Paradoxical [correction of parodoxical] effect of orexin A: hypophagia induced by hyperthermia

Orexins (hypocretins): The intersection between homeostatic and hedonic feeding

Orexins are hypothalamic neuropeptides originally discovered to play a role in the regulation of feeding behaviour. The broad connections of orexin neurons to mesocorticolimbic circuitry suggest they may play a role in mediating reward-related behaviour beyond homeostatic feeding. Here, we review the role of orexin in a variety of eating-related behaviour, with a focus on reward and motivation, and the neural circuits driving these effects. One emerging finding is the involvement of orexins in hedonic and appetitive behaviour towards palatable food, in addition to their role in homeostatic feeding. This review discusses the brain circuitry and possible mechanisms underlying the role of orexins in these behaviours. Overall, there is a marked bias in the literature towards studies involving male subjects. As such, future work needs to be done to involve female subjects. In summary, orexins play an important role in driving motivation for high salient rewards such as highly palatable food and may serve as the intersection between homeostatic and hedonic feeding.

Functional Changes Induced by Orexin A and Adiponectin on the Sympathetic/Parasympathetic Balance

Obesity and lifestyle-related diseases are major problems faced by people in developed nations. Although exercise training prevents the progression of diabetes and obesity, the motivation for exercise is generally low in obese animals and humans. The autonomic nervous system (SNA) plays a crucial role in the regulation of eating behavior. Moreover, the SNA is involved in the body temperature regulation that is strictly related to body weight control, in accordance with the “thermoregulatory hypothesis” of food intake. Some neuronal peptides and hormones, like orexins and adiponectin, are also involved in the regulation of locomotion activity as well as food intake and metabolic rate. Furthermore, adiponectin as well as orexin A are involved in the control of body temperature, food intake and therefore in obesity-related diseases. The aim of this study was to investigate the changes in body temperature (Tc), and heart rate (HR) after an intracerebroventricular (ICV) injection of orexin A and adiponectin in animal model. The results of this study show that the orexin A levels are likely involved in the increase of Tc and HR. It is also clear that there is not a correlation between these parameters and adiponectin levels. Further studies are needed to assess adiponectin actions and outcome in the central nervous system in terms of energy expenditure, body temperature, heart rate and physical activity performance regulation.

Functional Changes of Orexinergic Reaction to Psychoactive Substances

It is becoming increasingly apparent the importance of the central nervous system (CNS) as the major contributor to the regulation of systemic metabolism. Antipsychotic drugs are used often to treat several psychiatric disorders, including schizophrenia and bipolar disorder However, antipsychotic drugs prescription, particularly the second-generation ones (SGAs), such as clozapine and olanzapine, is related to a considerable weight gain which usually leads to obesity. The aim of this paper is to assess the influence of orexin A on sympathetic and hyperthermic reactions to several neuroleptic drugs. Orexin A is a neuropeptide which effects both body temperature and food intake by increasing sympathetic activity. Orexin A-mediated hyperthermia is reduced by haloperidol and is blocked by clozapine and olanzapine. Orexin A-mediated body temperature elevation is increased by risperidone. These hyperthermic effects are delayed by quietapine. In this paper, it is discussed the orexinergic pathway activation by neuroleptic drugs and its influence on human therapeutic strategies. With the aim to determine that neuroleptic drugs mediate body temperature control through to the orexinergic system, we summarized our previously published data. Psychiatric disorders increase the risk of developing metabolic disorders (e.g., weight gain, increased blood pressure, and glucose or lipid levels). Therefore, the choice of antipsychotic drug to be prescribed, based on the relevant risks and benefits of each individual drug, has an essential role in human health prevention.

Dose-Different Effects of Orexin-A on the Renal Sympathetic Nerve and Blood Pressure in Urethane-Anesthetized Rats

Previous studies have demonstrated that central injection of orexin-A affects renal sympathetic nerve activity (RSNA) and blood pressure (BP) in both anesthetized and unanesthetized rats. In the present study, we examined, using urethane-anesthetized rats, the dose-dependent effects of intravenous (iv) or intralateral cerebral ventricular (LCV) injection of various doses of orexin-A on RSNA and BP. We found that injection of a low dose of orexin-A (10 ng iv or 0.01 ng LCV) suppressed RSNA and BP significantly. Conversely, a high dose (1000 ng iv or 10 ng LCV) of orexin-A elevated both RSNA and BP significantly. Pretreatment with either iv or LCV injection of thioperamide, a histaminergic H3-receptor antagonist, eliminated the effects of a low dose of orexin-A on both RSNA and BP. Both iv and LCV injection of diphenhydramine, a histaminergic H1-receptor antagonist, abolished the effects of a high dose of orexin-A on RSNA and BP. Furthermore, bilateral lesions of the hypothalamic suprachiasmatic nucleus (SCN) abolished the effects of both low and high doses of orexin-A on RSNA and BP. These findings suggest that orexin-A affects RSNA and BP in a dose-dependent manner and that the SCN and histaminergic nerve may be involved in the dose-different effects of orexin-A in rats.

Thermoregulation under pressure: a role for orexin neurons

In the past, studies on stress responses and sleep/wake regulation were performed separately. The discovery of orexin (hypocretin) in 1998, however, dramatically changed the course of research and new findings regarding its role in these complex processes provided a better insight into their interactions and intricacies. Orexin-containing neuronal activity has been found to be minimal during sleep. It increases during the waking period and further increases during the active waking period, which includes stress responses and exploratory behaviors. Autonomic regulation of the body, which includes body temperature, blood flow, and ventilation, is also activated along with the change in vigilance states. Our recent findings suggest that orexin neurons act as a conductor of orchestration for vigilance states, behaviors, and autonomic functions. Body temperature regulation by orexin neurons seems to be mediated by one of its cotransmitters while cardiovascular and respiratory regulation are mediated by orexin itself.

Orexin-A controls sympathetic activity and eating behavior

It is extremely important for the health to understand the regulatory mechanisms of energy expenditure. These regulatory mechanisms play a central role in the pathogenesis of body weight alteration. The hypothalamus integrates nutritional information derived from all peripheral organs. This region of the brain controls hormonal secretions and neural pathways of the brainstem. Orexin-A is a hypothalamic neuropeptide involved in the regulation of feeding behavior, sleep-wakefulness rhythm, and neuroendocrine homeostasis. This neuropeptide is involved in the control of the sympathetic activation, blood pressure, metabolic status, and blood glucose level. This minireview focuses on relationship between the sympathetic nervous system and orexin-A in the control of eating behavior and energy expenditure. The “thermoregulatory hypothesis” of food intake is analyzed, underlining the role played by orexin-A in the control of food intake related to body temperature. Furthermore, the paradoxical eating behavior induced orexin-A is illustrated in this minireview.

Activity of autonomic nervous system is related to body weight in pre-menopausal, but not in post-menopausal women

This study analyzed vegetative modulation, expressed as heart rate variability (HRV) power spectral analysis, in lean and obese women at pre-menopausal or post-menopausal age to reveal possible differences in menopause-related autonomic activity in lean and obese subjects. Sedentary women (n = 40) were divided in four groups: pre-menopausal lean and obese women, post-menopausal lean and obese subjects. The HRV-power spectrum was evaluated on a 5-min long ECG recording. The absolute values of the spectrum were summed in the following frequencies: a low-frequency (0.04-0.15 Hz; LF) and high-frequency (0.15-0.40; HF) range. LF and HF were values used to estimate the sympathetic and parasympathetic activity. LF and HF values of pre-menopausal obese women are lower than values of lean women. The menopause induced a same decrease in LF and HF values in lean and obese subjects, so that no difference was found in post-menopausal groups. This experiment indicates that modifications of autonomic modulation can be included among factors related to obesity in pre-menopausal, but not post-menopausal women.

Behavioral responses to orexin, orexin receptor gene expression, and spontaneous physical activity contribute to individual sensitivity to obesity

There is significant variability in diet-induced obesity (DIO) among humans and rodents, which has been associated with differences in intrinsic spontaneous physical activity (SPA). The orexin neuropeptides positively modulate SPA through multiple brain sites, but the effects of DIO on orexin's activity are not well understood. In this study, we tested the hypothesis that DIO sensitivity is mediated by decreased SPA and changes in the function of the orexins. As a DIO model, we used male Sprague-Dawley rats fed a high-fat (HF; 45% kcal from fat) or a low-fat (LF; 10% kcal from fat) diet for 10 wk. We measured SPA before and after HF or LF feeding and expression of orexin receptors by real-time PCR after dietary treatments. We tested DIO effects on orexin signaling by measuring SPA after injection of orexin A in the rostral lateral hypothalamus (RLH) before and after 10 wk of HF feeding. Finally, we tested whether daily orexin A RLH injections prevent DIO caused by HF feeding. Our results show that resistance to DIO is associated with an increase in SPA, SPA after injection of orexin A in RLH, and orexin receptor expression in sites that mediate orexin's effect on SPA, including RLH. We show that daily injections of orexin peptide in RLH prevent DIO without altering food intake. We estimate that the energetic cost of SPA after orexin A RLH injection accounts for approximately 61% of the extra caloric intake associated with HF intake, suggesting additional effects of orexins. In summary, our results suggest that variability in DIO sensitivity is mediated through adaptations in the activity of the orexin peptides and their receptors.

Orexin modulates brown adipose tissue thermogenesis

Non-shivering thermogenesis in brown adipose tissue (BAT) plays an important role in thermoregulation. In addition, activations of BAT have important implications for energy homeostasis due to the metabolic consumption of energy reserves entailed in the production of heat in this tissue. In this conceptual overview we describe the role of orexins/hypocretins within the central nervous system in the modulation of thermogenesis in BAT under several physiological conditions. Within this framework, we consider potential neural mechanisms underlying the pathological conditions associated with the absence of the central orexinergic modulation of BAT thermogenesis and energy expenditure. Overall, the experimental basis for our understanding of the role of central orexin in regulating body temperature and energy homeostasis provides an illustrative example that highlights several general principles and caveats that should help to guide future investigations of the neurochemical regulation of thermogenesis and metabolism.

An orexinergic projection from perifornical hypothalamus to raphe pallidus increases rat brown adipose tissue thermogenesis

Non-shivering thermogenesis in brown adipose tissue (BAT) plays an important role in thermoregulatory cold-defense and, through its metabolic consumption of energy reserves to produce heat, can affect the long-term regulation of adiposity. An orexinergic pathway from the perifornical lateral hypothalamus (PeF/LH) to the rostral raphe pallidus (rRPa) has been demonstrated to increase the gain of the excitatory drives to medullary sympathetic premotor neurons controlling BAT sympathetic outflow and BAT thermogenesis. With this background, we consider neural mechanisms that could underlie orexin’s modulation of the excitability of BAT sympathetic premotor neurons in rRPa and the potential role of altered BAT thermogenesis in pathological conditions associated with the absence of the central orexin system. Overall, these new data enhance our understanding of the role of central orexin in regulating body temperature and energy homeostasis and provide further insight into the neurochemical regulation of BAT thermogenesis and metabolism.

Energy expenditure: role of orexin

The orexins/hypocretins are endogenous, modulatory and multifunctional neuropeptides with prominent influence on several physiological processes. The influence of orexins on energy expenditure is highlighted with focus on orexin action on individual components of energy expenditure. As orexin stabilizes and maintains normal states of arousal and the sleep/wake cycle, we also highlight orexin mediation of sleep and how sleep interacts with energy expenditure.

Orexin neurons and emotional stress

Stress increases cardiac function, ventilation, and body temperature and induces analgesia. These changes, which result in an increase in metabolic rate, oxygen supply, and the conduction velocity of nerve impulses, prepare the body for a fight-or-flight response. A part of the hypothalamus called the defense area has long been known to play a key role in these responses, but the precise mechanisms are largely unknown. Our recent findings suggest that orexin (hypocretin) neurons act as a master switch of the fight-or-flight response. In addition, our results, as well as those from other researchers, suggest that orexin neurons do not modulate specific behaviors such as the fight-or-flight responses but rather integrate the autonomic functions and behaviors in a broad sense or in a vigilance state-dependent manner. The orexin system seems to be a pivotal link between the subconscious and the conscious brain functions.

Neuropeptides controlling energy balance: orexins and neuromedins

In this chapter, we review the feeding and energy expenditure effects of orexin (also known as hypocretin) and neuromedin. Orexins are multifunctional neuropeptides that affect energy balance by participating in regulation of appetite, arousal, and spontaneous physical activity. Central orexin signaling for all functions originates in the lateral hypothalamus-perifornical area and is likely functionally differentiated based on site of action and on interacting neural influences. The effect of orexin on feeding is likely related to arousal in some ways but is nonetheless a separate neural process that depends on interactions with other feeding-related neuropeptides. In a pattern distinct from other neuropeptides, orexin stimulates both feeding and energy expenditure. Orexin increases in energy expenditure are mainly by increasing spontaneous physical activity, and this energy expenditure effect is more potent than the effect on feeding. Global orexin manipulations, such as in transgenic models, produce energy balance changes consistent with a dominant energy expenditure effect of orexin. Neuromedins are gut-brain peptides that reduce appetite. There are gut sources of neuromedin, but likely the key appetite-related neuromedin-producing neurons are in the hypothalamus and parallel other key anorectic neuropeptide expression in the arcuate to paraventricular hypothalamic projection. As with other hypothalamic feeding-related peptides, hindbrain sites are likely also important sources and targets of neuromedin anorectic action. Neuromedin increases physical activity in addition to reducing appetite, thus producing a consistent negative energy balance effect. Together with the other various neuropeptides, neurotransmitters, neuromodulators, and neurohormones, neuromedin and orexin act in the appetite network to produce changes in food intake and energy expenditure, which ultimately influences the regulation of body weight.

Orexin is required for brown adipose tissue development, differentiation, and function

Orexin (OX) neuropeptides stimulate feeding and arousal. Deficiency of orexin is implicated in narcolepsy, a disease associated with obesity, paradoxically in the face of reduced food intake. Here, we show that obesity in orexin-null mice is associated with impaired brown adipose tissue (BAT) thermogenesis. Failure of thermogenesis in OX-null mice is due to inability of brown preadipocytes to differentiate. The differentiation defect in OX-null neonates is circumvented by OX injections to OX-null dams. In vitro, OX, triggers the full differentiation program in mesenchymal progenitor stem cells, embryonic fibroblasts and brown preadipocytes via p38 mitogen activated protein (MAP) kinase and bone morphogenetic protein receptor-1a (BMPR1A)-dependent Smad1/5 signaling. Our study suggests that obesity associated with OX depletion is linked to brown-fat hypoactivity, which leads to dampening of energy expenditure. Thus, orexin plays an integral role in adaptive thermogenesis and body weight regulation via effects on BAT differentiation and function.

Hypocretin/orexin and energy expenditure

The hypocretins or orexins are endogenous neuropeptides synthesized in discrete lateral, perifornical and dorsal hypothalamic neurones. These multi-functional neuropeptides modulate energy homeostasis, arousal, stress, reward, reproduction and cardiovascular function. This review summarizes the role of hypocretins in modulating non-sleep-related energy expenditure with specific focus on the augmentation of whole body energy expenditure as well as hypocretin-induced physical activity and sympathetic outflow. We compare the efficacy of hypocretin-1 and 2 on energy expenditure and evaluate whether the literature implicates hypocretin signalling though the hypocretin-1 and -2 receptor as having shared and or functionally specific physiological effects. Thus far data suggest that hypocretin-1 has a more robust stimulatory effect relative to hypocretin-2. Furthermore, hypocretin-1 receptor predominantly mediates behaviours known to influence energy expenditure. Further studies on the hypocretin-2 receptor are needed.

Espresso coffee increases parasympathetic activity in young, healthy people

Caffeine induces modifications of activity of the autonomic nervous system. This study analyzed the effect of a cup of espresso coffee on the heart rate variability (HRV) power spectral analysis, which is a method providing evaluation of the sympathetic and parasympathetic discharge. In young, healthy sedentary subjects (10 male, 10 female; aged 25-30 years), the HRV-power spectrum was evaluated over a period of 150 min after the administration of espresso coffee (caffeine, 75 mg) or decaffeinated coffee (caffeine, < 18 mg) in supine and seated position. Absolute values of the spectrum were summed in low (LF) and high frequencies (HF). The LF and HF spectra were used to estimate the sympathetic and parasympathetic activity, respectively. In the supine position, coffee increases HF, while decaffeinated coffee causes little modifications of HF. In the seated position, HF is not modified by coffee or decaffeinated coffee. Coffee and decaffeinated coffee do not induce any modification of LF in both positions. This experiment indicates that espresso coffee influences parasympathetic activity in the supine position.

Melanin-concentrating hormone neurons discharge in a reciprocal manner to orexin neurons across the sleep-wake cycle

Neurons containing melanin-concentrating hormone (MCH) are codistributed with neurons containing orexin (Orx or hypocretin) in the lateral hypothalamus, a peptide and region known to be critical for maintaining wakefulness. Evidence from knockout and c-Fos studies suggests, however, that the MCH neurons might play a different role than Orx neurons in regulating activity and sleep-wake states. To examine this possibility, neurons were recorded across natural sleep-wake states in head-fixed rats and labeled by using the juxtacellular technique for subsequent immunohistochemical identification. Neurons identified as MCH+ did not fire during wake (W); they fired selectively during sleep, occasionally during slow wave sleep (SWS) and maximally during paradoxical sleep (PS). As W-Off/Sleep-On, the MCH neurons discharged in a reciprocal manner to the W-On/Sleep-Off Orx neurons and could accordingly play a complementary role to Orx neurons in sleep-wake state regulation and contribute to the pathophysiology of certain sleep disorders, such as narcolepsy with cataplexy.

Olanzapine blocks the sympathetic and hyperthermic reactions due to cerebral injection of orexin A

Since experiments regarding a possible relation between olanzapine and orexin A has been scarcely reported in international literature, this experiment tested the effect of olanzapine on the sympathetic and thermogenic effects induced by orexin A. The firing rates of the sympathetic nerves to interscapular brown adipose tissue (IBAT), along with IBAT, colonic temperatures and heart rate were monitored in urethane-anesthetized male Sprague-Dawley rats before an injection of orexin A (1.5 nmol) into the lateral cerebral ventricle and over a period of 150 min after the injection. The same variables were monitored in rats with an intraperitoneal administration of olanzapine (10mg/kg bw), injected 30 min before the orexin administration. The results show that orexin A increases the sympathetic firing rate, IBAT, colonic temperatures and heart rate. This increase is blocked by the injection of olanzapine. These findings indicate that olanzapine affects the complex reactions related to activation of orexinergic system.

Sympathetic and hyperthermic reactions by orexin A: role of cerebral catecholaminergic neurons

This experiment tested the effect of a lesion of cerebral catecholaminergic neurons on the sympathetic and thermogenic effects induced by an intracerebroventicular (icv) injection of orexin A. The firing rates of the sympathetic nerves to the interscapular brown adipose tissue (IBAT), along with IBAT, colonic temperatures and heart rate were monitored in urethane-anesthetized male Sprague-Dawley rats before an injection of orexin A (1.5 nmol) into the lateral cerebral ventricle and over a period of 150 min after the injection. Three days before the experiment, the rats were pre-treated with an icv injection of 6-hydroxydopamine (6-OHDA) or 6-OHDA plus desipramine or saline. The results show that orexin A increases the sympathetic firing rate, IBAT, colonic temperatures and heart rate in the rats pre-treated with saline. This increase is blocked by the pre-treatment with 6-OHDA alone or 6-OHDA plus desipramine. These findings indicate that cerebral catecholaminergic neurons (particularly the dopaminergic pathway) play a fundamental role in the complex reactions related to activation of the orexinergic system.

P2X2R purinergic receptor subunit mRNA and protein are expressed by all hypothalamic hypocretin/orexin neurons

Neurophysiologic data suggest that orexin neurons are directly excited by ATP through purinergic receptors (P2XR). Anatomical studies, though reporting P2XR in the hypothalamus, did not describe it in the perifornical hypothalamic area, where orexinergic neurons are located. Here we report the presence of the P2X(2)R subunit in the rat perifornical hypothalamus and demonstrate that hypothalamic orexin neurons express the P2X(2)R. Double immunohistochemistry showed that virtually all orexin-immunoreactive neurons are also P2X(2)R immunoreactive, whereas 80% of P2X(2)R-immunoreactive neurons are also orexin positive. Triple-labeling experiments, combining fluorescence in situ hybridization for P2X(2)R mRNA and P2X(2)R/orexin double immunofluorescence, confirmed these findings. In addition, in situ hybridization demonstrated that P2X(2)R mRNA is localized in cellular processes of orexinergic neurons. The present data support neurophysiologic findings on ATP modulation of orexinergic function and provide direct evidence that the entire population of orexin neurons expresses a P2XR subtype, namely, P2X(2)R. Thus, purinergic transmission might intervene in modulating key functions known to be controlled by the orexinergic system, such as feeding behavior and arousal.

Quetiapine lowers sympathetic and hyperthermic reactions due to cerebral injection of orexin A

Since no experiment regarding a possible relation between quetiapine and orexin A has been reported in international literature, this experiment tested the effect of quetiapine on the sympathetic and thermogenic effects induced by orexin A. The firing rates of the sympathetic nerves to interscapular brown adipose tissue (IBAT), along with IBAT, colonic temperatures and heart rate were monitored in urethane-anesthetized male Sprague-Dawley rats before an injection of orexin A (1.5 nmol) into the lateral cerebral ventricle and over a period of 150 min after the injection. The same variables were monitored in rats with an intraperitoneal administration of quetiapine (5 or 10 mg/kg bw), injected 30 min before the orexin administration. The results show that orexin A increases the sympathetic firing rate, IBAT, colonic temperatures and heart rate. This increase is delayed or reduced by the injection of quetiapine. These findings indicate that quetiapine affects the complex reactions related to activation of orexinergic system. Possible influences on the control of body weight and temperature are discussed.

Hyperthermic reactions induced by orexin A: role of the ventromedial hypothalamus

This experiment tested the involvement of the ventromedial hypothalamus (VMH) in the sympathetic and hyperthermic reactions induced by an intracerebroventricular (i.c.v.) injection of orexin A (1.5 nmol). In the first part of the experiment, the firing rate and cytochrome oxidase activity of the VMH neurons, and the colonic temperature were monitored in 12 urethane-anaesthetized rats before an i.c.v. injection of orexin and over a period of 2 h after the injection. Orexin induced an increase in the firing rate, colonic temperature and cytochrome oxidase activity. A group of 12 rats was used as a control: saline, but not orexin, was injected. No modifications in the firing rate, cytochrome oxidase reactivity and colonic temperature were noted. In the second part of the experiment, 12 rats were anaesthetized and lesioned bilaterally in the VMH with an injection of ibotenic acid. Sham lesions were carried out in 12 control rats. After 48 h, all animals were anaesthetized with ethyl-urethane. The firing rates of the sympathetic nerves to interscapular brown adipose tissue (IBAT), along with IBAT and colonic temperatures and heart rate were monitored before and over a period of 2 h after an i.c.v. injection of orexin or saline in the lesioned and sham-lesioned rats. Orexin increased the sympathetic firing rate, IBAT and colonic temperatures and heart rate in the sham-lesioned rats. These increases were reduced by lesion of VMH. Saline did not induce any modification. These findings indicate that the VMH is involved in the control of the orexin-induced hyperthermia.

Orexin and MCH neurons express c-Fos differently after sleep deprivation vs. recovery and bear different adrenergic receptors

Though overlapping in distribution within the posterior hypothalamus, neurons containing orexin (Orx) and melanin concentrating hormone (MCH) may play different roles in the regulation of behavioural state. In the present study in rats, we tested whether they express c-Fos differently after total sleep deprivation (SD) vs. sleep recovery (SR). Whereas c-Fos expression was increased in Orx neurons after SD, it was increased in MCH neurons after SR. We reasoned that Orx and MCH neurons could be differently modulated by noradrenaline (NA) and accordingly bear different adrenergic receptors (ARs). Of all Orx neurons (estimated at approximately 6700), substantial numbers were immunostained for the alpha1A-AR, including cells expressing c-Fos after SD. Yet, substantial numbers were also immunostained for the alpha2A-AR, also including cells expressing c-Fos after SD. Of all MCH neurons (estimated at approximately 12,300), rare neurons were immunostained for the alpha1A-AR, whereas significant numbers were immunostained for the alpha2A-AR, including cells expressing c-Fos after SR. We conclude that Orx neurons may act to sustain waking during sleep deprivation, whereas MCH neurons may act to promote sleep following sustained waking. Some Orx neurons would participate in the maintenance of waking during deprivation when excited by NA through alpha1-ARs, whereas MCH neurons would participate in sleep recovery after deprivation when released from inhibition by NA through alpha2-ARs. On the other hand, under certain conditions, Orx neurons may also be submitted to an inhibitory influence by NA through alpha2-ARs.

Discharge of identified orexin/hypocretin neurons across the sleep-waking cycle

Although maintained by multiple arousal systems, wakefulness falters if orexin (hypocretin), orexin receptors, or orexin neurons are deficient; narcolepsy results with hypersomnolence or sudden onset of rapid eye movement sleep [or paradoxical sleep (PS)] and loss of muscle tonus. To learn how orexin neurons maintain wakefulness, we recorded neurons in head-fixed rats across the sleep-waking cycle and then labeled them with Neurobiotin to identify them by immunohistochemistry. We show that identified orexin neurons discharge during active waking, when postural muscle tone is high in association with movement, decrease discharge during quiet waking in absence of movement, and virtually cease firing during sleep, when postural muscle tone is low or absent. During PS, they remain relatively silent in association with postural muscle atonia and most often despite phasic muscular twitches. They increase firing before the end of PS and thereby herald by several seconds the return of waking and muscle tone. The orexin neurons would thus stimulate arousal, while antagonizing sleep and muscle atonia.

Dual regulatory effects of orexins on sympathetic nerve activity innervating brown adipose tissue in rats

This study examined how orexin regulates the activity of the sympathetic nerves that innervate brown adipose tissue (BAT) in rats. Infusion of orexin A at a dose of 0.3 nmol into the third cerebral ventricle decreased BAT sympathetic nerve activity, compared with the effect of PBS (P < 0.05), whereas infusion of orexin B at the same dose caused a significant increase (P < 0.05). Pretreatment with a third cerebral ventricle injection of 2.24 micromol/kg alpha-fluoromethylhistidine, an irreversible inhibitor of the histamine-synthesizing enzyme histidine decarboxylase, attenuated the orexin B-induced response of BAT sympathetic nerve activity, but not that induced by orexin A. These results indicate that orexins may regulate both BAT energy expenditure and thermogenesis through their dual effects on sympathetic nerve activity. In particular, orexin B regulates BAT sympathetic nerve activity via neuronal histamine in the hypothalamus.

Acute, subacute and chronic effects of central neuropeptide Y on energy balance in rats

Central neuropeptide Y (NPY) injection has been reported to cause hyperphagia and in some cases also hypometabolism or hypothermia. Chronic central administration induced a moderate rise of short duration in body weight, without consistent metabolic/thermal changes. In the present studies the acute and subsequent subacute ingestive and metabolic/thermal changes were studied following intracerebroventricular (i.c.v.) injections of NPY in cold-adapted and non-adapted rats, or the corresponding chronic changes following i.c.v. NPY infusion. Besides confirming basic earlier data, we demonstrated novel findings: a temporal relationship for the orexigenic and metabolic/thermal effects, and differences of coordination in acute/subacute/chronic phases or states. The acute phase (30-60 min after injection) was anabolic: coordinated hyperphagia and hypometabolism/hypothermia. NPY evoked a hypothermia by suppressing any (hyper)metabolism in excess of basal metabolic rate, without enhancing heat loss. Thus, acute hypothermia was observed in sub-thermoneutral but not thermoneutral environments. The subsequent subacute catabolic phase exhibited opposite effects: slight increase in metabolic rate, rise in body temperature, reaching a plateau within 3-4 h after injection -- this was maintained for at least 24 h; meanwhile the food intake decreased and the normal daily weight gain stopped. This rebound is only indirectly related to NPY. Chronic (7-day long) i.c.v. NPY infusion induced an anabolic phase for 2-3 days, followed by a catabolic phase and fever, despite continued infusion. In cold-adaptation environment the primary metabolic effect of the infusion induced a moderate hypothermia with lower daytime nadirs and nocturnal peaks of the circadian temperature rhythm, while at near-thermoneutral environments in non-adapted rats the infusion attenuated only the nocturnal temperature rise by suppressing night-time hypermetabolism. Further finding is that in cold-adapted animals, the early feeding effect of NPY-infusion was enhanced, whereas the early hypothermic effect in cold was limited by interference with competing thermoregulatory mechanisms.

Differential effects of infralimbic cortical lesions on temperature and locomotor activity responses to feeding in rats

The time of food availability induces important behavioral and metabolic adaptations. Animals subjected to feeding restricted to a few daytime hours show increased locomotor activity and body temperature in anticipation of mealtime. In addition, animals under ad libitum feeding show a marked postprandial raise in body temperature and in thermogenesis. The areas of the brain commanding these responses to food are partially known. We investigated in the rat the role of the infralimbic area, located in the medial prefrontal cortex, and considered a visceral-autonomic motor area, in the responses to ad libitum or restricted feeding schedule. We performed infralimbic cortex excitotoxic lesions using injections of ibotenic acid, and measured body temperature and locomotor activity by telemetry in rats under ad libitum and restricted feeding conditions. We found that bilateral infralimbic area lesions prevented both the anticipatory and the postprandial increases in core temperature, decreased mean temperature by nearly 0.3 degrees C during both light/dark phases, and increased daily temperature variability. In contrast, the lesion caused a rapid induction of the anticipatory locomotor activity. These results show that behavioral and metabolic responses to the time of food availability are commanded separately and that the infralimbic area is a key structure to adjust the body temperature to an upcoming meal.

Extracellular GABA in the medial hypothalamus is increased following hypocretin-1 administration

AIM

Hypocretin 1 is an hypothalamic neuropeptide that induces an increase in food intake when administered into the cerebral lateral ventricle. As it is well known that the medial hypothalamus (MH) is involved in the feeding behaviour also through GABAergic circuits, the aim of this experiment was to investigate the effect of an hypocretin 1 intracerebroventricular (icv) injection on the extracellular levels of GABA in the MH.

METHODS

GABA levels in the MH were evaluated in six rats by microdialysis and high performance liquid chromatography-electrochemical detection 30 min before and every 30 min for an over all period of 6 h after an icv injection of hypocretin 1. The same procedure was used in another group of six rats but saline was injected into the lateral ventricle as control.

RESULTS

The results show that extracellular GABA increases in the MH after the injection of hypocretin 1 at 60 min and at 3 h after the injection.

CONCLUSION

This finding suggests a possible mechanism by which hypocretin 1 should induce hyperphagia in the first hour after injection. As it is already known that the inhibition of the MH by injection of GABA causes an increase of food intake, it is possible that hypocretin 1 causes an increase in food intake by increasing the GABA release in the MH. The lack of an increase in the GABA level after the fourth hour is consistent with the lack of an increase in food intake at this time, as we observed in previous experiments. The finding of a biphasic increase in the GABA level, at 60 min and at 3 h, was unexpected and should be further investigated.

Injection of orexin A into the diagonal band of Broca induces symphatetic and hyperthermic reactions

This experiment tested the effect of an injection of orexin A into the diagonal band of Broca on the sympathetic activity and body temperature. Concentration of glycerol into white fat of lumbar region, firing rates of sympathetic nerves to interscapular brown adipose tissue (IBAT), IBAT and colonic temperatures, and heart rate were monitored in urethane-anesthetized male Sprague-Dawley rats for 30 min before and 150 min after injections of orexin A (0.4 and 0.7 nmol) into the diagonal band of Broca. The same variables were monitored in control rats with an injection of saline. The results show that orexin A increases glycerol concentration, sympathetic firing rate, IBAT and colonic temperatures, and heart rate. The saline injection did not induce any modification. These findings suggest that the diagonal band of Broca is a cerebral structure involved in the induction of the hyperthermia due to orexin A.

Cortical spreading depression blocks the hyperthermic reaction induced by orexin a

This experiment tested the effect of cortical spreading depression on the sympathetic and thermogenic effects induced by orexin A. The firing rates of the sympathetic nerves to interscapular brown adipose tissue (IBAT), along with IBAT and colonic temperatures and heart rate were monitored in urethane-anesthetized male Sprague-Dawley rats before and 5 h after an injection of orexin A (1.5 nmol) into the lateral cerebral ventricle. The same variables were monitored in rats with cortical spreading depression, induced by an application of cotton pellets soaked with 2 M KCl to the frontal cortex. The results show that orexin A increases the sympathetic firing rate, IBAT and colonic temperatures and heart rate. The increases in firing rate, IBAT and colonic temperatures are blocked by cortical spreading depression, while the increase in heart rate is not affected by cortical spreading depression. These findings suggest that the cerebral cortex is involved in the control of the orexin A-induced hyperthermia. Furthermore, we suggested the name "hyperthermine A," as additional denomination of "orexin A."