The NPY intergeniculate leaflet projections to the suprachiasmatic nucleus transmit metabolic conditions

Irisin/BDNF signaling in the muscle-brain axis and circadian system: A review

In mammals, the timing of physiological, biochemical and behavioral processes over a 24-h period is controlled by circadian rhythms. To entrain the master clock located in the suprachiasmatic nucleus of the hypothalamus to a precise 24-h rhythm, environmental zeitgebers are used by the circadian system. This is done primarily by signals from the retina via the retinohypothalamic tract, but other cues like exercise, feeding, temperature, anxiety, and social events have also been shown to act as non-photic zeitgebers. The recently identified myokine irisin is proposed to serve as an entraining non-photic signal of exercise. Irisin is a product of cleavage and modification from its precursor membrane fibronectin type Ⅲ domain-containing protein 5 (FNDC5) in response to exercise. Apart from well-known peripheral effects, such as inducing the "browning" of white adipocytes, irisin can penetrate the blood-brain barrier and display the effects on the brain. Experimental data suggest that FNDC5/irisin mediates the positive effects of physical activity on brain functions. In several brain areas, irisin induces the production of brain-derived neurotrophic factor (BDNF). In the master clock, a significant role in gating photic stimuli in the retinohypothalamic synapse for BDNF is suggested. However, the brain receptor for irisin remains unknown. In the current review, the interactions of physical activity and the irisin/BDNF axis with the circadian system are reconceptualized.

The interplay between macronutrients and sleep: focus on circadian and homeostatic processes

Sleep disturbances are an emerging risk factor for metabolic diseases, for which the burden is particularly worrying worldwide. The importance of sleep for metabolic health is being increasingly recognized, and not only the amount of sleep plays an important role, but also its quality. In this review, we studied the evidence in the literature on macronutrients and their influence on sleep, focusing on the mechanisms that may lay behind this interaction. In particular, we focused on the effects of macronutrients on circadian and homeostatic processes of sleep in preclinical models, and reviewed the evidence of clinical studies in humans. Given the importance of sleep for health, and the role of circadian biology in healthy sleep, it is important to understand how macronutrients regulate circadian clocks and sleep homeostasis.

Metabolic cues impact non-oscillatory intergeniculate leaflet and ventral lateral geniculate nucleus: standard versus high-fat diet comparative study

The intergeniculate leaflet and ventral lateral geniculate nucleus (IGL/VLG) are subcortical structures involved in entrainment of the brain's circadian system to photic and non-photic (e.g. metabolic and arousal) cues. Both receive information about environmental light from photoreceptors, exhibit infra-slow oscillations (ISO) in vivo, and connect to the master circadian clock. Although current evidence demonstrates that the IGL/VLG communicate metabolic information and are crucial for entrainment of circadian rhythms to time-restricted feeding, their sensitivity to food intake-related peptides has not been investigated yet. We examined the effect of metabolically relevant peptides on the spontaneous activity of IGL/VLG neurons. Using ex vivo and in vivo electrophysiological recordings as well as in situ hybridisation, we tested potential sensitivity of the IGL/VLG to anorexigenic and orexigenic peptides, such as cholecystokinin, glucagon-like peptide 1, oxyntomodulin, peptide YY, orexin A and ghrelin. We explored neuronal responses to these drugs during day and night, and in standard vs. high-fat diet conditions. We found that IGL/VLG neurons responded to all the substances tested, except peptide YY. Moreover, more neurons responded to anorexigenic drugs at night, while a high-fat diet affected the IGL/VLG sensitivity to orexigenic peptides. Interestingly, ISO neurons responded to light and orexin A, but did not respond to the other food intake-related peptides. In contrast, non-ISO cells were activated by metabolic peptides, with only some being responsive to light. Our results show for the first time that peptides involved in the body's energy homeostasis stimulate the thalamus and suggest functional separation of the IGL/VLG cells. KEY POINTS: The intergeniculate leaflet and ventral lateral geniculate nucleus (IGL/VLG) of the rodent thalamus process various signals and participate in circadian entrainment. In both structures, cells exhibiting infra-slow oscillatory activity as well as non-rhythmically firing neurons being observed. Here, we reveal that only one of these two groups of cells responds to anorexigenic (cholecystokinin, glucagon-like peptide 1 and oxyntomodulin) and orexigenic (ghrelin and orexin A) peptides. Neuronal responses vary depending on the time of day (day vs. night) and on the diet (standard vs. high-fat diet). Additionally, we visualised receptors to the tested peptides in the IGL/VLG using in situ hybridisation. Our results suggest that two electrophysiologically different subpopulations of IGL/VLG neurons are involved in two separate functions: one related to the body's energy homeostasis and one associated with the subcortical visual system.

Suprachiasmatic to paraventricular nuclei interaction generates normal food searching rhythms in mice

Searching for food follows a well-organized decision process in mammals to take up food only if necessary. Moreover, scavenging is preferred during their activity phase. Various time-dependent regulatory processes have been identified originating from the suprachiasmatic nuclei (SCN), which convert external light information into synchronizing output signals. However, a direct impact of the SCN on the timing of normal food searching has not yet been found. Here, we revisited the function of the SCN to affect when mice look for food. We found that this process was independent of light but modified by the palatability of the food source. Surprisingly, reducing the output from the SCN, in particular from the vasopressin releasing neurons, reduced the amount of scavenging during the early activity phase. The SCN appeared to transmit a signal to the paraventricular nuclei (PVN) via GABA receptor A1. Finally, the interaction of SCN and PVN was verified by retrograde transport-mediated complementation. None of the genetic manipulations affected the uptake of more palatable food. The data indicate that the PVN are sufficient to produce blunted food searching rhythms and are responsive to hedonistic feeding. Nevertheless, the search for normal food during the early activity phase is significantly enhanced by the SCN.

Timed restricted feeding cycles drive daily rhythms in female rats maintained in constant light but only partially restore the estrous cycle

Light at night is an emergent problem for modern society. Rodents exposed to light at night develop a loss of circadian rhythms, which leads to increased adiposity, altered immune response, and increased growth of tumors. In female rats, constant light (LL) eliminates the estrous cycle leading to a state of persistent estrus. The suprachiasmatic nucleus (SCN) drives circadian rhythms, and it interacts with the neuroendocrine network necessary for reproductive function. Timed restricted feeding (RF) exerts a powerful entraining influence on the circadian system, and it can influence the SCN activity and can restore rhythmicity or accelerate re-entrainment in experimental conditions of shift work or jet lag. The present study explored RF in female rats exposed to LL, with the hypothesis that this cyclic condition can rescue or prevent the loss of daily rhythms and benefit the expression of the estrous cycle. Two different feeding schedules were explored: 1. A 12-h food/12-h fasting schedule applied to arrhythmic rats after 3 weeks in LL, visualized as a rescue strategy (LL + RFR, 3 weeks), or applied simultaneously with the first day of LL as a preventive strategy (LL + RFP, 6 weeks). 2. A 12-h window of food intake with food given in four distributed pulses (every 3 h), applied after 3 weeks in LL, as a rescue strategy (LL + PR, 3 weeks) or applied simultaneously with the first day of LL as a preventive strategy (LL + PP, 6 weeks). Here, we present evidence that scheduled feeding can drive daily rhythms of activity and temperature in rats exposed to LL. However, the protocol of distributed feeding pulses was more efficient to restore the day–night activity and core temperature as well as the c-Fos day–night change in the SCN. Likewise, the distributed feeding partially restored the estrous cycle and the ovary morphology under LL condition. Data here provided indicate that the 12-h feeding/12-h fasting window determines the rest-activity cycle and can benefit directly the circadian and reproductive function. Moreover, this effect is stronger when food is distributed along the 12 h of subjective night.

Cyrcadian Rhythm, Mood, and Temporal Patterns of Eating Chocolate: A Scoping Review of Physiology, Findings, and Future Directions

This paper discusses the effect of chrononutrition on the regulation of circadian rhythms; in particular, that of chocolate on the resynchronization of the human internal biological central and peripheral clocks with the main external synchronizers, light–dark cycle and nutrition-fasting cycle. The desynchronization of internal clocks with external synchronizers, which is so frequent in our modern society due to the tight rhythms imposed by work, social life, and technology, has a negative impact on our psycho-physical performance, well-being, and health. Taking small amounts of chocolate, in the morning at breakfast at the onset of the active phase, helps speed up resynchronization time. The high flavonoid contents in chocolate promote cardioprotection, metabolic regulation, neuroprotection, and neuromodulation with direct actions on brain function, neurogenesis, angiogenesis, and mood. Although the mechanisms of action of chocolate compounds on brain function and mood as well as on the regulation of circadian rhythms have yet to be fully understood, data from the literature currently available seem to agree in suggesting that chocolate intake, in compliance with chrononutrition, could be a strategy to reduce the negative effects of desynchronization. This strategy appears to be easily implemented in different age groups to improve work ability and daily life.

Effects of prenatal exposure to fluoxetine on circadian rhythmicity in the locomotor activity and neuropeptide Y and 5-HT expression in male and female adult Wistar rats

Serotonin (5-HT) reuptake inhibitors, such as fluoxetine, are the most prescribed antidepressant for maternal depression. In this sense, it exposes mothers and the brains of infants to increased modulatory and trophic effects of serotonergic neurotransmission. 5-HT promotes essential brain changes throughout its development, which include neuron migration, differentiation, and organization of neural circuitries related to emotional, cognitive, and circadian behavior. Early exposure to the SSRIs induces long-term effects on behavioral and neural serotonergic signalization. We have aimed to evaluate the circadian rhythm of locomotor activity and the neurochemical content, neuropeptide Y (NPY) and 5-HT in three brain areas: intergeniculate leaflet (IGL), suprachiasmatic nuclei (SCN) and raphe nuclei (RN), at two zeitgebers (ZT6 and ZT18), in male and female rat's offspring early exposed (developmental period GD13-GD21) to fluoxetine (20mg/kg). First, we have conducted daily records of the locomotor activity rhythm using activity sensors coupled to individual cages over four weeks. We have lastly evaluated the immunoreactivity of NPY in both SCN and IGL, and as well the 5-HT expression in the dorsal and medial RN. In summary, our results showed that (1) prenatal fluoxetine affects phase entrainment of the rest/activity rhythm at ZT6 and ZT18, more in male than female specimens, and (2) modulates the NPY and 5-HT expression. Here, we show male rats are more susceptible to phase entrainment and the NPY and 5-HT misexpression compared to female ones. The sex differences induced by early exposure to fluoxetine in both the circadian rhythm of locomotor activity and the neurochemical expression into SCN, IGL, and midbrain raphe are an important highlight in the present work. Thus, our results may help to improve the knowledge on neurobiological mechanisms of circadian rhythms and are relevant to understanding the "broken brains" and behavioral abnormalities of offspring early exposed to antidepressants.

Systematic review of drugs that modify the circadian system's phase-shifting responses to light exposure

We searched PubMed for primary research quantifying drug modification of light-induced circadian phase-shifting in rodents. This search, conducted for work published between 1960 and 2018, yielded a total of 146 papers reporting results from 901 studies. Relevant articles were those with any extractable data on phase resetting in wildtype (non-trait selected) rodents administered a drug, alongside a vehicle/control group, near or at the time of exposure. Most circadian pharmacology experiments were done using drugs thought to act directly on either the brain's central pacemaker, the suprachiasmatic nucleus (SCN), the SCN's primary relay, the retinohypothalamic tract, secondary pathways originating from the medial/dorsal raphe nuclei and intergeniculate leaflet, or the brain's sleep-arousal centers. While the neurotransmitter systems underlying these circuits were of particular interest, including those involving glutamate, gamma-aminobutyric acid, serotonin, and acetylcholine, other signaling modalities have also been assessed, including agonists and antagonists of receptors linked to dopamine, histamine, endocannabinoids, adenosine, opioids, and second-messenger pathways downstream of glutamate receptor activation. In an effort to identify drugs that unduly influence circadian responses to light, we quantified the net effects of each drug class by ratioing the size of the phase-shift observed after administration to that observed with vehicle in a given experiment. This allowed us to organize data across the literature, compare the relative efficacy of one mechanism versus another, and clarify which drugs might best suppress or potentiate phase resetting. Aggregation of the available data in this manner suggested that several candidates might be clinically relevant as auxiliary treatments to suppress ectopic light responses during shiftwork or amplify the circadian effects of timed bright light therapy. Future empirical research will be necessary to validate these possibilities.

Temporal dysregulation of hypothalamic integrative and metabolic nuclei in rats fed during the rest phase

Temporal coordination of organisms according to the daytime allows a better performance of physiological processes. However, modern lifestyle habits, such as food intake during the rest phase, promote internal desynchronization and compromise homeostasis and health. The hypothalamic suprachiasmatic nucleus (SCN) synchronizes body physiology and behavior with the environmental light-dark cycle by transmitting time information to several integrative hypothalamic nuclei, such as the paraventricular nucleus (PVN), dorsomedial hypothalamic nucleus (DMH) and median preoptic area (MnPO). The SCN receives metabolic information mainly via Neuropeptide Y (NPY) inputs from the intergeniculate nucleus of the thalamus (IGL). Nowadays, there is no evidence of the response of the PVN, DMH and MnPO when the animals are subjected to internal desynchronization by restricting food access to the rest phase of the day. To explore this issue, we compared the circadian activity of the SCN, PVN, DMH and MnPO. In addition, we analyzed the daily activity of the satiety centers of the brainstem, the nucleus of the tractus solitarius (NTS) and area postrema (AP), which send metabolic information to the SCN, directly or via the thalamic intergeniculate leaflet (IGL). For that, male Wistar rats were assigned to three meal protocols: fed during the rest phase (Day Fed); fed during the active phase (Night Fed); free access to food (ad libitum). After 21 d, the daily activity patterns of these nuclei were analyzed by c-Fos immunohistochemistry, as well as NPY immunohistochemistry, in the SCN. The results show that eating during the rest period produces a phase advance in the activity of the SCN, changes the daily activity pattern in the MnPO, NTS and AP and flattens the c-Fos rhythm in the PVN and DMH. Altogether, these results validate previous observations of circadian dysregulation that occurs within the central nervous system when meals are consumed during the rest phase, a behavior that is involved in the metabolic alterations described in the literature.

Induction of internal circadian desynchrony by misaligning zeitgebers

24-h rhythms in physiology and behaviour are orchestrated by an endogenous circadian clock system. In mammals, these clocks are hierarchically organized with a master pacemaker residing in the hypothalamic suprachiasmatic nucleus (SCN). External time signals-so-called zeitgebers-align internal with geophysical time. During shift work, zeitgeber input conflicting with internal time induces circadian desynchrony which, in turn, promotes metabolic and psychiatric disorders. However, little is known about how internal desynchrony is expressed at the molecular level under chronodisruptive environmental conditions. We here investigated the effects of zeitgeber misalignment on circadian molecular organisation by combining 28-h light-dark (LD-28) cycles with either 24-h (FF-24) or 28-h feeding-fasting (FF-28) regimes in mice. We found that FF cycles showed strong effects on peripheral clocks, while having little effect on centrally coordinated activity rhythms. Systemic, i.e., across-tissue internal circadian desynchrony was profoundly induced within four days in LD-28/FF-24, while phase coherence between tissue clocks was maintained to a higher degree under LD-28/FF-28 conditions. In contrast, temporal coordination of clock gene activity across tissues was reduced under LD-28/FF-28 conditions compared to LD-28/FF-24. These results indicate that timed food intake may improve internal synchrony under disruptive zeitgeber conditions but may, at the same time, weaken clock function at the tissue level.

Modulation of the Rat Intergeniculate Leaflet of the Thalamus Network by Norepinephrine

Circadian rhythms are regulated by a set of brain structures, one of which is the Intergeniculate Leaflet of the Thalamus (IGL). The most recognised role of the IGL is the integration of a variety of stimuli affecting rhythmicity, such as lighting conditions, received by the eye, or light-independent (non-photic) cues, the information about which is delivered via the activation of the non-specific projections. One of them is the norepinephrinergic system originating in the brainstem Locus Coeruleus (LC). In order to investigate the effect of norepinephrine (NE) on the IGL neurons we have performed ex vivo recordings using the extracellular multi-electrode array technique as well as the intracellular whole-cell patch clamp. Using both agonists and antagonists of specific NE receptor subtypes, we confirmed the presence of functional α₁-, α₂- and β-adrenergic receptors within the investigated structure, allowing NE to exert multiple types of effects on different IGL neurons, mainly depolarisation of the neurons projecting to the Suprachiasmatic Nuclei - the master circadian pacemaker, and various responses exhibited by the cells creating the connection with the contralateral IGL. Moreover, NE was shown to affect IGL cells both directly and via modulation of the synaptic network, in particular the miniature inhibitory postsynaptic currents. To the best of our knowledge, these are the first studies to confirm the effects of NE on the activity of the IGL network.

Circadian actions of orexins on the retinorecipient lateral geniculate complex in rat

Key points

Rhythmic processes in living organisms are controlled by biological clocks. The orexinergic system of the lateral hypothalamus carries circadian information to provide arousal for the brain during the active phase.

Here, we show that orexins exert an excitatory action in three parts of the lateral geniculate nucleus (LGN), in particular upon directly retinorecipient neurons in the non‐image forming visual structures.

We provide evidence for the high nocturnal levels of orexins with stable circadian expression of predominant orexin receptor 2 in the LGN.

Our data additionally establish the convergence of orexinergic and pituitary adenylate cyclase (PAC)‐activating peptide/PAC1 receptor systems (used by melanopsin‐expressing retinal ganglion cells), which directly regulates responses to the retinal input.

These results help us better understand circadian orexinergic control over the non‐image forming subcortical visual system, forming the animal's preparedness for the behaviourally active night.

Abstract

The orexinergic system of the lateral hypothalamus is tightly interlinked with the master circadian clock and displays daily variation in activity to provide arousal‐related excitation for the plethora of brain structures in a circadian manner. Here, using a combination of electrophysiological, optogenetic, histological, molecular and neuronal tracing methods, we explore a particular link between orexinergic and visual systems in rat. The results of the present study demonstrate that orexinergic fibre density at the area of subcortical visual system exerts a clear day to night variability, reaching a maximum at behaviourally active night. We also show pronounced electrophysiological activations of neurons in the lateral geniculate nucleus by orexin A through 24 h, via identified distinct orexin receptors, with the ventrolateral geniculate displaying a daily cycle of responsiveness. In addition, for the first time, we provide a direct evidence for orexins to act on retinorecipient neurons with a high convergence of orexinergic and putatively retinal pituitary adenylate cyclase (PAC)‐activating peptide/PAC1 receptor systems. Altogether, the present study ties orexins to non‐image forming visual structures with implications for circadian orexinergic modulation of neurons, which process information on ambient light levels.

Rhythmic processes in living organisms are controlled by biological clocks. The orexinergic system of the lateral hypothalamus carries circadian information to provide arousal for the brain during the active phase.

Here, we show that orexins exert an excitatory action in three parts of the lateral geniculate nucleus (LGN), in particular upon directly retinorecipient neurons in the non‐image forming visual structures.

We provide evidence for the high nocturnal levels of orexins with stable circadian expression of predominant orexin receptor 2 in the LGN.

Our data additionally establish the convergence of orexinergic and pituitary adenylate cyclase (PAC)‐activating peptide/PAC1 receptor systems (used by melanopsin‐expressing retinal ganglion cells), which directly regulates responses to the retinal input.

These results help us better understand circadian orexinergic control over the non‐image forming subcortical visual system, forming the animal's preparedness for the behaviourally active night.

Blue light at night acutely impairs glucose tolerance and increases sugar intake in the diurnal rodent Arvicanthis ansorgei in a sex-dependent manner

In our modern society, the exposure to light at night (LAN) has increased considerably, which may impact human health negatively. Especially exposure to light at night containing short wavelength emissions (~450–500 nm) can disrupt the normal function of the biological clock, altering sleep‐wake cycles and inducing metabolic changes. Recently, we reported that light at night acutely impairs glucose tolerance in nocturnal rats. However, light at night in nocturnal rodents coincides with their activity period, in contrast to artificial light at night exposure in humans. The aim of this study was to evaluate the acute effects of blue (λ = 490 ± 20 nm) artificial light at night (bALAN) on glucose metabolism and food intake in both male and female diurnal Sudanian grass rats (Arvicanthis ansorgei) fed either regular chow or a free choice high‐fat high sucrose diet (HFHS). In both chow and HFHS fed male Arvicanthis, 1‐hour of bALAN exposure induced a higher glucose response in the oral glucose tolerance test (OGTT) accompanied by a significant decrease in plasma insulin. Furthermore, in HFHS fed animals, bALAN induced an increase in sucrose intake during the dark phase in males but not in females. Additionally, 1‐h of bALAN increased the nonfasted glucose levels together with plasma corticosterone in female grass rats. These results provide new and further evidence for the deleterious effects of exposure to short wavelength emission‐containing artificial light at night on glucose metabolism in a diurnal rodent in a sex‐dependent manner.

Suprachiasmatic Nucleus-Arcuate Nucleus Axis: Interaction Between Time and Metabolism Essential for Health

In mammals, time and metabolism are tightly coupled variables; this relationship can be illustrated by numerous examples, such as the circadian variation in food intake or the circadian response to a glucose bolus. We review evidence that the interaction between the suprachiasmatic nucleus and the arcuate nucleus plays a key role in the execution of these functions. The nuclei are reciprocally connected via different projections, and this interaction provides an ideal anatomical framework to modify the temporal output of the hypothalamus to metabolic organs as a consequence of the feedback from the periphery. The suprachiasmatic nucleus-arcuate nucleus relationship is essential to integrate metabolic information into the circadian system and thus adapt circadian rhythms in core body temperature, locomotor activity, food intake, and circulating molecules such as glucose and corticosterone. With the rise in obesity-associated diseases in the world population, gaining knowledge about this relationship, and the consequences of disturbing this liaison, is essential to understand the pathogenesis of obesity.

Metabolic Effects of Light at Night are Time- and Wavelength-Dependent in Rats

OBJECTIVE

Intrinsically photosensitive retinal ganglion cells are most sensitive to short wavelengths and reach brain regions that modulate biological rhythms and energy metabolism. The increased exposure nowadays to artificial light at night (ALAN), especially short wavelengths, perturbs our synchronization with the 24-hour solar cycle. Here, the time- and wavelength dependence of the metabolic effects of ALAN are investigated.

METHODS

Male Wistar rats were exposed to white, blue, or green light at different time points during the dark phase. Locomotor activity, energy expenditure, respiratory exchange ratio (RER), and food intake were recorded. Brains, livers, and blood were collected.

RESULTS

All wavelengths decreased locomotor activity regardless of time of exposure, but changes in energy expenditure were dependent on the time of exposure. Blue and green light reduced RER at Zeitgeber time 16-18 without changing food intake. Blue light increased period 1 (Per1) gene expression in the liver, while green and white light increased Per2. Blue light decreased plasma glucose and phosphoenolpyruvate carboxykinase (Pepck) expression in the liver. All wavelengths increased c-Fos activity in the suprachiasmatic nucleus, but blue and green light decreased c-Fos activity in the paraventricular nucleus.

CONCLUSIONS

ALAN affects locomotor activity, energy expenditure, RER, hypothalamic c-Fos expression, and expression of clock and metabolic genes in the liver depending on the time of day and wavelength.

Retinal innervation tunes circuits that drive nonphotic entrainment to food

Daily changes in light and food availability are major time cues that influence circadian timing¹. However, little is known about the circuits that integrate these time cues to drive a coherent circadian output^1-3. Here we investigate whether retinal inputs modulate entrainment to nonphotic cues such as time-restricted feeding. Photic information is relayed to the suprachiasmatic nucleus (SCN)-the central circadian pacemaker-and the intergeniculate leaflet (IGL) through intrinsically photosensitive retinal ganglion cells (ipRGCs)⁴. We show that adult mice that lack ipRGCs from the early postnatal stages have impaired entrainment to time-restricted feeding, whereas ablation of ipRGCs at later stages had no effect. Innervation of ipRGCs at early postnatal stages influences IGL neurons that express neuropeptide Y (NPY) (hereafter, IGL^NPY neurons), guiding the assembly of a functional IGL^NPY-SCN circuit. Moreover, silencing IGL^NPY neurons in adult mice mimicked the deficits that were induced by ablation of ipRGCs in the early postnatal stages, and acute inhibition of IGL^NPY terminals in the SCN decreased food-anticipatory activity. Thus, innervation of ipRGCs in the early postnatal period tunes the IGL^NPY-SCN circuit to allow entrainment to time-restricted feeding.

Chocolate for breakfast prevents circadian desynchrony in experimental models of jet-lag and shift-work

Night-workers, transcontinental travelers and individuals that regularly shift their sleep timing, suffer from circadian desynchrony and are at risk to develop metabolic disease, cancer, and mood disorders, among others. Experimental and clinical studies provide evidence that food intake restricted to the normal activity phase is a potent synchronizer for the circadian system and can prevent the detrimental metabolic effects associated with circadian disruption. As an alternative, we hypothesized that a timed piece of chocolate scheduled to the onset of the activity phase may be sufficient stimulus to synchronize circadian rhythms under conditions of shift-work or jet-lag. In Wistar rats, a daily piece of chocolate coupled to the onset of the active phase (breakfast) accelerated re-entrainment in a jet-lag model by setting the activity of the suprachiasmatic nucleus (SCN) to the new cycle. Furthermore, in a rat model of shift-work, a piece of chocolate for breakfast prevented circadian desynchrony, by increasing the amplitude of the day-night c-Fos activation in the SCN. Contrasting, chocolate for dinner prevented re-entrainment in the jet-lag condition and favored circadian desynchrony in the shift-work models. Moreover, chocolate for breakfast resulted in low body weight gain while chocolate for dinner boosted up body weight. Present data evidence the relevance of the timing of a highly caloric and palatable meal for circadian synchrony and metabolic function.

Interaction Between Circadian Rhythms, Energy Metabolism, and Cognitive Function

The interaction between meal timing and light regulates circadian rhythms in mammals and not only determines the sleep-wake pattern but also the activity of the endocrine system. Related with that, the necessity to fulfill energy needs is a driving force that requires the participation of cognitive skills whose performance has been shown to undergo circadian variations. These facts have led to the concept that cognition and feeding behaviour can be analysed from a chronobiological perspective. In this context, research carried out during the last two decades has evidenced the link between feeding behaviour/nutritional habits and cognitive processes, and has highlighted the impact of circadian disorders on cognitive decline. All that has allowed hypothesizing a tight relationship between nutritional factors, chronobiology, and cognition. In this connection, experimental diets containing elevated amounts of fat and sugar (high-fat diets; HFDs) have been shown to alter in rodents the circadian distribution of meals, and to have a negative impact on cognition and motivational aspects of behaviour that disappear when animals are forced to adhere to a standard temporal eating pattern. In this review, we will present relevant studies focussing on the effect of HFDs on cognitive aspects of behaviour, paying particular attention to the influence that chronobiological alterations caused by these diets may have on hippocampaldependent cognition.

The suprachiasmatic nucleus; a responsive clock regulating homeostasis by daily changing the setpoints of physiological parameters

The suprachiasmatic nucleus (SCN) is responsible for determining circadian variations in physiological setpoints. The SCN achieves such control through projections to different target structures within and outside the hypothalamus. Thus the SCN prepares the physiology of the body every 24 h via hormones and autonomic nervous system (ANS), to coming changes in behavior. Resulting rhythms in hormones and ANS activity transmit a precise message to selective organs, adapting their sensitivity to coming hormones, metabolites or other essentials. Thus the SCN as autonomous clock gives rhythm to physiological processes. However when the body is challenged by infections, low or high temperature, food shortage or excess: physiological setpoints need to be changed. For example, under fasting conditions, setpoints for body temperature and glucose levels are lowered at the beginning of the sleep (inactive) phase. However, starting the active phase, a normal increase in glucose and temperature levels take place to support activities associated with the acquisition of food. Thus, the SCN adjusts physiological setpoints in agreement with time of the day and according to challenges faced by the body. The SCN is enabled to do this by receiving extensive input from brain areas involved in sensing the condition of the body. Therefore, when the body receives stimuli contradicting normal physiology, such as eating or activity during the inactive period, this information reaches the SCN, adapting its output to correct this disbalance. As consequence frequent violations of the SCN message, such as by shift work or night eating, will result in development of disease.

The bovine anterior hypothalamus: Characterization of the vasopressin-oxytocin containing nucleus and changes in relation to sexual differentiation

In an effort to systematically describe the neurochemical anatomy of the bovine anterior hypothalamus, we used a series of immunocytochemical markers such as acetylcholine esterase (AChE), arginine-vasopressin (AVP), calbindin (Calb), galanin (Gal), neuropeptide-Y (NPY), oxytocin (OXT), somatostatin (SST), and vasoactive intestinal peptide (VIP). We also investigated the potential sex difference present in the suprachiasmatic nucleus (SCN) and the vasopressin-oxytocin containing nucleus (VON) of six male and six female Bos taurus. Our study revealed that the cytochemical structure of the cattle anterior hypothalamus follows the blueprint of other mammals. The VON, which was never described before in cattle, showed a sex difference with a 33.7% smaller volume and 23.2% fewer magnocellular neurons (approximately 20-30 μm) in the male. The SCN also did show a sex difference in VIP neurons and volume with a 36.1% larger female nucleus with 28.1% more cells. Additionally, we included five heifers with freemartin syndrome as a new animal model relevant to sexual differentiation in the brain. This is, to the best of our knowledge, the first freemartin study in relation to the brain. Surprisingly, the SCN of freemartin heifers was 32.5% larger than its control male and female counterparts with 29% more VIP cells. Conversely, the freemartin VON had an intermediary size between male and female. To analyze our data, a classical statistical analysis and a novel multivariate and multi-aspect approach were applied. These findings shed new light on sexual dimorphism in the bovine brain and present this species with freemartins as a valuable animal model in neuroscience.

A central-acting connexin inhibitor, INI-0602, prevents high-fat diet-induced feeding pattern disturbances and obesity in mice

A high-fat diet (HFD) causes obesity by promoting excessive energy intake, and simultaneously, by disturbing the timing of energy intake. Restoring the feeding pattern is sufficient to prevent HFD-induced obesity in mice. However, the molecular mechanism(s) underlying HFD-induced feeding pattern disturbances remain elusive. Saturated fatty acids activate microglia and cause hypothalamic inflammation. Activated microglia cause neuroinflammation, which spreads via inflammatory cytokines and gap-junction hemichannels. However, the role of gap-junction hemichannels in HFD-induced obesity remains unaddressed. We used a novel, central-acting connexin inhibitor, INI-0602, which has high affinity for gap junction hemichannels and does not affect the induction of inflammatory cytokines. We analyzed ad libitum feeding behavior and locomotor activity in mice that were fed normal chow (NC), a HFD with elevated saturated fatty acids (SFAs), or a HFD with very high SFAs. We found that HFD feeding induced acute hyperphagia, mainly during the light cycle. Feeding pattern disturbances were more pronounced in mice that consumed the HFD with very high SFAs than in mice that consumed the HFD with elevated SFAs. When INI-0602 was administered before the HFD was introduced, it blocked the feeding pattern disturbance, but not locomotor activity disturbances; moreover, it prevented subsequent diet-induced obesity. However, when INI-0602 was administered after the HFD had disturbed the feeding pattern, it failed to restore the normal feeding pattern. Therefore, we propose that SFAs in HFDs played a major role in disrupting feeding patterns in mice. Moreover, the feeding pattern disturbance required the function of central, gap junction hemichannels at the initiation of a HFD. However, altering hemichannel function after the feeding pattern disturbance was established had no effect. Thus, preventing the occurrence of a feeding pattern disturbance by blocking the hemichannel pathway was associated with the prevention of the HFD-induced obesity in mice.

ES. The Suprachiasmatic Nucleus and the Intergeniculate Leaflet of the Flat-Faced Fruit-Eating Bat (Artibeus planirostris): Retinal Projections and Neurochemical Anatomy

In mammals, the suprachiasmatic nucleus (SCN) and the intergeniculate leaflet (IGL) are the main components of the circadian timing system. The SCN, classically known as the master circadian clock, generates rhythms and synchronizes them to environmental cues. The IGL is a key structure that modulates SCN activity. Strategies on the use of time by animals can provide important clues about how some species are adapted to competitive process in nature. Few studies have provided information about temporal niche in bats with special attention on the neural substrate underlies circadian rhythms. The aim of this study was to investigate these circadian centers with respect to their cytoarchitecture, chemical content and retinal projections in the flat-faced fruit-eating bat (Artibeus planirostris), a chiropteran endemic to South America. Unlike other species of phyllostomid bats, the flat-faced fruit-eating bat's peak of activity occurs 5 h after sunset. This raises several questions about the structure and function of the SCN and IGL in this species. We carried out a mapping of the retinal projections and cytoarchitectural study of the nuclei using qualitative and quantitative approaches. Based on relative optical density findings, the SCN and IGL of the flat-faced fruit-eating bat receive bilaterally symmetric retinal innervation. The SCN contains vasopressin (VP) and vasoactive intestinal polypeptide (VIP) neurons with neuropeptide Y (NPY), serotonin (5-HT) and glutamic acid decarboxylase (GAD) immunopositive fibers/terminals and is marked by intense glial fibrillary acidic protein (GFAP) immunoreactivity. The IGL contains NPY perikarya as well as GAD and 5-HT immunopositive terminals and is characterized by dense GFAP immunostaining. In addition, stereological tools were combined with Nissl stained sections to estimate the volumes of the circadian centers. Taken together, the present results in the flat-faced fruit-eating bat reveal some differences compared to other bat species which might explain the divergence in the hourly activity among bats in order to reduce the competitive potential and resource partitioning in nature.

Circadian clock-gastrointestinal peptide interaction in peripheral tissues and the brain

Food intake and sleep are two mutually exclusive behaviors and both are normally confined to opposing phases of the diurnal cycle. The temporal coordination of behavior and physiology along the 24-h day-night cycle is organized by a network of circadian clocks that orchestrate transcriptional programs controlling cellular physiology. Many of the peptide hormones of the gastrointestinal tract are not only secreted in a circadian fashion, they can also affect circadian clock function in peripheral metabolic tissues and the brain, thus providing metabolic feedback to metabolic and neurobehavioral circuits. In this review, we summarize the current knowledge on this gastrointestinal peptide crosstalk and its potential role in the coordination of nutrition and the maintenance of metabolic homeostasis.

Neural and Molecular Mechanisms Involved in Controlling the Quality of Feeding Behavior: Diet Selection and Feeding Patterns

We are what we eat. There are three aspects of feeding: what, when, and how much. These aspects represent the quantity (how much) and quality (what and when) of feeding. The quantitative aspect of feeding has been studied extensively, because weight is primarily determined by the balance between caloric intake and expenditure. In contrast, less is known about the mechanisms that regulate the qualitative aspects of feeding, although they also significantly impact the control of weight and health. However, two aspects of feeding quality relevant to weight loss and weight regain are discussed in this review: macronutrient-based diet selection (what) and feeding pattern (when). This review covers the importance of these two factors in controlling weight and health, and the central mechanisms that regulate them. The relatively limited and fragmented knowledge on these topics indicates that we lack an integrated understanding of the qualitative aspects of feeding behavior. To promote better understanding of weight control, research efforts must focus more on the mechanisms that control the quality and quantity of feeding behavior. This understanding will contribute to improving dietary interventions for achieving weight control and for preventing weight regain following weight loss.

Suprachiasmatic Nucleus Interaction with the Arcuate Nucleus; Essential for Organizing Physiological Rhythms

The suprachiasmatic nucleus (SCN) is generally considered the master clock, independently driving all circadian rhythms. We recently demonstrated the SCN receives metabolic and cardiovascular feedback adeptly altering its neuronal activity. In the present study, we show that microcuts effectively removing SCN-arcuate nucleus (ARC) interconnectivity in Wistar rats result in a loss of rhythmicity in locomotor activity, corticosterone levels, and body temperature in constant dark (DD) conditions. Elimination of these reciprocal connections did not affect SCN clock gene rhythmicity but did cause the ARC to desynchronize. Moreover, unilateral SCN lesions with contralateral retrochiasmatic microcuts resulted in identical arrhythmicity, proving that for the expression of physiological rhythms this reciprocal SCN-ARC interaction is essential. The unaltered SCN c-Fos expression following glucose administration in disconnected animals as compared to a significant decrease in controls demonstrates the importance of the ARC as metabolic modulator of SCN neuronal activity. Together, these results indicate that the SCN is more than an autonomous clock, and forms an essential component of a larger network controlling homeostasis. The present novel findings illustrate how an imbalance between SCN and ARC communication through circadian disruption could be involved in the etiology of metabolic disorders.

The Circadian System: A Regulatory Feedback Network of Periphery and Brain

Circadian rhythms are generated by the autonomous circadian clock, the suprachiasmatic nucleus (SCN), and clock genes that are present in all tissues. The SCN times these peripheral clocks, as well as behavioral and physiological processes. Recent studies show that frequent violations of conditions set by our biological clock, such as shift work, jet lag, sleep deprivation, or simply eating at the wrong time of the day, may have deleterious effects on health. This infringement, also known as circadian desynchronization, is associated with chronic diseases like diabetes, hypertension, cancer, and psychiatric disorders. In this review, we will evaluate evidence that these diseases stem from the need of the SCN for peripheral feedback to fine-tune its output and adjust physiological processes to the requirements of the moment. This feedback can vary from neuronal or hormonal signals from the liver to changes in blood pressure. Desynchronization renders the circadian network dysfunctional, resulting in a breakdown of many functions driven by the SCN, disrupting core clock rhythms in the periphery and disorganizing cellular processes that are normally driven by the synchrony between behavior and peripheral signals with neuronal and humoral output of the hypothalamus. Consequently, we propose that the loss of synchrony between the different elements of this circadian network as may occur during shiftwork and jet lag is the reason for the occurrence of health problems.

Suprachiasmatic Nucleus Neuropeptides and Their Control of Endogenous Glucose Production

Defective control of endogenous glucose production is an important factor responsible for hyperglycaemia in the diabetic individual. During the past decade, progressively more evidence has appeared indicating a strong and potentially causal relationship between disturbances of the circadian system and defects of metabolic regulation, including glucose metabolism. The detrimental effects of disturbed circadian rhythms may have their origin in disturbances of the molecular clock mechanisms in peripheral organs, such as the pancreas and liver, or in the central brain clock in the hypothalamic suprachiasmatic nuclei (SCN). To assess the role of SCN output per se on glucose metabolism, we investigated (i) the effect of several SCN neurotransmitters on endogenous glucose production and (ii) the effect of SCN neuronal activity on hepatic and systemic insulin sensitivity. We show that silencing of SCN neuronal activity results in decreased hepatic insulin sensitivity and increased peripheral insulin sensitivity. Furthermore, both oxytocin neurones in the paraventricular nucleus of the hypothalamus (PVN) and orexin neurones in the lateral hypothalamus may be important targets for the SCN control of glucose metabolism. These data further highlight the role of the central clock in the pathophysiology of insulin resistance.

Age-related changes in neurochemical components and retinal projections of rat intergeniculate leaflet

Aging leads to several anatomical and functional deficits in circadian timing system. In previous works, we observed morphological alterations with age in hypothalamic suprachiasmatic nuclei, one central component of this system. However, there are few data regarding aging effects on other central components of this system, such as thalamic intergeniculate leaflet (IGL). In this context, we studied possible age-related alterations in neurochemical components and retinal projections of rat IGL. For this goal, young (3 months), adult (13 months), and aged (23 months) Wistar rats were submitted to an intraocular injection of neural tracer, cholera toxin subunit b (CTb), 5 days before a tissue fixation process by paraformaldehyde perfusion. Optical density measurements and cell count were performed at digital pictures of brain tissue slices processed by immunostaining for glutamic acid decarboxylase (GAD), enkephalin (ENK), neuropeptide Y (NPY) and CTb, characteristic markers of IGL and its retinal terminals. We found a significant age-related loss in NPY immunoreactive neurons, but not in immunoreactivity to GAD and ENK. We also found a decline of retinal projections to IGL with age. We conclude aging impairs both a photic environmental clue afferent to IGL and a neurochemical expression which has an important modulatory circadian function, providing strong anatomical correlates to functional deficits of the aged biological clock.

Neural activity in the suprachiasmatic circadian clock of nocturnal mice anticipating a daytime meal

Circadian rhythms in mammals are regulated by a system of circadian oscillators that includes a light-entrainable pacemaker in the suprachiasmatic nucleus (SCN) and food-entrainable oscillators (FEOs) elsewhere in the brain and body. In nocturnal rodents, the SCN promotes sleep in the day and wake at night, while FEOs promote an active state in anticipation of a predictable daily meal. For nocturnal animals to anticipate a daytime meal, wake-promoting signals from FEOs must compete with sleep-promoting signals from the SCN pacemaker. One hypothesis is that FEOs impose a daily rhythm of inhibition on SCN output that is timed to permit the expression of activity prior to a daytime meal. This hypothesis predicts that SCN activity should decrease prior to the onset of anticipatory activity and remain suppressed through the scheduled mealtime. To assess the hypothesis, neural activity in the SCN of mice anticipating a 4-5-h daily meal in the light period was measured using FOS immunohistochemistry and in vivo multiple unit electrophysiology. SCN FOS, quantified by optical density, was significantly reduced at the expected mealtime in food-anticipating mice with access to a running disk, compared to ad libitum-fed and acutely fasted controls. Group differences were not significant when FOS was quantified by other methods, or in mice without running disks. SCN electrical activity was markedly decreased during locomotion in some mice but increased in others. Changes in either direction were concurrent with locomotion, were not specific to food anticipation, and were not sustained during longer pauses. Reduced FOS indicates a net suppression of SCN activity that may depend on the intensity or duration of locomotion. The timing of changes in SCN activity relative to locomotion suggests that any effect of FEOs on SCN output is mediated indirectly, by feedback from neural or systemic correlates of locomotion.

Plasticity of circadian clocks and consequences for metabolism

The increased prevalence of metabolic disorders and obesity in modern society, together with the widespread use of artificial light at night, have led researchers to investigate whether altered patterns of light exposure contribute to metabolic disorders. This article discusses the experimental evidence that perturbed environmental cycles induce rhythm disorders in the circadian system, thus leading to metabolic disorders. This notion is generally supported by animal studies. Distorted environmental cycles, including continuous exposure to light, affect the neuronal organization of the central circadian pacemaker in the suprachiasmatic nucleus (SCN), its waveform and amplitude of the rhythm in electrical activity. Moreover, repeated exposure to a shifted light cycle or the application of dim light at night are environmental cues that cause a change in SCN function. The effects on the SCN waveform are the result of changes in synchronization among the SCN's neuronal cell population, which lead consistently to metabolic disturbances. Furthermore, we discuss the effects of sleep deprivation and the time of feeding on metabolism, as these factors are associated with exposure to disturbed environmental cycles. Finally, we suggest that these experimental studies reveal a causal relationship between the rhythm disorders and the metabolic disorders observed in epidemiological studies performed in humans.

Serotonin, a possible intermediate between disturbed circadian rhythms and metabolic disease

It is evident that eating in misalignment with the biological clock (such as in shift work, eating late at night and skipping breakfast) is associated with increased risk for obesity and diabetes. The biological clock located in the suprachiasmatic nucleus dictates energy balance including feeding behavior and glucose metabolism. Besides eating and sleeping patterns, glucose metabolism also exhibits clear diurnal variations with higher blood glucose concentrations, glucose tolerance and insulin sensitivity prior to waking up. The daily variation in plasma glucose concentrations in rats, is independent of the rhythm in feeding behavior. On the other hand, feeding itself has profound effects on glucose metabolism, but differential effects occur depending on the time of the day. We here review data showing that a disturbed diurnal eating pattern results in alterations in glucose metabolism induced by a disrupted circadian clock. We first describe the role of central serotonin on feeding behavior and glucose metabolism and subsequently describe the effects of central serotonin on the circadian system. We next explore the interaction between the serotonergic system and the circadian clock in conditions of disrupted diurnal rhythms in feeding and how this might be involved in the metabolic dysregulation that occurs with chronodisruption.

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.

Impact of nutrients on circadian rhythmicity

The suprachiasmatic nucleus (SCN) in the mammalian hypothalamus functions as an endogenous pacemaker that generates and maintains circadian rhythms throughout the body. Next to this central clock, peripheral oscillators exist in almost all mammalian tissues. Whereas the SCN is mainly entrained to the environment by light, peripheral clocks are entrained by various factors, of which feeding/fasting is the most important. Desynchronization between the central and peripheral clocks by, for instance, altered timing of food intake can lead to uncoupling of peripheral clocks from the central pacemaker and is, in humans, related to the development of metabolic disorders, including obesity and Type 2 diabetes. Diets high in fat or sugar have been shown to alter circadian clock function. This review discusses the recent findings concerning the influence of nutrients, in particular fatty acids and glucose, on behavioral and molecular circadian rhythms and will summarize critical studies describing putative mechanisms by which these nutrients are able to alter normal circadian rhythmicity, in the SCN, in non-SCN brain areas, as well as in peripheral organs. As the effects of fat and sugar on the clock could be through alterations in energy status, the role of specific nutrient sensors will be outlined, as well as the molecular studies linking these components to metabolism. Understanding the impact of specific macronutrients on the circadian clock will allow for guidance toward the composition and timing of meals optimal for physiological health, as well as putative therapeutic targets to regulate the molecular clock.

Immunohistochemical mapping of neuropeptide Y in the tree shrew brain

Day-active tree shrews are promising animals as research models for a variety of human disorders. Neuropeptide Y (NPY) modulates many behaviors in vertebrates. Here we examined the distribution of NPY in the brain of tree shrews (Tupaia belangeri chinensis) using immunohistochemical techniques. The differential distribution of NPY-immunoreactive (-ir) cells and fibers were observed in the rhinencephalon, telencephalon, diencephalon, mesencephalon, metencephalon, and myelencephalon of tree shrews. Most NPY-ir cells were multipolar or bipolar in shape with triangular, fusiform, and/or globular perikarya. The densest cluster of NPY-ir cells were found in the mitral cell layer of the main olfactory bulb (MOB), arcuate nucleus of the hypothalamus, and pretectal nucleus of the thalamus. The MOB presented a unique pattern of NPY immunoreactivity. Laminar distribution of NPY-ir cells was observed in the MOB, neocortex, and hippocampus. Compared to rats, the tree shrews exhibited a particularly robust and widespread distribution of NPY-ir cells in the MOB, bed nucleus of the stria terminalis, and amygdala as well as the ventral lateral geniculate nucleus and pretectal nucleus of the thalamus. By contrast, a low density of neurons were scattered in the striatum, neocortex, polymorph cell layer of the dentate gyrus, superior colliculus, inferior colliculus, and dorsal tegmental nucleus. These findings provide the first detailed mapping of NPY immunoreactivity in the tree shrew brain and demonstrate species differences in the distribution of this neuropeptide, providing an anatomical basis for the participation of the NPY system in the regulation of numerous physiological and behavioral processes.

Central injection of relaxin-3 receptor (RXFP3) antagonist peptides reduces motivated food seeking and consumption in C57BL/6J mice

Behavioural arousal in mammals is regulated by various interacting central monoamine- and peptide-neurotransmitter/receptor systems, which function to maintain awake, alert and active states required for performance of goal-directed activities essential for survival, including food seeking. Existing anatomical and functional evidence suggests the highly-conserved neuropeptide, relaxin-3, which signals via its cognate Gi/o-protein coupled receptor, RXFP3, contributes to behavioural arousal and feeding behaviour in rodents. In studies to investigate this possibility further, adult male C57BL/6J mice were treated with the selective RXFP3 antagonist peptides, R3(B1-22)R/I5(A) and R3(B1-22)R, and motivated food seeking and consumption was assessed as a reflective output of behavioural arousal. Compared to vehicle treatment, intracerebroventricular (icv) injection of RXFP3 antagonists reduced: (i) food anticipatory activity before meal time during food restriction; (ii) consumption of highly palatable food; (iii) consumption of regular chow during the initial dark phase, and; (iv) consumption of regular chow after mild (∼4-h) food deprivation. Effects were not due to sedation and appeared to be specifically mediated via antagonism of relaxin-3/RXFP3 signalling, as RXFP3 antagonist treatment did not alter locomotor activity in wild-type mice or reduce palatable food intake in relaxin-3 deficient (knock-out) mice. Notably, in contrast to similar studies in the rat, icv injection of RXFP3 agonists and infusion into the paraventricular hypothalamic nucleus did not increase food consumption in mice, suggesting species differences in relaxin-3/RXFP3-related signalling networks. Together, our data provide evidence that endogenous relaxin-3/RXFP3 signalling promotes motivated food seeking and consumption, and in light of the established biological and translational importance of other arousal systems, relaxin-3/RXFP3 networks warrant further experimental investigation.

The suprachiasmatic nucleus is part of a neural feedback circuit adapting blood pressure response

The suprachiasmatic nucleus (SCN) is typically considered our autonomous clock synchronizing behavior with physiological parameters such as blood pressure (BP), just transmitting time independent of physiology. Yet several studies show that the SCN is involved in the etiology of hypertension. Here, we demonstrate that the SCN is incorporated in a neuronal feedback circuit arising from the nucleus tractus solitarius (NTS), modulating cardiovascular reactivity. Tracer injections into the SCN of male Wistar rats revealed retrogradely filled neurons in the caudal NTS, where BP information is integrated. These NTS projections to the SCN were shown to be glutamatergic and to terminate in the ventrolateral part of the SCN where light information also enters. BP elevations not only induced increased neuronal activity as measured by c-Fos in the NTS but also in the SCN. Lesioning the caudal NTS prevented this activation. The increase of SCN neuronal activity by hypertensive stimuli suggested involvement of the SCN in counteracting BP elevations. Examining this possibility we observed that elevation of BP, induced by α1-agonist infusion, was more than twice the magnitude in SCN-lesioned animals as compared to in controls, indicating indeed an active involvement of the SCN in short-term BP regulation. We propose that the SCN receives BP information directly from the NTS enabling it to react to hemodynamic perturbations, suggesting the SCN to be part of a homeostatic circuit adapting BP response. We discuss how these findings could explain why lifestyle conditions violating signals of the biological clock may, in the long-term, result in cardiovascular disease.