Contrary to other non-photic cues, acute melatonin injection does not induce immediate changes of clock gene mRNA expression in the rat suprachiasmatic nuclei

Does a melatonin-dependent circadian oscillator in the pars tuberalis drive prolactin seasonal rhythmicity?

The pars tuberalis (PT) of the adenohypophysis expresses a high density of melatonin receptors and is thought to be a crucial relay for the actions of melatonin on seasonal rhythmicity of prolactin secretion by the pars distalis (PD). In common with the suprachiasmatic nucleus of the hypothalamus and most other peripheral tissues, the PT rhythmically expresses a range of 'clock genes'. Interestingly, this expression is highly dependent upon melatonin/photoperiod, with several aspects unique to the PT. These observations led to the establishment of a conceptual framework for the encoding of seasonal timing in this tissue. This review summarises current knowledge of the morphological, functional and molecular aspects of the PT and considers its role in seasonal timing. The strengths and weaknesses of current hypotheses that link melatonin action in the PT to its seasonal effect on lactotrophs of the PD are discussed and alternative working hypotheses are suggested.

Rhythmic expression, light entrainment and α-MSH modulation of rhodopsin mRNA in a teleost pigment cell line

To investigate whether teleost fish GEM-81 erythrophoroma cells were photosensitive, the cells were submitted to constant darkness (DD), 14 h of light and 10 h of darkness (14L:10D), and 10 h of light and 14 h of darkness (10L:14L). The doubling times (hours) were: DD 35.33+/-0.05; 14L:10D 67.85+/-0.04; and 10L:14D 49.60+/-0.08. In order to verify whether proliferation was dependent on light phase length, GEM-81 cells were submitted to 7L: 5D. The proliferation curves and doubling times were similar in 14L:10D and 7L:5D (respectively 69.44+/-0.03 and 67.85+/-0.04), suggesting that the cell cycle was regulated by the length of the light phase within 24 h, or by the light/dark ratio. We have also demonstrated the expression of Carassius retinal rhodopsin mRNA in GEM-81 cells, which cycles in a circadian rhythm, entrained by light. In addition, we showed that alpha-melanocyte stimulating hormone (alpha-MSH, 10(-10) to 10(-8) M), a conspicuous hormone that exerts mitogenic and melanogenic activity in most vertebrates, decreased rhodopsin mRNA in the first 3 days; after 4 days the inhibition was reversed, and after 5 days an increase in rhodopsin mRNA level was elicited. This is the first report of rhythmic expression of extra-ocular rhodopsin and its modulation by light and hormones.

ORL1 receptor-mediated down-regulation of mPER2 in the suprachiasmatic nucleus accelerates re-entrainment of the circadian clock following a shift in the environmental light/dark cycle

The circadian pacemaker in the suprachiasmatic nucleus (SCN) generates the near 24-h period of the circadian rhythm and is entrained to the 24-h daily cycle by periodic environmental signals, such as the light/dark cycle (photic signal), and can be modulated by various drugs (non-photic signals). The mechanisms by which non-photic signals modulate the circadian clock are not well understood in mice. In mice, many reportedly non-photic stimuli have little effect on the circadian rhythm in vivo. Herein, we investigated the molecular mechanism in W-212393-induced phase advance using mice. W-212393 caused a significant phase advance of locomotor activity rhythm in mice at subjective day. Injection of W-212393 during subjective day elicited down-regulation of mPER2 protein in the SCN shell region, but not mPer2 mRNA. Administration of W-212393 during subjective day failed to produce phase advance in mPer2-mutant mice as well as in ORL1 receptor deficient mice. Furthermore, we show that such inhibition of mPER2 accelerates re-entrainment of the circadian clock following an abrupt shift in the environmental light/dark cycle, such as occurs with transmeridian flight. The present results suggest that post-translational down-regulation of mPER2 protein in the shell region of mouse SCN may be involved in W-212393-induced non-photic phase advance.

Promoting adjustment of the sleep–wake cycle by chronobiotics

Chronobiotics are substances that adjust the timing of internal biological rhythms. Many classes of drugs have been claimed to possess such properties and arouse growing interest as the circumstances for their use in sleep disturbances caused by circadian rhythms alterations (delayed or advanced sleep-phase syndromes, non-24-h sleep-wake disorders, jet lag, shift work sleep disorders and so on) have become progressively more frequent. Amongst the substances potentially presenting chronobiotic properties, a consensus seems to be reached on the possible use of melatonin or its agonists to shift the phase of the human circadian clock, but optimizing the dose, formulation and especially the time of administration require further studies.

Melatonin affects nuclear orphan receptors mRNA in the rat suprachiasmatic nuclei

The pineal hormone melatonin nocturnal synthesis feeds back on the suprachiasmatic nuclei (SCN), the central circadian clock. Indeed, daily melatonin injections in free-running rats resynchronize their locomotor activity to 24 h. However, the molecular mechanisms underlying this chronobiotic effect of the hormone are poorly understood. The endogenous circadian machinery involves positive and negative transcriptional feedback loops implicating different genes (particularly period (Per) 1-3, Clock, Bmal1, cryptochrome (Cry) 1-2). While CLOCK:BMAL1 heterodimer activates the rhythmic transcription of per and cry genes, the PER and CRY proteins inhibit the CLOCK:BMAL1 complex. In previous studies, we observed that the immediate resetting effect of a melatonin injection at the end of the subjective day on the SCN circadian activity did not directly involve the above-mentioned clock genes. Recently, nuclear orphan receptors (NORs) have been presented as functional links between the regulatory loops of the molecular clock. These NORs bind to a retinoic acid receptor-related orphan receptor response element (RORE) domain and activate (RORalpha) or repress (REV-ERBalpha) bmal1 expression. In this study, we investigated whether melatonin exerts its chronobiotic effects through transcriptional regulation of these transcription factors. We monitored roralpha, rorbeta and rev-erbalpha messenger RNA (mRNA) expression levels by quantitative in situ hybridization, up to 36 h following a melatonin injection at circadian time (CT) 11.5. Results clearly showed that, while roralpha was not affected by melatonin, the hormone partially prevented the decrease of the rorbeta mRNA expression observed in control animals during the first hours following the injection. The major result is that the rev-erbalpha mRNA expression rhythm was 1.3+/-0.8-h phase-advanced in melatonin-treated animals during the first subjective night following the melatonin administration. Moreover, the bmal1 mRNA expression was 1.9+/-0.9-h phase-shifted in the second subjective night following the melatonin injection. These results clearly suggest that the NOR genes could be the link between the chronobiotic action of melatonin and the core of the molecular circadian clock.

Melatonin

Melatonin is a ubiquitous molecule and widely distributed in nature, with functional activity occurring in unicellular organisms, plants, fungi and animals. In most vertebrates, including humans, melatonin is synthesized primarily in the pineal gland and is regulated by the environmental light/dark cycle via the suprachiasmatic nucleus. Pinealocytes function as 'neuroendocrine transducers' to secrete melatonin during the dark phase of the light/dark cycle and, consequently, melatonin is often called the 'hormone of darkness'. Melatonin is principally secreted at night and is centrally involved in sleep regulation, as well as in a number of other cyclical bodily activities. Melatonin is exclusively involved in signaling the 'time of day' and 'time of year' (hence considered to help both clock and calendar functions) to all tissues and is thus considered to be the body's chronological pacemaker or 'Zeitgeber'. Synthesis of melatonin also occurs in other areas of the body, including the retina, the gastrointestinal tract, skin, bone marrow and in lymphocytes, from which it may influence other physiological functions through paracrine signaling. Melatonin has also been extracted from the seeds and leaves of a number of plants and its concentration in some of this material is several orders of magnitude higher than its night-time plasma value in humans. Melatonin participates in diverse physiological functions. In addition to its timekeeping functions, melatonin is an effective antioxidant which scavenges free radicals and up-regulates several antioxidant enzymes. It also has a strong antiapoptotic signaling function, an effect which it exerts even during ischemia. Melatonin's cytoprotective properties have practical implications in the treatment of neurodegenerative diseases. Melatonin also has immune-enhancing and oncostatic properties. Its 'chronobiotic' properties have been shown to have value in treating various circadian rhythm sleep disorders, such as jet lag or shift-work sleep disorder. Melatonin acting as an 'internal sleep facilitator' promotes sleep, and melatonin's sleep-facilitating properties have been found to be useful for treating insomnia symptoms in elderly and depressive patients. A recently introduced melatonin analog, agomelatine, is also efficient for the treatment of major depressive disorder and bipolar affective disorder. Melatonin's role as a 'photoperiodic molecule' in seasonal reproduction has been established in photoperiodic species, although its regulatory influence in humans remains under investigation. Taken together, this evidence implicates melatonin in a broad range of effects with a significant regulatory influence over many of the body's physiological functions.

GABAAreceptor activation suppressesPeriod 1mRNA andPeriod 2mRNA in the suprachiasmatic nucleus during the mid-subjective day

The mammalian circadian clock can be entrained by photic and nonphotic environmental time cues. gamma-aminobutyric acid (GABA) is a nonphotic stimulus that induces phase advances in the circadian clock during the middle of the subjective day. Several nonphotic stimuli suppress Period 1- and Period 2 mRNA expression in the suprachiasmatic nucleus (SCN); however, the effect of GABA on Period mRNA is unknown. In the present study we demonstrate that microinjection of the GABA(A) receptor agonist muscimol into the SCN region suppresses the expression of Period 1 mRNA in the hamster. A significant suppression of Period 2 mRNA following microinjection of muscimol was not observed in free-running conditions. However, Period 2 mRNA was significantly reduced following muscimol treatment when animals were maintained under a light cycle and transferred to constant darkness 42 h prior to treatment. An additional study investigated the maximum behavioural phase advance inducible by GABA(A) receptor activation.Together, these data indicate that, like other nonphotic stimuli, GABA suppresses Period 1- and Period 2 mRNA in the SCN.

A functional subdivision of the circadian clock is revealed by differential effects of melatonin administration

The biological clock of the suprachiasmatic nuclei drives numerous physiological and behavioural circadian rhythms. In this study, we addressed the question as to whether different components of the clock may control separately various circadian functions. Using the rat transpineal microdialysis tool, we analysed the effect of clock perturbation by exogenous melatonin injection on two hormonal clock outputs: pineal melatonin and adrenal corticosterone secretions. As already reported, a single melatonin injection at the light/dark transition induces a marked increase in the endogenous pineal melatonin peak for the two following days. In the same animals, by contrast, the amplitude of the corticosterone rhythm was not altered following melatonin injection. These data show that the melatonin injection does not display an overall effect on the circadian clock, but rather influences a subpopulation of melatonin-sensitive neurons involved, among other functions, in the circadian control of the pineal pathway.

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

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

Melatonin: characteristics, concerns, and prospects

Melatonin is of great importance to the investigation of human biological rhythms. Its rhythm in plasma or saliva provides the best available measure of the timing of the internal circadian clock. Its major metabolite 6-sulphatoxymelatonin is robust and easily measured in urine. It thus enables long-term monitoring of human rhythms in real-life situations where rhythms may be disturbed, and in clinical situations where invasive procedures are difficult. Melatonin is not only a "hand of the clock"; endogenous melatonin acts to reinforce the functioning of the human circadian system, probably in many ways. Most is known about its relationship to sleep and the decline in core body temperature and alertness at night. Current perspectives also include a possible influence on major disease risk, arising from circadian rhythm disruption. Melatonin clearly has the ability to induce sleepiness and lower core body temperature during "biological day" and to change the timing of human rhythms when treatment is appropriately timed. It can entrain free-running rhythms and maintain entrainment in most blind and some sighted people. Used therapeutically it has proved a successful treatment for circadian rhythm disorder, particularly the non-24-h sleep wake disorder of the blind. Numerous other clinical applications are under investigation. There are, however, areas of controversy, large gaps in knowledge, and insufficient standardization of experimental conditions and analysis for general conclusions to be drawn with regard to most situations. The future holds much promise for melatonin as a therapeutic treatment. Most interesting, however, will be the dissection of its effects on human genes.

Timed hypocaloric feeding and melatonin synchronize the suprachiasmatic clockwork in rats, but with opposite timing of behavioral output

Temporal organization of the molecular clockwork and behavioral output were investigated in nocturnal rats housed in constant darkness and synchronized to nonphotic cues (daily normocaloric or hypocaloric feeding and melatonin infusion) or light (light-dark cycle and daily 1-h light exposure). Clock gene (Per1, Per2 and Bmal1) and clock-controlled gene (Vasopressin) expression in the suprachiasmatic nuclei was assessed over 24 h. Light and exogenous melatonin synchronized the molecular clock, signaling, respectively, 'daytime' and 'nighttime', without affecting temporal organization of behavioral output (rest/activity rhythm). By contrast, synchronization to hypocaloric feeding led to a striking temporal change between gene expression in the suprachiasmatic clock and waveform of locomotor activity rhythm, rats then becoming active during the subjective day (diurnal-like temporal organization). When the time of feeding coincided with activity offset, normocaloric feeding also synchronized the locomotor activity rhythm with no apparent switch in temporal organization. Peak of Per2 expression in the piriform cortex occurred between the beginning and the middle of the activity/feeding period, depending on the synchronizer. These data demonstrate that even though the suprachiasmatic clockwork can be synchronized to nonphotic cues, hypocaloric feeding likely acts downstream from clock gene oscillations in the suprachiasmatic nuclei to yield a stable yet opposite organization of the rest/activity cycle.

The opioid fentanyl affects light input, electrical activity andPergene expression in the hamster suprachiasmatic nuclei

The suprachiasmatic nuclei (SCN) contain a major circadian pacemaker, which is regulated by photic and nonphotic stimuli. Although enkephalins are present in the SCN, their role in phase regulation of the pacemaker is largely unknown. The opioid agonist fentanyl, a homologue of morphine, is an addictive drug that induces phase shifts of circadian rhythms in hamsters. We observed that these phase shifts are blocked by naloxone, which is a critical test for true opioid receptor involvement, and conclude that opioid receptors are the sole mediators of the actions of fentanyl on the circadian timing system. A strong interaction between opioids and light input was shown by the ability of fentanyl and light to completely block each other's phase shifts of behavioural activity rhythms. Neuronal ensemble recordings in vitro provide first evidence that SCN cells show direct responses to fentanyl and react with a suppression of firing rate. Moreover, we show that fentanyl induces a strong attenuation of light-induced Syrian hamster Period 1 (shPer1) gene expression during the night. During the subjective day, we found no evidence for a role of shPer1 in mediation of fentanyl-induced phase shifts. Based on the present results, however, we cannot exclude the involvement of shPer2. Our data indicate that opioids can strongly modify the photic responsiveness of the circadian pacemaker and may do so via direct effects on SCN electrical activity and regulation of Per genes. This suggests that the pathways regulating addictive behaviour and the circadian clock intersect.

Activation of 5-HT2C receptors acutely induces Per gene expression in the rat suprachiasmatic nucleus at night

The suprachiasmatic nucleus (SCN) of the hypothalamus receives dense serotonergic projections from the raphe nuclei and this input has been implicated in the modulation of circadian rhythms. In the present study, we investigated the effect of 5-HT2C receptor activation on various clock genes within the suprachiasmatic nucleus, including Per1 and Per2, which have previously been demonstrated as necessary for phase shifts. Rats were exposed to light (400 lx, 15 min), administered 5-HT2C receptor agonists (+/-)-1-(4-iodo-2,5-dimethoxy-phenyl)-2-aminopropane (DOI) (2 mg/kg) or RO 60-0175 (10 mg/kg) or vehicle 4 or 10 h after dark onset (ZT16 and ZT22). The expression of Per1, Per2, Cry1, Clock, Bmal1, Dec1, Dec2 and c-fos was determined 30 and 120 min after treatment in suprachiasmatic nucleus punches by real time reverse transcription-polymerase chain reaction (RT-PCR). Light exposure induced a 7-fold increase in c-fos expression within 30 min of treatment at both ZT16 and ZT22. Per1 expression was increased 2-fold following light exposure at ZT22, whereas treatment at ZT16 had no significant effect. Per2 expression was significantly induced following light at ZT16, but was not affected at ZT22. RO 60-0175 or DOI administration induced a 5-fold change in c-fos expression at ZT16 and a 3-fold change at ZT22 within 30 min of treatment. The drug increased both Per1 and Per2 expression at ZT16, but had no effect at ZT22. These results provide evidence for 5-HT2C receptors being involved in the modulation of circadian rhythms during early night.

Melatonin induces Cry1 expression in the pars tuberalis of the rat

In mammals, interacting transcriptional/post-translational feedback loops involving 'clock genes' and their protein products control circadian organisation. These genes are not only expressed in the master circadian clock of the suprachiasmatic nuclei (SCN) but also in many peripheral tissues where they exhibit similar but not identical dynamic to that seen in the SCN. Among these peripheral tissues, the pars tuberalis (PT) of the pituitary expresses clock genes. We show here that the PT of the rat, like that of other rodents, rhythmically expresses Per1. We also report rhythmic expression of another clock gene, Cry1. The peak of Cry1 mRNA expression occurs during the night concomitantly with rising blood plasma melatonin concentrations. Using an acute injection paradigm, we demonstrate that Cry1 expression is directly induced by melatonin in the PT. Melatonin injection at the end of the subjective day also affects Per1 expression, leading to diminished mRNA levels. These data support the existence of a time-measurement model in the PT based on direct opposite actions of melatonin on Per1 and Cry1 expression.