gland were previously identified and characterized. The data indicate that the density of D1-dopamine receptors far exceeded that of D2-dopamine receptors. In our previous study, the mRNA for both the D1- and D2-dopamine receptors which elucidated the status of dopamine and its possible involvement in the pineal function was identified. A selective D1-agonist enhanced N-acetyltransferase (NAT) activity and increased the melatonin level, whereas, a selective D2-agonist inhibited NAT activity and decreased the melatonin level. An attempt has been made in the present study to clarify the mechanism of dopamine in controlling melatonin production in bovine pineal. The level of intracellular cyclic 3',5'-adenosine monophosphate (cAMP) was determined after a 2-hr incubation of bovine pinealocytes with selected combinations of drugs. SKF 38393, a selective D1-agonist, enhanced intracellular level of cAMP, and its effect was blocked by SCH 23390, a D1-selective antagonist. In contrast quinpirole, a selective D2-agonist, inhibited forskolin-stimulated intracellular level of cAMP while its effect was blocked by a D2-selective antagonist, spiperone. In addition, the dopamine-dependent phosphorylation of the transcription factors, cAMP responsive element-binding protein (CREB) was investigated. Immunoblots showed that SKF 38393 enhanced CREB phosphorylation and the stimulatory effect was abolished by SCH 23390 whereas quinpirole inhibited forskolin-stimulated phosphorylated CREB production and the inhibitory effect was prevented by spiperone. Taken together with our previous data, the results indicate that activation of D1-dopamine receptor in bovine pinealocyte stimulates NAT activity and enhances melatonin level whereas activation of D2-dopamine receptor leads to an inhibitory effect and these stimulatory and inhibitory effects act, in part, via a cAMP-dependent transcription mechanism.

Posthatching developmental changes in noradrenaline content in the chicken pineal gland

Noradrenaline (NA) levels in pineal gland of chickens at various posthatching stages (P2, P4, P8, P15, P30 and P57) were determined by high-performance liquid chromatography with electrochemical detection. Pineal NA content markedly increased between P2 and P30. P30 and P57 chickens, kept from the day of hatching under a 12:12 hr light-dark (LD) illumination cycle, exhibited rhythmic changes in pineal NA, with levels in the dark period being markedly higher than in the light period. In younger birds pineal NA concentrations did not show pronounced daily variations. In 4-wk-old chickens (P28-30) kept under constant darkness (DD), the rhythmic pattern of pineal NA persisted for 1 day (with higher values during the subjective dark phase than during the subjective light phase), but this disappeared 24 hr after the introduction of DD. In contrast, NA content in pineal glands isolated from birds maintained for 2 days under continuous light was similar to that found during the light phase of the LD cycle, and did not exhibit significant rhythmicity. In P30 chickens, pretreated with alpha-methyl-p-tyrosine (AMPT, an inhibitor of tyrosine hydroxylase, the key regulatory enzyme in the biosynthesis of catecholamines), pineal NA content declined slowly and monophasically during the light phase. During the dark phase the AMPT-induced decay of NA was biphasic--namely an initial rapid decline over the first 15 min which was followed by a slow-rate decline--an observation indicating that NA turnover was higher in the dark. Acute exposure of the dark-adapted P30 and P57 chickens to light significantly decreased pineal NA content, but did not affect pineal NA concentrations in younger birds. Our results suggest that the NA rhythm in the chicken pineal gland and its sensitivity to light regulation progressively develop during the first month of life.

Just the two of us: melatonin and adenosine in rodent pituitary function

The function of the pituitary gland is tightly controlled by neuronal and hormonal afferents of the brain. In this review, the role of the neurohormone melatonin and the neuromodulator adenosine for rodent pituitary function will be elucidated. Adenosine is known as an important paracrine modulator for pituitary endocrine and folliculostellate cells, with availability regulated by local metabolic cellular activity. In general, adenosine inhibits the cyclic adenosine monophosphate (AMP) pathway in pituitary cells by binding to A1-, and A3-adenosinergic receptors, and activates it via A2-adenosinergic receptors. The neurohormone melatonin integrates time-of-day and time-of-year into pituitary function via binding to MT1-melatonin receptors. Melatonin impacts at the hypothalamic level neurons that synthesize releasing and release-inhibiting hormones, and at the pituitary level only cells of the hypophyseal pars tuberalis (PT). Thereby, the daily changes in the duration of the nocturnal melatonin surge are decoded and subsequently relayed to the pars distalis to adapt gonadotropin and prolactin release, respectively, to season. An exciting integration of time within the regulation of pituitary function was deciphered by analysing transmembrane signalling events in cells of the hypophyseal PT: a consecutive daily impact of initially the neurohormone melatonin and later the neuromodulator adenosine on rodent PT cells leads to a circadian rhythm in the transcription of cyclic-AMP-sensitive genes.

Distribution of transcription factor inducible cyclicAMP early repressor (ICER) in rodent brain and pituitary

In morphogenetic dynamics of neurons, and in adaptive physiology of brain function, transcription factors of the cyclicAMP signaling pathway, such as activator cyclicAMP responsive element binding protein (CREB) and inhibitor inducible cyclicAMP early repressor (ICER), play an important role. In particular, the presence of the transcription factor ICER in neurons or neuroendocrine cells suggests the need for the gating of an up-regulated gene expression. Little is known, however, about the natural distribution of the inhibitory transcription factor ICER. We, therefore, mapped the rodent brain and pituitary for an ICER immunoreaction and found a nuclear staining for this transcription factor. ICER-positive glial cells were found throughout the brain. ICER-positive neurons were found in sensory input centers, like the olfactory bulb, or sensory brain stem nuclei, and in hypothalamic nuclei involved in central neuroendocrine control. In addition, neuroendocrine/endocrine transducers, like the pituitary and the pineal gland showed a high basal presence of ICER. Our data show that a basic ICER level is required by many cell systems and can be seen as an anticipatory and/or a protective measure in systems with superior reactive dynamics.

Rhythmic melatonin secretion does not correlate with the expression of arylalkylamine N-acetyltransferase, inducible cyclic amp early repressor, period1 or cryptochrome1 mRNA in the sheep pineal

The pineal gland, through nocturnal melatonin, acts as a neuroendocrine transducer of daily and seasonal time. Melatonin synthesis is driven by rhythmic activation of the rate-limiting enzyme, arylalkylamine N-acetyltransferase (AA-NAT). In ungulates, AA-NAT mRNA is constitutively high throughout the 24-h cycle, and melatonin production is primarily controlled through effects on AA-NAT enzyme activity; this is in contrast to dominant transcriptional control in rodents. To determine whether there has been a selective loss of circadian control of AA-NAT mRNA expression in the sheep pineal, we measured the expression of other genes known to be rhythmic in rodents (inducible cAMP early repressor ICER, the circadian clock genes Period1 and Cryptochrome1, as well as AA-NAT). We first assayed gene expression in pineal glands collected from Soay sheep adapted to short days (Light: dark, 8-h: 16-h), and killed at 4-h intervals through 24-h. We found no evidence for rhythmic expression of ICER, AA-NAT or Cryptochrome1 under these conditions, whilst Period1 showed a low amplitude rhythm of expression, with higher values during the dark period. In a second group of animals, lights out was delayed by 8-h during the final 24-h sampling period, a manipulation that causes an immediate shortening of the period of melatonin secretion. This did not significantly affect the expression of ICER, AA-NAT or Cryptochrome1 in the pineal, whilst a slight suppressive effect on overall Per1 levels was observed. The attenuated response to photoperiod change appears to be specific to the ovine pineal, as the first long day induced rapid changes of Period1 and ICER expression in the hypothalamic suprachiasmatic nuclei and pituitary pars tuberalis, respectively. Overall, our data suggest a general reduction of circadian control of transcript abundance in the ovine pineal gland, consistent with a marked evolutionary divergence in the mechanism regulating melatonin production between terrestrial ruminants and fossorial rodents.

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

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

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

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

Norepinephrine-dependent phosphorylation of the transcription factor cyclic adenosine monophosphate responsive element-binding protein in bovine pinealocytes

Norepinephrine (NE)-dependent activation of transcription factors is of central importance for the rhythmic production of melatonin in the rodent pineal gland. At variance with rodents, NE regulates melatonin biosynthesis through post-translational mechanisms in ungulates, and it is not yet known whether transcription factors play any role in ungulate pineal functions. Here, we investigated in isolated bovine pinealocytes the NE-dependent phosphorylation of the transcription factor cyclic adenosine monophosphate (cAMP) responsive element-binding protein (CREB) and compared the effects of NE with those of vasoactive intestinal peptide (VIP) and pituitary adenylate cyclase-activating polypeptide (PACAP). Treatment with 10(-7) m NE for 30 min induced a strong nuclear phosphorylated CREB (pCREB) immunoreaction in cells that were identified as pinealocytes by immunocytochemical demonstration of serotonin, a pinealocyte-specific marker. Immunoblots showed that the NE-induced immunoreaction was due to phosphorylation of the transcription factor CREB and another protein, presumably the activating transcription factor 1 (ATF-1). 10(-7) m isoproterenol (ISO) or 10(-5) m forskolin mimicked the response to NE indicating that NE acts through the beta-adrenergic/cAMP pathway. Also 10(-7) m PACAP, but not 10(-7) m VIP-enhanced CREB phosphorylation; however, only a subpopulation of cells was responsive to PACAP. Our results suggest that, irrespective of whether or not melatonin production is controlled via transcriptional mechanisms, NE-induced CREB phosphorylation represents a very conserved element in pineal physiology of mammals because NE increases pCREB levels in all mammalian species investigated so far. However, the genes targeted by pCREB may vary from one mammalian species to the other. Our results also suggest that transcription factors other than pCREB, like ATF-1, may play a role in pineal functions of mammals.

Melatonin modulates the light-induced sympathoexcitation and vagal suppression with participation of the suprachiasmatic nucleus in mice

In mammals, the autonomic nervous system mediates the central circadian clock oscillation from the suprachiasmatic nucleus (SCN) to the peripheral organs, and controls cardiovascular, respiratory and gastrointestinal functions. The present study was conducted in mice to address whether light signals conveyed to the SCN can control peripheral autonomic functions, and further examined the impact of centrally administered melatonin on peripheral autonomic functions via activation of melatonin receptor signalling. In vivo electrophysiological techniques were performed in anaesthetised, open-chest and artificially ventilated mice whilst monitoring the arterial blood pressure and heart rate. Light induced an increase of the renal sympathetic nerve activity, arterial blood pressure and heart rate immediately after lights on. Conversely, light rapidly suppressed the gastric vagal parasympathetic nerve activity, which was affected neither by hepatic vagotomy nor by total subdiaphragmatic vagotomy. These autonomic responses were mediated by the SCN since bilateral SCN lesion totally abolished the light-evoked neuronal and cardiovascular responses. Melatonin administered intracerebroventricularly (I.C.V.) attenuated the sympathetic and vagal nerve activities in a dose-dependent manner with a threshold of 0.1 ng and these effects were blocked by I.C.V. pre-treatment of the competitive melatonin receptor antagonist luzindole. These results suggest that light induces sympathoexcitation and vagal suppression through the SCN and that melatonin modulates the light-induced autonomic responses via activation of the central melatonin receptor signalling.