Putative excitatory amino acid projections to the suprachiasmatic nucleus in the rat

Synchronization of Biological Clock Neurons by Light and Peripheral Feedback Systems Promotes Circadian Rhythms and Health

In mammals, the suprachiasmatic nucleus (SCN) functions as a circadian clock that drives 24-h rhythms in both physiology and behavior. The SCN is a multicellular oscillator in which individual neurons function as cell-autonomous oscillators. The production of a coherent output rhythm is dependent upon mutual synchronization among single cells and requires both synaptic communication and gap junctions. Changes in phase-synchronization between individual cells have consequences on the amplitude of the SCN’s electrical activity rhythm, and these changes play a major role in the ability to adapt to seasonal changes. Both aging and sleep deprivation negatively affect the circadian amplitude of the SCN, whereas behavioral activity (i.e., exercise) has a positive effect on amplitude. Given that the amplitude of the SCN’s electrical activity rhythm is essential for achieving robust rhythmicity in physiology and behavior, the mechanisms that underlie neuronal synchronization warrant further study. A growing body of evidence suggests that the functional integrity of the SCN contributes to health, well-being, cognitive performance, and alertness; in contrast, deterioration of the 24-h rhythm is a risk factor for neurodegenerative disease, cancer, depression, and sleep disorders.

Leptin normalizes photic synchronization in male ob/ob mice, via indirect effects on the suprachiasmatic nucleus

Mounting evidence indicates a strong link between metabolic diseases and circadian dysfunctions. The metabolic hormone leptin, substantially increased in dietary obesity, displays chronobiotic properties. Here we investigated whether leptin is involved in the alteration of timing associated with obesity, via direct or indirect effects on the suprachiasmatic nucleus (SCN), the site of the master clock. Photic synchronization was studied in obese ob/ob mice (deficient in leptin), either injected or not with high doses of recombinant murine leptin (5 mg/kg). This was performed first at a behavioral level, by shifting the light-dark cycle and inducing phase shifts by 30-minute light pulses and then at molecular levels (c-FOS and P-ERK1/2). Moreover, to characterize the targets mediating the chronomodulatory effects of leptin, we studied the induction of phosphorylated signal transducer and activator of transcription 3 (P-STAT3) in the SCN and in different structures projecting to the SCN, including the medial hypothalamus. Ob/ob mice showed altered photic synchronization, including augmented light-induced phase delays. Acute leptin treatment normalized the photic responses of the SCN at both the behavioral and molecular levels (decrease of light-induced c-FOS). Leptin-induced P-STAT3 was modulated by light in the arcuate nucleus and both the ventromedial and dorsomedial hypothalamic nuclei, whereas its expression was independent of the presence of leptin in the SCN. These results suggest an indirect action of leptin on the SCN, possibly mediated by the medial hypothalamus. Taken together, these results highlight a central role of leptin in the relationship between metabolic disturbances and circadian disruptions.

Dimorphic effects of leptin on the circadian and hypocretinergic systems of mice

The hormone leptin controls food intake and body weight through its receptor in the hypothalamus, and may modulate physiological functions such as reproduction, sleep or circadian timing. In the present study, the effects of leptin on the resetting of the circadian clock, the hypothalamic suprachiasmatic nucleus (SCN) and on the activity of the hypocretinergic system were examined in vivo, with comparative analysis between male and female mice. A single leptin injection (5 mg/kg) at both the onset and offset of the activity period did not alter locomotion of mice housed under a 12 : 12 h light/dark cycle and did not shift the circadian behavioral rhythm of mice housed in constant darkness. By contrast, leptin potentiated the phase-shifting effect of a 30-min light-pulse on behavioural rhythms during the late subjective night, although only in females. This was accompanied by a higher induction of the clock genes Per1 and Per2 in the SCN. A 2-week chronic exposure to a physiological dose of leptin (100 μg/kg per day) decreased locomotor activity, expression of hypocretin receptor 1 and 2, as well as the number of hypocretin-immunoreactive neurones only in female mice, whereas the number of c-fos-positive hypocretinergic neurones was reduced in both genders. These results highlight a dimorphic effect of leptin on the hypocretinergic system and on the response of the circadian clock to light. Leptin may thus modulate the sleep/wake cycle and circadian system beside its well-established action on food intake and regulation of body weight.

Glutamate and GABA neurotransmission from the paraventricular thalamus to the suprachiasmatic nuclei in the rat

The anterior paraventricular thalamus (aPVT) projects to the SCN, and a lesion of the aPVT leads to phase delays of circadian rhythms, instead of advances, produced by light pulses at CT23. As a first step to understanding the underlying mechanism, the authors characterized the monosynaptic responses of SCN neurons to aPVT in whole-cell recordings from brain slices in rats. Stimulation of aPVT evoked excitatory and inhibitory postsynaptic potentials in SCN neurons. Pharmacological isolation of such components indicated that the excitatory postsynaptic potential (EPSP) involves AMPA and NMDA glutamate receptors while the inhibitory postsynaptic potential (IPSP) involves GABA( A) receptors. Since the SCN comprises mostly GABA neurons, the persistence of IPSP after the blockade of glutamate receptors ruled out the possibility that GABA was released from SCN interneurons responsive to glutamate released from the paraventricular thalamus. Altogether, the present evidence demonstrates that glutamate and GABA are released in synapses between aPVT and the SCN.

Interactions between light, mealtime and calorie restriction to control daily timing in mammals

Daily variations in behaviour and physiology are controlled by a circadian timing system consisting of a network of oscillatory structures. In mammals, a master clock, located in the suprachiasmatic nuclei (SCN) of the hypothalamus, adjusts timing of other self-sustained oscillators in the brain and peripheral organs. Synchronisation to external cues is mainly achieved by ambient light, which resets the SCN clock. Other environmental factors, in particular food availability and time of feeding, also influence internal timing. Timed feeding can reset the phase of the peripheral oscillators whilst having almost no effect in shifting the phase of the SCN clockwork when animals are exposed (synchronised) to a light-dark cycle. Food deprivation and calorie restriction lead not only to loss of body mass (>15%) and increased motor activity, but also affect the timing of daily activity, nocturnal animals becoming partially diurnal (i.e. they are active during their usual sleep period). This change in behavioural timing is due in part to the fact that metabolic cues associated with calorie restriction affect the SCN clock and its synchronisation to light.

Robust food anticipatory circadian rhythms in rats with complete ablation of the thalamic paraventricular nucleus

Rats can anticipate a fixed daily mealtime by entrainment of a circadian timekeeping mechanism anatomically separate from the light-entrainable circadian pacemaker located in the suprachiasmatic nucleus. Neural substrates of this food-entrainable circadian system have not yet been fully elucidated. A role for the thalamic paraventricular nucleus (PVT) is suggested by observations that scheduled feeding synchronizes daily rhythms of glucose utilization and immediate early gene and circadian clock gene expression in this area. One study has reported absence of food anticipatory circadian activity rhythms in rats with PVT ablations. To determine whether this effect extends to other behavioral measures of food anticipation, rats received large radiofrequency lesions aimed at the PVT and were maintained on a 3-h meal provided each day 6 h after lights-on. Rats with unambiguously complete PVT ablation exhibited increased total daily activity, a change in the waveform of the nocturnal activity rhythm, but no change in the amplitude, duration, latency to appearance or persistence during total food deprivation of food anticipatory activity measured by activity at or near a food bin accessible via a small window in the recording cage. These results indicate that, while the PVT may modulate light-entrainable rhythms, it is not a critical input, oscillator or output component of the circadian system by which rats behaviorally anticipate a daily mealtime.

The circadian visual system, 2005

The primary mammalian circadian clock resides in the suprachiasmatic nucleus (SCN), a recipient of dense retinohypothalamic innervation. In its most basic form, the circadian rhythm system is part of the greater visual system. A secondary component of the circadian visual system is the retinorecipient intergeniculate leaflet (IGL) which has connections to many parts of the brain, including efferents converging on targets of the SCN. The IGL also provides a major input to the SCN, with a third major SCN afferent projection arriving from the median raphe nucleus. The last decade has seen a blossoming of research into the anatomy and function of the visual, geniculohypothalamic and midbrain serotonergic systems modulating circadian rhythmicity in a variety of species. There has also been a substantial and simultaneous elaboration of knowledge about the intrinsic structure of the SCN. Many of the developments have been driven by molecular biological investigation of the circadian clock and the molecular tools are enabling novel understanding of regional function within the SCN. The present discussion is an extension of the material covered by the 1994 review, "The Circadian Visual System."

A daily palatable meal without food deprivation entrains the suprachiasmatic nucleus of rats

Food is considered a potent Zeitgeber for peripheral oscillators but not for the suprachiasmatic nucleus (SCN), which is entrained principally by the light-dark cycle. However, when food attains relevant properties in quantity and quality, it can be a potent Zeitgeber even for the SCN. Here we evaluated the entrainment influence of a daily palatable meal, without regular food deprivation, on the circadian rhythm of locomotor activity and the c-Fos and PER-1 protein expression in the SCN. Rats fed ad libitum, in constant darkness, received a palatable meal for 6 weeks starting in the middle of the subjective day. Locomotor activity showed entrainment when the offset of activity coincided with the palatable meal-time. In the SCN, the peak expression of c-Fos was observed at palatable meal-time and PER-1 showed a peak during the onset of subjective night, as predicted according to the behavioural entrained pattern. In addition, c-Fos and PER-1 expression in the paraventricular thalamic nucleus (PVT) showed increased expression at palatable meal-time, while the intergeniculate leaflet did not, suggesting that the PVT may be involved as an input pathway of palatable food-entrainment to the SCN. These results demonstrate that daily access to a palatable meal can entrain the SCN; several stimuli can be implicated in this process, including motivation and arousal.

In search of the pathways for light-induced pacemaker resetting in the suprachiasmatic nucleus

Within the suprachiasmatic nucleus (SCN) of the mammalian hypothalamus is a circadian pacemaker that functions as a clock. Its endogenous period is adjusted to the external 24-h light-dark cycle, primarily by light-induced phase shifts that reset the pacemaker's oscillation. Evidence using a wide variety of neurobiological and molecular genetic tools has elucidated key elements that comprise the visual input pathway for SCN photoentrainment in rodents. Important questions remain regarding the intracellular signals that reset the autoregulatory molecular loop within photoresponsive cells in the SCN's retino-recipient subdivision, as well as the intercellular coupling mechanisms that enable SCN tissue to generate phase shifts of overt behavioral and physiological circadian rhythms such as locomotion and SCN neuronal firing rate. Multiple neurotransmitters, protein kinases, and photoinducible genes add to system complexity, and we still do not fully understand how dawn and dusk light pulses ultimately produce bidirectional, advancing and delaying phase shifts for pacemaker entrainment.

Anterior paraventricular thalamus modulates light-induced phase shifts in circadian rhythmicity in rats

The reciprocal connections between the paraventricular thalamic nucleus (PVT) and the suprachiasmatic nuclei suggest that PVT may participate in the regulation of circadian rhythms. We studied in rats the effect of lesions of the anterior and midposterior regions of the PVT on phase shifts of drinking circadian rhythm induced by light pulses at circadian times 6, 12, and 23, as well as the phase shifts produced by electrical or glutamatergic stimulation of the anterior PVT at the same circadian times. Lesion of the anterior PVT abolishes the advances induced by light during late subjective night, whereas midposterior PVT lesions did not affect the phase shifts. Electrical stimulation or glutamate injections in the anterior PVT mimic the phase-shifting effects of light pulses. These results indicate the participation of the anterior PVT as a modulator of entrainment of circadian rhythms to light.

Olfactory stimulation enhances light-induced phase shifts in free-running activity rhythms and Fos expression in the suprachiasmatic nucleus

There is evidence to suggest that the olfactory and circadian systems are linked, functionally, and that olfactory stimuli can modulate circadian rhythms in mammals. Furthermore, olfactory bulb removal can alter free-running rhythms in animals housed in constant darkness and can attenuate the effect of social stimuli on photic entrainment of circadian rhythms. The mechanisms through which olfactory stimuli influence circadian rhythms are not known. One possibility is that olfactory stimuli influence circadian rhythms by modulating the activity of the circadian clock located in the hypothalamic suprachiasmatic nucleus. To study this, we assessed the effect of olfactory stimulation on free-running rhythms and on photic resetting of the circadian clock in rats using phase shifts in wheel-running rhythms and expression of the transcription factor Fos in the suprachiasmatic nucleus. We found that brief exposure to an olfactory stimulus, cedar wood essence, in the subjective day or subjective night had no effect on either free-running rhythms or Fos expression in the suprachiasmatic nucleus, but that when presented in combination with light, the odor dramatically enhanced light-induced phase shifts and Fos expression in the suprachiasmatic nucleus. Olfactory stimulation alone induced Fos expression in several structures that innervate the suprachiasmatic nucleus, pointing to ways by which stimulus information transmitted in the olfactory pathways could gain access to the suprachiasmatic nucleus to modulate photic resetting. These findings, showing that clock resetting by light can be facilitated by olfactory stimulation, point to a mechanism by which olfactory cues can modulate entrainment of circadian rhythms.

Gold-thioglucose-induced hypothalamic lesions inhibit metabolic modulation of light-induced circadian phase shifts in mice

The circadian clock located in the suprachiasmatic nuclei is entrained by the 24-h variation in light intensity. The clock's responses to light can, however, be reduced when glucose availability is decreased. We tested the hypothesis that the ventromedial hypothalamus, a key area in the integration of metabolic and hormonal signals, mediates the metabolic modulation of circadian responses to light by injecting C57BL/6J mice with gold-thioglucose (0.6 g/kg) which damages glucose-receptive neurons, primarily located in the ventromedial hypothalamus. Light pulses applied during the mid-subjective night induce phase delays in the circadian rhythm of locomotor activity in mice kept in constant darkness. As previously observed, light-induced phase delays were significantly attenuated in fed mice pre-treated with 500 mg/kg i.p. 2-deoxy-D-glucose and in hypoglycemic mice fasted for 30 h, pre-treated with 5 IU/kg s.c. insulin or saline, compared to control mice fed ad libitum. In contrast, similar metabolic challenges in mice with gold-thioglucose-induced hypothalamic lesions did not significantly affect light-induced phase delays compared to mice treated with gold-thioglucose and fed ad libitum. These results indicate that destruction of gold-thioglucose-sensitive neurons in the ventromedial hypothalamus prevent metabolic regulation of circadian responses to light during shortage of glucose availability. Therefore, the ventromedial hypothalamus may be a central site coordinating the metabolic modulation of light-induced phase shifts of the circadian clock.

Serotonin and the regulation of mammalian circadian rhythmicity

The suprachiasmatic nucleus (SCN), the site of the primary mammalian circadian clock, contains one of the densest serotonergic terminal plexes in the brain. Although this fact has been appreciated for some time, only in the last decade has there been substantial approach toward the understanding of the function of serotonin in the circadian rhythm system. The intergeniculate leaflet, which projects to the SCN via the geniculohypothalamic tract, receives serotonergic innervation from the dorsal raphe nucleus, and the SCN receives its serotonergic input from the median raphe nucleus. This separation of serotonergic origins provides the opportunity to investigate the function of the two projections. Loss of serotonergic neurones of the median raphe yields earlier onset and later offset of the nocturnal activity phase, longer duration of the activity phase, and increased sensitivity of circadian rhythm response to light. Despite the simplicity of the origins of serotonergic anatomy with respect to the circadian rhythm system, the actual involvement of serotonin in rhythm modulation is not so obvious. A variety of pharmacological studies have clearly implicated serotonin as a direct regulator of circadian rhythm phase, but others employing different methods suggest that simple elevation of SCN serotonin concentrations does not modify rhythm phase. The most convincing role of serotonin is its apparent ability to modulate sensitivity of the circadian rhythm to light. The putative method for such modulation is via a presynaptic 5-HT1B receptor on the retinohypothalamic tract, the activation of which attenuates photic input to the SCN thereby reducing phase response to light. Serotonin may modulate phase response to benzodiazepines, but does not appear to modify such response to environmentally induced locomotor activity. Current interest in serotonergic modulation of circadian rhythmicity is strong and the research is vigorous. There is an abundance of information about serotonin and circadian rhythm function that lacks a satisfactory framework for its interpretation. The next decade is likely to see the gradual evolution of this framework as the role of serotonin in circadian rhythm regulation is further elucidated.

Lesions of glucose-responsive neurons impair synchronizing effects of calorie restriction in mice

Calorie restriction can induce phase-advances of daily rhythms in rodents exposed to light-dark cycles. To test whether glucose-responsive neurons are involved in the synchronizing effects of calorie restriction, C57BL/6J mice were injected with gold-thioglucose (GTG; 0.6 g/kg) which damages glucose-responsive neurons, primarily located in the ventromedial hypothalamus. From the day of injection, GTG-treated and control mice received a hypocaloric diet (66% of ad libitum food intake) 2 h after lights on. When mice were transferred to constant darkness after 4 weeks and fed ad libitum, the onset of circadian rhythm of locomotor activity was phase-advanced by 1 h in control but not in GTG-treated mice. Therefore, glucose-responsive neurons in the ventromedial hypothalamus may play a role in the synchronizing effects of calorie restriction on circadian rhythmicity.

Immunocytochemical localization of the mGluR1b metabotropic glutamate receptor in the rat hypothalamus

The mGluR1 metabotropic glutamate receptor is a G-protein-coupled receptor that exists as different C-terminal splice variants. When expressed in mammalian cells, the mGluR1 splice variants exhibit diverse transduction mechanisms and also slightly differ in their apparent agonist affinities. In the present study, we used an affinity-purified antiserum, specifically reactive to the mGluRlb splice variant, in combination with a highly sensitive preembedding immunocytochemical method for light microscopy to investigate the distribution of this receptor in the rat hypothalamus. An intense immunoreactivity for mGluRlb was observed in distinct hypothalamic nuclei. Thus, neuronal cell bodies and dendrites were stained in the preoptic area, suprachiasmatic nucleus, dorsal hypothalamus, lateral hypothalamus, dorsomedial nucleus, tuberomammilary nucleus, and lateral mammilary body. The ventromedial nucleus exhibited neuropil immunostaining but neuronal cell bodies were not labeled. Strong mGluRlb immunoreactivity was observed in magnocellular neurons of the neuroendocrine supraoptic, paraventricular, and arcuate nuclei. Also, neuronal cell bodies were heavily labeled in the retrochiasmatic nucleus, anterior commissural nucleus, and periventricular nucleus. These immunocytochemical observations, together with previous studies, suggest that mGluRlb is coexpressed with other class I mGluRs in some nuclei throughout the hypothalamus. However, mGluRlb is so far the only receptor of this class strongly expressed in the supraoptic, paraventricular, and arcuate nuclei, which might have relevant implications in the physiological control of the neuroendocrine hypothalamic-pituitary system.

Organization of neural inputs to the suprachiasmatic nucleus in the rat

The circadian timing of the suprachiasmatic nucleus (SCN) is modulated by its neural inputs. In the present study, we examine the organization of the neural inputs to the rat SCN using both retrograde and anterograde tracing methods. After Fluoro-Gold injections into the SCN, retrogradely labeled neurons are present in a number of brain areas, including the infralimbic cortex, the lateral septum, the medial preoptic area, the subfornical organ, the paraventricular thalamus, the subparaventricular zone, the ventromedial hypothalamic nucleus, the posterior hypothalamic area, the intergeniculate leaflet, the olivary pretectal nucleus, the ventral subiculum, and the median raphe nuclei. In the anterograde tracing experiments, we observe three patterns of afferent termination within the SCN that correspond to the photic/raphe, limbic/hypothalamic, and thalamic inputs. The median raphe projection to the SCN terminates densely within the ventral subdivision and sparsely within the dorsal subdivision. Similarly, areas that receive photic input, such as the retina, the intergeniculate leaflet, and the pretectal area, densely innervate the ventral SCN but provide only minor innervation of the dorsal SCN. A complementary pattern of axonal labeling, with labeled fibers concentrated in the dorsal SCN, is observed after anterograde tracer injections into the hypothalamus and into limbic areas, such as the ventral subiculum and infralimbic cortex. A third, less common pattern of labeling, exemplified by the paraventricular thalamic afferents, consists of diffuse axonal labeling throughout the SCN. Our results show that the SCN afferent connections are topographically organized. These hodological differences may reflect a functional heterogeneity within the SCN.