Exercise reverses the effects of early life stress on orexin cell reactivity in male but not female rats

Hypocretin modulation of behavioral coping strategies for social stress

Best known for promoting wakefulness and arousal, the neuropeptide hypocretin (Hcrt) also plays an important role in mediating stress responses, including social stress. However, central and systemic manipulation of the Hcrt system has produced diverse behavioral outcomes in animal models. In this review, we first focus on studies where similar manipulations of the Hcrt system led to divergent coping behaviors. We hypothesize that Hcrt differentially facilitates active and passive coping behaviors in response to social stress by acting in different brain regions and on different cell types. We then focus on region and cell type-specific effects of Hcrt in the ventral pallidum, lateral habenula, ventral tegmental area, nucleus accumbens, amygdala, and bed nucleus of the stria terminalis. Overall, the evidence suggests that rather than enhancing or inhibiting behavioral responses to social stress, Hcrt may signal the heightened arousal associated with stressful contexts. The resulting behavioral effects depend on which circuits Hcrt release occurs in and which receptor types are activated. Further study is needed to determine how and why circuit specific activation of Hcrt neurons occurs.

Contribution of hypothalamic orexin (hypocretin) circuits to pathologies of motivation

The orexin (also known as hypocretin) system, consisting of neuropeptides orexin-A and orexin-B, was discovered over 25 years ago and was immediately identified as a central regulator of sleep and wakefulness. These peptides interact with two G-protein coupled receptors, orexin 1 (OX1) and orexin 2 (OX2) receptors which are capable of coupling to all heterotrimeric G-protein subfamilies, but primarily transduce increases in calcium signalling. Orexin neurons are regulated by a variety of transmitter systems and environmental stimuli that signal reward availability, including food and drug related cues. Orexin neurons are also activated by anticipation, stress, cues predicting motivationally relevant information, including those predicting drugs of abuse, and engage neuromodulatory systems, including dopamine neurons of the ventral tegmental area (VTA) to respond to these signals. As such, orexin neurons have been characterized as motivational activators that coordinate a range of functions, including feeding and arousal, that allow the individual to respond to motivationally relevant information, critical for survival. This review focuses on the role of orexins in appetitive motivation and highlights a role for these neuropeptides in pathologies characterized by inappropriately high levels of motivated arousal (overeating, anxiety and substance use disorders) versus those in which motivation is impaired (depression).

Interaction between corticotropin-releasing factor, orexin, and dynorphin in the infralimbic cortex may mediate exacerbated alcohol-seeking behavior

A major challenge for the treatment of alcohol use disorder (AUD) is relapse to alcohol use, even after protracted periods of self-imposed abstinence. Stress significantly contributes to the chronic relapsing nature of AUD, given its long-lasting ability to elicit intense craving and precipitate relapse. As individuals transition to alcohol dependence, compensatory allostatic mechanisms result in insults to hypothalamic-pituitary-adrenal axis function, mediated by corticotropin-releasing factor (CRF), which is subsequently hypothesized to alter brain reward pathways, influence affect, elicit craving, and ultimately perpetuate problematic drinking and relapse vulnerability. Orexin (OX; also called hypocretin) plays a well-established role in regulating diverse physiological processes, including stress, and has been shown to interact with CRF. Interestingly, most hypothalamic cells that express Ox mRNA also express Pdyn mRNA. Both dynorphin and OX are located in the same synaptic vesicles, and they are co-released. The infralimbic cortex (IL) of the medial prefrontal cortex (mPFC) has emerged as being directly involved in the compulsive nature of alcohol consumption during dependence. The IL is a CRF-rich region that receives OX projections from the hypothalamus and where OX receptor mRNA has been detected. Although not thoroughly understood, anatomical and behavioral pharmacology data suggest that CRF, OX, and dynorphin may interact, particularly in the IL, and that functional interactions between these three systems in the IL may be critical for the etiology and pervasiveness of compulsive alcohol seeking in dependent subjects that may render them vulnerable to relapse. The present review presents evidence of the role of the IL in AUD and discusses functional interactions between CRF, OX, and dynorphin in this structure and how they are related to exacerbated alcohol drinking and seeking.

Pain-Insomnia-Depression Syndrome: Triangular Relationships, Pathobiological Correlations, Current Treatment Modalities, and Future Direction

Pain-insomnia-depression syndrome (PIDS) is a complex triad of chronic pain, insomnia, and depression that has profound effects on an individual's quality of life and mental health. The pathobiological context of PIDS involves complex neurobiological and physiological mechanisms, including alterations in neurotransmitter systems and impaired pain processing pathways. The first-line therapeutic approaches for the treatment of chronic pain, depression, and insomnia are a combination of pharmacological and non-pharmacological therapies. In cases where patients do not respond adequately to these treatments, additional interventions such as deep brain stimulation (DBS) may be required. Despite advances in understanding and treatment, there are still gaps in knowledge that need to be addressed. To improve our understanding, future research should focus on conducting longitudinal studies to uncover temporal associations, identify biomarkers and genetic markers associated with PIDS, examine the influence of psychosocial factors on treatment responses, and develop innovative interventions that address the complex nature of PIDS. The aim of this study is to provide a comprehensive overview of these components and to discuss their underlying pathobiological relationships.

Possible role of exercise therapy on depression: Effector neurotransmitters as key players

About 280 million people suffer from depression as the most common neurological disorder and the most common cause of death worldwide. Exercise with serotonin released in the brain by the 5-HT3-IGF-1 mechanism can lead to antidepressant effects. Swimming exercise has antidepressant effects by increasing the sensitivity of serotonin 5-HT2 receptors and postsynaptic 5-HT1A receptors, increasing 5-HT and 5HIAA levels, increasing TPH and serotonin, and decreasing inflammatory levels of IFN-γ and TNF-α. Anaerobic and aerobic exercises increase beta-endorphin, enkephalin, and dynorphin and have antidepressant effects. Exercise by increasing dopamine, D1R, and D2R leads to the expression of BDNF and activation of TrkB and has antidepressant behavior. Exercise leads to a significant increase in GABAAR (γ2 and α2 subunits) and reduces neurodegenerative disorders caused by GABA imbalance through anti-inflammatory pathways. By increasing glutamate and PGC1α and reducing glutamatergic neurotoxicity, exercise enhances neurogenesis and synaptogenesis and prevents neurodegeneration and the onset of depression. Irisin release during exercise shows an important role in depression by increasing dopamine, BDNF, NGF, and IGF-1 and decreasing inflammatory mediators such as IL-6 and IL-1β. In addition, exercise-induced orexin and NPY can increase hippocampal neurogenesis and relieve depression. After exercise, the tryptophan to large neutral amino acids (TRP/LNAA) ratio and the tryptophan to branched-chain amino acids (BCAA) ratio increase, which may have antidepressant effects. The expression of M5 receptor and nAChR α7 increases after exercise and significantly increases dopamine and acetylcholine and ameliorates depression. It appears that during exercise, muscarinic receptors can reduce depression through dopamine in the absence of acetylcholine. Therefore, exercise can be used to reduce depression by affecting neurotransmitters, neuromodulators, cytokines, and/or neurotrophins.

Prolonged stress alters the PC1/PC2 ratio in the rat lateral hypothalamus, implicating impaired orexin maturation

Objectives

Stress elicits physiological and neuroendocrine responses mediated by the hypothalamic-pituitary-adrenal (HPA) axis and lateral hypothalamus (LH). However, prolonged stress can dysregulate neuropeptide systems like orexin. This study investigated the effects of temporary and prolonged stress on HPA activity and orexin processing in the rat LH.

Materials and Methods

Male Wistar rats were exposed to various stress repetitions. The stress paradigm is defined as short (acute; 1 day and mild; 3 days) and long (sub-chronic; 10 days and chronic; 21 days)-term 6 hr daily restraint stress. Plasma corticosterone (CORT) served as an index of HPA function. Expression of prepro-orexin and its processing enzymes prohormone convertases (PC) 1 and 2 was measured in LH tissues using semiquantitative RT-PCR.

Results

The plasma level of CORT was elevated following mild, sub-chronic, and chronic, but not acute stress versus unstressed controls. The expression of prepro-orexin was heightened following all stress exposures. However, PC1 increased and PC2 decreased only after prolonged stress. The PC1/PC2 ratio was also selectively augmented with sub-chronic and chronic stress, implying impaired orexin maturation.

Conclusion

Together, these data demonstrate that the HPA axis and lateral hypothalamic orexin system respond to stress based on stress repetition. Changes in orexin processing enzyme mRNA, exclusively after chronic stress, imply potential effects on peptide maturation, requiring confirmation of the orexin production at the protein level.

Targeting the orexin/hypocretin system for the treatment of neuropsychiatric and neurodegenerative diseases: from animal to clinical studies

Orexins (also known as hypocretins) are neuropeptides located exclusively in hypothalamic neurons that have extensive projections throughout the central nervous system and bind two different G protein-coupled receptors (OX1R and OX2R). Since its discovery in 1998, the orexin system has gained the interest of the scientific community as a potential therapeutic target for the treatment of different pathological conditions. Considering previous basic science research, a dual orexin receptor antagonist, suvorexant, was the first orexin agent to be approved by the US Food and Drug Administration to treat insomnia. In this review, we discuss and update the main preclinical and human studies involving the orexin system with several psychiatric and neurodegenerative diseases. This system constitutes a nice example of how basic scientific research driven by curiosity can be the best route to the generation of new and powerful pharmacological treatments.

Early Life Stress Affects Bdnf Regulation: A Role for Exercise Interventions

Early life stress (ELS) encompasses exposure to aversive experiences during early development, such as neglect or maltreatment. Animal and human studies indicate that ELS has maladaptive effects on brain development, leaving individuals more vulnerable to developing behavioral and neuropsychiatric disorders later in life. This result occurs in part to disruptions in Brain derived neurotrophic factor (Bdnf) gene regulation, which plays a vital role in early neural programming and brain health in adulthood. A potential treatment mechanism to reverse the effects of ELS on Bdnf expression is aerobic exercise due to its neuroprotective properties and positive impact on Bdnf expression. Aerobic exercise opens the door to exciting and novel potential treatment strategies because it is a behavioral intervention readily and freely available to the public. In this review, we discuss the current literature investigating the use of exercise interventions in animal models of ELS to reverse or mitigate ELS-induced changes in Bdnf expression. We also encourage future studies to investigate sensitive periods of exercise exposure, as well as sufficient duration of exposure, on epigenetic and behavioral outcomes to help lead to standardized practices in the exercise intervention field.

From Molecule to Behavior: Hypocretin/orexin Revisited From a Sex-dependent Perspective

The hypocretin/orexin (Hcrt/Orx) system in the perifornical lateral hypothalamus has been recognized as a critical node in a complex network of neuronal systems controlling both physiology and behavior in vertebrates. Our understanding of the Hcrt/Orx system and its array of functions and actions has grown exponentially in merely 2 decades. This review will examine the latest progress in discerning the roles played by the Hcrt/Orx system in regulating homeostatic functions and in executing instinctive and learned behaviors. Furthermore, the gaps that currently exist in our knowledge of sex-related differences in this field of study are discussed.

Physiological Role of Orexinergic System for Health

Orexins, or hypocretins, are excitatory neuropeptides involved in the regulation of feeding behavior and the sleep and wakefulness states. Since their discovery, several lines of evidence have highlighted that orexin neurons regulate a great range of physiological functions, giving it the definition of a multitasking system. In the present review, we firstly describe the mechanisms underlining the orexin system and their interactions with the central nervous system (CNS). Then, the system's involvement in goal-directed behaviors, sleep/wakefulness state regulation, feeding behavior and energy homeostasis, reward system, and aging and neurodegenerative diseases are described. Advanced evidence suggests that the orexin system is crucial for regulating many physiological functions and could represent a promising target for therapeutical approaches to obesity, drug addiction, and emotional stress.

Insulin-like Growth Factor I Couples Metabolism with Circadian Activity through Hypo-Thalamic Orexin Neurons

Uncoupling of metabolism and circadian activity is associated with an increased risk of a wide spectrum of pathologies. Recently, insulin and the closely related insulin-like growth factor I (IGF-I) were shown to entrain feeding patterns with circadian rhythms. Both hormones act centrally to modulate peripheral glucose metabolism; however, whereas central targets of insulin actions are intensely scrutinized, those mediating the actions of IGF-I remain less defined. We recently showed that IGF-I targets orexin neurons in the lateral hypothalamus, and now we evaluated whether IGF-I modulates orexin neurons to align circadian rhythms with metabolism. Mice with disrupted IGF-IR activity in orexin neurons (Firoc mice) showed sexually dimorphic alterations in daily glucose rhythms and feeding activity patterns which preceded the appearance of metabolic disturbances. Thus, Firoc males developed hyperglycemia and glucose intolerance, while females developed obesity. Since IGF-I directly modulates orexin levels and hepatic expression of KLF genes involved in circadian and metabolic entrainment in an orexin-dependent manner, it seems that IGF-I entrains metabolism and circadian rhythms by modulating the activity of orexin neurons.

Orexin, serotonin, and energy balance

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 Diseases > Molecular and Cellular Physiology

Effects of early life adversity on immediate early gene expression: Systematic review and 3-level meta-analysis of rodent studies

Early-life adversity (ELA) causes long-lasting structural and functional changes to the brain, rendering affected individuals vulnerable to the development of psychopathologies later in life. Immediate-early genes (IEGs) provide a potential marker for the observed alterations, bridging the gap between activity-regulated transcription and long-lasting effects on brain structure and function. Several heterogeneous studies have used IEGs to identify differences in cellular activity after ELA; systematically investigating the literature is therefore crucial for comprehensive conclusions. Here, we performed a systematic review on 39 pre-clinical studies in rodents to study the effects of ELA (alteration of maternal care) on IEG expression. Females and IEGs other than cFos were investigated in only a handful of publications. We meta-analyzed publications investigating specifically cFos expression. ELA increased cFos expression after an acute stressor only if the animals (control and ELA) had experienced additional hits. At rest, ELA increased cFos expression irrespective of other life events, suggesting that ELA creates a phenotype similar to naïve, acutely stressed animals. We present a conceptual theoretical framework to interpret the unexpected results. Overall, ELA likely alters IEG expression across the brain, especially in interaction with other negative life events. The present review highlights current knowledge gaps and provides guidance to aid the design of future studies.

Reviewing the role of the orexinergic system and stressors in modulating mood and reward-related behaviors

In this review study, we aimed to introduce the orexinergic system as an important signaling pathway involved in a variety of cognitive functions such as memory, motivation, and reward-related behaviors. This study focused on the role of orexinergic system in modulating reward-related behavior, with or without the presence of stressors. Cross-talk between the reward system and orexinergic signaling was also investigated, especially orexinergic signaling in the ventral tegmental area (VTA), the nucleus accumbens (NAc), and the hippocampus. Furthermore, we discussed the role of the orexinergic system in modulating mood states and mental illnesses such as depression, anxiety, panic, and posttraumatic stress disorder (PTSD). Here, we narrowed down our focus on the orexinergic signaling in three brain regions: the VTA, NAc, and the hippocampus (CA1 region and dentate gyrus) for their prominent role in reward-related behaviors and memory. It was concluded that the orexinergic system is critically involved in reward-related behavior and significantly alters stress responses and stress-related psychiatric and mood disorders.

Maternal separation alters the reward system of activity-based anorexia rats

Maternal separation (MS) is a known chronic stressor in the postnatal period and when associated with another paradigm like the activity-based anorexia (ABA) rat model, causes different effects in the two sexes. In ABA females, the separation leads to increased hyperactivity and anxiety reduction, whereas, in males, the separation induces decreased locomotor activity without similar reduction of anxiety-like behaviors as observed in females. To understand the mechanisms altered by MS in synergy with the induction of the anorexic-like phenotype, we considered the reward system, which involves neurons synthesizing dopamine (DA) in the ventral tegmental area (VTA), substantia nigra pars compacta, and serotoninergic neurons in the dorsal raphe nucleus. Moreover, we analyzed the orexin circuit in the lateral hypothalamic area (LHA), which affects DA synthesis in the VTA and is also known to regulate food consumption and locomotor activity. Rats of both sexes were exposed to the two paradigms (MS and ABA), leading to four experimental groups for each sex: non-separated control (CON), non-separated ABA groups (ABA), MS control (MSCON), and MS plus ABA groups (MSABA). Immunohistochemistry analysis was performed to determine quantitative differences in the number of cells expressing DA, orexin, and serotonin (5-HT) among the experimental groups. The results showed that, in the DA system, the effect of MS was more evident in females than in males, with a substantial increase in DA cells in the VTA of MSABA. However, the analysis of the orexin system revealed a similar cellular increment in the LHA in the non-separated ABA groups of both sexes. Regarding 5-HT, there was an opposite effect in males and females of the MSABA groups, with only females showing a greater density of 5-HT cells. The changes in the reward system could partially explain the behavioral data: the hyperactivity, weight loss, and decreased anxiety levels of the MSABA females could be linked to an increase in DA and 5-HT cells, whereas in males, MS could mitigate the behavioral effects of the ABA protocol affecting the anxiety levels and locomotor activity through a lack of increased activation of the reward system.

Sex Dimorphic Changes in Trh Gene Methylation and Thyroid-Axis Response to Energy Demands in Maternally Separated Rats

The hypothalamus-pituitary-thyroid (HPT) axis regulates energy balance through the pleiotropic action of thyroid hormones. HPT basal activity and stimulation by cold or voluntary exercise are repressed by previous chronic stress in adults. Maternal separation (MS) modifies HPT basal activity; we thus studied the response of the axis to energy demands and analyzed possible epigenetic changes on Trh promoter. Nonhandled (NH) or MS male Wistar rats were cold exposed 1 h at adulthood; Trh expression in the hypothalamic paraventricular nucleus (PVN) and serum thyrotropin (TSH) concentration were increased only in NH rats. Two weeks of voluntary exercise decreased fat mass and increased Trh expression, and thyroid hormones concentration changed proportionally to running distance in NH male rats and MS male rats. Although NH females ran more than MS and much more than males, exercise decreased body weight and fat mass only in NH rats with no change on any parameter of the HPT axis but increased Pomc expression in arcuate-nucleus of NH and Npy in MS females. Overall, the methylation pattern of PVN Trh gene promoter was similar in NH males and females; MS modified methylation of specific CpG sites, a thyroid hormone receptor (THR)-binding site present after the initiation site was hypomethylated in MS males; in MS females, the THR binding site of the proximal promoter (site 4) and 2 sites in the first intron were hypermethylated. Our studies showed that, in a sex-dimorphic manner, MS blunted the responses of HPT axis to energy demands in adult animals and caused methylation changes on Trh promoter that could alter T3 feedback.

Hippocampal neurogenesis mediates sex-specific effects of social isolation and exercise on fear extinction in adolescence

Impaired extinction of conditioned fear is associated with anxiety disorders. Common lifestyle factors, like isolation stress and exercise, may alter the ability to extinguish fear. However, the effect of and interplay between these factors on adolescent fear extinction, and the relevant underlying neural mechanisms are unknown. Here we examined the effects of periadolescent social isolation and physical activity on adolescent fear extinction in rats and explored neurogenesis as a potential mechanism. Isolation stress impaired extinction recall in male adolescents, an effect prevented by exercise. Extinction recall in female adolescents was unaffected by isolation stress. However, exercise disrupted extinction recall in isolated females. Extinction recall in isolated females was positively correlated to the number of immature neurons in the ventral hippocampus, suggesting that exercise affected extinction recall via neurogenesis in females. Pharmacologically suppressing cellular proliferation in isolated adolescents using temozolomide blocked the effect of exercise on extinction recall in both sexes. Together, these findings highlight sex-specific outcomes of isolation stress and exercise on adolescent brain and behavior, and highlights neurogenesis as a potential mechanism underlying lifestyle effects on adolescent fear extinction.

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Periadolescent isolation stress disrupted extinction recall in male adolescents.

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Running prevented isolation-induced extinction recall deficit in male adolescents.

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Exercise impaired extinction recall in isolated female adolescents.

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Exercise increased hippocampal neurogenesis, except in isolated males.

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Suppression of neurogenesis blocked exercise effects in isolated adolescents.

The Insomnia-Addiction Positive Feedback Loop: Role of the Orexin System

Significant sleep impairments often accompany substance use disorders (SUDs). Sleep disturbances in SUD patients are associated with poor clinical outcomes and treatment adherence, emphasizing the importance of normalizing sleep when treating SUDs. Orexins (hypocretins) are neuropeptides exclusively produced by neurons in the posterior hypothalamus that regulate various behavioral and physiological processes, including sleep-wakefulness and motivated drug taking. Given its dual role in sleep and addiction, the orexin system represents a promising therapeutic target for treating SUDs and their comorbid sleep deficits. Here, we review the literature on the role of the orexin system in sleep and drug addiction and discuss the therapeutic potential of orexin receptor antagonists for SUDs. We argue that orexin receptor antagonists may be effective therapeutics for treating addiction because they target orexin's regulation of sleep (top-down) and motivation (bottom-up) pathways.

Influences of Stress and Sex on the Paraventricular Thalamus: Implications for Motivated Behavior

The paraventricular nucleus of the thalamus (PVT) is a critical neural hub for the regulation of a variety of motivated behaviors, integrating stress and reward information from environmental stimuli to guide discrete behaviors via several limbic projections. Neurons in the PVT are activated by acute and chronic stressors, however several roles of the PVT in behavior modulation emerge only following repeated stress exposure, pointing to a role for hypothalamic pituitary adrenal (HPA) axis modulation of PVT function. Further, there may be a reciprocal relationship between the PVT and HPA axis in which chronic stress-induced recruitment of the PVT elicits an additional role for the PVT to regulate motivated behavior by modulating HPA physiology and thus the neuroendocrine response to stress itself. This complex interaction may make the PVT and its role in influencing motivated behavior particularly susceptible to chronic stress-induced plasticity in the PVT, especially in females who display increased susceptibility to stress-induced maladaptive behaviors associated with neuropsychiatric diseases. Though literature is describing the sex-specific effects of acute and chronic stress exposure on HPA axis activation and motivated behaviors, the impact of sex on the role of the PVT in modulating the behavioral and neuroendocrine response to stress is less well established. Here, we review what is currently known regarding the acute and chronic stress-induced activation and behavioral role of the PVT in male and female rodents. We further explore stress hormone and neuropeptide signaling mechanisms by which the HPA axis and PVT interact and discuss the implications for sex-dependent effects of chronic stress on the PVT's role in motivated behaviors.

Orexin deficiency modulates cognitive flexibility in a sex-dependent manner

Cognitive flexibility is an important executive function and refers to the ability to adapt behaviors in response to changes in the environment. Of note, many brain disorders are associated with impairments in cognitive flexibility. Several classical neurotransmitter systems including dopamine, acetylcholine and noradrenaline are shown to be important for cognitive flexibility, however, there is not much known about the role of neuropeptides. The neuropeptide orexin, which is brain-widely released by neurons in the lateral hypothalamus, is a major player in maintaining sleep/wake cycle, feeding behavior, arousal, and motivational behavior. Recent studies showed a role of orexin in attention, cognition and stress-induced attenuation of cognitive flexibility by disrupting orexin signaling locally or systemically. However, it is not known so far whether brain-wide reduction or loss of orexin affects cognitive flexibility. We investigated this question by testing male and female orexin-deficient mice in the attentional set shifting task (ASST), an established paradigm of cognitive flexibility. We found that orexin deficiency impaired the intra-dimensional shift phase of the ASST selectively in female homozygous orexin-deficient mice and improved the first reversal learning phase selectively in male homozygous orexin-deficient mice. We also found that these orexin-mediated sex-based modulations of cognitive flexibility were not correlated with trait anxiety, narcoleptic episodes, and reward consumption. Our findings highlight a sexually dimorphic role of orexin in regulating cognitive flexibility and the need for further investigations of sex-specific functions of the orexin circuitry.

The orexin (hypocretin) neuropeptide system is a target for novel therapeutics to treat cocaine use disorder with alcohol coabuse

An estimated 50-90% of individuals with cocaine use disorder (CUD) also report using alcohol. Cocaine users report coabusing alcohol to 'self-medicate' against the negative emotional side effects of the cocaine 'crash', including the onset of anxiety. Thus, pharmaceutical strategies to treat CUD would ideally reduce the motivational properties of cocaine, alcohol, and their combination, as well as reduce the onset of anxiety during drug withdrawal. The hypothalamic orexin (hypocretin) neuropeptide system offers a promising target, as orexin neurons are critically involved in activating behavioral and physiological states to respond to both positive and negative motivators. Here, we seek to describe studies demonstrating efficacy of orexin receptor antagonists in reducing cocaine, alcohol- and stress-related behaviors, but note that these studies have largely focused on each of these phenomena in isolation. For orexin-based compounds to be viable in the clinical setting, we argue that it is imperative that their efficacy be tested in animal models that account for polysubstance use patterns. To begin to examine this, we present new data showing that rats' preferred level of cocaine intake is significantly increased following chronic homecage access to alcohol. We also report that cocaine intake and motivation are reduced by a selective orexin-1 receptor antagonist when rats have a history of cocaine + alcohol, but not a limited history of cocaine alone. In light of these proof-of-principle data, we outline what we believe to be the key priorities going forward with respect to further examining the orexin system in models of polysubstance use. This article is part of the special issue on Neurocircuitry Modulating Drug and Alcohol Abuse.

The Role of Positive Health Behaviors in the Relationship Between Early Life Stress and Migraine

BACKGROUND

The use of diet and physical activity in alleviating the burden of migraine is unclear, therefore, more scientific study on the role of positive health behaviors in migraine management is needed.

OBJECTIVE

To explore the potential moderating influence of positive health behaviors on the early life stress-migraine relationship in a sample of the Canadian adult population.

METHODS

This study used data from the 2011-2012 Canadian Community Health Survey - Annual Component, a National cross-sectional survey. Adults ≥18 years of age residing in British Columbia were surveyed about stressors experienced as children/teenagers (eg, parental divorce and substance abuse exposure). Migraine was self-report of health-professional diagnosis. Positive health behaviors were measured using an index of transportation and leisure time physical activity, and self-reported frequency of vegetable and fruit consumption. Population survey weights were employed throughout and analytic models were adjusted for known confounders (n_unweighted  = 11,910).

RESULTS

The weighted prevalence of migraine was 9.7% (95% CI: 9.1%, 10.2%). Odds of migraine increased as number of stressors increased: 1 early life stressor (OR = 1.18, 95% CI: 1.01, 1.39), and ≥ 2 early life stressors (OR = 1.51, 95% CI: 1.29, 1.76), compared to no stress. There was evidence of effect modification by physical activity (χ² (4) = 16.02, P < .001). The association between 1 early life stressor and migraine was lower for those who were physically active (moderately active: OR = 1.20, 95% CI: 0.90, 1.61; active: OR = 0.82, 95% CI: 0.61, 1.11). This difference was not apparent for those who experienced 2 or more early life stressors (moderately active: OR = 1.67, 95% CI: 1.28, 2.17; active: OR = 1.55, 95% CI: 1.21, 1.99). Across all levels of early life stress, physically active respondents had lower odds of migraine than moderately active respondents. There was no effect modification by vegetable, fruit, and total fruit/vegetable consumption.

CONCLUSIONS

Stressors experienced early in life are associated with migraine, and physical activity may ameliorate this relationship, particularly for those with exposure to a minimal level of early life stress. A higher dose of physical activity may yield greater benefit than a less intensive dose (ie, moderate activity level). However, the cross-sectional nature of this study precludes us from inferring causality, and future investigation requires prospectively collected data.

Newly identified sleep-wake and circadian circuits as potential therapeutic targets

Optogenetics and chemogenetics are powerful tools, allowing the specific activation or inhibition of targeted neuronal subpopulations. Application of these techniques to sleep and circadian research has resulted in the unveiling of several neuronal populations that are involved in sleep-wake control, and allowed a comprehensive interrogation of the circuitry through which these nodes are coordinated to orchestrate the sleep-wake cycle. In this review, we discuss six recently described sleep-wake and circadian circuits that show promise as therapeutic targets for sleep medicine. The parafacial zone (PZ) and the ventral tegmental area (VTA) are potential druggable targets for the treatment of insomnia. The brainstem circuit underlying rapid eye movement sleep behavior disorder (RBD) offers new possibilities for treating RBD and neurodegenerative synucleinopathies, whereas the parabrachial nucleus, as a nexus linking arousal state control and breathing, is a promising target for developing treatments for sleep apnea. Therapies that act upon the hypothalamic circuitry underlying the circadian regulation of aggression or the photic regulation of arousal and mood pathway carry enormous potential for helping to reduce the socioeconomic burden of neuropsychiatric and neurodegenerative disorders on society. Intriguingly, the development of chemogenetics as a therapeutic strategy is now well underway and such an approach has the capacity to lead to more focused and less invasive therapies for treating sleep-wake disorders and related comorbidities.

The contribution of orexins to sex differences in the stress response

Women are twice as likely as men to suffer from stress-related psychiatric disorders, such as post-traumatic stress disorder (PTSD) and Major Depressive Disorder (MDD), however, the biological basis of these sex differences is not fully understood. Interestingly, orexins are known to be dysregulated in these disorders. This review first discusses the important role of orexins regulating the response to stress. Next, we review the evidence for sex differences in the orexin system, in which the majority of both preclinical and clinical studies have reported higher orexin system expression in females. Finally, we discuss the functional consequences of these sex differences in orexin expression. Most importantly, the preclinical literature reveals that higher orexin system activity in females contributes to exaggerated neuroendocrine and behavioral responses to stress. In sum, the available data suggests that orexins may be important in the etiology of stress-related psychiatric disorders that present differently in men and women. Thus, targeting orexins could potentially ameliorate many phenotypes of stress-related illness in a sex-specific way.

Possible Role of CRF-Hcrt Interaction in the Infralimbic Cortex in the Emergence and Maintenance of Compulsive Alcohol-Seeking Behavior

Alcohol use disorder (AUD) is a chronic, relapsing disorder that is characterized by the compulsive use of alcohol despite numerous health, social, and economic consequences. Initially, the use of alcohol is driven by positive reinforcement. Over time, however, alcohol use can take on a compulsive quality that is driven by the desire to avoid the negative consequences of abstinence, including negative affect and heightened stress/anxiety. This transition from positive reinforcement- to negative reinforcement-driven consumption involves the corticotropin-releasing factor (CRF) system, although mounting evidence now suggests that the CRF system interacts with other neural systems to ultimately produce behaviors that are symptomatic of compulsive alcohol use, such as the hypocretin (Hcrt) system. Hypocretins are produced exclusively in the hypothalamus, but Hcrt neurons project widely throughout the brain and reach regions that perform regulatory functions for numerous behavioral and physiological responses-including the infralimbic cortex (IL) of the medial prefrontal cortex (mPFC). Although the entire mPFC undergoes neuroadaptive changes following prolonged alcohol exposure, the IL appears to undergo more robust changes compared with other mPFC substructures. Evidence to date suggests that the IL is likely involved in EtOH-seeking behavior, but ambiguities with respect to the specific role of the IL in this regard make it difficult to draw definitive conclusions. Furthermore, the manner in which CRF interacts with Hcrt in this region as it pertains to alcohol-seeking behavior is largely unknown, although immunohistochemical and electrophysiological experiments have shown that CRF and Hcrt directly interact in the mPFC, suggesting that the interaction between CRF and Hcrt in the IL may be critically important for the development and subsequent maintenance of compulsive alcohol seeking. This review aims to consolidate recent literature regarding the role of the IL in alcohol-seeking behavior and to discuss evidence that supports a functional interaction between Hcrt and CRF in the IL.

Significant Quantitative Differences in Orexin Neuronal Activation After Pain Assessments in an Animal Model of Sickle Cell Disease

Sickle cell disease is a hemoglobinopathy that causes sickling of red blood cells, resulting in vessel blockage, stroke, anemia, inflammation, and extreme pain. The development and treatment of pain, in particular, neuropathic pain in sickle cell disease patients is poorly understood and impedes our progress toward the development of novel therapies to treat pain associated with sickle cell disease. The orexin/hypocretin system offers a novel approach to treat chronic pain and hyperalgesia. These neuropeptides are synthesized in three regions: perifornical area (PFA), lateral hypothalamus (LH), and dorsomedial hypothalamus (DMH). Data suggest that orexin-A neuropeptide has an analgesic effect on inflammatory pain and may affect mechanisms underlying the maintenance of neuropathic pain. The purpose of this study was to determine whether there are neuronal activation differences in the orexin system as a result of neuropathic pain testing in a mouse model of sickle cell disease. Female transgenic sickle mice that express exclusively (99%) human sickle hemoglobin (HbSS-BERK) and age-/gender-matched controls (HbAA-BERK mice; n = 10/group, 20-30 g) expressing normal human hemoglobin A were habituated to each test protocol and environment before collecting baseline measurements and testing. Four measures were used to assess pain-related behaviors: thermal/heat hyperalgesia, cold hyperalgesia, mechanical hyperalgesia, and deep-tissue hyperalgesia. Hypothalamic brain sections from HbAA-BERK and HbSS-BERK mice were processed to visualize orexin and c-Fos immunoreactivity and quantified. The percentage of double labeled neurons in the PFA was significantly higher than the percentage of double labeled neurons in the LH orexin field of HbAA-BERK mice (* p < 0.05). The percentages of double labeled neurons in PFA and DMH orexin fields are significantly higher than those neurons in the LH of HbSS-BERK mice (* p < 0.05). These data suggest that DMH orexin neurons were preferentially recruited during neuropathic pain testing and a more diverse distribution of orexin neurons may be required to produce analgesia in response to pain in the HbSS-BERK mice. Identifying specific orexin neuronal populations that are integral in neuropathic pain processing will allow us to elucidate mechanisms that provide a more selective, targeted approach in treating of neuropathic pain in sickle cell disease.

Chronic ketamine abuse is associated with orexin-A reduction and ACTH elevation

BACKGROUND

Ketamine has emerged as a major substance of abuse worldwide. Evidence suggests a role of orexin system in reward processing, withdrawal, and stress response. It also interacts with the stress mechanisms of hypothalamic-pituitary-adrenal (HPA) axis to regulate drug-taking behavior. The study aimed to explore the relevance of orexin and stress hormones to chronic ketamine abuse.

METHODS

We enrolled 67 ketamine-dependent (KD) patients and 64 controls. The levels of orexin-A, adrenocorticotropic hormone (ACTH), and cortisol were measured at baseline, 1 week, and 2 weeks after ketamine discontinuation. KD patients were assessed by Beck Depression Inventory, Beck Anxiety Inventory, and Visual Analogue Scale for ketamine craving at baseline.

RESULTS

Compared with the controls, KD patients had significantly lower orexin-A (0.65 ± 0.12 vs. 0.74 ± 0.10 ng/mL, p < 0.001) and increased ACTH (32.3 ± 16.3 vs. 22.3 ± 11.0 pg/mL, p = 0.008) levels at baseline, whereas cortisol levels were similar between two groups. Levels of the three markers did not correlate with ketamine use variables, craving, depression, or anxiety symptoms. The levels did not alter after 1 or 2 weeks of ketamine discontinuation. Notably, those with higher anxiety had lower orexin-A but increased cortisol levels than did those with lower anxiety.

CONCLUSIONS

This study showed that KD patients had persistent orexin-A reduction and stress hormone dysregulation in early abstinence. The anxious phenotype of KD might be associated with a lower orexin-A expression. These results point to a promising pathway to investigate the neurochemical mechanisms of ketamine addiction.

Anterior Insular Cortex is Critical for the Propensity to Relapse Following Punishment-Imposed Abstinence of Alcohol Seeking

Humans with alcohol use disorder typically abstain because of the negative consequences associated with excessive drinking, and exposure to contexts previously associated with alcohol use can trigger relapse. We used a rat model that captures a characteristic of this human condition: namely voluntary abstinence from alcohol use because of contingent punishment. There is substantial variability in the propensity to relapse following extended periods of abstinence, and this is a critical feature preventing the successful treatment of alcohol use disorder. Here we examined relapse following acute or prolonged abstinence. In male alcohol preferring P rats, we found an increased propensity to relapse in Context B, the punishment context after prolonged abstinence. Next, we found that neither alcohol intake history nor the motivational strength of alcohol predicted the propensity to relapse. We next examined the putative circuitry of context-induced relapse to alcohol seeking following prolonged abstinence using Fos as a marker of neuronal activation. The anterior insular cortex (AI) was the only brain region examined where Fos expression correlated with alcohol seeking behavior in Context B after prolonged abstinence. Finally, we used local infusion of GABAA and GABAB receptor agonists (muscimol + baclofen) to show a causal role of the AI in context-induced relapse in Context B, the punishment context after prolonged abstinence. Our results show that there is substantial individual variability in the propensity to relapse in the punishment-associated context after prolonged abstinence, and this is mediated by activity in the AI.SIGNIFICANCE STATEMENT A key feature of alcohol use disorder is that sufferers show an enduring propensity to relapse throughout their lifetime. Relapse typically occurs despite the knowledge of adverse consequences including health complications or relationship breakdowns. Here we use a recently developed rodent model that recapitulates this behavior. After an extended period of abstinence, relapse propensity is markedly increased in the "adverse consequence" environment, akin to humans with alcohol use disorder relapsing in the face of adversity. From a circuitry perspective, we demonstrate a causal role of the anterior insular cortex in relapse to alcohol seeking after extended abstinence following punishment imposed voluntary cessation of alcohol use.

Orexin/hypocretin receptor modulation of anxiolytic and antidepressive responses during social stress and decision-making: Potential for therapy

Hypothalmic orexin/hypocretin (Orx) neurons in the lateral and dorsomedial perifornical region (LH-DMH/PeF) innervate broadly throughout the brain, and receive similar inputs. This wide distribution, as well as two Orx peptides (Orx_A and Orx_B) and two Orx receptors (Orx₁ and Orx₂) allow for functionally related but distinctive behavioral outcomes, that include arousal, sleep-wake regulation, food seeking, metabolism, feeding, reward, addiction, and learning. These are all motivational functions, and tie the orexin systems to anxiety and depression as well. We present evidence, that for affective behavior, Orx₁ and Orx₂ receptors appear to have opposing functions. The majority of research on anxiety- and depression-related outcomes has focused on Orx₁ receptors, which appear to have primarily anxiogenic and pro-depressive actions. Although there is significant research suggesting contrary findings, the primary potential for pharmacotherapies linked to the Orx₁ receptor is via antagonists to block anxious and depressive behavior. Dual orexin receptor antagonists have been approved for treatment of sleep disorders, and are likely candidates for adaptation for affect disorder treatments. However, we present evidence here that demonstrates the Orx₂ receptors are anxiolytic and antidepressive. Using a new experimental pre-clinical model of anxious and depressive behavior stimulated by social stress and decision-making that produces two stable behavioral phenotypes, Escape/Resilient and Stay/Susceptible, we tested the effects of intracerebroventricular injections of Orx₂ agonist and antagonist drugs. Over ten behavioral measures, we have demonstrated that Orx₂ agonists promote resilience, as well as anxiolytic and antidepressive behavior. In contrast, Orx₂ antagonists or knockdown kindle anxious and pro-depressive behavior plus increase susceptibility. The results suggest that the Orx₂ receptor may be a useful target for pharmacotherapies to treat anxiety and depression.

The effect of acute or repeated stress on the corticotropin releasing factor system in the CRH-IRES-Cre mouse: A validation study

Corticotropin releasing factor (CRF) is a key component of stress responsivity, modulating related behaviors including anxiety and reward. Difficulties identifying CRF neurons, using traditional approaches including immunohistochemistry, has led to the development of a number of transgenic CRF reporter mice. The Crh-IRES-Cre::Ai14 (tdTomato) reporter mouse is increasing in popularity as a useful tool to assess the localization, connectivity and function of CRF neurons in various stress-related behaviors. However, without proper characterization of reporter expression, the in vivo and in vitro manifestations resulting from the manipulation of these cells must be interpreted with caution. Here we mapped the distribution of tdTomato-expressing CRF cells throughout the rostro-caudal extent of the Crh-IRES-Cre::Ai14 mouse brain. To determine if reporter expression faithfully reproduced native CRF expression, we assessed the colocalization of CRF expression with tdTomato reporter expression across several brain regions. Good concordance was observed in the extended amygdala and paraventricular nucleus of the hypothalamus (PVN), while discrepancies were observed within the lateral hypothalamus and hippocampus. Finally, we examined the activation of CRF neurons in Crh-IRES-Cre::Ai14 mice in response to different types of stressors using Fos immunohistochemistry. Acute psychological (swim) and pharmacological (yohimbine) stress stimulated Fos-protein expression in PVN CRF neurons. Interestingly though, exposure to four daily restraint stress sessions followed by a novel acute stressor did not further recruit CRF neurons across any brain region examined. Our results highlight the importance of thoroughly characterizing reporter mice before use and suggest that acute versus repeated stress may differentially impact the CRF system. This article is part of the Special Issue entitled 'Hypothalamic Control of Homeostasis'.

The role of the orexin system in stress response

Orexins are neuropeptides that are exclusively produced by hypothalamic neurons, which project throughout the entire brain. Orexin, also known as hypocretins, were initially identified to play a fundamental role in food intake, arousal and the regulation of sleep and wakefulness. Recent studies identified orexins to be critical for diverse physiological processes including motivation, reward, attention, emotional regulation, stress and anxiety. Here, I review recent findings that indicate orexin has an important role in acute and chronic stress. I also summarize the recent optogenetic and chemogenetic studies that have advanced our understanding of the orexin system. I will conclude by discussing clinical studies that implicate orexins in mental health disorders. This article is part of the Special Issue entitled 'Hypothalamic Control of Homeostasis'.

Involvement of dopamine system in the regulation of the brain corticotropin-releasing hormone in paraventricular nucleus in a rat model of chronic visceral pain

Objective We aimed to investigate the mechanism of paraventricular nucleus (PVN) and ventral tegmental area (VTA) circuit in the pathogenesis of visceral pain-depression with a rat model induced by neonatal and adult colorectal distension (CRD). Methods Neonate male Sprague-Dayley (SD) rats underwent CRD on postnatal days 8, 10, and 12, and when matured, were tested for adult abdominal withdrawal reflex (AWR) scores to assess visceral hypersensitivity. The forced swimming test was employed to evaluate depression-like behaviors. The rats exhibiting visceral pain-depressive behaviors underwent lidocaine injection in the VTA to explore the relationship between VTA and visceral pain. Moreover, double immunofluorescence was employed to evaluate the qualitative and quantitative expression of dopamine/ c-Fos in CRD rats. After verifying the existed fiber projection from PVN to VTA, the intra-PVN microinjection of CRH-RNAi lentivirus to inhibit corticotropin-releasing hormone (CRH) expression, behavioral changes were assessed by AWR score and FST. Thereafter, with the sacrifice of the rats, the variations of TH protein in rats were evaluated by immunofluorescence and Western blot. Results Intra-VTA microinjection of lidocaine increased the pain threshold of CRD group. After intra-VTA microinjection of green retrograde tracer, immunofluorescence photomicrographs visualized the PVN with a typical green retrograde tracer. Intra-PVN microinjection of CRH-RNAi lentivirus alleviated the visceral pain-depression behaviors and decreased the TH protein expression in the VTA. Conclusion These data demonstrated that the VTA played a functional role in chronic visceral pain and depression, and the CRH-containing neurons in hypothalamic PVN may be implicated in the onset and maintenance of the chronic visceral pain and depression via the activation of dopamine in the VTA.

Chronic light exposure alters serotonergic and orexinergic systems in the rat brain and reverses maternal separation-induced increase in orexin receptors in the prefrontal cortex

Maternal separation (MS) is a well-established rodent model of depression. Chronic constant light (CCL) treatment during adolescence has been shown to reverse the depression-like behaviour induced by MS. We aimed to further delineate the antidepressant effect of light by investigating the involvement of the dopaminergic, serotonergic and orexinergic systems. MS was used to induce changes in adult male Sprague-Dawley rats, some of whom were also treated with CCL for 3 weeks during adolescence. At P80, rats were decapitated and brain tissue collected for analysis of glutamate- and potassium-stimulated dopamine release in the nucleus accumbens (NAc) using an in vitro superfusion technique. Enzyme-linked immunosorbent assays were employed to measure 5-hydroxytryptamine (5-HT) levels in the hypothalamus and prefrontal cortex (PFC). Western blotting was used to measure orexin receptor 1 (OXR-1) and 2 (OXR-2) in the PFC. MS did not affect 5-HT levels in these rats. However, CCL increased hypothalamic 5-HT and reduced 5-HT levels in the PFC. CCL had opposite effects on OXR levels in the PFC of maternally separated and non-separated rats. MS increased OXR-1 and OXR-2 levels in the PFC, an effect that was normalized by CCL treatment. MS reduced glutamate-stimulated dopamine release in the NAc, an effect that was not reversed by CCL. The present results suggest that CCL treatment affects 5-HT and orexinergic systems in the MS model while not affecting the MS-induced decrease in dopamine release in the NAc. The reversal of changes in the orexinergic system may be of particular relevance to the antidepressant effect of CCL in depression.

Testosterone boosts physical activity in male mice via dopaminergic pathways

Low testosterone (T) in men, especially its free fraction, has been associated with loss of energy. In accordance, orchidectomy (ORX) in rodents results in decreased physical activity. Still, the mechanisms through which T stimulates activity remain mostly obscure. Here, we studied voluntary wheel running behavior in three different mouse models of androgen deficiency: ORX, androgen receptor (AR) knock-out (ARKO) and sex hormone binding globulin (SHBG)-transgenic mice, a novel mouse model of “low free T”. Our results clearly show a fast and dramatic action of T stimulating wheel running, which is not explained by its action on muscle, as evidenced by neuromuscular studies and in a muscle-specific conditional ARKO mouse model. The action of T occurs via its free fraction, as shown by the results in SHBG-transgenic mice, and it implies both androgenic and estrogenic pathways. Both gene expression and functional studies indicate that T modulates the in vivo sensitivity to dopamine (DA) agonists. Furthermore, the restoration of wheel running by T is inhibited by treatment with DA antagonists. These findings reveal that the free fraction of T, both via AR and indirectly through aromatization into estrogens, stimulates physical activity behavior in male mice by acting on central DA pathways.

Early-life adversity facilitates acquisition of cocaine self-administration and induces persistent anhedonia

Early-life adversity increases the risk for emotional disorders such as depression and schizophrenia. Anhedonia, thought to be a core feature of these disorders, is provoked by our naturalistic rodent model of childhood adversity (i.e., rearing pups for one week in cages with limited bedding and nesting, LBN). Drug use and addiction are highly comorbid with psychiatric disorders featuring anhedonia, yet effects of LBN on drug-seeking behavior and the reward and stress-related circuits that underlie it remain unknown. Here we examined the effects of LBN on cocaine intake and seeking, using a battery of behavioral tests measuring distinct aspects of cocaine reward, and for comparison, chocolate intake. We also examined activation of neurons within the pleasure/reward and stress circuits following cocaine in LBN and control rats. Early-life adversity reduced spontaneous intake of palatable chocolate, extending prior reports of sucrose and social-play anhedonia. In a within-session cocaine behavioral economic test, LBN rats self-administered lower dosages of cocaine under low-effort conditions, consistent with a reduced hedonic set-point for cocaine, and potentially anhedonia. In contrast, cocaine demand elasticity was not consistently affected, indicating no major changes in motivation to maintain preferred cocaine blood levels. These changes were selective, as LBN did not cause an overt anxiety-like phenotype, nor did it affect sensitivity to self-administered cocaine dose, responding for cocaine under extinction conditions, cocaine- or cue-induced reinstatement of cocaine seeking, or locomotor response to acute cocaine. However, high Fos expression was seen after cocaine in both reward- and stress-related brain regions of LBN rats, including nucleus accumbens core, central amygdala, and lateral habenula. In contrast, hypothalamic orexin neuron activation after cocaine was significantly attenuated in LBN rats. Together, these findings demonstrate enduring effects of early-life adversity on both reward- and fear/anxiety-related neural circuits, as well as anhedonia-like reductions in consumption of natural and drug rewards.

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Adversity during a sensitive developmental period provokes persistent anhedonia.

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This adversity reduces cocaine hedonic set point, but not motivation.

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Cocaine-associated Fos is altered in reward- and anxiety/fear circuits.

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Cocaine-dose sensitivity, reinstatement, and locomotion are unchanged.

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Effects are selective, as anxiety-related behaviors were unaltered.

Chemogenetic activation of the lateral hypothalamus reverses early life stress-induced deficits in motivational drive

Altered motivated behaviour is a cardinal feature of several neuropsychiatric conditions including mood disorders. One well-characterized antecedent to the development of mood disorders is exposure to early life stress (ELS). A key brain substrate controlling motivated behaviour is the lateral hypothalamus (LH). Here, we examined the effect of ELS on LH activation and the motivation to self-administer sucrose. We tested whether chemogenetic activation of LH circuits could modify sucrose responding in ELS rats and examined the impact on LH cell populations. Male rat pups were maternally separated for 0 or 3 h on postnatal days 2-14. During adolescence, rats received bilateral injections of hM3D(Gq), the excitatory designer receptor exclusively activated by designer drugs, into LH. In adulthood, rats were trained to self-administer sucrose and tested under a progressive ratio schedule to determine their motivation for reward following injection with either vehicle or 5 mg/kg clozapine-N-oxide. Brains were processed for Fos-protein immunohistochemistry. ELS significantly suppressed lever responding for sucrose, indicating a long-lasting impact of ELS on motivation circuits. hM3D(Gq) activation of LH increased responding, normalizing deficits in ELS rats, and increased Fos-positive orexin and MCH cell numbers within LH. Our findings indicate that despite being susceptible to environmental stressors, LH circuits retain the capacity to overcome ELS-induced deficits in motivated behaviour.

New directions for the treatment of depression: Targeting the photic regulation of arousal and mood (PRAM) pathway

Both preclinical and clinical studies demonstrate that depression is strongly associated with reduced light availability, which in turn contributes to decreased function of brain regions that control mood. Here, we review findings that support a critical pathway for the control of mood that depends upon ambient light. We put forward a novel hypothesis, functionally linking retina to locus coeruleus (LC) in depression, and discuss the role of norepinephrine in affective disease. Finally, we discuss how utilizing the chemogenetic tool Designer Receptors Exclusively Activated by Designer Drugs (DREADDs) to precisely control this retina-LC circuit may be used as a novel therapeutic to treat depression.

Role of the Orexin/Hypocretin System in Stress-Related Psychiatric Disorders

Orexins (hypocretins) are critically involved in coordinating appropriate physiological and behavioral responses to aversive and threatening stimuli. Acute stressors engage orexin neurons via direct projections from stress-sensitive brain regions. Orexin neurons, in turn, facilitate adaptive behavior via reciprocal connections as well as via direct projections to the hypophysiotropic neurons that coordinate the hypothalamic-pituitary-adrenal (HPA) axis response to stress. Consequently, hyperactivity of the orexin system is associated with increased motivated arousal and anxiety, and is emerging as a key feature of panic disorder. Accordingly, there has been significant interest in the therapeutic potential of pharmacological agents that antagonize orexin signaling at their receptors for the treatment of anxiety disorders. In contrast, disorders characterized by inappropriately low levels of motivated arousal, such as depression, generally appear to be associated with hypoactivity of the orexin system. This includes narcolepsy with cataplexy, a disorder characterized by the progressive loss of orexin neurons and increased rates of moderate/severe depression symptomology. Here, we provide a comprehensive overview of both clinical and preclinical evidence highlighting the role of orexin signaling in stress reactivity, as well as how perturbations to this system can result in dysregulated behavioral phenotypes.

A Decade of Orexin/Hypocretin and Addiction: Where Are We Now?

One decade ago, our laboratory provided the first direct evidence linking orexin/hypocretin signaling with drug seeking by showing that activation of these neurons promotes conditioned morphine-seeking behavior. In the years since, contributions from many investigators have revealed roles for orexins in addiction for all drugs of abuse tested, but only under select circumstances. We recently proposed that orexins play a fundamentally unified role in coordinating "motivational activation" under numerous behavioral conditions, and here we unpack this hypothesis as it applies to drug addiction. We describe evidence collected over the past 10 years that elaborates the role of orexin in drug seeking under circumstances where high levels of effort are required to obtain the drug, or when motivation for drug reward is augmented by the presence of external stimuli like drug-associated cues/contexts or stressors. Evidence from studies using traditional self-administration and reinstatement models, as well as behavioral economic analyses of drug demand elasticity, clearly delineates a role for orexin in modulating motivational, rather than the primary reinforcing aspects of drug reward. We also discuss the anatomical interconnectedness of the orexin system with wider motivation and reward circuits, with a particular focus on how orexin modulates prefrontal and other glutamatergic inputs onto ventral tegmental area dopamine neurons. Last, we look ahead to the next decade of the research in this area, highlighting the recent FDA approval of the dual orexin receptor antagonist suvorexant (Belsomra®) for the treatment of insomnia as a promising sign of the potential clinical utility of orexin-based therapies for the treatment of addiction.

Altered neural connectivity in adult female rats exposed to early life social stress

The use of a variety of neuroanatomical techniques has led to a greater understanding of the adverse effects of stress on psychiatric health. One recent advance that has been particularly valuable is the development of resting state functional connectivity (RSFC) in clinical studies. The current study investigates changes in RSFC in F1 adult female rats exposed to the early life chronic social stress (ECSS) of the daily introduction of a novel male intruder to the cage of their F0 mothers while the F1 pups are in the cage. This ECSS for the F1 animals consists of depressed maternal care from their F0 mothers and exposure to conflict between their F0 mothers and intruder males. Analyses of the functional connectivity data in ECSS exposed adult females versus control females reveal broad changes in the limbic and reward systems, the salience and introspective socioaffective networks, and several additional stress and social behavior associated nuclei. Substantial changes in connectivity were found in the prefrontal cortex, nucleus accumbens, hippocampus, and somatosensory cortex. The current rodent RSFC data support the hypothesis that the exposure to early life social stress has long term effects on neural connectivity in numerous social behavior, stress, and depression relevant brain nuclei. Future conscious rodent RSFC studies can build on the wealth of data generated from previous neuroanatomical studies of early life stress and enhance translational connectivity between animal and human fMRI studies in the development of novel preventative measures and treatments.