In Vivo Imaging of the Central and Peripheral Effects of Sleep Deprivation and Suprachiasmatic Nuclei Lesion on PERIOD-2 Protein in Mice.

STUDY OBJECTIVES: That sleep deprivation increases the brain expression of various clock genes has been well documented. Based on these and other findings we hypothesized that clock genes not only underlie circadian rhythm generation but are also implicated in sleep homeostasis. However, long time lags have been reported between the changes in the clock gene messenger RNA levels and their encoded proteins. It is therefore crucial to establish whether also protein levels increase within the time frame known to activate a homeostatic sleep response. We report on the central and peripheral effects of sleep deprivation on PERIOD-2 (PER2) protein both in intact and suprachiasmatic nuclei-lesioned mice. DESIGN: In vivo and in situ PER2 imaging during baseline, sleep deprivation, and recovery. SETTINGS: Mouse sleep-recording facility. PARTICIPANTS: Per2::Luciferase knock-in mice. INTERVENTIONS: N/A. MEASUREMENTS AND RESULTS: Six-hour sleep deprivation increased PER2 not only in the brain but also in liver and kidney. Remarkably, the effects in the liver outlasted those observed in the brain. Within the brain the increase in PER2 concerned the cerebral cortex mainly, while leaving suprachiasmatic nuclei (SCN) levels unaffected. Against expectation, sleep deprivation did not increase PER2 in the brain of arrhythmic SCN-lesioned mice because of higher PER2 levels in baseline. In contrast, liver PER2 levels did increase in these mice similar to the sham and partially lesioned controls. CONCLUSIONS: Our results stress the importance of considering both sleep-wake dependent and circadian processes when quantifying clock-gene levels. Because sleep deprivation alters PERIOD-2 in the brain as well as in the periphery, it is tempting to speculate that clock genes constitute a common pathway mediating the shared and well-known adverse effects of both chronic sleep loss and disrupted circadian rhythmicity on metabolic health.

Disruption of the suprachiasmatic nucleus blunts a time of day-dependent variation in systemic anaphylactic reaction in mice.

Anaphylaxis is a severe systemic allergic reaction which is rapid in onset and potentially fatal, caused by excessive release of mediators including histamine and cytokines/chemokines from mast cells and basophils upon allergen/IgE stimulation. Increased prevalence of anaphylaxis in industrialized countries requires urgent needs for better understanding of anaphylaxis. However, the pathophysiology of the disease is not fully understood. Here we report that the circadian clock may be an important regulator of anaphylaxis. In mammals, the central clock located in the suprachiasmatic nucleus (SCN) of the hypothalamus synchronizes and entrains peripheral circadian clock present in virtually all cell types via neural and endocrine pathways, thereby driving the daily rhythms in behavior and physiology. We found that mechanical disruption of the SCN resulted in the absence of a time of day-dependent variation in passive systemic anaphylactic (PSA) reaction in mice, associated with loss of daily variations in serum histamine, MCP-1 (CCL2), and IL-6 levels. These results suggest that the central SCN clock controls the time of day-dependent variation in IgE-mediated systemic anaphylactic reaction, which may provide a novel insight into the pathophysiology of anaphylaxis.

Sleep-wake pattern following gunshot suprachiasmatic damage.

BACKGROUND: The suprachiasmatic nucleus (SCN) plays a critical role in maintaining melatonin and sleep-wake cycles. METHODS/PATIENT: We report a case of 38-year-old woman who, after gunshot wound to the right temple, developed a sleep complaint of multiple nocturnal awakenings and several naps throughout the day. RESULTS: Computerized tomography and magnetic resonance imaging revealed bilateral optic nerve and optic chiasm damage. Diagnostic polysomnography and actigraphy revealed an irregular sleep wake rhythm. CONCLUSIONS: We speculate concurrent damage of the SCN and optic nerves bilaterally resulted in the posttraumatic irregular sleep-wake rhythm.

The complex relationship between the light-entrainable and methamphetamine-sensitive circadian oscillators: evidence from behavioral studies of Period-mutant mice.

The methamphetamine-sensitive circadian oscillator (MASCO) is an enigmatic circadian clock whose output is observed during continuous consumption of low-dose methamphetamine. The MASCO rhythm persists when the light-entrainable pacemaker in the suprachiasmatic nucleus (SCN) is lesioned, but the anatomical location of MASCO is unknown. We recently found that the period of the MASCO rhythm is unusually short (21 h) in mice with disruption of all three paralogs of the canonical clock gene, Period. In this study, we investigated the contribution of each Period paralog to timekeeping in MASCO. We measured wheel-running activity rhythms in intact and SCN-lesioned Per1-, 2- and 3-mutant mice administered methamphetamine, and found that none of the mice displayed a short (21-h) period, demonstrating that no single Period gene is responsible for the short-period MASCO rhythm of Per1(-/-) /Per2(-/-) /Per3(-/-) mice. We also found that the periods of activity rhythms in constant darkness were lengthened by methamphetamine treatment in intact wild-type, Per1(-/-) and Per3(-/-) mice but not Per2(-/-) mice, and Per2(-/-) mice had two distinct activity rhythms upon release to constant light. These data suggest that the SCN and MASCO are not coupled in Per2(-/-) mice. The MASCO rhythm in Per1(-/-) /Per2(-/-) mice in constant darkness alternated between a short (22-h) and a long (27-h) period. This pattern could result from two coupled oscillators that are not synchronised to each other, or from a single oscillator displaying birhythmicity. Finally, we propose a working model of the in vivo relationship between MASCO and the SCN that poses testable hypotheses for future studies.

Site of origin of and sex differences in the vasopressin innervation of the mouse (Mus musculus) brain.

Defining how arginine vasopressin (AVP) acts centrally to regulate homeostasis and behavior is problematic, as AVP is made in multiple nuclei in the hypothalamus (i.e., paraventricular [PVN], supraoptic [SON], and suprachiasmatic [SCN]) and extended amygdala (i.e., bed nucleus of the stria terminalis [BNST] and medial amygdala [MeA]), and these groups of neurons have extensive projections throughout the brain. To understand the function of AVP, it is essential to know the site of origin of various projections. In mice, we used gonadectomy to eliminate gonadal steroid hormone-dependent expression of AVP in the BNST and MeA and electrolytic lesions to eliminate the SCN, effectively eliminating those AVP-immunoreactive projections; we also quantified AVP-immunoreactive fiber density in gonadectomized and sham-operated male and female mice to examine sex differences in AVP innervation. Our results suggest that the BNST/MeA AVP system innervates regions containing major modulatory neurotransmitters (e.g., serotonin and dopamine) and thus may be involved in regulating behavioral state. Furthermore, this system may be biased toward the regulation of male behavior, given the numerous regions in which males have a denser AVP-immunoreactive innervation than females. AVP from the SCN is found in regions important for the regulation of hormone output and behavior. Innervation from the PVN and SON is found in brain regions that likely work in concert with the well-known peripheral AVP actions of controlling homeostasis and stress response; female-biased sex differences in this system may be related to the heightened stress response observed in females.

Role of L-carnosine in the control of blood glucose, blood pressure, thermogenesis, and lipolysis by autonomic nerves in rats: involvement of the circadian clock and histamine.

L-carnosine (ß-alanyl-L-histidine; CAR) is synthesized in mammalian skeletal muscle. Although the physiological roles of CAR have not yet been clarified, there is evidence that the release of CAR from skeletal muscle during physical exercise affects autonomic neurotransmission and physiological functions. In particular, CAR affects the activity of sympathetic and parasympathetic nerves innervating the adrenal glands, liver, kidney, pancreas, stomach, and white and brown adipose tissues, thereby causing changes in blood pressure, blood glucose, appetite, lipolysis, and thermogenesis. CAR-mediated changes in neurotransmission and physiological functions were eliminated by histamine H1 or H3 receptor antagonists (diphenhydramine or thioperamide) and bilateral lesions of the hypothalamic suprachiasmatic nucleus (SCN), a master circadian clock. Moreover, a carnosine-degrading enzyme (carnosinase 2) was shown to be localized to histamine neurons in the hypothalamic tuberomammillary nucleus (TMN). Thus, CAR released from skeletal muscle during exercise may be transported into TMN-histamine neurons and hydrolyzed. The resulting L-histidine may subsequently be converted into histamine, which could be responsible for the effects of CAR on neurotransmission and physiological function. Thus, CAR appears to influence hypoglycemic, hypotensive, and lipolytic activity through regulation of autonomic nerves and with the involvement of the SCN and histamine. These findings are reviewed and discussed in the context of other recent reports, including those on carnosine synthetases, carnosinases, and carnosine transport.

Circadian modulation of passive avoidance is not eliminated in arrhythmic hamsters with suprachiasmatic nucleus lesions.

The expression of passive avoidance (PA) learning in rats displays a daily or circadian rhythm in that optimal performance is displayed when the time of testing matches the time of training. Lesions of the suprachiasmatic nucleus (SCN) were later shown to abolish this rhythm. Using golden hamsters, we have since demonstrated similar rhythms of performance in a conditioned place avoidance (CPA) task but unlike the PA results in rats, the rhythmic expression of CPA was maintained in arrhythmic hamsters with lesions of the SCN. We determined whether PA performance in hamsters is dependent on the SCN (as in the rat) or independent (as in the hamster CPA). Performance on the PA task was rhythmic in intact control animals with optimal performance occurring when training and testing time matched and significantly diminished at both 6h before and 6h after training time. SCN-lesions, verified by the loss of behavioral circadian rhythms, had no effect on the rhythmic expression. Therefore, time of day modulation of PA performance in the hamster does not depend on the SCN circadian clock.

The diurnal oscillation of MAP (mitogen-activated protein) kinase and adenylyl cyclase activities in the hippocampus depends on the suprachiasmatic nucleus.

Consolidation of hippocampus-dependent memory is dependent on activation of the cAMP/Erk/MAPK (mitogen-activated protein kinase) signal transduction pathway in the hippocampus. Recently, we discovered that adenylyl cyclase and MAPK activities undergo a circadian oscillation in the hippocampus and that inhibition of this oscillation impairs contextual memory. This suggests the interesting possibility that the persistence of hippocampus-dependent memory depends upon the reactivation of MAPK in the hippocampus during the circadian cycle. A key unanswered question is whether the circadian oscillation of this signaling pathway is intrinsic to the hippocampus or is driven by the master circadian clock in the suprachiasmatic nucleus (SCN). To address this question, we ablated the SCN of mice by electrolytic lesion and examined hippocampus-dependent memory as well as adenylyl cyclase and MAPK activities. Electrolytic lesion of the SCN 2 d after training for contextual fear memory reduced contextual memory measured 2 weeks after training, indicating that maintenance of contextual memory depends on the SCN. Spatial memory was also compromised in SCN-lesioned mice. Furthermore, the diurnal oscillation of adenylyl cyclase and MAPK activities in the hippocampus was destroyed by lesioning of the SCN. These data suggest that hippocampus-dependent long-term memory is dependent on the SCN-controlled oscillation of the adenylyl cyclase/MAPK pathway in the hippocampus.

Circadian variations of prostaglandin E2 and F2 alpha release in the golden hamster retina.

Circadian variations of prostaglandin E2 and F2alpha release were examined in the golden hamster retina. Both parameters showed significant diurnal variations with maximal values at midnight. When hamsters were placed under constant darkness for 48 h, the differences in prostaglandin release between subjective mid-day and subjective midnight persisted. Western blot analysis showed that cyclooxygenase (COX)-1 levels were significantly higher at midnight than at mid-day, and at subjective midnight than at subjective mid-day, whereas no changes in COX-2 levels were observed among these time points. Immunohistochemical studies indicated the presence of COX-1 and COX-2 in the inner (but not outer) retina. Circadian variations of retinal prostaglandin release were also assessed in suprachiasmatic nuclei (SCN)-lesioned animals. Significant differences in retinal prostaglandin release between subjective mid-day and subjective midnight were observed in SCN-lesioned animals. These results indicate that hamster retinal prostaglandin release is regulated by a retinal circadian clock independent from the SCN. Thus, the present results suggest that the prostaglandin/COX-1 system could be a retinal clock output or part of the retinal clock mechanism.

Day-night difference in thermoregulatory responses to olfactory stimulation.

Previously, we observed that olfactory stimulation with scent of grapefruit oil (SGFO) or scent of lavender oil (SLVO) affected, elevated or lowered brown adipose tissue temperature (BAT-T) in conscious mice, respectively. In the present study, to test the day-night difference in the actions of olfactory stimulations, we examined the responses of BAT-T and body temperature (BT) measured as the abdominal temperature to SGFO or SLVO during day-time at 14:00 and night-time at 2:00 in conscious rats. In the light period, BAT-T and BT were suppressed after SLVO and elevated after SGFO whereas in the dark period, these parameters remained unchanged with olfactory stimulations. Bilateral lesions of the hypothalamic suprachiasmatic nucleus (SCN) eliminated the effects of olfactory stimulations with SGFO and SVLO on BAT-T and BT. Moreover, sympathetic nerve activity innervating brown adipose tissue (BAT-SNA) changes after SGFO or SLVO were abolished in SCN-lesioned rats. Thus, we concluded that there is day-night difference in the effects of SGFO or SLVO on BAT-T and BT, and that the SCN might be involved in these effects.

Effects of suprachiasmatic nucleus lesions on habituation of the head-shake response.

Habituation is a form of non-associative learning that is characterized by a decrease in responsiveness to a repeatedly presented stimulus. A useful model of mammalian habituation is the head-shake response (HSR), a rapid twisting of the head about the anterior-to-posterior axis elicited by a stream of air to the ear. The behavioral properties of HSR habituation include sensitivity to rate of stimulus presentation and a very predictable pattern of spontaneous recovery, suggesting that a neural timing mechanism is involved. One possible candidate is the suprachiasmatic nucleus (SCN) of the hypothalamus which utilizes "clock genes" to generate daily rhythms in behavior. To test this hypothesis, the effects of SCN lesions on habituation and recovery of the HSR were assessed across four inter-session intervals (ISI: 5 min, 2, 24, and 48 h) in rats. SCN-lesioned animals showed a significant decrease in responsiveness within sessions and impaired spontaneous recovery with the 24h ISI condition. The present findings suggest that the SCN may mediate temporal patterning of spontaneous recovery from habituation and is necessary in order to appropriately reset the animal to its pre-habituation level of responsiveness.

Effects of olfactory stimulations with scents of grapefruit and lavender oils on renal sympathetic nerve and blood pressure in Clock mutant mice.

Previously, we observed that in mice, olfactory stimulation with scent of grapefruit oil elevates renal sympathetic nerve activity and blood pressure. In contrast, olfactory stimulation with scent of lavender oil has opposite effects in mice. Moreover, electrolytic lesions of the mouse hypothalamic suprachiasmatic nucleus eliminated changes in renal sympathetic nerve activity and blood pressure induced by either scent of grapefruit oil or scent of lavender oil. Here, we show that grapefruit oil-induced elevations in renal sympathetic nerve activity and blood pressure were not observed in Clock mutant mice, which harbor mutations in Clock and lack normal circadian rhythms, whereas lavender oil-suppressions were preserved in Clock mutant mice. In addition, responses of c-Fos inductions in the suprachiasmatic nucleus and paraventricular nucleus of the hypothalamus to scent of grapefruit oil observed in wild-type mice were not observed in Clock mutant mice. These findings suggest that the Clock gene might be implicated in elevating responses of autonomic and cardiovascular functions to olfactory stimulation with scent of grapefruit oil.

Temporal profile of circadian clock gene expression in a transplanted suprachiasmatic nucleus and peripheral tissues.

The mammalian hypothalamic suprachiasmatic nucleus (SCN) is the master oscillator that regulates the circadian rhythms of the peripheral oscillators. Previous studies have demonstrated that the transplantation of embryonic SCN tissues into SCN-lesioned arrhythmic mice restores the behavioral circadian rhythms of these animals. In our present study, we examined the clock gene expression profiles in a transplanted SCN and peripheral tissues, and also analysed the circadian rhythm of the locomotor activity in SCN-grafted mice. These experiments were undertaken to elucidate whether the transplanted SCN generates a dynamic circadian oscillation and maintains the phase relationships that can be detected in intact mice. The grafted SCN indeed showed dynamic circadian expression rhythms of clock genes such as mPeriod1 (mPer1) and mPeriod2 (mPer2). Furthermore, the phase differences between the expression rhythms of these genes in the grafted SCN and the locomotor activity rhythms of the transplanted animals were found to be very similar to those in intact animals. Moreover, in the liver, kidney and skeletal muscles of the transplanted animals, the phase angles between the circadian rhythm of the grafted SCN and that of the peripheral tissues were maintained as in intact animals. However, in the SCN-grafted animals, the amplitudes of the mPer1 and mPer2 rhythms were attenuated in the peripheral tissues. Our current findings therefore indicate that a transplanted SCN has the capacity to generate a dynamic intrinsic circadian oscillation, and can also lock the normal phase angles among the SCN, locomotor activity and peripheral oscillators in a similar manner as in intact control animals.

Mechanism of changes induced in plasma glycerol by scent stimulation with grapefruit and lavender essential oils.

In a previous study, we found that stimulation with scent of grapefruit oil (SGFO) elevated plasma glycerol levels in rats. However, stimulation with scent of lavender oil (SLVO) triggered a negative effect. To identify the mechanism of these changes during lipolysis, we examined the role of autonomic blockers and bilateral lesions of the hypothalamic suprachiasmatic nucleus (SCN) in the modification of plasma glycerol in rats exposed to SGFO and SLVO. We found that intraperitoneal injection of propranolol hydrochloride and atropine sulfate eliminated the changes in plasma glycerol levels induced by SGFO and SLVO, respectively. Bilateral lesions of the SCN completely abolished the effects of SGFO and SLVO on lipolysis. In addition, we investigated tyrosine phosphorylation of the transmembrane glycoprotein BIT (a brain immunoglobulin-like molecule with tyrosine-based activation motifs, a member of the signal-regulator protein family), which was found to be involved in the activation of renal sympathetic nerves and increase in body temperature on cold exposure. SGFO was found to enhance the immunoreactivity of BIT to the 4G10 anti-phosphotyrosine antibody in the SCN, whereas SLVO decreased the immunoreactivity. The changes in BIT phosphorylation resulting from the exposure to SGFO and SLVO were eliminated by the corresponding histamine receptor antagonists, which eliminated the changes in plasma glycerol concentration. The results suggest that SGFO and SLVO affect the autonomic neurotransmission and lipolysis. The SCN and histamine neurons are involved in the lipolytic responses to SGFO and SLVO, and tyrosine phosphorylation of BIT is implicated in the relevant signaling pathways.

In vivo effects of leptin on autonomic nerve activity and lipolysis in rats.

Leptin, a 16-kDa protein, is produced by white adipose tissue (WAT), and is thought to serve as a feedback signal indicating the size of fat stores. Considerable amount of data have shown that leptin can mediate lipid metabolism. However, its possible direct effects on the metabolism of lipids in vivo and the mechanisms involved have not been fully characterized. In this study, we investigated the in vivo effects of leptin on the autonomic nerve activity and lipolysis. We found that intravenous administration of leptin (10 microg/rat) excited the sympathetic nerves innervating WAT, and this effect was abolished by the pretreatment with diphenhydramine, a histamine H(1) receptor antagonist. Moreover, intraperitoneal administration of leptin (130 microg/kg) elevated the levels of plasma glycerol and free fatty acid (FFA). The effect of leptin on plasma FFA was eliminated by pretreatment with diphenhydramine and propranolol, a beta-adrenergic receptor blocker, and disappeared in suprachiasmatic nucleus (SCN)-lesioned rats. Our results suggest that leptin might regulate the lipolytic processes in adipose tissue through facilitation of the sympathetic nerves, driven by histamine neurons through the H(1) receptor, and a beta-adrenergic receptor, probably the beta(3)-receptor, is involved in the lipolytic response to leptin. The actions of leptin in this study are supposed to be controlled by the SCN.

Physiological importance of a circadian clock outside the suprachiasmatic nucleus.

Circadian clocks are widely distributed in mammalian tissues, but little is known about the physiological functions of clocks outside the suprachiasmatic nucleus of the brain. The retina has an intrinsic circadian clock, but its importance for vision is unknown. Here, we show that mice lacking Bmal1, a gene required for clock function, had abnormal retinal transcriptional responses to light and defective inner retinal electrical responses to light, but normal photoreceptor responses to light and retinas that appeared structurally normal as observed by light and electron microscopy. We generated mice with a retina-specific genetic deletion of Bmal1, and they had defects of retinal visual physiology essentially identical to those of mice lacking Bmal1 in all tissues and lacked a circadian rhythm of inner retinal electrical responses to light. Our findings indicate that the intrinsic circadian clock of the retina regulates retinal visual processing in vivo.

A circadian clock in the olfactory bulb controls olfactory responsivity.

Recently, it has been shown that multiple mammalian cell types express daily rhythms in vitro. Although the suprachiasmatic nucleus (SCN) of the hypothalamus is known to regulate a wide range of circadian behaviors, the role for intrinsic rhythmicity in other tissues is unknown. We tested whether the main olfactory bulb (OB) of mice mediates daily changes in olfaction. We found circadian rhythms in cedar oil-induced c-Fos, a protein marker of cellular excitation, in the mitral and granular layers of the OB and in the piriform cortex (PC). These oscillations persisted in constant darkness with a fourfold change in amplitude and a peak approximately 4 h after the onset of daily locomotor activity. Electrolytic lesions of the SCN abolished circadian locomotor rhythms, but not odor-induced c-Fos rhythms in the OB or PC. Furthermore, removal of the OB abolished spontaneous circadian cycling of c-Fos in the PC, shortened the free-running period of locomotor rhythms, and accelerated re-entrainment after a 6 h advance and slowed re-entrainment after a 6 h delay in the light schedule. OB ablation or odorant altered the amplitude of c-Fos rhythms in the SCN and ablation of one OB abolished c-Fos rhythms in the ipsilateral PC, but not in the contralateral OB and PC. We conclude that the OB comprises a master circadian pacemaker, which enhances olfactory responsivity each night, drives rhythms in the PC, and interacts with the SCN to coordinate other daily behaviors.

Effect of lesioning the suprachiasmatic nuclei on behavioral despair in rats.

The suprachiasmatic nucleus (SCN) is involved in regulating many biological rhythms. Several lines of research implicate the SCN in affective behavior. The SCN is directly involved in regulating the daily rhythms of the hypothalamo-pituitary-adrenal (HPA) axis hormones involved in stress. Bilateral lesions of the SCN disrupt both the rhythms and the basal levels of the HPA axis hormones involved in coping with stress. Moreover, stress can affect the biological rhythms regulated by the SCN, and disruption of biological rhythms in turn can cause stress. The present study assessed the effect of bilateral destruction of the SCN on behavioral despair, an animal model of depression sensitive to antidepressant treatment. The results indicate that bilateral destruction of the SCN results in reduced immobility in the second forced swimming test (FST) compared to sham controls and animals with incomplete lesions. These results indicate that bilateral destruction of the SCN has a protective effect in the induction of behavioral despair which may arise out of disruption of the secretion of the HPA axis hormones and/or of the neural connections between the SCN and the limbic structures that modulate the response to swim stress.

Disruption of circadian coordination accelerates malignant growth in mice.

An animal model (mice B6D2F1) was developed to study the consequence of suprachiasmatic nuclei (SCN) destruction on tumor growth. SCN destruction abolished the rest-activity and body temperature rhythms and markedly altered the rhythms in serum corticosterone concentration and lymphocyte count. Tumor growth was faster in mice with lesioned SCN than in controls for both tumor models studied, Glasgow osteosarcoma (GOS) and pancreatic adenocarcinoma (P03). This shows that disruption of circadian coordination accelerates malignant growth in mice, suggesting that the host circadian clock controls tumor progression.

Suprachiasmatic control of melatonin synthesis in rats: inhibitory and stimulatory mechanisms.

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).