Effects of vivo morpholino knockdown of lateral hypothalamus orexin/hypocretin on renewal of alcohol seeking

Orexin Reserve: A Mechanistic Framework for the Role of Orexins (Hypocretins) in Addiction

In 2014, we proposed that orexin signaling transformed motivationally relevant states into adaptive behavior directed toward exploiting an opportunity or managing a threat, a process we referred to as motivational activation. Advancements in animal models since then have permitted higher-resolution measurements of motivational states; in particular, the behavioral economics approach for studying drug demand characterizes conditions that lead to the enhanced motivation that underlies addiction. This motivational plasticity is paralleled by persistently increased orexin expression in a topographically specific manner-a finding confirmed across species, including in humans. Normalization of orexin levels also reduces drug motivation in addiction models. These new advancements lead us to update our proposed framework for the orexin function. We now propose that the capacity of orexin neurons to exhibit dynamic shifts in peptide production contributes to their role in adaptive motivational regulation and that this is achieved via a pool of reserve orexin neurons. This reserve is normally bidirectionally recruited to permit motivational plasticity that promotes flexible, adaptive behavior. In pathological states such as addiction, however, we propose that the orexin system loses capacity to adaptively adjust peptide production, resulting in focused hypermotivation for drug, driven by aberrantly and persistently high expression in the orexin reserve pool. This mechanistic framework has implications for the understanding and treatment of several psychiatric disorders beyond addiction, particularly those characterized by motivational dysfunction.

Ventral pallidum cellular and pathway specificity in drug seeking

The ventral pallidum (VP) is central to the reinforcing effects across a variety of drugs and relapse to drug seeking. Emerging studies from animal models of reinstatement reveal a complex neurobiology of the VP that contributes to different aspects of relapse to drug seeking. This review builds on classical understanding of the VP as part of the final common pathway of relapse but also discusses the properties of the VP as an independent structure. These include VP neural anatomical subregions, cellular heterogeneity, circuitry, neurotransmitters and peptides. Collectively, this review provides a current understanding of the VP from molecular to circuit level architecture that contributes to both the appetitive and aversive symptoms of drug addiction. We show the complex neurobiology of the VP in drug seeking, emphasizing its critical role in addiction, and review strategic approaches that target the VP to reduce relapse rates.

Orexin gene expression is downregulated in alcohol dependent rats during acute alcohol withdrawal

Alcohol use disorders (AUD) are chronic relapsing brain disorder characterized by compulsive and heavy alcohol consumption. During acute withdrawal, patients with AUD display excessive daytime sleepiness, a condition linked to serious life-threatening complications, however, the mechanism is not known. Orexin and melanin-concentrating hormone (MCH) are the two hypothalamic neuropeptides that regulate many behaviors including sleep-wakefulness, and alcohol consumption, reinforcement, and reinstatement. Importantly, loss of orexin neurons causes narcolepsy, a severe sleep disorder with excessive daytime sleepiness. Does acute alcohol withdrawal reduce orexin gene expression? To investigate this, male Sprague-Dawley rats were divided in two groups: Rats were either administered with alcohol, mixed with infant formula (alcohol group) or control mixture containing water and infant formula (Controls) by gastric intubation every 8 h for 4 days using Majchrowicz's chronic binge drinking protocol. The doses of alcohol were adjusted depending on degree of intoxication, exhibited by animals, prior to each dose. The animals were euthanized after 12 h of last alcohol/water administration. During withdrawal, the hypothalamus was rapidly dissected out, and the expressions of orexin and MCH genes were examined by Real-time PCR. There was a significant reduction in orexin gene expression in rats subjected to alcohol withdrawal as compared to controls. No such change was observed in the MCH gene expression. These results suggest that downregulation of orexin gene expression may be a possible mechanism responsible for excessive daytime sleepiness associated with alcohol withdrawal in patients with AUD.

Orexin-1 receptor signaling within the lateral hypothalamus, but not bed nucleus of the stria terminalis, mediates context-induced relapse to alcohol seeking

BACKGROUND

The lateral hypothalamic orexin (hypocretin) system has a well-established role in the motivation for reward. This has particular relevance to substance use disorders since orexin-1 receptors play a critical role in alcohol-seeking behavior, acting at multiple nodes in relapse-associated networks.

AIMS

This study aimed to further our understanding of the role of orexin-1 receptor signaling within the lateral hypothalamus and bed nucleus of the stria terminalis, specifically in context-induced relapse to alcohol-seeking following punishment-imposed abstinence.

METHODS

We trained inbred male alcohol-preferring rats to self-administer alcohol in one environment or context (Context A) and subsequently punished their alcohol-reinforced lever presses in a different environment (Context B) using contingent foot shock punishment. Finally, we tested rats for relapse-like behavior in either context following systemic, intra-lateral hypothalamus or intra-bed nucleus of the stria terminalis orexin-1 receptor antagonism with SB-334867.

RESULTS/OUTCOMES

We found that systemic orexin-1 receptor antagonism significantly reduced alcohol-seeking in both contexts. Intra-lateral hypothalamus orexin-1 receptor antagonism significantly reduced alcohol-seeking in Context A whereas intra-bed nucleus of the stria terminalis orexin-1 receptor antagonism had no effect on alcohol-seeking behavior.

CONCLUSIONS/INTERPRETATION

Our results suggest a role for the orexin-1 receptor system in context-induced relapse to alcohol-seeking. Specifically, intra-lateral hypothalamus orexin microcircuits contribute to alcohol-seeking.

Role of melanin-concentrating hormone in drug use disorders

Melanin-concentrating hormone (MCH) is a neuropeptide primarily transcribed in the lateral hypothalamus (LH), with vast projections to many areas throughout the central nervous system that play an important role in motivated behaviors and drug use. Anatomical, pharmacological and genetic studies implicate MCH in mediating the intake and reinforcement of commonly abused substances, acting by influencing several systems including the mesolimbic dopaminergic system, glutamatergic as well as GABAergic signaling and being modulated by inflammatory neuroimmune pathways. Further support for the role of MCH in controlling behavior related to drug use will be discussed as it relates to cerebral ventricular volume transmission and intracellular molecules including cocaine- and amphetamine-regulated transcript peptide, dopamine- and cAMP-regulated phosphoprotein 32 kDa. The primary goal of this review is to introduce and summarize current literature surrounding the role of MCH in mediating the intake and reinforcement of commonly abused drugs, such as alcohol, cocaine, amphetamine, nicotine and opiates.

The role of orexin-1 receptor signaling in demand for the opioid fentanyl

The orexin system is a potential treatment target for drug addiction. Orexin-1 receptor (OxR1) antagonism reduces demand for cocaine and remifentanil, indicating that orexin-based therapies may reduce demand for many classes of abused drugs. However, pharmacokinetics vary greatly among opioids and it is unclear if OxR1 antagonism would reduce demand for all opioids, particularly ones with high abuse liability. Here, we established a behavioral economics (BE) procedure to assess the effects of OxR1 antagonism on demand for the highly abused opioid fentanyl. We also investigated the utility of our procedure to predict OxR1 antagonism efficacy and relapse propensity. Demand parameters α (demand elasticity or price sensitivity of consumption, an inverse measure of drug motivation) and Q_o (drug consumption at null cost) were assessed. The OxR1 antagonist SB-334867 (SB) decreased motivation (increased α) for fentanyl without affecting Q_o. Baseline α values predicted SB efficacy, such that SB was most effective at reducing motivation (increasing α) in highly motivated rats. Baseline α values predicted the amount of cued reinstatement of fentanyl seeking; this reinstatement behavior was attenuated by SB administration. These results highlight the promise of the orexin system as a treatment target for opioid addiction and emphasize the usefulness of BE procedures in the study of opioid abuse.

Knockdown of hypocretin attenuates extended access of cocaine self-administration in rats

The hypocretin/orexin (HCRT) neuropeptide system regulates feeding, arousal state, stress responses, and reward, especially under conditions of enhanced motivational relevance. In particular, HCRT neurotransmission facilitates drug-seeking behavior in circumstances that demand increased effort and/or motivation to take the drug. The present study used a shRNA-encoding adeno-associated viral vector to knockdown Hcrt expression throughout the dorsal hypothalamus in adult rats and determine the role of HCRT in cocaine self-administration. Chronic Hcrt silencing did not impact cocaine self-administration under short-access conditions, but robustly attenuated cocaine intake under extended access conditions, a model that mimics key features of compulsive cocaine taking. In addition, Hcrt silencing decreased motivation for both cocaine and a highly palatable food reward (i.e., sweetened condensed milk; SCM) under a progressive ratio schedule of reinforcement, but did not alter responding for SCM under a fixed ratio schedule. Importantly, Hcrt silencing did not affect food or water consumption, and had no consequence for general measures of arousal and stress reactivity. At the molecular level, chronic Hcrt knockdown reduced the number of neurons expressing dynorphin (DYN), and to a smaller extent melanin-concentrating hormone (MCH), in the dorsal hypothalamus. These original findings support the hypothesis that HCRT neurotransmission promotes operant responding for both drug and non-drug rewards, preferentially under conditions requiring a high degree of motivation. Furthermore, the current study provides compelling evidence for the involvement of the HCRT system in cocaine self-administration also under low-effort conditions in rats allowed extended access, possibly via functional interactions with DYN and MCH signaling.

Contribution of Dynorphin and Orexin Neuropeptide Systems to the Motivational Effects of Alcohol

Understanding the neural systems that drive alcohol motivation and are disrupted in alcohol use disorders is of critical importance in developing novel treatments. The dynorphin and orexin/hypocretin neuropeptide systems are particularly relevant with respect to alcohol use and misuse. Both systems are strongly associated with alcohol-seeking behaviors, particularly in cases of high levels of alcohol use as seen in dependence. Furthermore, both systems also play a role in stress and anxiety, indicating that disruption of these systems may underlie long-term homeostatic dysregulation seen in alcohol use disorders. These systems are also closely interrelated with one another - dynorphin/kappa opioid receptors and orexin/hypocretin receptors are found in similar regions and hypocretin/orexin neurons also express dynorphin - suggesting that these two systems may work together in the regulation of alcohol seeking and may be mutually disrupted in alcohol use disorders. This chapter reviews studies demonstrating a role for each of these systems in motivated behavior, with a focus on their roles in regulating alcohol-seeking and self-administration behaviors. Consideration is also given to evidence indicating that these neuropeptide systems may be viable targets for the development of potential treatments for alcohol use disorders.

The Role of Orexins/Hypocretins in Alcohol Use and Abuse

Addiction is a chronic relapsing disorder characterized by compulsive drug seeking and drug taking despite negative consequences. Alcohol abuse and addiction have major social and economic consequences and cause significant morbidity and mortality worldwide. Currently available therapeutics are inadequate, outlining the need for alternative treatments. Detailed knowledge of the neurocircuitry and brain chemistry responsible for aberrant behavior patterns should enable the development of novel pharmacotherapies to treat addiction. Therefore it is important to expand our knowledge and understanding of the neural pathways and mechanisms involved in alcohol seeking and abuse. The orexin (hypocretin) neuropeptide system is an attractive target, given the recent FDA and PMDA approval of suvorexant for the treatment of insomnia. Orexin is synthesized exclusively in neurons located in the lateral (LH), perifornical (PEF), and dorsal medial (DMH) hypothalamus. These neurons project widely throughout the neuraxis with regulatory roles in a wide range of behavioral and physiological responses, including sleep-wake cycle neuroendocrine regulation, anxiety, feeding behavior, and reward seeking. Here we summarize the literature to date, which have evaluated the interplay between alcohol and the orexin system.

Local anaesthetic refinement of pentobarbital euthanasia reduces abdominal writhing without affecting immunohistochemical endpoints in rats

Sodium pentobarbital is a commonly used agent for euthanizing laboratory rats, however its high pH can cause abdominal discomfort after intraperitoneal injection. Previous studies suggest that the addition of a local anaesthetic may alleviate this discomfort, but the practice has not been widely adopted. We examined the effect of combining lidocaine with pentobarbital on abdominal writhing, defecation, ultrasonic vocalizations, the rat grimace scale and immunohistochemical staining for c-Fos in the nucleus accumbens and basolateral amygdala of the brain. We also compared the amount of abdominal writhing following intraperitoneal administration of pentobarbital–lidocaine with that of pentobarbital–bupivacaine. Our results show that lidocaine reduces abdominal writhing and defecation without affecting immunohistochemistry for c-Fos or latency to loss of posture. However, scores on the rat grimace scale were low in both situations and almost no ultrasonic vocalizations were recorded. Additionally, we found that the amount of abdominal writhing was not significantly different when bupivacaine was used rather than lidocaine. Our results suggest that pentobarbital-induced euthanasia can be refined with the addition of lidocaine or other local anaesthetics.

How contexts promote and prevent relapse to drug seeking

The contexts where drugs are self-administered play an important role in regulating persistent drug taking and in relapse to such taking after periods of abstinence. Here, we review the behavioral and brain mechanisms enabling contexts to promote and prevent relapse to drug seeking. We review the key brain structures, their neuropharmacology and their connectivity. We discuss the similarities and differences between the mechanisms for context-induced reinstatement of drug seeking vs. other forms of relapse to drug seeking in animal models and we highlight the numerous deficits in our understanding. We emphasize that current understanding, although significant, defies explanations in terms of models at the level of brain structures and their connectivity. Rather, we show that there is significant functional compartmentalization and segregation within these structures during reinstatement and extinction of drug seeking that parallels their anatomical segregation into circuits and channels. A key challenge is to recognize this complexity, understand how these circuits and channels are organized, as well as understand how different modes of activity of ensembles of neurons within them promote abstinence or relapse to drug seeking.

Hypothalamic neuropeptide signaling in alcohol addiction

The hypothalamus is now known to regulate alcohol intake in addition to its established role in food intake, in part through neuromodulatory neurochemicals termed neuropeptides. Certain orexigenic neuropeptides act in the hypothalamus to promote alcohol drinking, although they affect different aspects of the drinking response. These neuropeptides, which include galanin, the endogenous opioid enkephalin, and orexin/hypocretin, appear to stimulate alcohol intake not only through mechanisms that promote food intake but also by enhancing reward and reinforcement from alcohol. Moreover, these neuropeptides participate in a positive feedback relationship with alcohol, whereby they are upregulated by alcohol intake to promote even further consumption. They contrast with other orexigenic neuropeptides, such as melanin-concentrating hormone and neuropeptide Y, which promote alcohol intake under limited circumstances, are not consistently stimulated by alcohol, and do not enhance reward. They also contrast with neuropeptides that can be anorexigenic, including the endogenous opioid dynorphin, corticotropin-releasing factor, and melanocortins, which act in the hypothalamus to inhibit alcohol drinking as well as reward and therefore counter the ingestive drive promoted by orexigenic neuropeptides. Thus, while multiple hypothalamic neuropeptides may work together to regulate different aspects of the alcohol drinking response, excessive signaling from orexigenic neuropeptides or inadequate signaling from anorexigenic neuropeptides can therefore allow alcohol drinking to become dysregulated.