Despite a pronounced inhibitory effect of light on pineal melatonin synthesis, usually the daily melatonin rhythm is not a passive response to the surrounding world. In mammals, and almost every other vertebrate species studied so far, the melatonin rhythm is coupled to an endogenous pacemaker, i.e. a circadian clock. In mammals the principal circadian pacemaker is located in the suprachiasmatic nuclei (SCN), a bilateral cluster of neurons in the anterior hypothalamus. In the present paper we show in the rat that bilateral abolition of gamma-aminobutyric acid (GABA), but not vasopressin, neurotransmission in an SCN target area, i.e. the paraventricular nucleus of the hypothalamus, during (subjective) daytime results in increased pineal melatonin levels. The fact that complete removal of the SCN results in a pronounced increase of daytime pineal mRNA levels for arylalkylamine N-acetyltransferase (AA-NAT), i.e. the rate-limiting enzyme of melatonin synthesis, further substantiates the existence of a major inhibitory SCN output controlling the circadian melatonin rhythm.

The suprachiasmatic nucleus (SCN) controls the circadian rhythm of melatonin synthesis in the mammalian pineal gland by a multisynaptic pathway including, successively, preautonomic neurons of the paraventricular nucleus (PVN), sympathetic preganglionic neurons in the spinal cord and noradrenergic neurons of the superior cervical ganglion (SCG). In order to clarify the role of each of these structures in the generation of the melatonin synthesis rhythm, we first investigated the day- and night-time capacity of the rat pineal gland to produce melatonin after bilateral SCN lesions, PVN lesions or SCG removal, by measurements of arylalkylamine N-acetyltransferase (AA-NAT) gene expression and pineal melatonin content. In addition, we followed the endogenous 48 h-pattern of melatonin secretion in SCN-lesioned vs. intact rats, by microdialysis in the pineal gland. Corticosterone content was measured in the same dialysates to assess the SCN lesions effectiveness. All treatments completely eliminated the day/night difference in melatonin synthesis. In PVN-lesioned and ganglionectomised rats, AA-NAT levels and pineal melatonin content were low (i.e. 12% of night-time control levels) for both day- and night-time periods. In SCN-lesioned rats, AA-NAT levels were intermediate (i.e. 30% of night-time control levels) and the 48-h secretion of melatonin presented constant levels not exceeding 20% of night-time control levels. The present results show that ablation of the SCN not only removes an inhibitory input but also a stimulatory input to the melatonin rhythm generating system. Combination of inhibitory and stimulatory SCN outputs could be of a great interest for the mechanism of adaptation to day-length (i.e. adaptation to seasons).

Hibernation is a fascinating, yet enigmatic, physiological phenomenon during which body temperature and metabolism are reduced to save energy. During the harsh season, this strategy allows substantial energy saving by reducing body temperature and metabolism. Accordingly, biological processes are considerably slowed down and reduced to a minimum. However, the persistence of a temperature-compensated, functional biological clock in hibernating mammals has long been debated. Here, we show that the master circadian clock no longer displays 24-h molecular oscillations in hibernating European hamsters. The clock genes Per1, Per2, and Bmal1 and the clock-controlled gene arginine vasopressin were constantly expressed in the suprachiasmatic nucleus during deep torpor, as assessed by radioactive in situ hybridization. Finally, the melatonin rhythm-generating enzyme, arylalkylamine N-acetyltransferase, whose rhythmic expression in the pineal gland is controlled by the master circadian clock, no longer exhibits day/night changes of expression but constantly elevated mRNA levels over 24 h. Overall, these data provide strong evidence that in the European hamster the molecular circadian clock is arrested during hibernation and stops delivering rhythmic output signals.deep torpor ͉ suprachiasmatic ͉ euthermia

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Photoperiodic changes of pineal melatonin (MEL) profile are accompanied by parallel changes of arylalkylamine-N-acetyltransferase (AA-NAT) activity. In the present study, the authors investigated, for the first time, whether two other important variables of pineal metabolism, AA-NAT and hydroxyindole-O-methyltransferase (HIOMT) gene expression, also may be affected by the photoperiod. Evening rises in AA-NAT and HIOMT mRNA and in circulating MEL occurred concomitantly with an increased delay from dark onset as scotophase shortened. On the opposite, the morning declines of all three variables occurred with different kinetics but were locked to light onset. These observations demonstrate that the daily rhythms in AA-NAT and HIOMT gene expression are modulated by the photoperiod and bring further evidence in favor of nor adrenaline as the possible link between the endogenous clock and MEL. Interestingly, the duration of the nocturnal peak in HIOMT mRNA was positively correlated with HIOMT activity. In conclusion, this study adds two important links to the chain of mechanisms involved in the photoperiodic control of pineal metabolism. First, photoperiodic modulation of the MEL rhythm primarily results from changes in the AA-NAT gene expression. Second, the photoperiodic regulation of HIOMT activity occurs at the transcriptional level.

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Pineal melatonin synthesis is stimulated at night following an increase in arylalkylamine-N-acetyltransferase (AA-NAT) activity. Depending on the species, two mechanisms of enzyme activation have been described: a cAMP/phospho-cAMP response element-binding protein-dependent stimulation of Aa-nat gene transcription in the rat, presumed to occur in all rodents, or a posttranslational regulation of AA-NAT protein in ongulates. The present data obtained in the Syrian hamster indicate another route of AA-NAT regulation. Elevated nocturnal levels of Aa-nat mRNA were strongly suppressed following light exposure or adrenergic antagonist administration, demonstrating the involvement of norepinephrine in the stimulation of melatonin synthesis. However, administration of adrenergic agonists during the day did not increase Aa-nat mRNA unless a protein synthesis inhibitor was given during the previous night. This indicates that an inhibitory protein, synthesized at night, prevents melatonin synthesis during the day. By contrast, a protein synthesis inhibitor given at the beginning of the night markedly reduced Aa-nat mRNA, suggesting that a stimulatory protein (transcription factor?) is necessary for Aa-nat gene transcription at night. Noteworthy, hamsters raised in long photoperiod were responsive to adrenergic agonist injection only in the first hour after light onset, a response that may be important in this photoperiodic species in which the melatonin peak extends into the morning hours in a short photoperiod.