Blockade of orexin-1 receptors in the ventral tegmental area could attenuate the lateral hypothalamic stimulation-induced potentiation of rewarding properties of morphine

Whole Brain Mapping of Orexin Receptor mRNA Expression Visualized by Branched In Situ Hybridization Chain Reaction

Orexins, which are produced within neurons of the lateral hypothalamic area, play a pivotal role in the regulation of various behaviors, including sleep/wakefulness, reward behavior, and energy metabolism, via orexin receptor type 1 (OX1R) and type 2 (OX2R). Despite the advanced understanding of orexinergic regulation of behavior at the circuit level, the precise distribution of orexin receptors in the brain remains unknown. Here, we develop a new branched in situ hybridization chain reaction (bHCR) technique to visualize multiple target mRNAs in a semiquantitative manner, combined with immunohistochemistry, which provided comprehensive distribution of orexin receptor mRNA and neuron subtypes expressing orexin receptors in mouse brains. Only a limited number of cells expressing both Ox1r and Ox2r were observed in specific brain regions, such as the dorsal raphe nucleus and ventromedial hypothalamic nucleus. In many brain regions, Ox1r-expressing cells and Ox2r-expressing cells belong to different cell types, such as glutamatergic and GABAergic neurons. Moreover, our findings demonstrated considerable heterogeneity in Ox1r- or Ox2r-expressing populations of serotonergic, dopaminergic, noradrenergic, cholinergic, and histaminergic neurons. The majority of orexin neurons did not express orexin receptors. This study provides valuable insights into the mechanism underlying the physiological and behavioral regulation mediated by the orexin system, as well as the development of therapeutic agents targeting orexin receptors.

Restraint stress potentiates sensitivity to the antinociceptive effect of morphine through orexin receptors in the ventral tegmental area

Orexin signaling in the ventral tegmental area (VTA) plays a critical role in stress and addictive behaviors. On the other hand, exposure to stress potentiates behavioral sensitization to drugs of abuse such as morphine. This study aimed to elucidate the role of orexin receptors within the VTA in restraint stress (RS)-induced morphine sensitization. Adult male albino Wistar rats underwent stereotaxic surgery, and two stainless steel guide cannulae were bilaterally implanted into the VTA. Different doses of SB334867 or TCS OX2 29 as orexin-1 (OX1) and orexin-2 (OX2) receptor antagonists were microinjected into the VTA five min before exposure to RS, respectively. A duration of three hours was considered for applying the RS, and 10 min after RS exposure, animals received a subcutaneous injection of an ineffective dose of morphine (1 mg/kg) for three consecutive days followed by a five-day drug/stress-free period. On the ninth day, the tail-flick test evaluated the sensitivity to the antinociceptive effects of morphine. The results demonstrated that the sole application of RS or morphine (1 mg/kg) could not induce morphine sensitization; however, concurrent application of RS and morphine could induce morphine sensitization. Besides, intra-VTA administration of OX1 R or OX2 R antagonists before paired administration of morphine and RS blocked morphine sensitization. The role of OX1 R and OX2 R in the induction of stress-induced morphine sensitization was almost identical. This study provides new insight into the role of orexin signaling in the VTA in the potentiation of morphine sensitization induced by RS and morphine co-administration.

The role of orexin-1 receptors within the ventral tegmental area in the extinction and reinstatement of methamphetamine place preference

Targeting the orexin system has recently been identified as one of the promising options for treating drug addiction. It may be more feasible and achievable if we investigate the accurate function of the orexin system in brain areas implicated in reward and addiction, such as the ventral tegmental area (VTA) by animal reward models. This study investigated the contribution of the orexin system, mainly the orexin-1 receptors (OX1R) in the VTA, in the extinction and reinstatement of methamphetamine (METH) related memories in the conditioned place preference (CPP) model. Animals after the acquisition of METH place preference were subjected to two separate sets of extinction and reinstatement experiments to receive various concentrations of selective OX1R antagonist, SB334867 into the bilateral VTA before extinction sessions (1, 3, and 10 nmol/0.3 μl DMSO per side) or only on the reinstatement phase (3, 10, and 30 nmol/0.3 μl DMSO per side), respectively. Intra-VTA infusion of SB334867 throughout the extinction phase could remarkably facilitate the extinction process and decrease the maintenance of reinforcing effects of METH at the highest dosage (10 nmol; p < 0.0001). Data also indicated a single microinfusion of SB334867 into the VTA before reinstatement of the METH-seeking behavior could considerably prevent the relapse of previously formed reward-context memories (10 nmol; p < 0.01 and 30 nmol; p < 0.001). The present study provided evidence supporting the potential therapeutic effects of the orexin system modulation, specifically in the VTA, on different stages of METH-induced place preference.

The therapeutic potential of opioids in narcolepsy type 1: A systematic literature review and questionnaire study

OBJECTIVE

Narcolepsy type 1 is a primary sleep disorder caused by deficient hypocretin transmission leading to excessive daytime sleepiness and cataplexy. Opioids have been suggested to increase the number of hypocretin-producing neurons. We aimed to assess opioid use and its self-reported effect on narcolepsy type 1 symptom severity through a literature review and questionnaire study.

METHODS

We systematically reviewed literature on opioid use in narcolepsy. We also recruited 100 people with narcolepsy type 1 who completed an online questionnaire on opioid use in the previous three years. The main questionnaire topics were the indication for use, and the possible effects on narcolepsy symptom severity. Structured follow-up interviews were conducted when opioid use was reported.

RESULTS

The systematic literature review mainly showed improvements in narcolepsy symptom severity. Recent opioid use was reported by 16/100 questionnaire respondents, who had used 20 opioids (codeine: 7/20, tramadol: 6/20, oxycodone: 6/20, fentanyl: 1/20). Narcolepsy symptom changes were reported in 11/20. Positive effects on disturbed nocturnal sleep (9/20), excessive daytime sleepiness (4/20), hypnagogic hallucinations (3/17), cataplexy (2/18), and sleep paralysis (1/13) were most pronounced for oxycodone (4/6) and codeine (4/7).

CONCLUSIONS

Opioids were relatively frequently used compared to a similarly young general Dutch sample. Oxycodone and, to a lesser extent, codeine were associated with self-reported narcolepsy symptom severity improvements. Positive changes in disturbed nocturnal sleep and daytime sleepiness were most frequently reported, while cataplexy effects were less pronounced. Randomised controlled trials are now needed to verify the potential of opioids as therapeutic agents for narcolepsy.

Suvorexant ameliorated sleep disturbance, opioid withdrawal, and craving during a buprenorphine taper

Increased orexin/hypocretin signaling is implicated in opioid withdrawal, sleep disturbances, and drug-seeking behaviors. This study examined whether a dual-orexin receptor antagonist would improve sleep and withdrawal outcomes when compared with placebo during a buprenorphine/naloxone taper. Thirty-eight participants with opioid use disorder were recruited to a clinical research unit and maintained on 8/2 to 16/4 mg of buprenorphine/naloxone treatment for 3 days before being randomized to 20 mg of suvorexant (n = 14), 40 mg of suvorexant (n = 12), or placebo (n = 12); 26 individuals completed the study. After randomization, participants underwent a 4-day buprenorphine/naloxone taper and 4-day post-taper observation period. Total sleep time (TST) was collected nightly with a wireless electroencephalography device and wrist-worn actigraphy; opioid withdrawal symptoms were assessed via the Subjective Opiate Withdrawal Scale (SOWS); and abuse potential was assessed on a 0- to 100-point visual analog scale of "High" every morning. A priori outcomes included two-group (collapsing suvorexant doses versus placebo) and three-group comparisons of area-under-the-curve (AUC) scores for TST, SOWS, and High. In two-group comparisons, participants receiving suvorexant displayed increased TST during the buprenorphine/naloxone taper and decreased SOWS during the post-taper period. In three-group comparisons, participants receiving 20 mg of suvorexant versus placebo displayed increased AUC for TST during the buprenorphine/naloxone taper, but there was no difference in SOWS among groups. There was no evidence of abuse potential in two- or three-group analyses. The results suggest that suvorexant might be a promising treatment for sleep and opioid withdrawal in individuals undergoing a buprenorphine/naloxone taper.

Neurobiology of the Orexin System and Its Potential Role in the Regulation of Hedonic Tone

Orexin peptides comprise two neuropeptides, orexin A and orexin B, that bind two G-protein coupled receptors (GPCRs), orexin receptor 1 (OXR1) and orexin receptor 2 (OXR2). Although cell bodies that produce orexin peptides are localized in a small area comprising the lateral hypothalamus and adjacent regions, orexin-containing fibres project throughout the neuraxis. Although orexins were initially described as peptides that regulate feeding behaviour, research has shown that orexins are involved in diverse functions that range from the modulation of autonomic functions to higher cognitive functions, including reward-seeking, behaviour, attention, cognition, and mood. Furthermore, disruption in orexin signalling has been shown in mood disorders that are associated with low hedonic tone or anhedonia, including depression, anxiety, attention deficit hyperactivity disorder, and addiction. Notably, projections of orexin neurons overlap circuits involved in the modulation of hedonic tone. Evidence shows that orexins may potentiate hedonic behaviours by increasing the feeling of pleasure or reward to various signalling, whereas dysregulation of orexin signalling may underlie low hedonic tone or anhedonia. Further, orexin appears to play a key role in regulating behaviours in motivationally charged situations, such as food-seeking during hunger, or drug-seeking during withdrawal. Therefore, it would be expected that dysregulation of orexin expression or signalling is associated with changes in hedonic tone. Further studies investigating this association are warranted.

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.

Potential therapeutic targets for the treatment of opioid abuse and pain

Although μ-opioid peptide (MOP) receptor agonists are effective analgesics available in clinical settings, their serious adverse effects put limits on their use. The marked increase in abuse and misuse of prescription opioids for pain relief and opioid overdose mortality in the past decade has seriously impacted society. Therefore, safe analgesics that produce potent analgesic effects without causing MOP receptor-related adverse effects are needed. This review highlights the potential therapeutic targets for the treatment of opioid abuse and pain based on available evidence generated through preclinical studies and clinical trials. To ameliorate the abuse-related effects of opioids, orexin-1 receptor antagonists and mixed nociceptin/MOP partial agonists have shown promising results in translational aspects of animal models. There are several promising non-opioid targets for selectively inhibiting pain-related responses, including nerve growth factor inhibitors, voltage-gated sodium channel inhibitors, and cannabinoid- and nociceptin-related ligands. We have also discussed several emerging and novel targets. The current medications for opioid abuse are opioid receptor-based ligands. Although neurobiological studies in rodents have discovered several non-opioid targets, there is a translational gap between rodents and primates. Given that the neuroanatomical aspects underlying opioid abuse and pain are different between rodents and primates, it is pivotal to investigate the functional profiles of these non-opioid compounds compared to those of clinically used drugs in non-human primate models before initiating clinical trials. More pharmacological studies of the functional efficacy, selectivity, and tolerability of these newly discovered compounds in non-human primates will accelerate the development of effective medications for opioid abuse and pain.

Morphine-element interactions - The influence of selected chemical elements on neural pathways associated with addiction

Addiction is a pressing social problem worldwide and opioid dependence can be considered the strongest and most difficult addiction to treat. Mesolimbic and mesocortical dopaminergic pathways play an important role in modulation of cognitive processes and decision making and, therefore, changes in dopamine metabolism are considered the central basis for the development of dependence. Disturbances caused by excesses or deficiency of certain elements have a significant impact on the functioning of the central nervous system (CNS) both in physiological conditions and in pathology and can affect the cerebral reward system and therefore, may modulate processes associated with the development of addiction. In this paper we review the mechanisms of interactions between morphine and zinc, manganese, chromium, cadmium, lead, fluoride, their impact on neural pathways associated with addiction, and on antinociception and morphine tolerance and dependence.

The hypocretin (orexin) system: from a neural circuitry perspective

Hypocretin/orexin neurons are distributed restrictively in the hypothalamus, a brain region known to orchestrate diverse functions including sleep, reward processing, food intake, thermogenesis, and mood. Since the hypocretins/orexins were discovered more than two decades ago, extensive studies have accumulated concrete evidence showing the pivotal role of hypocretin/orexin in diverse neural modulation. New method of viral-mediated tracing system offers the possibility to map the monosynaptic inputs and detailed anatomical connectivity of Hcrt neurons. With the development of powerful research techniques including optogenetics, fiber-photometry, cell-type/pathway specific manipulation and neuronal activity monitoring, as well as single-cell RNA sequencing, the details of how hypocretinergic system execute functional modulation of various behaviors are coming to light. In this review, we focus on the function of neural pathways from hypocretin neurons to target brain regions. Anatomical and functional inputs to hypocretin neurons are also discussed. We further briefly summarize the development of pharmaceutical compounds targeting hypocretin signaling. This article is part of the special issue on Neuropeptides.

Intra-hippocampal administration of orexin receptor antagonists dose-dependently attenuates reinstatement of morphine seeking behavior in extinguished rats

It has been shown that the hippocampus plays an essential role in the regulation of reward and memory as indicated by the conditioned place preference (CPP) paradigm. Morphine-induced CPP is a common method to consider motivational properties of morphine in animals. Recently, this model has been used in many laboratories to investigate neuronal mechanisms underlying reinstatement of morphine seeking induced by drug re-exposure. Our previous studies indicate that the hippocampus especially CA1 region is involved in reinstatement of drug-seeking behaviors. Also, several studies have shown that orexin attenuates key functional and behavioral effects of its co-transmitter dynorphin. The present study evaluates the role of orexinergic receptors within the CA1 region of the hippocampus in the reinstatement of morphine-induced CPP. Therefore, after the extinction period, the different doses (SB 334867; 0.3, 3, and 30 nM/0.5 μl DMSO) of either orexin-1 or -2 receptor antagonists were bilaterally microinjected into the CA1, 15 min before receiving an effective priming dose of morphine (1 mg/kg). The results revealed that administration of both SB 334867 and TCS OX2 29 prior to injection of the priming dose of morphine significantly reduced the reinstatement of morphine-induced CPP without altering the animal's locomotor activity. Also, the 50% effective dose value of SB 334867 on the reinstatement of morphine seeking behavior was close three times more than that in TCS OX2 29 treatment group. Therefore, the consequences suggested that both orexin receptors in the CA1 play a considerable role in the reinstatement of morphine-induced CPP.

Identification of discrete, intermingled hypocretin neuronal populations

Neurons in the lateral hypothalamic area that express hypocretin (Hcrt) neuropeptides help regulate many behaviors including wakefulness and reward seeking. These neurons project throughout the brain, including to neural populations that regulate wakefulness, such as the locus coeruleus (LC) and tuberomammilary nucleus (TMN), as well as to populations that regulate reward, such as the nucleus accumbens (NAc) and ventral tegmental area (VTA). To address the roles of Hcrt neurons in seemingly disparate behaviors, it has been proposed that Hcrt neurons can be anatomically subdivided into at least two distinct subpopulations: a "medial group" that projects to the LC and TMN, and a "lateral group" that projects to the NAc and VTA. Here, we use a dual retrograde tracer strategy to test the hypotheses that Hcrt neurons can be classified based on their downstream projections and medial/lateral location within the hypothalamus. We found that individual Hcrt neurons were significantly more likely to project to both the LC and TMN or to both the VTA and NAc than would be predicted by chance. In contrast, we found that Hcrt neurons that projected to the LC or TMN were mostly distinct from Hcrt neurons that projected to the VTA or NAc. Interestingly, these two populations of Hcrt neurons are intermingled within the hypothalamus and cannot be classified into medial or lateral groups. These results suggest that Hcrt neurons can be distinguished based on their downstream projections but are intermingled within the hypothalamus.

Orexins and stress

The neuropeptides orexins are important in regulating the neurobiological systems that respond to stressful stimuli. Furthermore, orexins are known to play a role many of the phenotypes associated with stress-related mental illness such as changes in cognition, sleep-wake states, and appetite. Interestingly, orexins are altered in stress-related psychiatric disorders such as Major Depressive Disorder and Anxiety Disorders. Thus, orexins may be a potential target for treatment of these disorders. In this review, we will focus on what is known about the role of orexins in acute and repeated stress, in stress-induced phenotypes relevant to psychiatric illness in preclinical models, and in stress-related psychiatric illness in humans. We will also briefly discuss how orexins may contribute to sex differences in the stress response and subsequent phenotypes relevant to mental health, as many stress-related psychiatric disorders are twice as prevalent in women.

Orexin A in men with heroin use disorder undergoing methadone maintenance treatment

Orexins have played a role in reward-seeking and addiction-related behavior. There are few reports in the literature on serum levels of orexins in patients with heroin use disorder (HUD) undergoing methadone maintenance treatment (MMT). The aim of this study was to investigate the serum levels of orexin A in HUD patients undergoing MMT. Fifty male HUD patients undergoing MMT and 25 healthy males were enrolled for this study. Serum orexin A were measured with assay kits. Using analysis of covariance (ANCOVA) with body mass index (BMI) adjustments, the serum levels of orexin A in HUD men undergoing MMT were found to be significantly higher than in healthy controls. In conclusion, our results suggest that MMT might increase orexin A levels in HUD patients.

SB-334867 (an Orexin-1 Receptor Antagonist) Effects on Morphine-Induced Sensitization in Mice-a View on Receptor Mechanisms

The present study focused upon the role of SB-334867, an orexin-1 receptor antagonist, in the acquisition of morphine-induced sensitization to locomotor activity in mice. Behavioral sensitization is an enhanced systemic reaction to the same dose of an addictive substance, which assumingly increases both the desire for the drug and the risk of relapse to addiction. Morphine-induced sensitization in mice was achieved by sporadic doses (five injections every 3 days) of morphine (10 mg/kg, i.p.), while a challenge dose of morphine (10 mg/kg) was injected 7 days later. In order to assess the impact of orexin system blockade on the acquisition of sensitization, SB-334867 was administered before each morphine injection, except the morphine challenge dose. The locomotor activity test was performed on each day of morphine administration. Brain structures (striatum, hippocampus, and prefrontal cortex) were collected after behavioral tests for molecular experiments in which mRNA expression of orexin, dopamine, and adenosine receptors was explored by the qRT-PCR technique. Additionally, the mRNA expression of markers, such as GFAP and Iba-1, was also analyzed by the same technique. SB-334867 inhibited the acquisition of morphine-induced sensitization to locomotor activity of mice. Significant alterations were observed in mRNA expression of orexin, dopamine, and adenosine receptors and in the expression of GFAP and Iba-1, showing a broad range of interactions in the mesolimbic system among orexin, dopamine, adenosine, and glial cells during behavioral sensitization. Summing up, the orexin system may be an effective measure to inhibit morphine-induced behavioral sensitization.

Role of orexin receptors in the ventral tegmental area on acquisition and expression of morphine-induced conditioned place preference in the rats

The orexins are hypothalamic neuropeptides and their role in reward processing and drug addiction has been demonstrated. The extent of involvement of each orexin receptor in the acquisition and expression of conditioned place preference (CPP) for morphine is still a matter of controversy. We investigated the functional differences between orexin-1 and -2 receptor blockade in the ventral tegmental area (VTA) on the acquisition and expression of morphine CPP. A total of 86 adult male Wistar rats weighing 250±30g (age 7-8weeks) received intra-VTA microinjection of either SB334867 (0.1, 1 and 10nM), a selective orexin-1 receptor (OX1R) antagonist, or TCS-OX2-29 (1, 5 and 25nM), a selective orexin-2 receptor (OX2R) antagonist. To measure the acquisition, the animals received each antagonist (SB334867 or TCS-OX2-29) 5min prior to subcutaneous injection of morphine (5mg/kg) during the conditioning phase. To measure the CPP expression, the animals received each antagonist on the post-conditioning phase. The CPP conditioning score was recorded by Ethovision software. Data showed that intra-VTA microinjection of OX1-R antagonist significantly attenuated morphine CPP acquisition, during the conditioning phase, and expression, during the post-conditioning phase. Intra-VTA microinjection of OX2-R antagonist also significantly attenuated morphine CPP acquisition and expression in the mentioned phases. Our results showed the orexin role in learning and memory and indicate that orexin receptors (OX1R and OX2R) function in the VTA is essential for both acquisition and expression of morphine reward in rats in the CPP model.

Functional roles of orexin/hypocretin receptors in reward circuit

Since its first discovery in 1998, it has become clear that the orexinergic system plays an important role in regulating a number of functions including food, sex, social connections, and most prominently reward-related behaviors. Orexinergic neurons in the lateral hypothalamus project extensively to other brain areas, two most important of which are the ventral tegmental area and the nucleus accumbens that are involved in reward processing. In this review, we have presented the work in our laboratory along with the work of others and have discussed the possible functions we can infer from the research. We discuss the anatomy of the orexinergic system and its components followed by a presentation of other connected brain areas. The second part of this review discusses observed results from the morphine conditioned place preference test that sheds light on the possible role of the involved areas in reward processing. The complex circuits involved in reward processing are only beginning to be understood and we need to deepen our understanding regarding the nature of the interactions between all brain areas involved.

Endogenous Opiates and Behavior: 2015

This paper is the thirty-eighth consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2015 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior, and the roles of these opioid peptides and receptors in pain and analgesia, stress and social status, tolerance and dependence, learning and memory, eating and drinking, drug abuse and alcohol, sexual activity and hormones, pregnancy, development and endocrinology, mental illness and mood, seizures and neurologic disorders, electrical-related activity and neurophysiology, general activity and locomotion, gastrointestinal, renal and hepatic functions, cardiovascular responses, respiration and thermoregulation, and immunological responses.

Blockade of dorsal hippocampal orexin-1 receptors impaired morphine-induced state-dependent learning

Behavioral abnormalities associated with opiate addiction include memory and learning deficits, which are the result of some alterations in the neuromodulatory systems. Recently, orexin has shown to influence drug addiction neural circuitry, specifically in mediating reward-related perception and memory. To explore the possible interaction of orexinergic and opioidergic system on modulation of learning and memory, we have investigated the effects of intra-dorsal hippocampal (intra-CA1) administration of orexin-1 receptor agonist and the competitive orexin-1 antagonist, SB-334867, on morphine-induced memory impairment by using step-down passive avoidance task in mice. Pre-training injection of morphine (5mg/kg, i.p.) impaired memory, which was restored when 24h later the same dose of the drug was administered. Pre-test administration of orexin-1 (0.5, 5 and 50pmol, intra-CA1) had not a significant effect on the retention latency compared to the saline-treated animals, but it restored the memory impairment induced by pre-training morphine (5mg/kg, i.p.). Pre-test administration of SB-334867 (10, 20 and 40nmol, intra-CA1) by itself decreased the retention latencies of passive avoidance task. Co-administration of orexin-1 (0.5, 5 and 50pmol, intra-CA1) and morphine (1mg/kg, i.p.) on the test day induced morphine state-dependent memory. Conversely, pre-test injection of SB-334867 (10, 20 and 40nmol, intra-CA1) inhibited the orexin-1-induced potentiation of morphine state-dependent learning on the test day. It is concluded that dorsal hippocampal orexin-1 receptors may be involved, at least in part, in morphine state-dependent learning in mice.

Nigrostriatal Dopamine Acting on Globus Pallidus Regulates Sleep

Lesions of the globus pallidus externa (GPe) produce a profound sleep loss (∼45%) in rats, suggesting that GPe neurons promote sleep. As GPe neuronal activity is enhanced by dopamine (DA) from the substantia nigra pars compacta (SNc), we hypothesized that SNc DA via the GPe promotes sleep. To test this hypothesis, we selectively destroyed the DA afferents to the caudoputamen (CPu) using 6-hydroxydopamine and examined changes in sleep-wake profiles in rats. Rats with 80-90% loss of SNc neurons displayed a significant 33.7% increase in wakefulness (or sleep reduction). This increase significantly correlated with the extent of SNc DA neuron loss. Furthermore, these animals exhibited sleep-wake fragmentation and reduced diurnal variability of sleep. We then optogenetic-stimulated SNc DA terminals in the CPu and found that 20-Hz stimulation from 9 to 10 PM increased total sleep by 69% with high electroencephalograph (EEG) delta power. We finally directly optogenetic-stimulated GPe neurons and found that 20-Hz stimulation of the GPe from 9 to 10 PM increased total sleep by 66% and significantly increased EEG delta power. These findings elucidate a novel circuit for DA control of sleep and the mechanisms of abnormal sleep in BG disorders such as Parkinson's disease and Huntington's disease.

Hypocretins, Neural Systems, Physiology, and Psychiatric Disorders

The hypocretins (Hcrts), also known as orexins, have been among the most intensely studied neuropeptide systems since their discovery about two decades ago. Anatomical evidence shows that the hypothalamic neurons that produce hypocretins/orexins project widely throughout the entire brain, innervating the noradrenergic locus coeruleus, the cholinergic basal forebrain, the dopaminergic ventral tegmental area, the serotonergic raphe nuclei, the histaminergic tuberomammillary nucleus, and many other brain regions. By interacting with other neural systems, the Hcrt system profoundly modulates versatile physiological processes including arousal, food intake, emotion, attention, and reward. Importantly, interruption of the interactions between these systems has the potential to cause neurological and psychiatric diseases. Here, we review the modulation of diverse neural systems by Hcrts and summarize potential therapeutic strategies based on our understanding of the Hcrt system's role in physiology and pathophysiological processes.

Therapies in early development for the treatment of opiate addiction

INTRODUCTION

Opiate drugs are psychoactive substances used to manage severe pain. However, their chronic use is associated with the development of addiction. Opiate addiction represents a significant public health concern.

AREAS COVERED

This review focuses on the most recent advances in the pharmacological treatment of opiate addiction, from those being tested in clinical trials (Phase I and II), to preclinical studies that point to new targets. Readers will gain knowledge of the wide variety of treatments used to treat opiate addiction, including their strengths and weaknesses, and the promising pharmacological targets identified by preclinical research.

EXPERT OPINION

Among the currently available agonist therapies, new dosage forms of buprenorphine can increase patient acceptability and compliance. New extended-release forms of naltrexone are building hope of retaining opiate-dependent subjects in a drug-free state. Unfortunately, the review of the literature shows that successful preclinical studies are often followed by discouraging results in human clinical trials. Nevertheless, all targets of potential interest should be tested exhaustively. Indeed, a number of new targets and research lines (genetics and neuroinflammation approaches) may lead to breakthroughs in the future.