Differential orexin/hypocretin expression in addiction-prone and -resistant rats selectively bred for high (HiS) and low (LoS) saccharin intake

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.

Rev-erbα modulates the hypothalamic orexinergic system to influence pleasurable feeding behaviour in mice

The drive to eat is regulated by two compensatory brain pathways termed as homeostatic and hedonic. Hypothalamic orexinergic (ORX) neurons regulate metabolism, feeding and reward, thus controlling physiological and hedonic appetite. Circadian regulation of feeding, metabolism and rhythmic activity of ORX cells are driven by the brain suprachiasmatic clock. How the circadian clock impacts on ORX signalling and feeding-reward rhythms is, however, unknown. Here we used mice lacking the nuclear receptor REV-ERBα, a transcription repressor and a key component of the molecular clockwork, to study food-reward behaviour. Rev-Erbα mutant mice showed highly motivated behaviours to obtain palatable food, an increase in the intake and preference for tasty diets, and in the expression of the ORX protein in the hypothalamus. Palatable food intake was inhibited in animals treated with the ORX1R antagonist. Analyzing the Orx promoter, we found Retinoic acid-related Orphan receptor Response Element binding sites for Rev-Erbα. Furthermore, Rev-Erbα dampened the activation of Orx in vitro and in vivo. Our data provide evidence for a possible repressive role of Rev-Erbα in the regulation of ORX signalling, highlighting an implication of the circadian clockwork in modulating food-reward behaviours with an important impact for the central regulation of overeating.

How to study sex differences in addiction using animal models

The importance of studying sex as a biological variable in biomedical research is becoming increasingly apparent. There is a particular need in preclinical studies of addiction to include both sexes, as female animals are often excluded from studies, leaving large gaps in our knowledge of not only sex differences and potential prevention and treatment strategies but also with regard to the basic neurobiology of addiction. This review focuses on methodology that has been developed in preclinical studies to examine sex differences in the behavioral aspects and neurobiological mechanisms related to addiction across the full range of the addiction process, including initiation (acquisition), maintenance, escalation, withdrawal, relapse to drug seeking and treatment. This review also discusses strategic and technical issues that need to be considered when comparing females and males, including the role of ovarian hormones and how sex differences interact with other major vulnerability factors in addiction, such as impulsivity, compulsivity and age (adolescent versus adult). Novel treatments for addiction are also discussed, such as competing non-drug rewards, repurposed medications such as progesterone and treatment combinations. Practical aspects of conducting research comparing female and male animals are also considered. Making sex differences a point of examination requires additional effort and consideration; however, such studies are necessary given mounting evidence demonstrating that the addiction process occurs differently in males and females. These studies should lead to a better understanding of individual differences in the development of addiction and effective treatments for males and females.

Reduced emotional signs of opiate withdrawal in rats selectively bred for low (LoS) versus high (HiS) saccharin intake

RATIONALE

Rats bred for high (HiS) and low (LoS) saccharin intake exhibit divergent behavioral responses to multiple drugs of abuse, with HiS rats displaying greater vulnerability to drug taking. Previous research indicates that this effect may be due to increased sensitivity to reward in HiS rats and to the aversive effects of acute drug administration in LoS rats.

OBJECTIVE

The current study investigated whether HiS and LoS rats also exhibit different behavioral signs of withdrawal following one or repeated opiate exposures.

METHODS

Emotional signs of opiate withdrawal were assessed with potentiation of the acoustic startle reflex and conditioned place aversion (CPA) in male and female HiS and LoS rats. Startle was measured before and 4 h after a 10-mg/kg injection of morphine on days 1, 2, and 7 of opiate exposure. CPA was induced with a 2-day, naloxone-precipitated conditioning paradigm. Somatic signs of withdrawal and weight loss were also measured.

RESULTS

Male and female LoS rats exhibited lower startle potentiation than HiS rats on the seventh day of morphine exposure. LoS male rats also failed to develop a CPA to morphine withdrawal. No differences in physical withdrawal signs were observed between HiS and LoS rats, but males of both lines had more physical signs of withdrawal than females.

CONCLUSIONS

These results suggest that LoS rats are less vulnerable to the negative emotional effects of morphine withdrawal than HiS rats. A less severe withdrawal syndrome may contribute to decreased levels of drug taking in the LoS line.

Ascending orexinergic pathways and alcohol-seeking

Orexin (hypocretin) containing neurons reside in discrete regions of the lateral hypothalamus from where they innervate the entire neuroaxis. Via actions upon orexin receptors (OX1 and OX2), the orexin peptides (orexin A and orexin B) are thought to play a role in ethanol consumption and seeking. While a role for OX1 receptors in these behaviours is established, the case for OX2 receptors is less clear at present, although recent data certainly support an involvement of OX2 receptors in ethanol consumption. In terms of circuitry, orexin receptors the ventral tegmental area appear to contribute to ethanol consumption. Other loci remain to be characterised, and we suggest prefrontal cortical orexin receptors deserve attention in this respect.

The role of orexins/hypocretins in alcohol use and abuse: an appetitive-reward relationship

Orexins (hypocretins) are neuropeptides synthesized in neurons located in the lateral (LH), perifornical, and dorsomedial (DMH) hypothalamus. These neurons innervate many regions in the brain and modulate multiple other neurotransmitter systems. As a result of these extensive projections and interactions orexins are involved in numerous functions, such as feeding behavior, neuroendocrine regulation, the sleep-wake cycle, and reward-seeking. This review will summarize the literature to date which has evaluated a role of orexins in the behavioral effects of alcohol, with a focus on understanding the importance of this peptide and its potential as a clinical therapeutic target for alcohol use disorders.