Role of the Suprachiasmatic and Arcuate Nuclei in Diurnal Temperature Regulation in the Rat

Activation of oxytocinergic neurons enhances torpor in mice

Mus musculus enters a torpid state in response to caloric restriction in sub-thermoneutral ambient temperatures. This torpid state is characterized by an adaptive and controlled decrease in metabolic rate, heart rate, body temperature, and activity. Previous research has identified the paraventricular nucleus (PVN) within the hypothalamus, a region containing oxytocin neurons, as a location that is active during torpor onset. We hypothesized that oxytocin neurons within the PVN are part of this neural circuit and that activation of oxytocin neurons would deepen and lengthen torpor bouts. We report that activation of oxytocin neurons alone is not sufficient to induce a torpor-like state in the fed mouse, with no significant difference in body temperature or heart rate upon activation of oxytocin neurons. However, we found that activation of oxytocin neurons prior to the onset of daily torpor both deepens and lengthens the subsequent bout, with a 1.7 ± 0.4 °C lower body temperature and a 135 ± 32 min increase in length. We therefore conclude that oxytocin neurons are involved in the neural circuitry controlling daily torpor in the mouse.

Dim Light at Night Promotes Circadian Disruption in Female Rats, at the Metabolic, Reproductive, and Behavioral Level

Inhabitants of urban areas are constantly exposed to light at night, which is an important environmental factor leading to circadian disruption. Streetlights filtering light through the windows and night dim light lamps are common sources of dim light at night (DLAN). The female population is susceptible to circadian disruption. The present study is aimed to determine the impact of DLAN on female Wistar rats circadian rhythms, metabolism, reproductive physiology, and behavior. After 5 weeks of DLAN exposure daily, oscillations in activity and body temperature of female rats are abolished. DLAN also decreases nocturnal food ingestion, which results in a diminishment in total food consumption. These alterations in the temporal organization of the body are associated with a significant decrease in melatonin plasmatic levels, reproductive disruptions, decreased exploration times, and marked anhedonia. This study highlights the importance of avoiding exposure to light at night, even at low intensities, to maintain the circadian organization of physiology, and denotes the great necessity of increasing the studies in females since the sexual dimorphism within the effects of desynchronizing protocols has been poorly studied.

Suprachiasmatic nucleus promotes hyperglycemia induced by sleep delay

Short sleep is linked to disturbances in glucose metabolism and may induce a prediabetic condition. The biological clock in the suprachiasmatic nucleus (SCN) regulates the glucose rhythm in the circulation and the sleep-wake cycle. SCN vasopressin neurons (SCNVP) control daily glycemia by regulating the entrance of glucose into the arcuate nucleus (ARC). Thus, we hypothesized that sleep delay may influence SCN neuronal activity. We, therefore, investigated the role of SCNVP when sleep is disrupted by forced locomotor activity. After 2 h of sleep delay, rats exhibited decreased SCNVP neuronal activity, a decrease in the glucose transporter GLUT1 expression in tanycytes lining the third ventricle, lowered glucose entrance into the ARC, and developed hyperglycemia. The association between reduced SCNVP neuronal activity and hyperglycemia in sleep-delayed rats was evidenced by injecting intracerebroventricular vasopressin; this increased GLUT1 immunoreactivity in tanycytes, thus promoting normoglycemia. Following sleep recovery, glucose levels decreased, whereas SCNVP neuronal activity increased. These results imply that sleep-delay-induced changes in SCNVP activity lead to glycemic impairment, inferring that disruption of biological clock function might represent a critical step in developing type 2 diabetes.

Inputs and Outputs of the Mammalian Circadian Clock

Circadian rhythms in mammals are coordinated by the central circadian pacemaker, the suprachiasmatic nucleus (SCN). Light and other environmental inputs change the timing of the SCN neural network oscillator, which, in turn, sends output signals that entrain daily behavioral and physiological rhythms. While much is known about the molecular, neuronal, and network properties of the SCN itself, the circuits linking the outside world to the SCN and the SCN to rhythmic outputs are understudied. In this article, we review our current understanding of the synaptic and non-synaptic inputs onto and outputs from the SCN. We propose that a more complete description of SCN connectivity is needed to better explain how rhythms in nearly all behaviors and physiological processes are generated and to determine how, mechanistically, these rhythms are disrupted by disease or lifestyle.

Recent advances in mass spectrometry analysis of neuropeptides

Due to their involvement in numerous biochemical pathways, neuropeptides have been the focus of many recent research studies. Unfortunately, classic analytical methods, such as western blots and enzyme-linked immunosorbent assays, are extremely limited in terms of global investigations, leading researchers to search for more advanced techniques capable of probing the entire neuropeptidome of an organism. With recent technological advances, mass spectrometry (MS) has provided methodology to gain global knowledge of a neuropeptidome on a spatial, temporal, and quantitative level. This review will cover key considerations for the analysis of neuropeptides by MS, including sample preparation strategies, instrumental advances for identification, structural characterization, and imaging; insightful functional studies; and newly developed absolute and relative quantitation strategies. While many discoveries have been made with MS, the methodology is still in its infancy. Many of the current challenges and areas that need development will also be highlighted in this review.

The cold-sensing ion channel TRPM8 regulates central and peripheral clockwork and the circadian oscillations of body temperature

AIM

Physiological functions in mammals show circadian oscillations, synchronized by daily cycles of light and temperature. Central and peripheral clocks participate in this regulation. Since the ion channel TRPM8 is a critical cold sensor, we investigated its role in circadian function.

METHODS

We used TRPM8 reporter mouse lines and TRPM8-deficient mice. mRNA levels were determined by in situ hybridization or RT-qPCR and protein levels by immunofluorescence. A telemetry system was used to measure core body temperature (Tc).

RESULTS

TRPM8 is expressed in the retina, specifically in cholinergic amacrine interneurons and in a subset of melanopsin-positive ganglion cells which project to the central pacemaker, the suprachiasmatic nucleus (SCN) of the hypothalamus. TRPM8-positive fibres were also found innervating choroid and ciliary body vasculature, with a putative function in intraocular temperature, as shown in TRPM8-deficient mice. Interestingly, Trpm8^-/- animals displayed increased expression of the clock gene Per2 and vasopressin (AVP) in the SCN, suggesting a regulatory role of TRPM8 on the central oscillator. Since SCN AVP neurons control body temperature, we studied Tc in driven and free-running conditions. TRPM8-deficiency increased the amplitude of Tc oscillations and, under dim constant light, induced a greater phase delay and instability of Tc rhythmicity. Finally, TRPM8-positive fibres innervate peripheral organs, like liver and white adipose tissue. Notably, Trpm8^-/- mice displayed a dysregulated expression of Per2 mRNA in these metabolic tissues.

CONCLUSION

Our findings support a function of TRPM8 as a temperature sensor involved in the regulation of central and peripheral clocks and the circadian control of Tc.

Sex differences in thermoregulation in mammals: Implications for energy homeostasis

Thermal homeostasis is a fundamental process in mammals, which allows the maintenance of a constant internal body temperature to ensure an efficient function of cells despite changes in ambient temperature. Increasing evidence has revealed the great impact of thermoregulation on energy homeostasis. Homeothermy requires a fine regulation of food intake, heat production, conservation and dissipation and energy expenditure. A great interest on this field of research has re-emerged following the discovery of thermogenic brown adipose tissue and browning of white fat in adult humans, with a potential clinical relevance on obesity and metabolic comorbidities. However, most of our knowledge comes from male animal models or men, which introduces unwanted biases on the findings. In this review, we discuss how differences in sex-dependent characteristics (anthropometry, body composition, hormonal regulation, and other sexual factors) influence numerous aspects of thermal regulation, which impact on energy homeostasis. Individuals of both sexes should be used in the experimental paradigms, considering the ovarian cycles and sexual hormonal regulation as influential factors in these studies. Only by collecting data in both sexes on molecular, functional, and clinical aspects, we will be able to establish in a rigorous way the real impact of thermoregulation on energy homeostasis, opening new avenues in the understanding and treatment of obesity and metabolic associated diseases.

Obesity in male volcano mice Neotomodon alstoni affects the daily rhythm of metabolism and thermoregulation

The mouse N. alstoni spontaneously develops the condition of obesity in captivity when fed regular chow. We aim to study the differences in metabolic performance and thermoregulation between adult lean and obese male mice. The experimental approach included indirect calorimetry using metabolic cages for VO₂ intake and VCO₂ production. In contrast, the body temperature was measured and analyzed using intraperitoneal data loggers. It was correlated with the relative presence of UCP1 protein and its gene expression from interscapular adipose tissue (iBAT). We also explored in this tissue the relative presence of Tyrosine Hydroxylase (TH) protein, the rate-limiting enzyme for catecholamine biosynthesis present in iBAT. Results indicate that obese mice show a daily rhythm persists in estimated parameters but with differences in amplitude and profile. Obese mice presented lower body temperature, and a low caloric expenditure, together with lower VO₂ intake and VCO₂ than lean mice. Also, obese mice present a reduced thermoregulatory response after a cold pulse. Results are correlated with a low relative presence of TH and UCP1 protein. However, qPCR analysis of Ucp1 presents an increase in gene expression in iBAT. Histology showed a reduced amount of brown adipocytes in BAT. The aforementioned indicates that the daily rhythm in aerobic metabolism, thermoregulation, and body temperature control have reduced amplitude in obese mice Neotomodon alstoni.

Dopamine systems and biological rhythms: Let's get a move on

How dopamine signaling regulates biological rhythms is an area of emerging interest. Here we review experiments focused on delineating dopamine signaling in the suprachiasmatic nucleus, nucleus accumbens, and dorsal striatum to mediate a range of biological rhythms including photoentrainment, activity cycles, rest phase eating of palatable food, diet-induced obesity, and food anticipatory activity. Enthusiasm for causal roles for dopamine in the regulation of circadian rhythms, particularly those associated with food and other rewarding events, is warranted. However, determining that there is rhythmic gene expression in dopamine neurons and target structures does not mean that they are bona fide circadian pacemakers. Given that dopamine has such a profound role in promoting voluntary movements, interpretation of circadian phenotypes associated with locomotor activity must be differentiated at the molecular and behavioral levels. Here we review our current understanding of dopamine signaling in relation to biological rhythms and suggest future experiments that are aimed at teasing apart the roles of dopamine subpopulations and dopamine receptor expressing neurons in causally mediating biological rhythms, particularly in relation to feeding, reward, and activity.

Breathing Rhythm and Pattern and Their Influence on Emotion

Breathing is a vital rhythmic motor behavior with a surprisingly broad influence on the brain and body. The apparent simplicity of breathing belies a complex neural control system, the breathing central pattern generator (bCPG), that exhibits diverse operational modes to regulate gas exchange and coordinate breathing with an array of behaviors. In this review, we focus on selected advances in our understanding of the bCPG. At the core of the bCPG is the preBötzinger complex (preBötC), which drives inspiratory rhythm via an unexpectedly sophisticated emergent mechanism. Synchronization dynamics underlying preBötC rhythmogenesis imbue the system with robustness and lability. These dynamics are modulated by inputs from throughout the brain and generate rhythmic, patterned activity that is widely distributed. The connectivity and an emerging literature support a link between breathing, emotion, and cognition that is becoming experimentally tractable. These advances bring great potential for elucidating function and dysfunction in breathing and other mammalian neural circuits.

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.

Suprachiasmatic nucleus-mediated glucose entry into the arcuate nucleus determines the daily rhythm in blood glycemia

Glycemia is maintained within very narrow boundaries with less than 5% variation at a given time of the day. However, over the circadian cycle, glycemia changes with almost 50% difference. How the suprachiasmatic nucleus, the biological clock, maintains these day-night variations with such tiny disparities remains obscure. We show that via vasopressin release at the beginning of the sleep phase, the suprachiasmatic nucleus increases the glucose transporter GLUT1 in tanycytes. Hereby GLUT1 promotes glucose entrance into the arcuate nucleus, thereby lowering peripheral glycemia. Conversely, blocking vasopressin activity or the GLUT1 transporter at the daily trough of glycemia increases circulating glucose levels usually seen at the peak of the rhythm. Thus, biological clock-controlled mechanisms promoting glucose entry into the arcuate nucleus explain why peripheral blood glucose is low before sleep onset.

Arcuate proopiomelanocortin is part of a novel neural connection for short-term low-degree of high ambient temperature effects on food intake

High ambient temperature (HTa) is an important environmental factor influencing food intake (FI). We previously demonstrated that low-degree HTa exposure decreased FI earlier than activated physiological responses, and this effect was related to the median preoptic nucleus (MnPO) and arcuate nucleus (Arc) connection. The present study refines the condition of low-degree HTa exposure and focuses on the mechanism of Arc neural activation. We demonstrated in the first experiment that with the usual ambient temperature (Ta) at 23 °C, the low degree HTa condition is at a 7 °C temperature difference and with 90 min exposure. Rats exposed to this short-term low-degree HTa had significantly lower 1-h FI than those exposed to control Ta (CTa) without differences in rectal temperature and hematocrit. Under nonfeeding conditions, HTa could enhance c-Fos at the Arc without the activation of proopiomelanocortin (POMC) neurons. Under feeding conditions, HTa could enhance both c-Fos and POMC at Arc. In addition, the number of c-Fos and POMC colocalizations in the HTa group was higher than that in the CTa group. Finally, intracerebral preinfusion with a subthreshold dose of the melanocortin antagonist SHU9119 reversed the effect of low-degree HTa exposure on FI. Therefore, we conclude that the effect of short-term low-degree HTa exposure on FI in rats is mediated in part by activation of POMC neurons at the Arc. The results partially support the hypothesis that Arc is a crucial hypothalamic nucleus for the effect of low-degree HTa exposure on FI.

Circadian Rhythms in Bacterial Sepsis Pathology: What We Know and What We Should Know

Sepsis is a syndrome caused by a deregulated host response to infection, representing the primary cause of death from infection. In animal models, the mortality rate is strongly dependent on the time of sepsis induction, suggesting a main role of the circadian system. In patients undergoing sepsis, deregulated circadian rhythms have also been reported. Here we review data related to the timing of sepsis induction to further understand the different outcomes observed both in patients and in animal models. The magnitude of immune activation as well as the hypothermic response correlated with the time of the worst prognosis. The different outcomes seem to be dependent on the expression of the clock gene Bmal1 in the liver and in myeloid immune cells. The understanding of the role of the circadian system in sepsis pathology could be an important tool to improve patient therapies.

The Genomes of Two Billfishes Provide Insights into the Evolution of Endothermy in Teleosts

Endothermy is a typical convergent phenomenon which has evolved independently at least eight times in vertebrates, and is of significant advantage to organisms in extending their niches. However, how vertebrates other than mammals or birds, especially teleosts, achieve endothermy has not previously been fully understood. In this study, we sequenced the genomes of two billfishes (swordfish and sailfish), members of a representative lineage of endothermic teleosts. Convergent amino acid replacements were observed in proteins related to heat production and the visual system in two endothermic teleost lineages, billfishes and tunas. The billfish-specific genetic innovations were found to be associated with heat exchange, thermoregulation, and the specialized morphology, including elongated bill, enlarged dorsal fin in sailfish and loss of the pelvic fin in swordfish.

Social interaction masking contributes to changes in the activity of the suprachiasmatic nucleus and impacts on circadian rhythms

Light is the most powerful temporal cue that entrains physiology and behavior through modulation of the suprachiasmatic nucleus (SCN) of the hypothalamus. However, on a daily basis, individuals face a combination of light and several non-photic cues, such as social interaction. In order to investigate whether SCN activity and SCN-driven rhythms are altered by social interaction, adult male C57BLJ/6 mice were maintained in groups of 3-4 animals per cage or 1 animal per cage (socially isolated) under 12:12 h / light:dark (LD) cycles or constant darkness (DD). Analysis of the two anatomical subdivisions (ventral, v and dorsal, d) of the medial SCN revealed an effect of housing conditions on the d-SCN but not on the v-SCN on the number of c-Fos immunoreactive (ir) neurons. As such, 2 h after the light-phase onset d-SCN c-Fos-ir number was lower in single-housed mice under LD. Importantly, under DD there were no effect of housing conditions in the number of c-Fos-ir SCN neurons. Social isolation increased the amplitude and strength of SCN-driven rhythm of body temperature (Tc) entrained to LD and it advanced its onset, uncoupling with spontaneous locomotor activity (SLA) rhythm, without altering endogenous Tc and SLA rhythms expressed under DD. Associated with reduced Tc in the light phase, single-housed mice showed reduced body weight. However, these phenotypes were not accompanied by changes in the number of c-Fos-ir neurons in the preoptic area (POA), which are known to regulate energy metabolism and Tc. Altogether, these results imply that the social interaction masking effect on the d-SCN is added to that of light stimulus, in order to achieve full c-Fos expression in the SCN, which, in turn seems to be required to maintain daily-phase coherence between the photo-entrained rhythms of Tc and SLA. There might be an inter-relationship between masking (social interaction) and entrainment stimulus (light) that impacts the circadian parameters of the photo-entrained Tc rhythm. As such, in the absence of social interactions a more robust Tc rhythm is shown. This inter-relationship seems to occur in the dorsal subdivision of the SCN but not in the POA.

The circadian system: From clocks to physiology

The circadian system, composed of the central autonomous clock, the suprachiasmatic nucleus (SCN), and systems of the body that follow the signals of the SCN, continuously change the homeostatic set points of the body over the day-night cycle. Changes in the body's physiological state that do not agree with the time of the day feedback to the hypothalamus, and provide input to the SCN to adjust the condition, thus reaching another set point required by the changed conditions. This allows the adjustment of the set points to another level when environmental conditions change, which is thought to promote adaptation and survival. In fasting, the body temperature drops to a lower level only at the beginning of the sleep phase. Stressful conditions raise blood pressure relatively more during the active period than during the rest phase. Extensive, mostly reciprocal SCN interactions, with hypothalamic networks, induce these physiological adjustments by hormonal and autonomic control of the body's organs. More importantly, in addition to SCN's hormonal and autonomic influences, SCN induced behavior, such as rhythmic food intake, induces the oscillation of many genes in all tissues, including the so-called clock genes, which have an essential role as a transcriptional driving force for numerous cellular processes. Consequently, the light-dark cycle, the rhythm of the SCN, and the resulting rhythm in behavior need to be perfectly synchronized, especially where it involves synchronizing food intake with the activity phase. If these rhythms are not synchronous for extended periods of times, such as during shift work, light exposure at night, or frequent night eating, disease may develop. As such, our circadian system is a perfect illustration of how hypothalamic-driven processes depend on and interact with each other and need to be in seamless synchrony with the body's physiology.

Vasopressin: An output signal from the suprachiasmatic nucleus to prepare physiology and behaviour for the resting phase

Vasopressin (VP) is an important hormone produced in the supraoptic (SON) and paraventricular nucleus (PVN) with antidiuretic and vasoconstrictor functions in the periphery. As one of the first discovered peptide hormones, VP was also shown to act as a neurotransmitter, where VP is produced and released under the influence of various stimuli. VP is one of the core signals via which the biological clock, the suprachiasmatic nucleus (SCN), imposes its rhythm on its target structures and its production and release is influenced by the rhythm of clock genes and the light/dark cycle. This is contrasted with VP production and release from the bed nucleus of the stria terminalis and the medial amygdala, which is influenced by gonadal hormones, as well as with VP originating from the PVN and SON, which is released in the neural lobe and central targets. The release of VP from the SCN signals the near arrival of the resting phase in rodents and prepares their physiology accordingly by down-modulating corticosterone secretion, the reproductive cycle and locomotor activity. All these circadian variables are regulated within very narrow boundaries at a specific time of the day, where day-to-day variation is less than 5% at any particular hour. However, the circadian peak values can be at least ten times higher than the circadian trough values, indicating the need for an elaborate feedback system to inform the SCN and other participating nuclei about the actual levels reached during the circadian cycle. In short, the interplay between SCN circadian output and peripheral feedback to the SCN is essential for the adequate organisation of all circadian rhythms in physiology and behaviour.

Role of central kisspeptin and RFRP-3 in energy metabolism in the male Wistar rat

Kisspeptin (Kp) and (Arg)(Phe) related peptide 3 (RFRP-3) are two RF-amides acting in the hypothalamus to control reproduction. In the past 10 years, it has become clear that, apart from their role in reproductive physiology, both neuropeptides are also involved in the control of food intake, as well as glucose and energy metabolism. To investigate further the neural mechanisms responsible for these metabolic actions, we assessed the effect of acute i.c.v. administration of Kp or RFRP-3 in ad lib. fed male Wistar rats on feeding behaviour, glucose and energy metabolism, circulating hormones (luteinising hormone, testosterone, insulin and corticosterone) and hypothalamic neuronal activity. Kp increased plasma testosterone levels, had an anorexigenic effect and increased lipid catabolism, as attested by a decreased respiratory exchange ratio (RER). RFRP-3 also increased plasma testosterone levels but did not modify food intake or energy metabolism. Both RF-amides increased endogenous glucose production, yet with no change in plasma glucose levels, suggesting that these peptides provoke not only a release of hepatic glucose, but also a change in glucose utilisation. Finally, plasma insulin and corticosterone levels did not change after the RF-amide treatment. The Kp effects were associated with an increased c-Fos expression in the median preoptic area and a reduction in pro-opiomelanocortin immunostaining in the arcuate nucleus. No effects on neuronal activation were found for RFRP-3. Our results provide further evidence that Kp is not only a very potent hypothalamic activator of reproduction, but also part of the hypothalamic circuit controlling energy metabolism.

Diurnal and seasonal molecular rhythms in the human brain and their relation to Alzheimer disease

Diurnal and seasonal rhythms influence many aspects of human physiology including brain function. Moreover, altered diurnal and seasonal behavioral and physiological rhythms have been linked to Alzheimer's disease and related dementias (ADRD). Understanding the molecular basis for these links may lead to identification of novel targets to mitigate the negative impact of normal and abnormal diurnal and seasonal rhythms on ADRD or to alleviate the adverse consequences of ADRD on normal diurnal and seasonal rhythms. Diurnally and seasonally rhythmic gene expression and epigenetic modification in the human neocortex may be a key mechanism underlying these links. This chapter will first review the observed epidemiological links between normal and abnormal diurnal and seasonal rhythmicity, cognitive impairment, and ADRD. Then it will review normal diurnal and seasonal rhythms of brain epigenetic modification and gene expression in model organisms. Finally, it will review evidence for diurnal and seasonal rhythms of epigenetic modification and gene expression the human brain in aging, Alzheimer's disease, and other brain disorders.

The median preoptic nucleus: A major regulator of fluid, temperature, sleep, and cardiovascular homeostasis

Located in the midline lamina terminalis of the anterior wall of the third ventricle, the median preoptic nucleus is a thin elongated nucleus stretching around the rostral border of the anterior commissure. Its neuronal elements, composed of various types of excitatory glutamatergic and inhibitory GABAergic neurons, receive afferent neural signals from (1) neighboring subfornical organ and organum vasculosum of the lamina terminalis related to plasma osmolality and hormone concentrations, e.g., angiotensin II; (2) from peripheral sensors such as arterial baroreceptors and cutaneous thermosensors. Different sets of these MnPO glutamatergic and GABAergic neurons relay output signals to hypothalamic, midbrain, and medullary regions that drive homeostatic effector responses. Included in the effector responses are (1) thirst, antidiuretic hormone secretion and renal sodium excretion that subserve osmoregulation and body fluid homeostasis; (2) vasoconstriction or dilatation of skin blood vessels, and shivering and brown adipose tissue thermogenesis for core temperature homeostasis; (3) inhibition of hypothalamic and midbrain nuclei that stimulate wakefulness and arousal, thereby promoting both REM and non-REM sleep; and (4) activation of sympathetic pathways that drive vasoconstriction and heart rate to maintain arterial pressure and the perfusion of vital organs. The small size of MnPO belies its massive homeostatic significance.

PGC-1β-expressing POMC neurons mediate the effect of leptin on thermoregulation in the mouse

The arcuate nucleus (ARC) of the hypothalamus is a key regulator of food intake, brown adipose tissue (BAT) thermogenesis, and locomotor activity. Whole-body deficiency of the transcriptional coactivator peroxisome proliferator-activated receptor γ (PPARγ) coactivator-1β (PGC-1β) disrupts mouse circadian locomotor activity and BAT-regulated thermogenesis, in association with altered gene expression at the central level. We examined whether PGC-1β expression in the ARC is required for proper energy balance and locomotor behavior by generating mice lacking the PGC-1β gene specifically in pro-opiomelanocortin (POMC) neurons. POMC neuron-specific deletion of PGC-1β did not impact locomotor behavior, food intake, body composition, energy fuel utilization and metabolic rate in fed, 24-h fasted and 24-h refed conditions. In contrast, in the fed state, deletion of PGC-1β in POMC cells elevated core body temperature during the nighttime period. Importantly, this higher body temperature is not associated with changes in BAT function and gene expression. Conversely, we provide evidence that mice lacking PGC-1β in POMC neurons are more sensitive to the effect of leptin on heat dissipation. Our data indicate that PGC-1β-expressing POMC neurons are part of a circuit controlling body temperature homeostasis and that PGC-1β function in these neurons is involved in the thermoregulatory effect of leptin.

Monosynaptic Projections to Excitatory and Inhibitory preBötzinger Complex Neurons

The key driver of breathing rhythm is the preBötzinger Complex (preBötC) whose activity is modulated by various functional inputs, e.g., volitional, physiological, and emotional. While the preBötC is highly interconnected with other regions of the breathing central pattern generator (bCPG) in the brainstem, there is no data about the direct projections to either excitatory and inhibitory preBötC subpopulations from other elements of the bCPG or from suprapontine regions. Using modified rabies tracing, we identified neurons throughout the brain that send monosynaptic projections to identified excitatory and inhibitory preBötC neurons in mice. Within the brainstem, neurons from sites in the bCPG, including the contralateral preBötC, Bötzinger Complex, the nucleus of the solitary tract (NTS), parafacial region (pF L /pF V ), and parabrachial nuclei (PB), send direct projections to both excitatory and inhibitory preBötC neurons. Suprapontine inputs to the excitatory and inhibitory preBötC neurons include the superior colliculus, red nucleus, amygdala, hypothalamus, and cortex; these projections represent potential direct pathways for volitional, emotional, and physiological control of breathing.

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.

Regulation and function of extra-SCN circadian oscillators in the brain

Most organisms evolved endogenous, so called circadian clocks as internal timekeeping mechanisms allowing them to adapt to recurring changes in environmental demands brought about by 24-hour rhythms such as the light-dark cycle, temperature variations or changes in humidity. The mammalian circadian clock system is based on cellular oscillators found in all tissues of the body that are organized in a hierarchical fashion. A master pacemaker located in the suprachiasmatic nucleus (SCN) synchronizes peripheral tissue clocks and extra-SCN oscillators in the brain with each other and with external time. Different time cues (so called Zeitgebers) such as light, food intake, activity and hormonal signals reset the clock system through the SCN or by direct action at the tissue clock level. While most studies on non-SCN clocks so far have focused on peripheral tissues, several extra-SCN central oscillators were characterized in terms of circadian rhythm regulation and output. Some of them are directly innervated by the SCN pacemaker, while others receive indirect input from the SCN via other neural circuits or extra-brain structures. The specific physiological function of these non-SCN brain oscillators as well as their role in the regulation of the circadian clock network remains understudied. In this review we summarize our current knowledge about the regulation and function of extra-SCN circadian oscillators in different brain regions and devise experimental approaches enabling us to unravel the organization of the circadian clock network in the central nervous system.

Differential Thermoregulatory and Inflammatory Patterns in the Circadian Response to LPS-Induced Septic Shock

Sepsis is caused by a dysregulated host response to infection, and characterized by uncontrolled inflammation together with immunosuppression, impaired innate immune functions of phagocytes and complement activation. Septic patients develop fever or hypothermia, being the last one characteristic of severe cases. Both lipopolysaccharide (LPS) and Tumor Necrosis Factor (TNF)-α- induced septic shock in mice is dependent on the time of administration. In this study, we aimed to further characterize the circadian response to high doses of LPS. First, we found that mice injected with LPS at ZT11 developed a higher hypothermia than those inoculated at ZT19. This response was accompanied by higher neuronal activation of the preoptic, suprachiasmatic, and paraventricular nuclei of the hypothalamus. However, LPS-induced Tnf-α and Tnf-α type 1 receptor (TNFR1) expression in the preoptic area was time-independent. We also analyzed peritoneal and spleen macrophages, and observed an exacerbated response after ZT11 stimulation. The serum of mice inoculated with LPS at ZT11 induced deeper hypothermia in naïve animals than the one coming from ZT19-inoculated mice, related to higher TNF-α serum levels during the day. We also analyzed the response in TNFR1-deficient mice, and found that both the daily difference in the mortality rate, the hypothermic response and neuronal activation were lost. Moreover, mice subjected to circadian desynchronization showed no differences in the mortality rate throughout the day, and developed lower minimum temperatures than mice under light-dark conditions. Also, those injected at ZT11 showed increased levels of TNF-α in serum compared to standard light conditions. These results suggest a circadian dependency of the central thermoregulatory and peripheral inflammatory response to septic-shock, with TNF-α playing a central role in this circadian response.

The Concept of Coupling in the Mammalian Circadian Clock Network

The circadian clock network regulates daily rhythms in mammalian physiology and behavior to optimally adapt the organism to the 24-h day/night cycle. A central pacemaker, the hypothalamic suprachiasmatic nucleus (SCN), coordinates subordinate cellular oscillators in the brain, as well as in peripheral organs to align with each other and external time. Stability and coordination of this vast network of cellular oscillators is achieved through different levels of coupling. Although coupling at the molecular level and across the SCN is well established and believed to define its function as pacemaker structure, the notion of coupling in other tissues and across the whole system is less well understood. In this review, we describe the different levels of coupling in the mammalian circadian clock system - from molecules to the whole organism. We highlight recent advances in gaining knowledge of the complex organization and function of circadian network regulation and its significance for the generation of stable but plastic intrinsic 24-h rhythms.

Circadian blueprint of metabolic pathways in the brain

The circadian clock is an endogenous, time-tracking system that directs multiple metabolic and physiological functions required for homeostasis. The master or central clock located within the suprachiasmatic nucleus in the hypothalamus governs peripheral clocks present in all systemic tissues, contributing to their alignment and ultimately to temporal coordination of physiology. Accumulating evidence reveals the presence of additional clocks in the brain and suggests the possibility that circadian circuits may feed back to these from the periphery. Here, we highlight recent advances in the communications between clocks and discuss how they relate to circadian physiology and metabolism.

Resveratrol mitigates hypercholesterolemia exacerbated hyperthermia in chronically heat-stressed rats

BACKGROUND AND AIM

Hypercholesterolemia (HC) is the major leading cause of cardiovascular disease worldwide. Such atherogenic aberration deeply impacts blood circulation. Resveratrol (R) is a polyphenol that has received attention as a hypolipidemic, antioxidant, and vascular agility advocate. Efficient blood redistribution is a key element in mammalian thermoregulation. We hypothesized that R treatment may aid in mitigating hyperthermic responses under both acute and chronic heat stress (HS) conditions in HC male rats.

MATERIALS AND METHODS

All rats were initially fitted with miniaturized thermologgers to measure core body temperature (T_core). With a 2 × 2 factorial arrangement, four groups were randomly allotted, in which half of the animals ingested an HC diet (C+), while the other half ingested a control (C-) diet, throughout the whole study duration of 35 days. Seven rats from each dietary treatment, however, received R (R+; 13 mg/kg BW/day), while the rest received normal saline (R-) for 5 continuous days. All animals were maintained at thermoneutrality (TN; ambient temperature; T_a=23.15±0.04°C) for a period of 30 continuous days (days 0-29). On day 29, an acute HS (HS; T_a=35.86±0.37°C; for 9 nocturnal h) was imposed. Then, from day 29, a chronic HS protocol (T_a=32.28±1.00°C) was maintained until the past day of the trial (day 34), after which blood samples were drawn for analyses of platelet (PL) count, total antioxidant activity (TAO), total cholesterol (TC), triglycerides (TGs), and lipid peroxidation (LP).

RESULTS

Switching animals from TN to HS resulted in abrupt rises in T_core. The HC diet induced a significant (p<0.01) hyperlipidemia over the control of diet-consuming rats. Interestingly, the hyperthermic response to acute HS was highly pronounced in the rats consuming the C- diet, while the C+ diet exacerbated the chronic HS-induced hyperthermia. Despite failure to improve TAO in the C+ diet, R+ treatment caused a marked (p<0.05) decline in nighttime - hyperthermia in C+ rats, likely by enhancing blood flow to extremities (for heat dissipation) as delineated by drastic downregulations of C+ related rises in PL, TC, TG, and LP (HC diet by R+ interaction; p<0.03).

CONCLUSION

The hyperthermic response in C- groups was attributed to higher amount of feed intake than those consuming the C+ diet. Yet, the R+ improvement of thermoregulation in the C+ group was likely related to enhancement of vascular hemodynamics. Resveratrol intake mitigated chronic HS-evoked hyperthermia in rats. Such an approach is worthy to follow-up in other mammals and humans.

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.

Kisspeptin Neurons in the Arcuate Nucleus of the Hypothalamus Orchestrate Circadian Rhythms and Metabolism

Successful reproduction in female mammals is precisely timed and must be able to withstand the metabolic demand of pregnancy and lactation. We show that kisspeptin-expressing neurons in the arcuate hypothalamus (Kiss1^ARH) of female mice control the daily timing of food intake, along with the circadian regulation of locomotor activity, sleep, and core body temperature. Toxin-induced silencing of Kiss1^ARH neurons shifts wakefulness and food consumption to the light phase and induces weight gain. Toxin-silenced mice are less physically active and have attenuated temperature rhythms. Because the rhythm of the master clock in the suprachiasmatic nucleus (SCN) appears to be intact, we hypothesize that Kiss1^ARH neurons signal to neurons downstream of the master clock to modulate the output of the SCN. We conclude that, in addition to their well-established role in regulating fertility, Kiss1^ARH neurons are a critical component of the hypothalamic circadian oscillator network that times overt rhythms of physiology and behavior.

The increased expression of GABA receptors within the arcuate nucleus is associated with high intraocular pressure

PURPOSE

To investigate the relationship between intraocular pressure (IOP) and GABA receptors within the arcuate nucleus (ARC).

METHODS

In the chronic high IOP rat model, ibotenic acid (IBO) was injected to induce impairment of the ARC, and IOP was measured at the 0, 1, 2, 3, and 4 week time points with a Tono-Pen. To assess the expression of GABA-A/B receptors within the ARC under persistent high IOP, we performed immunofluorescence (IF) and immunohistochemical (IHC) staining at 2 weeks and 4 weeks. Furthermore, we treated the ARC with GABA-A/B receptor antagonists separately, and IOP was evaluated, as well as retinal ganglion cell apoptosis in the chronic high IOP rat model. In the following induced high IOP animal model, the expression of GABA-A/B receptors within the ARC was evaluated in DBA/2J mice which developed progressive eye abnormalities spontaneously that closely mimic human hereditary glaucoma.

RESULTS

Compared with the control group, statistically significant downregulation of IOP was noted due to the IBO injection into the ARC at the 2, 3, and 4 week time points (p<0.05). Persistent high IOP elicited increased expression of the GABA-A/B receptors in the ARC compared with the control group (p<0.01). In addition, treatment with GABA-A/B receptor antagonists separately caused a decrease in the IOP, along with reduced retinal ganglion cell apoptosis (p<0.01). In the DBA/2J mice, the expression of the GABA receptors was statistically significantly increased (p<0.01).

CONCLUSIONS

GABA-A/B receptors in the ARC may be involved in regulation of IOP, and pathologically high IOP affects the expression of GABA-A/B receptors in the ARC.

Sex differences in the ACTH and cortisol response to pharmacological probes are stressor-specific and occur regardless of alcohol dependence history

Women and men differ in their risk for developing stress-related conditions such as alcohol use and anxiety disorders and there are gender differences in the typical sequence in which these disorders co-occur. However, the neural systems underlying these gender-biased psychopathologies and clinical course modifiers in humans are poorly understood and may involve both central and peripheral mechanisms regulating the limbic-hypothalamic-pituitary-adrenal axis. In the present randomized, double blind, placebo-controlled, triple-dummy crossover study, we juxtaposed a centrally-acting, citalopram (2 mg/unit BMI) neuroendocrine stimulation test with a peripherally-acting, dexamethasone (Dex) (1.5 mg)/corticotropin-releasing factor (CRF) (1 μg/kg) test in euthymic women (N = 38) and men (N = 44) with (54%) and without histories of alcohol dependence to determine whether sex, alcohol dependence or both influenced the adrenocorticotropic hormone (ACTH) and cortisol responses to the pharmacological challenges and to identify the loci of these effects. We found that central serotonergic mechanisms, along with differences in pituitary and adrenal sensitivity, mediated sexually-diergic ACTH and cortisol responses in a stressor-specific manner regardless of a personal history of alcohol dependence. Specifically, women exhibited a greater response to the Dex/CRF test than they did the citalopram test while men exhibited the opposite pattern of results. Women also had more robust ACTH, cortisol and body temperature responses to Dex/CRF than men, and exhibited a shift in their adrenal glands' sensitivity to ACTH as measured by the cortisol/log (ACTH) ratio during that session in contrast to the other test days. Our findings indicate that central serotonergic and peripheral mechanisms both play roles in mediating sexually dimorphic, stressor-specific endocrine responses in humans regardless of alcohol dependence history.

Defective daily temperature regulation in a mouse model of amyotrophic lateral sclerosis

Current understanding of the pathogenesis of the familial form of amyotrophic lateral sclerosis has been aided by the study of transgenic mice that over-express mutated forms of the human CuZn-superoxide dismutase (SOD1) gene. While mutant SOD1 in motor neurons determines disease onset, other non-cell autonomous factors are critical for disease progression, and altered energy metabolism has been implicated as a contributing factor. Since most energy expended by laboratory mice is utilized to defend body temperature (T_b), we analyzed thermoregulation in transgenic mice carrying the G93A mutation of the human SOD1 gene, using implantable temperature data loggers to continuously record T_b for up to 85 days. At room (22 °C) ambient temperature, G93A mice exhibited a diminished amplitude of the daily T_b rhythm compared to C57BL/6J controls, secondary to decreased T_b values during the dark (behaviorally active) phase of the light-dark cycle. The defect arose at 85-99 days of age, around the age of symptom onset (as assessed by grip strength), well before observable weakness and weight loss, and could not be accounted for by decreased levels of locomotor activity or food consumption. Housing under thermoneutral (29 °C) ambient temperature partially rescued the defect, but age-dependently (only in animals >100 days of age), suggesting that the deficit in older mice was due in part to inadequate thermogenesis by "peripheral" thermogenic organs as the disease progressed. In younger mice, we found that cold-induced thermogenesis and energy expenditure were intact, hinting that an initial "central" defect might localize to the subparaventricular zone, involving neural output pathways from the circadian clock in the hypothalamic suprachiasmatic nucleus to forebrain thermoregulatory circuitry.

Diet-induced obesity reduces core body temperature across the estrous cycle and pregnancy in the rat

Obesity during pregnancy causes adverse maternal and fetal health outcomes and programs offspring for adult-onset diseases, including cardiovascular disease. Obesity also disrupts core body temperature (T_c) regulation in nonpregnant rodents; however, it is unknown whether obesity alters normal maternal T_c adaptations to pregnancy. Since T_c is influenced by the circadian system, and both obesity and pregnancy alter circadian biology, it was hypothesized that obesity disrupts the normal rhythmic patterns of T_c before and during gestation. Obesity was induced by cafeteria (CAF) feeding in female Wistar rats for 8 weeks prior to and during gestation, whereas control (CON) animals had free access to chow. Intraperitoneal temperature loggers measured daily T_c profiles throughout the study, while maternal body composition and leptin levels were assessed near term. Daily temperature profiles were examined for rhythmic features (mesor, amplitude and acrophase) by cosine regression analysis. CAF animals exhibited increased fat mass (93%) and associated hyperleptinemia (3.2-fold increase) compared to CON animals. CAF consumption reduced the average T_c (by up to 0.29°C) across the estrous cycle and most of pregnancy; however, T_c for CAF and CON animals converged toward the end of gestation. Obesity reduced the amplitude of T_c rhythms at estrus and proestrus and on day 8 of pregnancy, but increased the amplitude at day 20 of pregnancy. Photoperiod analysis revealed that obesity reduced T_c exclusively in the light period during pre-pregnancy but only during the dark period in late gestation. In conclusion, obesity alters rhythmic T_c profiles and reduces the magnitude of the T_c decline late in rat gestation, which may have implications for maternal health and fetal development.

Central Circadian Clock Regulates Energy Metabolism

Our body not only responds to environmental changes but also anticipates them. The light and dark cycle with the period of about 24 h is a recurring environmental change that determines the diurnal variation in food availability and safety from predators in nature. As a result, the circadian clock is evolved in most animals to align locomotor behaviors and energy metabolism with the light cue. The central circadian clock in mammals is located at the suprachiasmatic nucleus (SCN) of the hypothalamus in the brain. We here review the molecular and anatomic architecture of the central circadian clock in mammals, describe the experimental and observational evidence that suggests a critical role of the central circadian clock in shaping systemic energy metabolism, and discuss the involvement of endocrine factors, neuropeptides, and the autonomic nervous system in the metabolic functions of the central circadian clock.

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.

Interaction between the hypothalamus and the immune system

NEW FINDINGS

What is the topic of this review? Both branches of the autonomic nervous system are involved in the regulation of the inflammatory response. We explore how the hypothalamus may influence this process. What advances does it highlight? We analyse how a lipopolysaccharide signal is transmitted to the brain and which areas participate in the response of the brain to lipopolysaccharide. Recent studies show that the hypothalamus can influence the inflammatory response by modifying the autonomic output. The biological clock, the suprachiasmatic nucleus, is integrated into this circuit, putting a time stamp on the intensity of the inflammatory response. The brain is responsible for maintaining homeostasis of the organism, constantly adjusting its output via hormones and the autonomic nervous system to reach an optimal setting in every compartment of the body. Also, the immune system is under strong control of the brain. Apart from the conventional systemic responses evoked by the brain during inflammation, such as hypothalamic-pituitary-adrenal axis activation and the induction of sickness behaviour, the autonomic nervous system is now recognized to exert regulatory effects on the inflammatory response. Both branches of the autonomic nervous system are proposed to influence the inflammatory process. Here, we focus on those areas of the brain that might be involved in sensing inflammatory stimuli, followed by how that sensing could change the output of the autonomic nervous system in order to regulate the inflammatory response. Finally, we will discuss how the defenses of the body against a lipopolysaccharide challenge are organized by the hypothalamus.

The Suprachiasmatic Nucleus Modulates the Sensitivity of Arcuate Nucleus to Hypoglycemia in the Male Rat

The suprachiasmatic nucleus (SCN) and arcuate nucleus (ARC) have reciprocal connections; catabolic metabolic information activates the ARC and inhibits SCN neuronal activity. Little is known about the influence of the SCN on the ARC. Here, we investigated whether the SCN modulated the sensitivity of the ARC to catabolic metabolic conditions. ARC neuronal activity, as determined by c-Fos immunoreactivity, was increased after a hypoglycemic stimulus by 2-deoxyglucose (2DG). The highest ARC neuronal activity after 2DG was found at the end of the light period (zeitgeber 11, ZT11) with a lower activity in the beginning of the light period (zeitgeber 2, ZT2), suggesting the involvement of the SCN. The higher activation of ARC neurons after 2DG at ZT11 was associated with higher 2DG induced blood glucose levels as compared with ZT2. Unilateral SCN-lesioned animals, gave a mainly ipsilateral activation of ARC neurons at the lesioned side, suggesting an inhibitory role of the SCN on ARC neurons. The 2DG-induced counterregulatory glucose response correlated with increased ARC neuronal activity and was significantly higher in unilateral SCN-lesioned animals. Finally, the ARC as site where 2DG may, at least partly, induce a counterregulatory response was confirmed by local microdialysis of 2DG. 2DG administration in the ARC produced a higher increase in circulating glucose compared with 2DG administration in surrounding areas such as the ventromedial nucleus of the hypothalamus (VMH). We conclude that the SCN uses neuronal pathways to the ARC to gate sensory metabolic information to the brain, regulating ARC glucose sensitivity and counterregulatory responses to hypoglycemic conditions.

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.