Circadian rhythms and energy metabolism with special reference to the suprachiasmatic nucleus

The circadian rhythms regulated by Cx43-signaling in the pathogenesis of Neuromyelitis Optica

Introduction

Neuromyelitis Optica (NMO) is an inflammatory demyelinating disease of the central nervous system (CNS). NMO manifests as selective and severe attacks on axons and myelin of the optic nerve and spinal cord, resulting in necrotic cavities. The circadian rhythms are well demonstrated to profoundly impact cellular function, behavior, and disease. This study is aimed to explore the role and molecular basis of circadian rhythms in NMO.

Methods

We used an Aquaporin 4(AQP4) IgG-induced NMO cell model in isolated astrocytes. The expression of Cx43 and Bmal1 were detected by real-time PCR and Western Blot. TAT-Gap19 and DQP-1105 were used to inhibit Cx43 and glutamate receptor respectively. The knockdown of Bmal1 were performed with the shRNA containing adenovirus. The levels of glutamate, anterior visual pathway (AVP), and vasoactive intestinal peptide (VIP) were quantified by ELISA kits.

Results

We found that Bmal1 and Clock, two essential components of the circadian clock, were significantly decreased in NMO astrocytes, which were reversed by Cx43 activation (linoleic acid) or glutamate. Moreover, the expression levels of Bmal1 and Clock were also decreased by Cx43 blockade (TAT-Gap19) or glutamate receptor inhibition (DQP-1105). Furthermore, adenovirus-mediated Bmal1 knockdown by shRNA (Ad-sh-Bmal1) dramatically decreased the levels of glutamate, AVP, and VIP from neurons, and significantly down-regulated the protein level of Cx43 in NMO astrocytes with Cx43 activation (linoleic acid) or glutamate treatment. However, Bmal1 knockdown did not alter these levels in normal astrocytes with Cx43 blockade (TAT-Gap19) or glutamate receptor inhibition (DQP-1105).

Discussion

Collectively, these results suggest that Cx43-glutamate signaling would be a critical upstream regulator that contributes to the NMO-induced rhythmic damage in SCN astrocytes.

Molecular Mechanisms Underlying the Circadian Rhythm of Blood Pressure in Normotensive Subjects

PURPOSE OF REVIEW

Blood pressure (BP) follows a circadian rhythm (CR) in normotensive subjects. BP increases in the morning and decreases at night. This review aims at providing an up-to-date overview regarding the molecular mechanisms underlying the circadian regulation of BP.

RECENT FINDINGS

The suprachiasmatic nucleus (SCN) is the regulatory center for CRs. In SCN astrocytes, the phosphorylated glycogen synthase kinase-3β (pGSK-3β) also follows a CR and its expression reaches a maximum in the morning and decreases at night. pGSK-3β induces the β-catenin migration to the nucleus. During the daytime, the nuclear β-catenin increases the expression of the glutamate excitatory amino acid transporter 2 (EAAT2) and glutamine synthetase (GS). In SCN, EAAT2 removes glutamate from the synaptic cleft of glutamatergic neurons and transfers it to the astrocyte cytoplasm where GS converts glutamate into glutamine. Thus, glutamate decreases in the synaptic cleft. This decreases the stimulation of the glutamate receptors AMPA-R and NMDA-R located on glutamatergic post-synaptic neurons. Consequently, activation of NTS is decreased and BP increases. The opposite occurs at night. Despite several studies resulting from animal studies, the circadian regulation of BP appears largely controlled in normotensive subjects by the canonical WNT/β-catenin pathway involving the SCN, astrocytes, and glutamatergic neurons.

Free-running circadian breathing rhythms are eliminated by suprachiasmatic nucleus lesion

It is widely agreed that breathing is subject to circadian regulation. Circadian differences in respiratory physiology significantly impact a number of diseases including sleep apnea, asthma, and seizure-induced death. The effect of time of day on breathing has been previously characterized; however, an endogenous free-running respiratory rhythm in mammals has not previously been described. Furthermore, it is assumed that circadian rhythms in breathing are dependent on the hypothalamic suprachiasmatic nucleus (SCN), the home of the mammalian central circadian oscillator, but this has not been shown experimentally. The breathing of mice was monitored during wakefulness using whole body plethysmography at six times of day while housed under light-dark conditions and at six circadian phases while housed under constant darkness. Respiratory frequency and minute ventilation, but not tidal volume, were significantly higher during the active phase in both entrained and free-running conditions. To determine whether circadian regulation of breathing requires the SCN, in separate sets of animals this structure was electrolytically lesioned bilaterally or a sham surgery was performed, and breathing was measured at six different time points. Time-dependent oscillations in breathing were lost in SCN-lesioned animals, but not those subjected to sham surgery. These results suggest that breathing is subject to circadian regulation via the SCN. Mechanistic insights into the circadian regulation of breathing may lead to targeted interventions to reduce the morbidity and mortality associated with diseases with respiratory pathophysiology.NEW & NOTEWORTHY It has long been appreciated that breathing is altered by time of day. This study demonstrates that rhythmicity in breathing persists in constant darkness but is dependent on the suprachiasmatic nucleus in the hypothalamus. Understanding circadian rhythms in breathing may be important for the treatment and prevention of diseases such as sleep apnea and sudden unexpected death in epilepsy.

Hypothalamic Dysfunction in Obesity and Metabolic Disorders

The hypothalamus is the brain region responsible for the maintenance of energetic homeostasis. The regulation of this process arises from the ability of the hypothalamus to orchestrate complex physiological responses such as food intake and energy expenditure, circadian rhythm, stress response, and fertility. Metabolic alterations such as obesity can compromise these hypothalamic regulatory functions. Alterations in circadian rhythm, stress response, and fertility further contribute to aggravate the metabolic dysfunction of obesity and contribute to the development of chronic disorders such as depression and infertility.At cellular level, obesity caused by overnutrition can damage the hypothalamus promoting inflammation and impairing hypothalamic neurogenesis. Furthermore, hypothalamic neurons suffer apoptosis and impairment in synaptic plasticity that can compromise the proper functioning of the hypothalamus. Several factors contribute to these phenomena such as ER stress, oxidative stress, and impairments in autophagy. All these observations occur at the same time and it is still difficult to discern whether inflammatory processes are the main drivers of these cellular dysfunctions or if the hypothalamic hormone resistance (insulin, leptin, and ghrelin) can be pinpointed as the source of several of these events.Understanding the mechanisms that underlie the pathophysiology of obesity in the hypothalamus is crucial for the development of strategies that can prevent or attenuate the deleterious effects of obesity.

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.

Circadian rhythms in glucose and lipid metabolism in nocturnal and diurnal mammals

Most aspects of energy metabolism display clear variations during day and night. This daily rhythmicity of metabolic functions, including hormone release, is governed by a circadian system that consists of the master clock in the suprachiasmatic nuclei of the hypothalamus (SCN) and many secondary clocks in the brain and peripheral organs. The SCN control peripheral timing via the autonomic and neuroendocrine system, as well as via behavioral outputs. The sleep-wake cycle, the feeding/fasting rhythm and most hormonal rhythms, including that of leptin, ghrelin and glucocorticoids, usually show an opposite phase (relative to the light-dark cycle) in diurnal and nocturnal species. By contrast, the SCN clock is most active at the same astronomical times in these two categories of mammals. Moreover, in both species, pineal melatonin is secreted only at night. In this review we describe the current knowledge on the regulation of glucose and lipid metabolism by central and peripheral clock mechanisms. Most experimental knowledge comes from studies in nocturnal laboratory rodents. Nevertheless, we will also mention some relevant findings in diurnal mammals, including humans. It will become clear that as a consequence of the tight connections between the circadian clock system and energy metabolism, circadian clock impairments (e.g., mutations or knock-out of clock genes) and circadian clock misalignments (such as during shift work and chronic jet-lag) have an adverse effect on energy metabolism, that may trigger or enhancing obese and diabetic symptoms.

Metabolism and the circadian clock converge

Circadian rhythms occur in almost all species and control vital aspects of our physiology, from sleeping and waking to neurotransmitter secretion and cellular metabolism. Epidemiological studies from recent decades have supported a unique role for circadian rhythm in metabolism. As evidenced by individuals working night or rotating shifts, but also by rodent models of circadian arrhythmia, disruption of the circadian cycle is strongly associated with metabolic imbalance. Some genetically engineered mouse models of circadian rhythmicity are obese and show hallmark signs of the metabolic syndrome. Whether these phenotypes are due to the loss of distinct circadian clock genes within a specific tissue versus the disruption of rhythmic physiological activities (such as eating and sleeping) remains a cynosure within the fields of chronobiology and metabolism. Becoming more apparent is that from metabolites to transcription factors, the circadian clock interfaces with metabolism in numerous ways that are essential for maintaining metabolic homeostasis.

The circadian clock: a framework linking metabolism, epigenetics and neuronal function

The circadian clock machinery is responsible for biological timekeeping on a systemic level. The central clock system controls peripheral clocks through a number of output cues that synchronize the system as a whole. There is growing evidence that changing cellular metabolic states have important effects on circadian rhythms and can thereby influence neuronal function and disease. Epigenetic control has also been implicated in the modulation of biological timekeeping, and cellular metabolism and epigenetic state seem to be closely linked. We discuss the idea that cellular metabolic state and epigenetic mechanisms might work through the circadian clock to regulate neuronal function and influence disease states.

Coordinated regulation of circadian rhythms and homeostasis by the suprachiasmatic nucleus

We have demonstrated that in rats activities of various enzymes related to gluconeogenesis and amino acid metabolism show circadian rhythms. Based on these results, we have explored the molecular mechanisms underlying circadian oscillation and phase response to light of the master clock located in the dorsomedial subdivision of the suprachiasmatic nucleus (SCN) and found various proteins closely related to phase response such as BIT/SHPS-1 and those of circadian oscillation, some of which are involved in protein-tyrosine phosphorylation.On the other hand, we have presented several lines of evidence that the ventrolateral subdivision of the SCN includes not only the control center of energy supply to the brain, but also that of homeostasis such as blood glucose, blood pressure, water balance, and body temperature. We have also shown that besides these functions, the latter subdivision is involved in the regulations of hormone secretions such as insulin, glucagon, corticosterone and vasopressin. It has been also shown by electrophysiological means that light exposure to rat eye enhances sympathetic nerve activity, whereas it depresses parasympathetic nerve activity. Thus, environmental light is implicated not only in the phase-shift through the retinohypthalamic tract (RHT), but also control of autonomic nerve activities through the RHT, It is also discussed in this review how the two divisions are interconnected and how environmental light is involved in this interconnection.

The daily rhythms of melatonin and free fatty acids in goats under varying photoperiods and constant darkness

The purpose of the study was to explore parallel and divergent features of the daily rhythms of melatonin and plasma free fatty acids (FFA) in goats exposed to different lighting conditions. From these features, we attempted to analyze whether the endogenous melatonin rhythm plays any role in the maintenance of the FFA rhythm. Seven Finnish landrace goats were kept under artificial lighting that simulated the annual changes of photoperiod at 60 degrees N (longest photoperiod, 18 h; shortest, 6 h). The ambient temperature and feeding regimen were kept constant. Blood samples were collected 6 times a year at 2 h intervals for 2 d, first in the prevailing light-dark (LD) conditions and then after 3 d in constant darkness (DD). In LD conditions, the melatonin levels always increased immediately after lights-off and declined around lights-on, except in winter (18 h darkness), when the low daytime levels were restored clearly before lights-on. The FFA levels also displayed a consistent rhythmicity, with low levels at night and a transient peak around lights-on. In DD conditions, the melatonin profiles were very similar to those found in the habitual LD conditions, but the rhythm tended to advance. The FFA rhythm persisted also in DD, and the morning peak tended to advance. There was an overall parallelism between the two rhythms, with one significant exception. In winter in LD conditions, the morning rise in FFA levels coincided with lights-on and not with the declining phase of melatonin, whereas in DD conditions, the FFA peak advanced several hours and coincided with the declining phase of melatonin. From this finding and comparisons of the calculated rhythm characteristics, i.e., phase-shifts, phase differences, and correlations, we conclude that the daily rhythm of FFA levels is most probably generated by an endogenous oscillator, primarily adjusted by dawn, whereas the melatonin rhythm in this species is regulated by an oscillator primarily adjusted by dusk. The results did not exclude a modulatory effect of melatonin on the daily FFA profiles, but melatonin secretion, alone, does not explain the patterns sufficiently.

Intermittent and rhythmic exposure to melatonin in primary cultured adipocytes enhances the insulin and dexamethasone effects on leptin expression

Considering the cyclic characteristic of production and secretion of pineal melatonin, it is reasonable to assume that this oscillation might be important in determining the variety of its circadian and seasonal effects. To simulate this physiological condition in vitro, isolated adipocytes were exposed to melatonin in a circadian-like pattern by adding the hormone to the incubating medium during 12 hr (mimicking the night), followed by an equal period without melatonin (mimicking the day). This intermittent procedure was interrupted when three cycles with melatonin were fulfilled (60-hr incubation). Here, we report the effects of melatonin (1 nM) added intermittently or continuously to the incubating medium alone or in combination with insulin (5 nM) and/or dexamethasone (7 nM) on leptin release and expression by rat adipocytes. After acute 12-hr incubation neither melatonin nor insulin alone affected leptin expression, but together they increased it by 105%. Dexamethasone increased leptin mRNA content and release (70%) but this effect was not enhanced by melatonin. Nevertheless, after 60 hr under intermittent melatonin, we observed a synergism between melatonin and dexamethasone. This interaction promoted an increment (75% compared with dexamethasone alone) in leptin release and expression. Our results suggest that circadian-like exposure to melatonin potentiates the dexamethasone action and is important to the effects promoted by insulin on leptin expression. Based on an in vitro approach, this work helps to clarify the physiological relevance and the repercussions of the in vivo circadian pattern of melatonin secretion.

Daily variations of blood glucose, acid-base state and PCO2 in rats: effect of light exposure

The suprachiasmatic nuclei (SCN) of the hypothalamus are the site of the main circadian clock in mammals. Synchronization of the SCN to light is achieved by direct retinal inputs. The present study performed in rats transferred to constant darkness shows that blood glucose, pH and PCO2 display significant diurnal changes when measurements were made during the subjective day, the early subjective night or the late subjective night. The effects of a 30-min light exposure (100 lx) on these metabolic parameters at each of these circadian times were assessed. Regardless of the circadian time, light induced an increase in blood glucose, but did not affect plasma pH and PCO2. This study suggests that blood glucose, PCO2 and acid-base state are under circadian control, most likely mediated by the SCN, while the hyperglycemic response to light seems not to be gated by a circadian clock and may thus involve retinal inputs to non-SCN retino-recipient areas.

Digging deep--structure-function relationships in the melatonin receptor family

The melatonin receptor family is a small group of receptors within the G protein-coupled receptor (GPCR) superfamily. The group comprises of three subtypes which bind melatonin and one member, the melatonin related receptor (MRR), that shares >40% sequence identity with the other melatonin receptors but does not bind melatonin. Identification of two subtypes expressed in the mouse suprachiasmatic nucleus, one of which (MT1) inhibits neuronal firing and the other (MT2) mediating the phase advancing properties of melatonin has given renewed interest to the development of subtype specific compounds for each of the mammalian melatonin receptors. Towards this goal site-directed and chimaeric receptor mutagenesis studies have been performed which have provided some insight into the structure-function relationships of the melatonin receptors. Furthermore, these studies may lead to the identification of the ligand for the orphan MRR.

Total parenteral nutrition entrains the central and peripheral circadian clocks

The administration schedule of total parenteral nutrition (TPN) ignores the human physiological food-intake rhythm. This study examined the effects of TPN on the central and peripheral circadian clocks. Male Wistar rats were divided into a control group, a nocturnal-TPN group, and a diurnal-TPN group. On the seventh day after TPN administration, expression of rat period2 (rPer2) and D-site binding protein (rDBP) mRNA were measured in the suprachiasmatic nucleus (SCN) and liver. While rPer2 and rDBP mRNA expression of the nocturnal-TPN rats showed similar oscillation patterns to those in the free-fed control rats both in the SCN and liver, they were shifted significantly in the diurnal-TPN rats. This phase shift occurred as early as day 1. TPN, which ignores physiological food-intake rhythms, alters the rhythm of the central and peripheral clocks.

Hibernation: when good clocks go cold

Hibernating animals have been a successful model system for elucidating fundamental properties of many physiological systems. Over the past 50 years, a diverse literature has emerged on the role of the circadian system in control and expression of winter torpor in several orders of birds and mammals. This body of research has also provided insights to circadian function in non-hibernating species. The aim of this review is to examine how this work applies to questions of general interest to chronobiologists, such as temperature compensation, the 2-oscillator model of entrainment, and suprachiasmatic nucleus (SCN) function. Convergent lines of evidence suggest a role for the SCN in timing daily torpor and controlling several parameters of hibernation. In addition to its role as a circadian pacemaker, the SCN may serve a noncircadian function in hibernators related to maintenance of energy balance.

Circadian rhythm of blood pressure is transformed from a dipper to a non-dipper pattern in shift workers with hypertension

Shift workers make great use of health care services because they are associated with increased cardiovascular morbidity and mortality. Whether the circadian rhythm of blood pressure rapidly adapts to shift work is controversial. It is unknown if shift work has adverse effects on blood pressure in patients with hypertension. To evaluate the effects of shift work, we examined 12 male shift workers with untreated hypertension aged 53.6 +/- 2.5 years. Twenty-four hour ambulatory blood pressure monitoring was performed three times as follows: the last day of a 4-day period of day shifts (09.00 to 21.00), the first day of a 4-day period of night shifts (21.00 to 09.00), and the fourth day of night shifts (21.00 to 09.00). Blood pressure at night-time dropped significantly in the day-shift workers, showing a dipper pattern. Average differences in blood pressure in the sleep-wake cycle were decreased by 8.5% at the beginning of night shift work showing a non-dipper pattern. After 4 days the pattern was completely reversed to a dipper pattern. The results indicate that the circadian blood pressure pattern is changed from a dipper to a non-dipper pattern on the first day of the night shift and reverses to a dipper pattern within a few days. We suggest that night shift work may have unfavourable effects on blood pressure in patients with hypertension.

The suprachiasmatic nucleus is essential for circadian body temperature rhythms in hibernating ground squirrels

Body temperature (T(b)) was recorded at 10 min intervals over 2.5 years in female golden-mantled ground squirrels that sustained complete ablation of the suprachiasmatic nucleus (SCNx). Animals housed at an ambient temperature (T(a)) of 6.5 degrees C were housed in a 12 hr light/dark cycle for 19 months followed by 11 months in constant light. The circadian rhythm of T(b) was permanently eliminated in euthermic and torpid SCNx squirrels, but not in those with partial destruction of the SCN or in neurologically intact control animals. Among control animals, some low-amplitude T(b) rhythms during torpor were driven by small (<0.1 degrees C) diurnal changes in T(a). During torpor bouts in which T(b) rhythms were unaffected by T(a), T(b) rhythm period ranged from 23.7 to 28.5 hr. Both SCNx and control squirrels were more likely to enter torpor at night and to arouse during the day in the presence of the light/dark cycle, whereas entry into and arousal from torpor occurred at random clock times in both SCNx and control animals housed in constant light. Absence of circadian rhythms 2.5 years after SCN ablation indicates that extra-SCN pacemakers are unable to mediate circadian organization in euthermic or torpid ground squirrels. The presence of diurnal rhythms of entry into and arousal from torpor in SCNx animals held under a light/dark cycle, and their absence in constant light, suggest that light can reach the retina of hibernating ground squirrels maintained in the laboratory and affect hibernation via an SCN-independent mechanism.

Parasympathetic and sympathetic control of the pancreas: a role for the suprachiasmatic nucleus and other hypothalamic centers that are involved in the regulation of food intake

To reveal brain regions and transmitter systems involved in control of pancreatic hormone secretion, specific vagal and sympathetic denervation were combined with injection of a retrograde transsynaptic tracer, pseudorabies virus (PRV), into the pancreas. After sympathetic or vagal transsection first-order neurons were revealed in the dorsal motor nucleus of the vagus (DMV) or in preganglionic spinal cord neurons (SPN), respectively. Careful timing of the survival of the animals allowed the detection of cell groups in immediate control of these DMV or SPN neurons. A far larger number of cell groups is involved in the control of DMV than of SPN neurons. Examples are given of a high level of interaction between the sympathetic and parasympathetic nervous system. Several cell groups project to both branches of the autonomic nervous system, sometimes even the same neurotransmitter is used, e.g., oxytocin neurons in the paraventricular nucleus and melanin-concentrating hormone and orexin neurons in the lateral hypothalamus project to both the DMV and SPN neurons. Moreover, the appearance of third-order neurons located in the sympathetic SPN after complete sympathectomy and in the DMV after complete vagotomy illustrates the possibility that motor neurons of the sympathetic and parasympathetic system may exchange information by means of interneurons. The presence of second-order neurons in prefrontal, gustatory, and piriform cortex may provide an anatomic basis for the involvement of these cortices in the cephalic insulin response. The observation that second-order neurons in both vagal and sympathetic control of the pancreas contain neuropeptides that are known to play a role in food intake indicates a direct association between behavioral and autonomic functions. Finally, the observation of third-order neurons in the suprachiasmatic nucleus and ventromedial hypothalamus shows the modulatory action of the time of the day and metabolic state, respectively.

Influence of melatonin administration on glucose tolerance and insulin sensitivity of postmenopausal women

OBJECTIVE

The effect of melatonin on human carbohydrate metabolism is not yet clear. We investigated whether melatonin influences glucose tolerance and insulin sensitivity in aged women.

PATIENTS

Twenty-two postmenopausal women of whom 14 were on hormone replacement therapy.

DESIGN

After an overnight fast, at 0800 hours on two nonconsecutive days, placebo or melatonin (1 mg) were administered randomly and in a double blind fashion. Forty-five minutes later, an oral glucose tolerance test (75 g; OGTT) was performed in 13 women. In another nine women insulin-dependent (Si) and -independent (Sg) glucose utilization was tested by a frequently sampled intravenous glucose tolerance test (FSIGT).

RESULTS

Areas under the response curve to OGTT (AUC) for glucose (1420 +/- 59 vs. 1250 +/- 55 mmol x min/l; P < 0.01), and C-peptide (42,0980 +/- 45,320 vs. 33,528 +/- 15,779 pmol x min/l; P < 0.02) were higher following melatonin than placebo, while Si values were lower (2.6 +/- 0.28 units vs. 3.49 +/- 0.4 units; P < 0.03). Si modifications induced by melatonin were inversely related to Si values of the placebo day (r(2) = 0.538; P < 0.025).

CONCLUSIONS

The present results indicate that in aged women administration of 1 mg of melatonin reduces glucose tolerance and insulin sensitivity. The present data may have both physiological and clinical implications.

The diurnal rhythm of energy expenditure differs between obese and glucose-intolerant rats and streptozotocin-induced diabetic rats

Otsuka Long Evans Tokushima Fatty (OLETF) rats were developed as a model of noninsulin-dependent diabetes mellitus (NIDDM) with mild obesity. Changes in carcass composition and in the daily profile of energy expenditure were examined before and after manifestation of diabetes (8 and 24 wk, respectively), and compared with the normal control Long Evans Tokushima (LETO) rats and streptozotocin (STZ)-induced diabetic LETO rats. OLETF rats had greater body weights than LETO rats and significantly greater absolute and relative fat weights. A diurnal rhythm of energy expenditure associated with two peaks was observed in LETO rats, but the two peaks were not apparent in OLETF rats at 24 wk of age. A diurnal rhythm associated with one peak was observed in STZ-induced diabetic LETO rats. Energy derived from fat constituted this peak; the pattern of the daily energy expenditure was significantly different from that of either nontreated LETO or OLETF rats at 24 wk of age. NIDDM in OLETF rats at 24 wk of age has only a small role in modification of the diurnal rhythm of energy expenditure, whereas STZ-induced diabetes significantly affected the rhythm.

Circadian rhythm of pain in male mice

1. Circadian rhythm of pain in response to the thermal stimuli was assessed in male mice. 2. The hot-plate method was used. Response latencies were measured every 2 hours and showed a sinusoidal rhythm. 3. Minimal latencies were observed at 10 and 20 h, the highest were obtained at 12 and 4 hours. 4. The circadian changes in pain sensitivity may play an important role in many experiments on stress and post-stress analgesia as well as on susceptibility to pharmacological agents.

Innervation of mammalian white adipose tissue: implications for the regulation of total body fat

We review the extensive physiological and neuroanatomical evidence for the innervation of white adipose tissue (WAT) by the sympathetic nervous system (SNS) as well as what is known about the sensory innervation of this tissue. The SNS innervation of WAT appears to be a part of the general SNS outflow from the central nervous system, consisting of structures and connections throughout the neural axis. The innervation of WAT by the SNS could play a role in the regulation of total body fat in general, most likely plays an important role in regional differences in lipid mobilization specifically, and may have a trophic affect on WAT. The exact nature of the SNS innervation of WAT is not known but it may involve contact with adipocytes and/or their associated vasculature. We hypothesize that the SNS innervation of WAT is an important contributor to the apparent "regulation" of total body fat.

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

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

Disappearance of diurnal rhythm of energy expenditure in genetically diabetic obese rats

The daily profile of energy expenditures was examined in the new animal model of genetically diabetic obese rats. The diurnal rhythm was observed at 8 weeks of age, with highest and lowest values for energy consumption per hour observed in the dark and light periods, respectively. However, at 24 weeks of age after the manifestation of noninsulin-dependent diabetes mellitus, the rhythm completely disappeared, but it did not in the control rats.

Suprachiasmatic nucleus neurons are glucose sensitive

The suprachiasmatic nucleus (SCN) in the hypothalamus serves as the pacemaker for mammalian circadian rhythms. In a hamster brain slice preparation, the authors were able to record spontaneous activity from SCN cells for up to 4 days in vitro and verify a self-sustained rhythm in firing. The phase of this rhythm was altered by the concentration of glucose in the bathing medium, with time of peak firing advanced for a 20 mM glucose condition and slightly delayed for a 5 mM glucose condition, relative to 10 mM. The advancing effect of 20 mM glucose and the delaying effect of 5 mM glucose were not maintained during a 2nd day in vitro after changing the bathing medium back to 10 mM glucose, thus indicating the effect was not a permanent phase shift of the underlying oscillation. In experiments recording from cell-attached membrane patches on acutely dissociated hamster SCN neurons, exchanging the bathing medium from high (20 mM) to zero glucose increased potassium (K+)-selective channel activity. With inside-out membrane patches, the authors revealed the presence of a glybenclamide-sensitive K+ channel (190 pS) and a larger conductance (260 pS) Ca(2+)-dependent K+ channel that were both reversibly inhibited by ATP at the cytoplasmic surface. Furthermore, 1 mM tetraethylammonium chloride was demonstrated to advance peak firing time in the brain slice in a similar manner to a high concentration of glucose (20 mM). The authors interpret the result to imply that SCNs are sensitive to glucose, most probably via ATP modulation of K+ channel activity in these neurons. Tonic modulation of K+ channel activity appears to alter output of the pacemaker but does not reset the phase.

Bilateral lesions of the hypothalamic suprachiasmatic nucleus eliminated sympathetic response to intracranial injection of 2-deoxy-D-glucose and VIP rescued this response

We previously found that bilateral lesions of the suprachiasmatic nucleus abolished hyperglycemic response to intracranial injection of 2-deoxy-D-glucose in rats. Because the hyperglycemia due to 2-deoxy-D-glucose was shown to be dependent on the functions of the adrenal medulla and sympathetic nervous system, the effect of bilateral lesions of the suprachiasmatic nucleus on changes in the nervous activity of sympathetic efferents to the adrenal after intracranial injection of 2-deoxy-D-glucose was examined in rats. It was found that bilateral lesions of the nucleus eliminated the increase in neural activity of the sympathetic efferent that occurred after the injection of 2-deoxy-D-glucose. Because the suprachiasmatic nucleus possesses neurons containing a vasoactive intestinal polypeptide-like substance, the effects of vasoactive intestinal polypeptide and 2-deoxy-D-glucose, administered alone or in combination, on the sympathetic activity were examined in intact control rats and in rats with bilateral lesions of the suprachiasmatic nucleus. It was found that in the normal control rats, vasoactive intestinal polypeptide alone increased the sympathetic activity, whereas it dramatically enhanced the sympathetic response to 2-deoxy-D-glucose. However, in rats with bilateral lesions of the suprachiasmatic nucleus, vasoactive intestinal polypeptide alone elicited no increase in the nervous activity of the sympathetic efferents to the adrenal, but combined administration of vasoactive intestinal polypeptide and 2-deoxy-D-glucose caused an increase in the nervous activity of sympathetic efferents to the adrenal. These findings suggest that the suprachiasmatic nucleus is involved in the enhancement of sympathetic activity caused by intracranial injection of 2-deoxy-D-glucose, and that neurons containing a vasoactive intestinal polypeptide-like substance in the suprachiasmatic nucleus play an important role in the sympathetic enhancement that occurs after intracranial injection of 2-deoxy-D-glucose. This role might be a permissive and facilitative one.

Involvement of cerebral neurotensin in hyperglycemic response caused by 2-deoxy-D-glucose in rats

Previous studies have indicated that neurons containing a vasoactive intestinal polypeptide (VIP)-like immunoreactive substance (VIP neurons) in the suprachiasmatic nucleus (SCN) are involved in regulating glucose metabolism in rats. In this connection, it has been suggested that in rats, VIP neurons in the SCN have neurotensin (NT) receptors. To clarify the role of NT, we examined the effects of intracranial injection of NT and an NT-antagonist on the hyperglycemic response to intracranial injection of 2-deoxy-D-glucose (2DG) in rats. The hyperglycemic and hyperglucagonemic responses caused by intracerebroventricular injection of 2DG were significantly enhanced by intracerebroventricular co-injection of NT, but suppressed by co-injection of the NT-antagonist. Intraperitoneal injection of the NT-antagonist did not affect the hyperglycemic and hyperglucagonemic responses to 2DG. These results suggest that intracranial NT plays an endogenously enhancive role in the hyperglycemic and hyperglucagonemic responses caused by 2DG.