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The regulation of sleep/wake behavior and energy homeostasis is maintained in part by the hypothalamic neuropeptide orexin A (OXA, hypocretin). Reduction in orexin signaling is associated with sleep disorders and obesity, whereas higher lateral hypothalamic (LH) orexin signaling and sensitivity promotes obesity resistance. Similarly, dysregulation of hypothalamic neural networks is associated with onset of age-related diseases, including obesity and several neurological diseases. Despite the association of obesity and aging, and that adult populations are the target for the majority of pharmaceutical and obesity studies, conventional models for neuronal networks utilize embryonic neural cultures rather than adult neurons. Synchronous activity describes correlated changes in neuronal activity between neurons and is a feature of normal brain function, and is a measure of functional connectivity and final output from a given neural structure. Earlier studies show alterations in hypothalamic synchronicity following behavioral perturbations in embryonic neurons obtained from obesity-resistant rats and following application of orexin onto embryonic hypothalamic cultures. Synchronous network dynamics in adult hypothalamic neurons remain largely undescribed. To address this, we established an adult rat hypothalamic culture in multi-electrode-array (MEA) dishes and recorded the field potentials. Then we studied the effect of exogenous orexin on network synchronization of these adult hypothalamic cultures. In addition, we studied the wake promoting effects of orexin in vivo when directly injected into the lateral hypothalamus (LH). Our results showed that the adult hypothalamic cultures are viable for nearly 3 mo in vitro, good quality MEA recordings can be obtained from these cultures in vitro, and finally, that cultured adult hypothalamus is responsive to orexin. These results support that adult rat hypothalamic cultures could be used as a model to study the neural mechanisms underlying obesity. In addition, LH administration of OXA enhanced wakefulness in rats, indicating that OXA enhances wakefulness partly by promoting neural synchrony in the hypothalamus. NEW & NOTEWORTHY This study, for the first time, demonstrates that adult hypothalamic cultures are viable in vitro for a prolonged duration and are electrophysiologically active. In addition, the study shows that orexin enhances neural synchronization in adult hypothalamic cultures.

Loss of orexin-producing neurons results in narcolepsy with cataplexy, and orexin agonists have been shown to increase wakefulness and alleviate narcolepsy symptoms in animal models. Several OX2R agonists have been reported but with little or no activity at OX1R. We conducted structure-activity relationship studies on the OX2R agonist YNT-185 (2) and discovered dual agonists such as RTOXA-43 (40) with EC50's of 24 nM at both OX2R and OX1R. Computational modeling studies based on the agonist-bound OX2R cryogenic electron microscopy structures showed that 40 bound in the same binding pocket and interactions of the pyridylmethyl group of 40 with OX1R may have contributed to its high OX1R potency. Intraperitoneal injection of 40 increased time awake, decreased time asleep, and increased sleep/wake consolidation in 12-month old mice. This work provides a promising dual small molecule agonist and supports development of orexin agonists as potential treatments for orexin-deficient disorders such as narcolepsy.

Synchronous neural activity is a feature of normal brain function, and altered synchronization is observed in several neurological diseases. Dysfunction in hypothalamic pathways leads to obesity, suggesting that hypothalamic neural synchrony is critical for energy homeostasis. The lateral hypothalamic orexin neurons are extensively interconnected with other brain structures and are important for energy balance. Earlier studies show that rats with higher orexin sensitivity are obesity resistant. Similarly, topiramate, an anti-epileptic drug, has been shown to reduce weight in humans. Since orexin enhances neuronal excitation, we hypothesized that obesity-resistant rats with higher orexin sensitivity may exhibit enhanced hypothalamic synchronization. We further hypothesized that anti-obesity agents such as orexin and topiramate will enhance hypothalamic synchronization. To test this, we examined neural synchronicity in primary embryonic hypothalamic cell cultures, obtained from embryonic day 18 (E18) obesity-susceptible Sprague-Dawley (SD) and obesity-resistant rats. Hypothalamic tissue was cultured in multielectrode array (MEA), and recordings were performed twice weekly, from 4th to 32nd day in vitro (DIV). Next, we tested the effects of orexin and topiramate application on neural synchronicity of hypothalamic cultures obtained from SD rat embryos. Signals were analyzed for synchronization using cross correlation. Our results showed that (1) obesity-resistant hypothalamus exhibits significantly higher synchronization compared to obesity-sensitive hypothalamus; and (2) orexin and topiramate enhance hypothalamic synchronization. These results support that enhanced orexin sensitivity is associated with greater neural synchronization, and that anti-obesity treatments enhance network synchronization, thus constrain variability in hypothalamic output signals, to extrahypothalamic structures involved in energy homeostasis.

Obesity is a leading cause of preventable death in the United States. Currently approved pharmacotherapies for the treatment of obesity are associated with rebound weight gain, negative side effects, and the potential for abuse. There is a need for new treatments with fewer side effects. Minor tobacco alkaloids (MTAs) are potential candidates for novel obesity pharmacotherapies. These alkaloids are structurally related to nicotine, which can help reduce body weight, but without the same addictive potential. The purpose of the current study was to examine the effects of three MTAs (nornicotine, anatabine, and anabasine) and nicotine on weight gain, body composition, chow intake, and physical activity. We hypothesized that the MTAs and nicotine would reduce weight gain through reductions in chow intake and increases in physical activity. To test this, male Sprague Dawley rats were housed in metabolic phenotyping chambers. Following acclimation to these chambers and to (subcutaneous (sc)) injections of saline, animals received daily injections (sc) of nornicotine, anabasine, anatabine, or nicotine for one week. Compared to saline-injected animals that gained body weight and body fat during the treatment phase, injections of nornicotine and anatabine prevented additional weight gain, alongside reductions in body fat. Rats receiving anabasine and nicotine gained body weight at a slower rate relative to rats receiving saline injections, and body fat remained unchanged. All compounds reduced the intake of chow pellets. Nornicotine and nicotine produced consistent increases in physical activity 6 h post-injection, whereas anabasine’s and anatabine’s effects on physical activity were more transient. These results show that short-term, daily administration of nornicotine, anabasine, and anatabine has positive effects on weight loss, through reductions in body fat and food intake and increases in physical activity. Together, these findings suggest that MTAs are worthy of further investigations as anti-obesity pharmacotherapies.

The lateral hypothalamus is critical for the control of ingestive behavior and spontaneous physical activity (SPA), as lesion or stimulation of this region alters these behaviors. Evidence points to lateral hypothalamic orexin neurons as modulators of feeding and SPA. These neurons affect a broad range of systems, and project to multiple brain regions such as the dorsal raphe nucleus, which contains serotoninergic neurons (DRN) important to energy homeostasis. Physical activity is comprised of intentional exercise and SPA. These are opposite ends of a continuum of physical activity intensity and structure. Non-goal-oriented behaviors, such as fidgeting, standing, and ambulating, constitute SPA in humans, and reflect a propensity for activity separate from intentional activity, such as high-intensity voluntary exercise. In animals, SPA is activity not influenced by rewards such as food or a running wheel. Spontaneous physical activity in humans and animals burns calories and could theoretically be manipulated pharmacologically to expend calories and protect against obesity. The DRN neurons receive orexin inputs, and project heavily onto cortical and subcortical areas involved in movement, feeding and energy expenditure (EE). This review discusses the function of hypothalamic orexin in energy-homeostasis, the interaction with DRN serotonin neurons, and the role of this orexin-serotonin axis in regulating food intake, SPA, and EE. In addition, we discuss possible brain areas involved in orexin-serotonin cross-talk; the role of serotonin receptors, transporters and uptake-inhibitors in the pathogenesis and treatment of obesity; animal models of obesity with impaired serotonin-function; single-nucleotide polymorphisms in the serotonin system and obesity; and future directions in the orexin-serotonin field. This article is categorized under: Metabolic DiseasesMolecular and Cellular Physiology.

The position statement is issued by The Obesity Society in response to published literature, as well as inquiries made to the Society by patients, providers, Society members, policy makers, and others regarding the efficacy of vaccines in persons with obesity against SARS‐CoV‐2, the virus that causes COVID‐19. The Obesity Society has critically evaluated data from published peer‐reviewed literature and briefing documents from Emergency Use Authorization applications submitted by Pfizer‐BioNTech, Moderna, and Johnson & Johnson. We conclude that these vaccines are highly efficacious, and their efficacy is not significantly different in people with and without obesity, based on scientific evidence available at the time of publication. The Obesity Society believes there is no definitive way to determine which of these three COVID‐19 vaccines is “best” for any weight subpopulation (because of differences in the trial design and outcome measures in the phase 3 trials, elapsed time between doses, and regional differences in the presence of SARS‐CoV‐2 variants [e.g., South Africa B.1.351 in Johnson & Johnson trial]). All three trials have demonstrated high efficacy against COVID‐19–associated hospitalization and death. Therefore, The Obesity Society encourages adults with obesity ≥18 years (≥16 years for Pfizer‐BioNTech) to undergo vaccination with any one of the currently available vaccines authorized for emergency use by the US Food and Drug Administration as soon as they are able.

Behavioral plasticity refers to changes occurring due to external influences on an organism, including adaptation, learning, memory and enduring influences from early life experience. There are 2 types of behavioral plasticity: "developmental", which refers to gene/environment interactions affecting a phenotype, and "activational" which refers to innate physiology and can involve structural physiological changes of the body. In this review, we focus on feeding behavior, and studies involving neuropeptides that influence behavioral plasticity - primarily opioids, orexin, neuropeptide Y, and oxytocin. In each section of the review, we include examples of behavioral plasticity as it relates to actions of these neuropeptides. It can be concluded from this review that eating behavior is influenced by a number of external factors, including time of day, type of food available, energy balance state, and stressors. The reviewed work underscores that environmental factors play a critical role in feeding behavior and energy balance, but changes in eating behavior also result from a multitude of non-environmental factors, such that there can be no single mechanism or variable that can explain ingestive behavior.

The orexin peptides promote hedonic intake and other reward behaviors through different brain sites. The opioid dynorphin peptides are co-released with orexin peptides but block their effects on reward in the ventral tegmental area (VTA). We previously showed that in the paraventricular hypothalamic nucleus (PVN), dynorphin and not orexin peptides enhance hedonic intake, suggesting they have brain-site-specific effects. Obesity alters the expression of orexin and dynorphin receptors, but whether their expression across different brain sites is important to hedonic intake is unclear. We hypothesized that hedonic intake is regulated by orexin and dynorphin peptides in PVN and that hedonic intake in obesity correlates with expression of their receptors. Here we show that in mice, injection of DYN-A1-13 (an opioid dynorphin peptide) in the PVN enhanced hedonic intake, whereas in the VTA, injection of OXA (orexin-A, an orexin peptide) enhanced hedonic intake. In PVN, OXA blunted the increase in hedonic intake caused by DYN-A1-13. In PVN, injection of norBNI (opioid receptor antagonist) reduced hedonic intake but a subsequent OXA injection failed to increase hedonic intake, suggesting that OXA activity in PVN is not influenced by endogenous opioid activity. In the PVN, DYN-A1-13 increased the intake of the less-preferred food in a two-food choice task. In obese mice fed a cafeteria diet, orexin 1 receptor mRNA across brain sites involved in hedonic intake correlated with fat preference but not caloric intake. Together, these data support that orexin and dynorphin peptides regulate hedonic intake in an opposing manner with brain-site-specific effects.

Synchronous neural activity is a feature of normal brain function, and altered synchronization is observed in several neurological diseases. Dysfunction in hypothalamic pathways leads to obesity, suggesting that hypothalamic neural synchrony is critical for energy homeostasis. The lateral hypothalamic orexin neurons are extensively interconnected with other brain structures and are important for energy balance. Earlier studies show that rats with higher orexin sensitivity are obesity resistant. Similarly, topiramate, an anti-epileptic drug, has been shown to reduce weight in humans. Since orexin enhances neuronal excitation, we hypothesized that obesity-resistant rats with higher orexin sensitivity may exhibit enhanced hypothalamic synchronization. We further hypothesized that anti-obesity agents such as orexin and topiramate will enhance hypothalamic synchronization. To test this, we examined neural synchronicity in primary embryonic hypothalamic cell cultures, obtained from embryonic day 18 (E18) obesity-susceptible Sprague-Dawley (SD) and obesity-resistant rats. Hypothalamic tissue was cultured in multielectrode array (MEA), and recordings were performed twice weekly, from 4th to 32nd day in vitro (DIV). Next, we tested the effects of orexin and topiramate application on neural synchronicity of hypothalamic cultures obtained from SD rat embryos. Signals were analyzed for synchronization using cross correlation. Our results showed that (1) obesity-resistant hypothalamus exhibits significantly higher synchronization compared to obesity-sensitive hypothalamus; and (2) orexin and topiramate enhance hypothalamic synchronization. These results support that enhanced orexin sensitivity is associated with greater neural synchronization, and that anti-obesity treatments enhance network synchronization, thus constrain variability in hypothalamic output signals, to extrahypothalamic structures involved in energy homeostasis.

Orexin A is produced in neurons of the lateral, perifornical and dorsomedial regions of the lateral hypothalamic area, which then project widely throughout the central nervous system to regulate arousal state, sleep-wake architecture, energy homeostasis and cognitive processes. Disruption of orexin signaling leads to sleep disturbances and increased body mass index, but recent studies also indicate that orexin neuron activation improves learning and memory. We hypothesized that hippocampal orexin receptor activation improves memory. To test this idea, we obtained orexin/ataxin-3 (O/A3) mice, which become deficient in orexin neurons by about 12 weeks of age. We first measured hippocampal orexin receptor 1 (OX1R) gene expression and protein levels, then tested acquisition and consolidation of two-way active avoidance (TWAA) memory, a hippocampal-dependent learning and memory task. Finally, we determined if exogenous intra-hippocampal OXA treatment could reverse cognitive impairment (as determined by TWAA) in OA/3 mice. We showed that OX1R mRNA expression and protein levels were significantly elevated in O/A3 mice, indicating the potential for preserved orexin responsiveness. The O/A3 mice were significantly impaired in TWAA memory vs. control mice, but OXA treatment (both acute and chronic) reversed these memory deficits. These results demonstrate that orexin plays an important role in hippocampal-dependent consolidation of two-way active avoidance memory, and orexin replacement can rescue the cognitive impairment.

Spontaneous physical activity (SPA) is a physical activity not motivated by a rewarding goal, such as that associated with food-seeking or wheel-running behavior. SPA is often thought of as only “fidgeting,” but that is a mischaracterization, since fidgety behavior can be linked to stereotypies in neurodegenerative disease and other movement disorders. Instead, SPA should be thought of as all physical activity behavior that emanates from an unconscious drive for movement. An example of this may be restless behavior, which can include fidgeting and gesticulating, frequent sit-to-stand movement, and more time spent standing and moving. All physical activity burns calories, and as such, SPA could be manipulated as a means to burn calories, and defend against weight gain and reduce excess adiposity. In this review, we discuss human and animal literature on the use of SPA in reducing weight gain, the neuromodulators that could be targeted to this end, and future directions in this field.

BACKGROUND: Identifying whether components of total energy expenditure (EE) are affected by orexin receptor (OXR1 and OXR2) stimulation or antagonism with dual orexin receptor antagonists (DORAs) has relevance for obesity treatment. Orexin receptor stimulation reduces weight gain by increasing total EE and EE during spontaneous physical activity (SPA). OBJECTIVE: The purpose of this study was to determine if a DORA (TCS-1102) in the ventrolateral preoptic area (VLPO) reduced orexin-A-induced arousal, SPA, total EE and EE during sleep, rest, wake and SPA and whether the DORA alone reduced total EE and its components. We hypothesized that: (1) a DORA would reduce orexin-A induced increases in arousal, SPA, components of total EE, reductions in sleep and the EE during sleep and (2) the DORA alone would reduce baseline (non-stimulated) SPA and total EE. SUBJECTS/METHODS: Sleep, wakefulness, SPA and EE were determined after microinjection of the DORA (TCS-1102) and orexin-A in the VLPO of male Sprague–Dawley rats with a unilateral cannula targeted towards the VLPO. Individual components of total EE were determined based on time-stamped data. RESULTS: The DORA reduced orexin-A-induced increases in arousal, SPA, total EE and EE during SPA, wake, rest and sleep 1 h post injection (P < 0.05). Orexin-A significantly reduced sleep and significantly increased EE during sleep 1 h post injection (P < 0.05). Furthermore, the DORA alone significantly reduced total EE, EE during sleep (NREM and REM) and resting EE 2 h post injection (P < 0.05). CONCLUSIONS: These data suggest that orexin-A reduces weight gain by stimulating total EE through increases in EE during SPA, rest and sleep. Residual effects of the DORA alone include decreases in total EE and EE during sleep and rest, which may promote weight gain.

Spontaneous physical activity (SPA) describes activity outside of formal exercise and shows large interindividual variability. The hypothalamic orexin/hypocretin peptides are key regulators of SPA. Orexins drive SPA within multiple brain sites, including rostral lateral hypothalamus (LH) and nucleus accumbens shell (NAcSh). Rats with high basal SPA (high activity, HA) show higher orexin mRNA expression and SPA after injection of orexin-A in rostral LH compared with low-activity (LA) rats. Here, we explored the contribution of orexin signaling in rostral LH and NAcSh to the HA/LA phenotype. We found that HA rats have higher sensitivity to SPA after injection of orexin-A in rostral LH, but not in NAcSh. HA and LA rats showed similar levels of orexin receptor expression in rostral LH, and activation of orexin-producing neurons after orexin-A injection in rostral LH. Also, in HA and LA rats, the coinjection of orexin-A in rostral LH and NAcSh failed to further increase SPA beyond the effects of orexin-A in rostral LH. Pretreatment with muscimol, a GABAA receptor agonist, in NAcSh potentiated SPA produced by orexin-A injection in rostral LH in HA but not in LA rats. Our results suggest that a feedback loop from orexin-responsive neurons in rostral LH to orexin neurons and a the NAcSh–orexin neuron–rostral LH circuit regulate SPA. Overall, our data suggest that differences in orexin sensitivity in rostral LH and its modulation by GABA afferents from NAcSh contribute to individual SPA differences.

Obesity, metabolic syndrome, and diabetes continue to be a scourge of modern society, despite much research and developed therapies. Research in these areas is ongoing, and in this review we discuss models of obesity, metabolic syndrome, and diabetes in the rat. There are many experimental and genetic ways to achieve these conditions, and these states can be either experimentally induced or spontaneously developed. Here we cover the various ways in which these disease states are achieved, and experimental uses of these disease models. The goal is to provide a practical and scientific resource for the experimenter who aims to study these disease states. First, we describe dietary methods to induce obesity, as well as sleep deprivation and physical activity models that lead to obesity. Next, aspects and models of metabolic syndrome are presented. Finally, models and caveats to the study of diabetes rat models are described.

The emerging obesity epidemic and accompanying health consequences led The Obesity Society (TOS) in 2008 to publish a position paper defining obesity as a disease. Since then, new information has emerged on the underlying mechanisms leading to excess adiposity and the associated structural, cardiometabolic, and functional disturbances. This report presents the updated TOS 2018 position statement on obesity as a noncommunicable chronic disease.

Parkinson’s disease (PD), classically defined as a progressive motor disorder accompanied with dopaminergic neuron loss and presence of Lewy bodies, is the second most common neurodegenerative disease. PD also has various non-classical symptoms, including cognitive impairments. In addition, inflammation and astrogliosis are recognized as an integral part of PD pathology. The hippocampus (Hipp) is a brain region involved in cognition and memory, and the neuropeptide orexin has been shown to enhance learning and memory. Previous studies show impairments in Hipp-dependent memory in a transgenic mouse model of Parkinson’s disease (A53T mice), and we hypothesized that increasing orexin tone will reverse this. To test this, we subjected 3, 5, and 7-month old A53T mice to a Barnes maze and a contextual object recognition test to determine Hipp dependent memory. Inflammation and astrogliosis markers in the Hipp were assessed by immuno-fluorescence densitometry. The data show that early cognitive impairment is coupled with an increase in expression of inflammatory and astrogliosis markers. Next, in two separate experiments, mice were given intra-hippocampal injections of orexin or chemogenetic viral injections of an orexin neuron specific Designer Receptor Exclusively Activated by Designer Drug (DREADD). For the pharmacological approach mice were intracranially treated with orexin A, whereas the chemogenetic approach utilized clozapine N-oxide (CNO). Both pharmacological orexin A intervention as well as chemogenetic activation of orexin neurons ameliorated Hipp-dependent early memory impairment observed in A53T mice. This study implicates orexin in PD-associated cognitive impairment and suggests that exogenous orexin treatment and/or manipulation of endogenous orexin levels may be a potential strategy for addressing early cognitive loss in PD.

Parkinson's disease (PD) is a multi-layered progressive neurodegenerative disease. Signature motor system impairments are accompanied by a variety of other symptoms such as mood, sleep, metabolic, and cognitive disorders. Interestingly, social cognition impairments can be observed from the earliest stages of the disease, prior to the onset of the motor symptoms. In this study, we investigated age-related reductions in sociability and social memory in the A53T mouse model of PD. Since inflammation and astrogliosis are an integral part of PD pathology and impair proper neuronal function, we examined astrogliosis and inflammation markers and parvalbumin expression in medial pre-frontal cortex (mPFC), part of the brain responsible for social cognition regulation. Finally, we used DREADDs (Designer Receptors Exclusively Activated by Designer Drugs) for the stimulation and inhibition of orexin neuronal activity to modulate sociability and social memory in A53T mice. We observed that social cognition impairment in A53T mice is accompanied by an increase in astrogliosis and inflammation markers, in addition to loss of parvalbumin neurons and inhibitory pre-synaptic terminals in the mPFC. Moreover, DREADD-induced activation of orexin neurons restores social cognition in the A53T mouse model of PD. SIGNIFICANCE STATEMENT: Social cognition is severely affected in the early stages of Parkinson's disease. In this study, we identified the A53T mouse as a model of social cognitive impairment in PD. Observed alterations in sociability and social memory are accompanied by loss of parvalbumin positive neurons and loss of inhibitory input to mPFC. Stimulating orexin neurons using a chemogenetic approach (DREADDs) ameliorated social cognitive impairment. This study identifies a role for orexin neurons in social cognition in PD and suggests potential therapeutic targets for PD-related social cognition impairments.

Aging affects numerous physiological processes, as well as behavior. A large number of these processes are regulated, at least partially, by hypothalamic orexin neurons, and orexin tone may decrease with normal aging. In this study, we hypothesized that designer receptors exclusively activated by designer drugs (DREADD) stimulation of orexin neuronal activity will ameliorate the effect of aging on behavioral and metabolic alterations in young and middle-aged mice. DREADD targeting was achieved by stereotaxic injection of AAV vectors (AAV2-hSyn-DIO-hM3D(Gq)-mCherry) into the lateral hypothalamus of 5- and 12-mo old orexin-cre female mice and was confirmed by immunohistochemistry (IHC) analysis of orexin A and mCherry expression. After recovery, animals were subjected to a behavioral test battery consisting of the elevated plus maze (EPM), open field (OFT), and novel object recognition tests (NORT) to assess effects of aging on anxiety-like behavior, general locomotion, and working memory. A comprehensive laboratory animal monitoring system (CLAMS) was used to measure spontaneous physical activity (SPA) and energy expenditure (EE). The results indicate that activation of orexin neurons mitigates aging-induced reductions in anxiety-like behavior in middle-aged mice ( P < 0.005) and increases locomotion in both young and middle-aged mice ( P < 0.05). Activation of orexin neurons increases SPA ( P < 0.01) and EE ( P < 0.005) in middle-aged mice, restoring the levels to that observed in young animals. Results from this study identify orexin neurons as potential therapeutic targets for age-related impairments in cognitive and anxiety-related behavior, and energy balance.

Aside from the classical motor symptoms, Parkinson&rsquo, s disease also has various non-classical symptoms. Interestingly, orexin neurons, involved in the regulation of exploratory locomotion, spontaneous physical activity, and energy expenditure, are affected in Parkinson&rsquo, s. In this study, we hypothesized that Parkinson&rsquo, s-disease-associated pathology affects orexin neurons and therefore impairs functions they regulate. To test this, we used a transgenic animal model of Parkinson&rsquo, s, the A53T mouse. We measured body composition, exploratory locomotion, spontaneous physical activity, and energy expenditure. Further, we assessed alpha-synuclein accumulation, inflammation, and astrogliosis. Finally, we hypothesized that chemogenetic inhibition of orexin neurons would ameliorate observed impairments in the A53T mice. We showed that aging in A53T mice was accompanied by reductions in fat mass and increases in exploratory locomotion, spontaneous physical activity, and energy expenditure. We detected the presence of alpha-synuclein accumulations in orexin neurons, increased astrogliosis, and microglial activation. Moreover, loss of inhibitory pre-synaptic terminals and a reduced number of orexin cells were observed in A53T mice. As hypothesized, this chemogenetic intervention mitigated the behavioral disturbances induced by Parkinson&rsquo, s disease pathology. This study implicates the involvement of orexin in early Parkinson&rsquo, s-disease-associated impairment of hypothalamic-regulated physiological functions and highlights the importance of orexin neurons in Parkinson&rsquo, s disease symptomology.

Orexins are involved in a variety of behaviors, including the ability to induce spontaneous physical activity (SPA). This type of activity includes all nonexercise physical activity (e.g., what could be considered restlessness) and represents an unconscious drive for movement. The calories expended via SPA are physiologically relevant to human energy balance and are controlled by a distributed brain network that includes several neural systems. The hypothalamic orexin neurons play a key role in generating SPA and NEAT (nonexercise activity thermogenesis or NEAT) ( Kotz et al., 2008 ; Teske et al., 2008 ). These neural systems are yet to be fully described, but are attractive therapeutic targets for obesity prevention and/or treatment. In this review, we focus on evidence supporting a positive effect of orexins on energy expenditure through SPA, in the context of the potential for orexin to have a therapeutic role in obesity.

Excess dietary saturated fatty acids such as palmitic acid (PA) induce peripheral and hypothalamic inflammation. Hypothalamic inflammation, mediated in part by microglial activation, contributes to metabolic dysregulation. In rodents, high fat diet-induced microglial activation results in nuclear translocation of nuclear factor-kappa B (NFκB), and increased central pro-inflammatory cytokines tumor necrosis factor alpha (TNF-α) and interleukin-6 (IL-6). The hypothalamic neuropeptide orexin A (OXA, hypocretin 1) is neuroprotective in brain. In cortex, OXA can also reduce inflammation and neurodegeneration through a microglial-mediated pathway. Whether hypothalamic orexin neuroprotection mechanisms depend upon microglia is unknown. To address this issue, we evaluated effects of OXA and PA on inflammatory response in immortalized murine microglial and hypothalamic neuronal cell lines. We demonstrate for the first time in microglial cells that exposure to PA increases gene expression of orexin-1 receptor but not orexin-2 receptor. Pro-inflammatory markers IL-6, TNF-α, and inducible nitric oxide synthase in microglial cells are increased following PA exposure, but are reduced by pretreatment with OXA. The anti-inflammatory marker arginase-1 is increased by OXA. Finally, we show hypothalamic neurons exposed to conditioned media from PA-challenged microglia have increased cell survival only when microglia were pretreated with OXA. These data support the concept that OXA may act as an immunomodulatory regulator of microglia, reducing pro-inflammatory cytokines and increasing anti-inflammatory factors to promote a favorable neuronal microenvironment.

Background/Objectives: Low levels of orexin are associated with obesity and reduced physical activity in humans and animals. Subjects/Methods: Designer Receptors Exclusively Activated by Designer Drugs (DREADDs) selectively activated orexin neurons in mouse lateral hypothalamus (LH) to measure effects on spontaneous physical activity (SPA). DREADD targeting was achieved by stereotaxic injection of AAV vectors into caudal lateral LH of heterozygous orexin-Cre or C57/B6J mice. In one set of studies, excitation of orexin neurons was examined (virus: AAV2-EF1a-DIO-hM3Dq-mCherry), and test sessions began 3-4 h after light cycle onset. In a study examining the inhibition of orexin neurons (virus: AAV2-hSyn-DIO-hM4Di-mCherry), testing began 15 min prior to dark cycle onset. Clozapine n-oxide (CNO; 1 or 5 mg/kg) or saline was injected intraperitoneally and time spent moving in open field chambers was recorded for 2 h. Follow-up studies in separate mouse cohorts quantified SPA in parallel with changes in energy expenditure (EE) and chow intake using indirect calorimetry chambers (SableSystem™). Following acclimation, testing sessions (saline and/or CNO) took place over the course of ~1 week, with injections administered every day. Changes in SPA, EE, chow intake, fecal boli, and body composition (EchoMRI™) were measured. Additional mice cohorts were fed a high-fat diet (HFD) and injected with CNO daily up to 10 days to assess the potential for orexin activation to prevent diet-induced obesity. Results: Activation of orexin resulted in increases in SPA in male and female mice, and was accompanied by increases in energy expenditure without changes in overall chow intake. When orexin activation occurred in the context of high fat diet, weight gain and adiposity were significantly attenuated. SPA was decreased when DREADDs were used to inhibit orexin activity. Conclusion: These results demonstrate that orexin neurons play a critical role in mediating physical activity and suggest a novel therapeutic target for treating obesity.

Overweight and obesity result from an imbalance between caloric intake and energy expenditure, including expenditure from spontaneous physical activity (SPA). Changes in SPA and resulting changes in non-exercise activity thermogenesis (NEAT) likely interact with diet to influence risk for obesity. However, previous research on the relationship between diet, physical activity, and energy expenditure has been mixed. The neuropeptide orexin is a driver of SPA, and orexin neuron activity can be manipulated using DREADDs (Designer Receptors Exclusively Activated by Designer Drugs). We hypothesized that HFD decreases SPA and NEAT, and that DREADD-mediated activation of orexin neuron signaling would abolish this decrease and produce an increase in NEAT instead. To test these ideas, we characterized behaviors to determine the extent to which access to a high-fat diet (HFD) influences the proportion and probability of engaging in food intake and activity. We then measured NEAT following access to HFD and following a DREADD intervention targeting orexin neurons. Two cohorts of orexin-cre male mice were injected with an excitatory DREADD virus into the caudal hypothalamus, where orexin neurons are concentrated. Mice were then housed in continuous metabolic phenotyping cages (Sable Promethion). Food intake, indirect calorimetry, and SPA were automatically measured every second. For cohort 1 (n=8), animals were given access to chow, then switched to HFD. For cohort 2 (n=4/group), half of the animals were given access to HFD, the other access to chow. Then, among animals on HFD, orexin neurons were activated following injections of clozapine n-oxide (CNO). Mice on HFD spent significantly less time eating (p

When exploring biological determinants of spontaneous physical activity (SPA), it is critical to consider whether methodological factors differentially affect rodents and the measured SPA. We determined whether acclimation time, sensory stimulation, vendor, or chamber size affected measures in rodents with varying propensity for SPA. We used principal component analysis to determine which SPA components (ambulatory and vertical counts, time in SPA, and distance traveled) best described the variability in SPA measurements. We compared radiotelemetry and infrared photobeams used to measure SPA and exploratory activity. Acclimation time, sensory stimulation, vendor, and chamber size independently influenced SPA, and the effect was moderated by the propensity for SPA. A 24-h acclimation period prior to SPA measurement was sufficient for habituation. Principal component analysis showed that ambulatory and vertical measurements of SPA describe different dimensions of the rodent's SPA behavior. Smaller testing chambers and a sensory attenuation cubicle around the chamber reduced SPA. SPA varies between rodents purchased from different vendors. Radiotelemetry and infrared photobeams differ in their sensitivity to detect phenotypic differences in SPA and exploratory activity. These data highlight methodological considerations in rodent SPA measurement and a need to standardize SPA methodology.

Obesity has increased in prevalence worldwide, attributed in part to the influences of an obesity-promoting environment and genetic factors. While obesity and overweight increasingly seem to be the norm, there remain individuals who resist obesity. We present here an overview of data supporting the idea that hypothalamic neuropeptide orexin A (OXA; hypocretin 1) may be a key component of brain mechanisms underlying obesity resistance. Prior work with models of obesity and obesity resistance in rodents has shown that increased orexin and/or orexin sensitivity is correlated with elevated spontaneous physical activity (SPA), and that orexin-induced SPA contributes to obesity resistance via increased non-exercise activity thermogenesis (NEAT). However, central hypothalamic orexin signaling mechanisms that regulate SPA remain undefined. Our ongoing studies and work of others support the hypothesis that one such mechanism may be upregulation of a hypoxia-inducible factor 1 alpha (HIF-1α)-dependent pathway, suggesting that orexin may promote obesity resistance both by increasing SPA and by influencing the metabolic state of orexin-responsive hypothalamic neurons. We discuss potential mechanisms based on both animal and in vitro pharmacological studies, in the context of elucidating potential molecular targets for obesity prevention and therapy.

Objective—Sleep-restriction in humans increases risk for obesity, but previous rodent studies show weight loss following sleep deprivation, possibly due to stressful-methods used to prevent sleep. Obesity-resistant (OR) rats exhibit consolidated-sleep and resistance to weight-gain. We hypothesized that sleep disruption by a less-stressful method would increase body weight, and examined effect of partial sleep deprivation (PSD) on body weight in OR and Sprague-Dawley (SD) rats. Design and Methods—OR and SD rats (n=12/group) were implanted with transmitters to record sleep/wake. After baseline recording, six SD and six OR rats underwent 8 h PSD during light-phase for 9 d. Sleep was reduced using recordings of random noise. Sleep/wake states were scored as wakefulness (W), slow-wave-sleep (SWS) and rapid-eye-movement-sleep (REMS). Total number of transitions between stages, SWS-delta-power, food intake and body weight were documented. Results—Exposure to noise decreased SWS and REMS time, while increasing W time. Sleepdeprivation increased number of transitions between stages and SWS-delta-power. Further, PSD during the rest phase increased recovery-sleep during active phase. The PSD SD and OR rats had greater food intake and body weight compared to controls Conclusions—PSD by less-stressful means increases body weight in rats. Also, PSD during rest phase increases active period sleep.

This study examined how changes in wildland firefighters' mood relate to cytokine and cortisol levels in response to simulated physical firefighting work and sleep restriction. Firefighters completed 3 days of simulated wildfire suppression work separated by an 8-h (control condition; n = 18) or 4-h sleep opportunity (sleep restriction condition; n = 17) each night. Firefighters' mood was assessed daily using the Mood Scale II and Samn-Perelli fatigue scale. Participants also provided samples for the determination of salivary cortisol and pro- (IL-6, IL-8, IL-1β, TNF-α) and anti-inflammatory (IL-4, IL-10) cytokine levels. An increase in the positive mood dimension Happiness was related to a rise in IL-8 and TNF-α in the sleep restriction condition. A rise in the positive mood dimension Activation among sleep restricted firefighters was also related to higher IL-6 levels. An increase in the negative mood dimension Fatigue in the sleep restriction condition was associated with increased IL-6, TNF-α, IL-10 and cortisol levels. In addition, an increase in Fear among sleep restricted firefighters was associated with a rise in TNF-α. Elevated positive mood and immune activation may reflect an appropriate response by the firefighters to these stressors. To further understand this relationship, subsequent firefighting-based research is needed that investigates whether immune changes are a function of affective arousal linked to the expression of positive moods. Positive associations between negative mood and inflammatory and cortisol levels to physical work and restricted sleep provide useful information to fire agencies about subjective fire-ground indicators of physiological changes.

A major side effect of insulin treatment of diabetes is weight gain, which limits patient compliance and may pose additional health risks. Although the mechanisms responsible for this weight gain are poorly understood, it has been suggested that there may be a link to the incidence of recurrent episodes of hypoglycemia. Here we present a rodent model of marked weight gain associated with weekly insulin-induced hypoglycemic episodes in the absence of diabetes. Insulin treatment caused a significant increase in both body weight and fat mass, accompanied by reduced motor activity, lowered thermogenesis in response to a cold challenge, and reduced brown fat uncoupling protein mRNA. However, there was no effect of insulin treatment on total food intake nor on hypothalamic neuropeptide Y or proopiomelanocortin mRNA expression, and insulin-treated animals did not become insulin-resistant. Our results suggest that repeated iatrogenic hypoglycemia leads to weight gain, and that such weight gain is associated with a multifaceted deficit in metabolic regulation rather than to a chronic increase in caloric intake.

Humans and rodents show large variability in their individual sensitivity to diet-induced obesity (DIO), which has been associated with differences in intrinsic spontaneous physical activity (SPA). Evidence from genetic and out-bred rat obesity models shows that higher activity of the orexin peptides results in higher intrinsic SPA and protection against DIO. Based on this, we hypothesized that naturally occurring variation in SPA and orexin signaling is sufficient to drive differences in sensitivity to DIO.Orexin expression, behavioral responses to orexin-A, basal energy expenditure and sensitivity to DIO were measured in in non-manipulated male Sprague-Dawley rats selected for high and low intrinsic SPA.Male Sprague-Dawley rats were classified as high-activity or low-activity based on differences in intrinsic SPA. High-activity rats showed higher expression of prepro-orexin mRNA, higher sensitivity to behavioral effects of orexin injection, higher basal energy expenditure and were more resistant to obesity caused by high-fat diet consumption than low-activity rats.Our results define a new model of differential DIO sensitivity, the high-activity and low-activity rats, and suggest that naturally occurring variations in intrinsic SPA cause differences in energy expenditure that are mediated by orexin signaling and alter DIO sensitivity.

Current data support the idea that hypothalamic neuropeptide orexin A (OxA; hypocretin 1) mediates resistance to high fat diet-induced obesity. We previously demonstrated that OxA elevates spontaneous physical activity (SPA), that rodents with high SPA have higher endogenous orexin sensitivity, and that OxA-induced SPA contributes to obesity resistance in rodents. Recent reports show that OxA can confer neuroprotection against ischemic damage, and may decrease lipid peroxidation. This is noteworthy as independent lines of evidence indicate that diets high in saturated fats can decrease SPA, increase hypothalamic apoptosis, and lead to obesity. Together data suggest OxA may protect against obesity both by inducing SPA and by modulation of anti-apoptotic mechanisms. While OxA effects on SPA are well characterized, little is known about the short- and long-term effects of hypothalamic OxA signaling on intracellular neuronal metabolic status, or the physiological relevance of such signaling to SPA. To address this issue, we evaluated the neuroprotective effects of OxA in a novel immortalized primary embryonic rat hypothalamic cell line. We demonstrate for the first time that OxA increases cell viability during hydrogen peroxide challenge, decreases hydrogen peroxide-induced lipid peroxidative stress, and decreases caspase 3/7 induced apoptosis in an in vitro hypothalamic model. Our data support the hypothesis that OxA may promote obesity resistance both by increasing SPA, and by influencing survival of OxA-responsive hypothalamic neurons. Further identification of the individual mediators of the anti-apoptotic and peroxidative effects of OxA on target neurons could lead to therapies designed to maintain elevated SPA and increase obesity resistance.

Resistance to obesity is becoming an exception rather than the norm, and understanding mechanisms that lead some to remain lean in spite of an obesigenic environment is critical if we are to find new ways to reverse this trend. Levels of energy intake and physical activity both contribute to body weight management, but it is challenging for most to adopt major long-term changes in either factor. Physical activity outside of formal exercise, also referred to as activity of daily living, and in stricter form, spontaneous physical activity (SPA), may be an attractive modifiable variable for obesity prevention. In this review, we discuss individual variability in SPA and NEAT (nonexercise thermogenesis, or the energy expended by SPA) and its relationship to obesity resistance. The hypothalamic neuropeptide orexin (hypocretin) may play a key role in regulating SPA and NEAT. We discuss how elevated orexin signaling capacity, in the context of a brain network modulating SPA, may play a major role in defining individual variability in SPA and NEAT. Greater activation of this SPA network leads to a lower propensity for fat mass gain and therefore may be an attractive target for obesity prevention and therapy.

Butorphanol ([BT] an opioid receptor agonist/antagonist) is different from other opioid agonists in that a single dose of BT can elicit up to 12 g of chow intake in a satiated rat whereas most opioid agonists induce a mild feeding response (2-3 g). Here, we first examined whether the effectiveness of BT to elicit feeding was affected by dose, method of infusion and possible tachyphylaxis following administration. Secondly, we examined whether BT administration influenced hypothalamic NPY gene expression and peptide levels. A single dose administration of BT (4 mg/kg) significantly increased food intake at 2, 3 and 6 hours after administration. However following repeated injections of BT at 4 mg/kg, the cumulative long-term intake of BT-treated rats did not differ from that of controls, indicating that the animals compensate for the increased feeding following BT injection by decreased feeding at a later time. An ascending dose schedule of repeated BT injections resulted in additional feeding. NPY gene expression in the ARC was influenced by how much food had been consumed, but not by BT. The amount of food consumed and the level of NPY mRNA were inversely correlated. This is consistent with NPY’s role in normal feeding. BT treatment did not affect either NPY or leptin RIA levels. We conclude that the feeding produced by BT is sensitive to dose and dosing paradigm. Further, its mechanism of action does not appear to be mediated by NPY or leptin pathways.

It is unclear whether elevated spontaneous physical activity (SPA, very low-intensity physical activity) positively influences body composition long-term. Objective We determined whether SPA and caloric intake were differentially related to the growth curve trajectories of body weight, FM and FFM between obesity resistant and Sprague-Dawley rats at specific age intervals. Design and Subjects Body composition, SPA and caloric intake were measured in selectively-bred obesity resistant and out-bred Sprague-Dawley rats from 1-18 mo. Data from development throughout maturation were analyzed by longitudinal growth curve modeling to determine the rate and acceleration of body weight, fat mass (FM) and fat-free mass (FFM) gain. Results Obesity resistant rats had a lower rate of FM gain overall, a lower acceleration in body weight early in life, significantly greater SPA and lower cumulative caloric intake. Greater SPA in obesity resistant rats was significantly associated with a lower rate of FM gain overall and lower acceleration in body weight early in life. Obesity resistant rats lost less FFM compared to Sprague-Dawley rats despite that obesity resistant rats had a lower acceleration in FFM gain early in life. Obesity resistant rats gained less FM and more FFM per gram body weight and were less energy efficient than Sprague-Dawley rats. Caloric intake was significantly and positively related to body weight, FM and FFM gain in both groups. Circadian patterns of caloric intake were group and age-dependent. Our data demonstrate that elevated and sustained SPA during development and over the lifespan are related to the reduced the rate of FM gain and may preserve FFM. Conclusion These data support the idea that SPA level is a reproducible marker that reliably predicts propensity for obesity in rats, and that elevated levels of SPA maintained during the lifespan promote a lean phenotype.

We evaluated the EchoMRI-900 combination rat and mouse quantitative magnetic resonance (QMR) body composition method in comparison to traditional whole-body chemical carcass composition analysis (CCA) for measurements of fat and fat-free mass in rodents. Live and postmortem (PM) QMR fat and lean mass measurements were obtained for lean, obese and outbred strains of rats and mice, and compared with measurements obtained using CCA. A second group of rats was measured before and after 18 h food or water deprivation. Significant positive correlations between QMR and CCA fat and lean mass measurements were shown for rats and mice. Although all live QMR fat and lean measurements were more precise than CCA for rats, values obtained for mice significantly differed from CCA for lean mass only. QMR performed PM slightly overestimated fat and lean values relative to live QMR but did not show lower precision than live QMR. Food deprivation reduced values for both fat and lean mass; water deprivation reduced estimates of lean mass only. In summary, all measurements using this QMR system were comparable to those obtained by CCA, but with higher overall precision, similar to previous reports for the murine QMR system. However, PM QMR measurements slightly overestimated live QMR values, and lean and fat mass measurements in this QMR system are influenced by hydration status and animal size, respectively. Despite these caveats, we conclude that the EchoMRI QMR system offers a fast in vivo method of body composition analysis, well correlated to but with greater overall precision than CCA.

Objective To determine if resistance to weight gain is associated with alterations in sleep/wake states and orexin receptor gene expression. Design Three-month old obesity susceptible Sprague-Dawley (SD) and obesity resistant (OR) rats were fed standard rodent chow. Sleep/wake cycle was measured by radiotelemetry and orexin receptor profiles in sleep/wake regulatory areas of the brain were quantified by quantitative RT-PCR. Subjects Adult male obesity susceptible SD and selectively-bred OR rats. Measurements Body weight, food intake, energy efficiency, percent time spent in active wake, quiet wake, slow-wave sleep (SWS), rapid eye movement (REM) sleep, number and mean duration of sleep/wake episodes, number of stage transitions, SWS sleep delta power and orexin receptor mRNA levels were measured. Results Obesity resistant rats weighed significantly less and had lower energy efficiency than SD rats. Food intake was not different between SD and OR rats. Time spent in quiet wake was similar between groups, and therefore active wake and quiet wake were combined and are referred to as ‘wakefulness’. Obesity resistant rats spent significantly more time in wakefulness and less time in SWS compared to SD rats during the 24 h recording period. Relative to SD rats, OR rats had significantly fewer sleep/wake episodes and the duration of the episodes were prolonged, indicating less fragmented sleep. Further, OR rats had fewer transitions between sleep stages, which indicates that OR rats were behaviorally more stable and had more consolidated sleep than obesity susceptible SD rats. Obesity resistant rats exhibited lower delta power during SWS sleep, indicating a lower sleep drive. Our results demonstrated greater orexin receptor gene expression in sleep regulatory brain areas in OR rats. Conclusion These results demonstrate that prolonged wakefulness, better sleep quality, lower sleep drive and greater orexin signaling may confer protection against obesity.

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.

The hypocretin (hcrt) system has been implicated in addiction-relevant effects of several drugs, but its role in nicotine dependence has been little studied.These experiments examined the role of the hcrt system in nicotine reinforcement.Rats were trained for nicotine self-administration (NSA) on fixed-ratio schedules. The effects of acute, presession treatments with the hcrtR1 antagonist SB334867 and the hcrtR1/2 antagonist almorexant were examined on NSA maintained on a fixed-ratio (FR) 5 schedule. Gene expression for the hcrt system (mRNA for hcrt, hcrtR1, and hcrtR2) was measured in animals following NSA on a FR 1 schedule for a 19-day period.The hcrtR1 antagonist SB334867 and the hcrtR1/2 antagonist almorexant both reduced NSA dose-dependently (significantly at doses of 30 and 300 mg/kg, respectively); SB334867 did not affect food-maintained responding whereas almorexant (at the 300 mg/kg) did. Tissue from animals collected 5 h after self-administration showed an increase in hcrtR1 mRNA in the arcuate nucleus compared to control subjects. In tissue collected immediately after a similar 19-day self-administration period, mRNA for hcrtR1 was decreased in the rostral lateral hypothalamus compared to controls.These data confirm a previous report (Hollander et al., Proc Natl Acad Sci U S A 105:19480-19485, 2008) that the hypocretin receptor hcrtR1 is activated in nicotine reinforcement and in addition show that both the arcuate nucleus and lateral hypothalamus are sites at which hcrt receptor mechanisms may influence reinforcement. Different patterns of mRNA expression at different times after NSA suggest that changes in the hcrt system may be labile with time.

Prior work has shown that adiabatic T(1rho) and T(2rho) relaxation time constants may have sensitivity to cellular changes and the presence of iron, respectively, in Parkinson's disease (PD). Further understanding of these magnetic resonance imaging (MRI) methods and how they relate to measures of disease severity and progression in PD is needed. Using T(1rho) and T(2rho) on a 4T MRI scanner, we assessed the substantia nigra (SN) of nine non-demented moderately affected PD and ten gender- and age-matched control participants. When compared to controls, the SN of PD subjects had increased T(1rho) and reduced T(2rho). We also found a significant correlation between asymmetric motor features and asymmetry based on T(1rho). This study provides additional validation of T(1rho) and T(2rho) as a means to separate PD from control subjects, and T(1rho) may be a useful marker of asymmetry in PD.

Caloric restriction (CR) and metabolic glucoprivation affect spontaneous physical activity (SPA), but it's unknown whether these treatments similarly affect SPA in selectively bred obesity-prone (OP) and -resistant (OR) rats. OR rats have greater basal SPA and are more responsive to treatments that modulate SPA, such as orexin A administration. We hypothesized that OR rats would be more sensitive to other treatments modulating SPA. To test this, continuous 24-h SPA was measured before and during acute (24 h) and chronic (8 wk) CR in OR, OP, and Sprague-Dawley rats. Pharmacological glucoprivation was produced by injection of 2-deoxyglucose (2-DG), and SPA was measured 5 h postinjection. Acute CR increased SPA in all groups; however, the effect was dependent on the index of SPA and time interval during the 24-h time period. In contrast to OR rats, chronic CR increased distance traveled, ambulatory episodes, and time spent in ambulation and stereotypy during the time interval preceding anticipation of food in OP and Sprague-Dawley rats. Although the effects of 2-DG treatment on SPA were minimal, OR rats had significantly greater SPA than OP and Sprague-Dawley rats independent of treatment. That chronic CR failed to result in significant changes in SPA in OR rats suggests that these rats may be especially unresponsive to treatments modulating feeding. This insensitivity coupled with elevated basal SPA levels may in part mediate phenotypic traits of lean rats.

High levels of spontaneous physical activity in lean people and the nonexercise activity thermogenesis (NEAT) derived from that activity appear to protect lean people from obesity during caloric challenge, while obesity in humans is characterized by dramatically reduced spontaneous physical activity. We have similarly demonstrated that obesity-resistant rats have significantly greater spontaneous physical activity than obesity-prone rats, and that spontaneous physical activity predicts body weight gain. Although the energetic cost of activity varies between types of activity and may be regulated, individual level of spontaneous physical activity is important in determining propensity for obesity. We review the current status of knowledge about the brain mechanisms involved in controlling the level of spontaneous physical activity and the NEAT so generated. Focus is on potential neural mediators of spontaneous physical activity and NEAT, including orexin A (also known as hypocretin 1), agouti-related protein, ghrelin, and neuromedin U, in addition to brief mention of neuropeptide Y, corticotrophin releasing hormone, cholecystokinin, estrogen, leptin, and dopamine effects on spontaneous physical activity. We further review evidence that strain differences in orexin stimulation pathways for spontaneous physical activity and NEAT appear to track with the body weight phenotype, thus providing a potential mechanistic explanation for reduced activity and weight gain.