Role of 5-hydroxytryptamine 1B (5-HT1B) receptors in the regulation of ethanol intake in rodents

The Relationship Between Oxytocin and Alcohol Dependence

The hypothalamic neuropeptide oxytocin (OT) is well known for its prosocial, anxiolytic, and ameliorating effects on various psychiatric conditions, including alcohol use disorder (AUD). In this chapter, we will first introduce the basic neurophysiology of the OT system and its interaction with other neuromodulatory and neurotransmitter systems in the brain. Next, we provide an overview over the current state of research examining the effects of acute and chronic alcohol exposure on the OT system as well as the effects of OT system manipulation on alcohol-related behaviors in rodents and humans. In rodent models of AUD, OT has been repeatedly shown to reduce ethanol consumption, particularly in models of acute alcohol exposure. In humans however, the results of OT administration on alcohol-related behaviors are promising but not yet conclusive. Therefore, we further discuss several physiological and methodological limitations to the effective application of OT in the clinic and how they may be mitigated by the application of synthetic OT receptor (OTR) agonists. Finally, we discuss the potential efficacy of cutting-edge pharmacology and gene therapies designed to specifically enhance endogenous OT release and thereby rescue deficient expression of OT in the brains of patients with severe forms of AUD and other incurable mental disorders.

Role of the serotonergic system in ethanol-induced aggression and anxiety: A pharmacological approach using the zebrafish model

Acute ethanol (EtOH) consumption exerts a biphasic effect on behavior and increases serotonin levels in the brain. However, the molecular mechanisms underlying alcohol-mediated behavioral responses still remain to be fully elucidated. Here, we investigate pharmacologically the involvement of the serotonergic pathway on acute EtOH-induced behavioral changes in zebrafish. We exposed zebrafish to 0.25, 0.5, 1.0% (v/v) EtOH for 1 h and analyzed the effects on aggression, anxiety-like behaviors, and locomotion. EtOH concentrations that changed behavioral responses were selected to the subsequent experiments. As a pharmacological approach, we used pCPA (inhibitor of tryptophan hydroxylase), WAY100135 (5-HT_1A antagonist), buspirone (5-HT_1A agonist), CGS12066A and CGS12066B (5-HT_1B antagonist and agonist, respectively), ketanserin (5-HT_2A antagonist) and (±)-DOI hydrochloride (5-HT_2A agonist). All serotonergic receptors tested modulated aggression, with a key role of 5-HT_2A in aggressive behavior following 0.25% EtOH exposure. Because CGS12066B mimicked 0.5% EtOH anxiolysis, which was antagonized by CGS12066A, we hypothesized that anxiolytic-like responses are possibly mediated by 5-HT_1B receptors. Conversely, the depressant effects of EtOH are probably not related with direct changes on serotonergic pathway. Overall, our novel findings demonstrate a role of the serotonergic system in modulating the behavioral effects of EtOH in zebrafish. These data also reinforce the growing utility of zebrafish models in alcohol research and help elucidate the neurobiological mechanisms underlying alcohol abuse and associated complex behavioral phenotypes.

Pindolol Rescues Anxiety-Like Behavior and Neurogenic Maladaptations of Long-Term Binge Alcohol Intake in Mice

Long-term binge alcohol consumption alters the signaling of numerous neurotransmitters in the brain including noradrenaline (NE) and serotonin (5-HT). Alterations in the signaling of these neuronal pathways result in dysfunctional emotional states like anxiety and depression which are typically seen during alcohol withdrawal. Interestingly, studies have demonstrated that the development of alcohol-induced negative affective states is linked to disrupted neurogenesis in the dentate gyrus (DG) region of the hippocampus in alcohol-dependent animals. We have previously shown that modulation of NE and 5-HT activity by pharmacological targeting of β-adrenoreceptors (β-ARs) and 5-HT1A/1B receptors with pindolol reduces consumption in long-term alcohol-consuming mice. Since these receptors are also involved in emotional homeostasis and hippocampal neurogenesis, we investigated the effects of pindolol administration on emotional and neurogenic deficits in mice consuming long-term alcohol (18 weeks). We report that acute administration of pindolol (32 mg/kg) reduces anxiety-like behavior in mice at 24 h withdrawal in the marble-burying test (MBT) and the elevated plus-maze (EPM). We also show that chronic (2 weeks) pindolol treatment (32 mg/kg/day) attenuates alcohol-induced impairments in the density of immature neurons (DCX⁺) but not newborn cells (BrdU⁺) in the hippocampal DG. Pindolol treatment also restores the normal proportion of newborn proliferating cells (BrdU⁺/Ki67⁺/DCX⁻), newborn proliferating immature neurons (BrdU⁺/Ki67⁺/DCX⁺) and newborn non-proliferating immature neurons (BrdU⁺/Ki67⁻/DCX⁺) following long-term alcohol intake. These results suggest that pindolol, through its unique pharmacology may rescue some but not all deficits of long-term alcohol abuse on the brain, adding further value to its properties as a strong pharmaceutical option for alcohol use disorders (AUDs).

There is no association between rs6296 and alcoholism: a meta-analysis

Previous studies have reported controversial results about the association between rs6296 and alcoholism. Thus, a meta-analysis was performed to further explore this association. A comprehensive search was conducted to identify relevant case-control or cohort studies (up to December 1, 2017). A fixed- or random-effect model was selected as a pooling method depending on the heterogeneity among studies. The heterogeneity was measured by Q test and I² statistic. The Harbord and Peters test was used to estimate publication bias. Fifteen English articles with 16 outcomes and 5,429 participants were included in this meta-analysis. A fixed-effect model was chosen, and the pooled result showed that rs6296 was not related to alcoholism (z = 1.93, p = .053). The Harbord and Peters test showed that there was no publication bias. This meta-analysis indicated that rs6296 may be not be significantly associated with alcoholism, which needs to be further confirmed by future research.

The antihypertensive drug pindolol attenuates long-term but not short-term binge-like ethanol consumption in mice

Alcohol dependence is a debilitating disorder with current therapies displaying limited efficacy and/or compliance. Consequently, there is a critical need for improved pharmacotherapeutic strategies to manage alcohol use disorders (AUDs). Previous studies have shown that the development of alcohol dependence involves repeated cycles of binge-like ethanol intake and abstinence. Therefore, we used a model of binge-ethanol consumption (drinking-in-the-dark) in mice to test the effects of compounds known to modify the activity of neurotransmitters implicated in alcohol addiction. From this, we have identified the FDA-approved antihypertensive drug pindolol, as a potential candidate for the management of AUDs. We show that the efficacy of pindolol to reduce ethanol consumption is enhanced following long-term (12 weeks) binge-ethanol intake, compared with short-term (4 weeks) intake. Furthermore, pindolol had no effect on locomotor activity or consumption of the natural reward sucrose. Because pindolol acts as a dual beta-adrenergic antagonist and 5-HT_1A/1B partial agonist, we examined its effect on spontaneous synaptic activity in the basolateral amygdala (BLA), a brain region densely innervated by serotonin and norepinephrine-containing fibres. Pindolol increased spontaneous excitatory post-synaptic current frequency of BLA principal neurons from long-term ethanol-consuming mice but not naïve mice. Additionally, this effect was blocked by the 5-HT_1A/1B receptor antagonist methiothepin, suggesting that altered serotonergic activity in the BLA may contribute to the efficacy of pindolol to reduce ethanol intake following long-term exposure. Although further mechanistic investigations are required, this study demonstrates the potential of pindolol as a new treatment option for AUDs that can be fast-tracked into human clinical studies.

The 5-HT7 receptor as a potential target for treating drug and alcohol abuse

Alcohol and drug abuse take a large toll on society and affected individuals. However, very few effective treatments are currently available to treat alcohol and drug addiction. Basic and clinical research has begun to provide some insights into the underlying neurobiological systems involved in the addiction process. Several neurotransmitter pathways have been implicated and distinct reward neurocircuitry have been proposed—including the mesocorticolimbic dopamine (MCL-DA) system and the extended amygdala. The serotonin (5-HT) neurotransmitter system is of particular interest and multiple 5-HT receptors are thought to play significant roles in alcohol and drug self-administration and the development of drug dependence. Among the 5-HT receptors, the 5-HT7 receptor is currently undergoing characterization as a potential target for the treatment of several psychiatric disorders. Although this receptor has received only limited research regarding addictive behaviors, aspects of its neuroanatomical, biochemical, physiological, pharmacological, and behavioral profiles suggest that it could play a key role in the addiction process. For instance, genomic studies in humans have suggested a link between variants in the gene encoding the 5-HT7 receptor and alcoholism. Recent behavioral testing using high-affinity antagonists in mice and preliminary tests with alcohol-preferring rats suggest that this receptor could mediate alcohol consumption and/or reinforcement and play a role in seeking/craving behavior. Interest in the development of new and more selective pharmacological agents for this receptor will aid in examining the 5-HT7 receptor as a novel target for treating addiction.

The role of serotonin in drug use and addiction

The use of psychoactive drugs is a wide spread behaviour in human societies. The systematic use of a drug requires the establishment of different drug use-associated behaviours which need to be learned and controlled. However, controlled drug use may develop into compulsive drug use and addiction, a major psychiatric disorder with severe consequences for the individual and society. Here we review the role of the serotonergic (5-HT) system in the establishment of drug use-associated behaviours on the one hand and the transition and maintenance of addiction on the other hand for the drugs: cocaine, amphetamine, methamphetamine, MDMA (ecstasy), morphine/heroin, cannabis, alcohol, and nicotine. Results show a crucial, but distinct involvement of the 5-HT system in both processes with considerable overlap between psychostimulant and opioidergic drugs and alcohol. A new functional model suggests specific adaptations in the 5-HT system, which coincide with the establishment of controlled drug use-associated behaviours. These serotonergic adaptations render the nervous system susceptible to the transition to compulsive drug use behaviours and often overlap with genetic risk factors for addiction. Altogether we suggest a new trajectory by which serotonergic neuroadaptations induced by first drug exposure pave the way for the establishment of addiction.

Dopamine D3 and 5-HT1B receptor dysregulation as a result of psychostimulant intake and forced abstinence: Implications for medications development

Addiction to psychostimulants, including cocaine and amphetamine, is associated with dysregulation of dopamine and serotonin (5-HT) neurotransmitter systems. Neuroadaptations in these systems vary depending on the stage of the drug taking-abstinence-relapse cycle. Consequently, the effects of potential treatments that target these systems may vary depending on whether they are given during abstinence or relapse. In this review, we discuss evidence that dopamine D3 receptors (D3Rs) and 5-HT1B receptors (5-HT1BRs) are dysregulated in response to both chronic psychostimulant use and subsequent abstinence. We then review findings from preclinical self-administration models which support targeting D3Rs and 5-HT1BRs as potential medications for psychostimulant dependence. Potential side effects of the treatments are discussed and attention is given to studies reporting positive treatment outcomes that depend on: 1) whether testing occurs during self-administration versus abstinence, 2) whether escalation of drug self-administration has occurred, 3) whether the treatments are given repeatedly, and 4) whether social factors influence treatment outcomes. We conclude that D3/D2 agonists may decrease psychostimulant intake; however, side effects of D3/D2R full agonists may limit their therapeutic potential, whereas D3/D2R partial agonists have fewer undesirable side effects. D3-selective antagonists may not reduce psychostimulant intake during relapse, but nonetheless, may decrease motivation for seeking psychostimulants with relatively few side-effects. 5-HT1BR agonists provide a striking example of treatment outcomes that are dependent on the stage of the addiction cycle. Specifically, these agonists initially increase cocaine's reinforcing effects during maintenance of self-administration, but after a period of abstinence they reduce psychostimulant seeking and the resumption of self-administration. In conclusion, we suggest that factors contributing to dysregulation of monoamine systems, including drug history, abstinence, and social context, should be considered when evaluating potential treatments to better model treatment effects in humans. This article is part of a Special Issue entitled 'NIDA 40th Anniversary Issue'.