The effects of chronic ethanol self-administration on hippocampal 5-HT1A receptors in monkeys

Changes in serotonin neurotransmission as assayed by microdialysis after acute, intermittent or chronic ethanol administration and withdrawal

BACKGROUND

The serotonergic neurotransmitter system is involved in many ethanol-induced changes, including many behavioural alterations, as well as contributing to alcohol dependence and its withdrawal.

AIMS

This review has evaluated microdialysis studies where alterations in the serotonin system, that is, serotonin, 5-HT, or its metabolite 5-hydroxyindoleacetic acid, 5-HIAA, have been reported during different ethanol intoxication states, as well as in animals showing alcohol preference or not. Changes in 5-HT receptors and the 5-HT transporter are briefly reviewed to comprehend the significance of changes in microdialysate 5-HT concentrations.

MATERIALS AND METHODS

Changes in 5-HT content following acute, chronic and during ethanol withdrawal states are evaluated. In addition, the serotoninergic system was assessed in animals that have been genetically selected for alcohol preference to ascertain whether changes in this monoamine microdialysate content may contribute to alcohol preference.

RESULTS AND DISCUSSION

Changes occurred in 5-HT signalling in the limbic brain regions, increasing after acute ethanol administration in specific brain regions, particularly at higher doses, while chronic alcohol exposure essentially decreased serotonergic transmission. Such changes may play a pivotal role in emotion-driven craving and relapse. Depending on the dosage, mode of administration and consumption rate, ethanol affects specific brain regions in different ways, enhancing or reducing 5-HT microdialysate content, thereby inducing behavioural and cognitive functions and enhancing ethanol consumption.

CONCLUSION

Microdialysis studies demonstrated that ethanol induces several alterations in 5-HT content as well as its metabolites, 5-HIAA and 5-HTOL, not only in its release from a specific brain region but also in the modifications of its different receptor subtypes and its transporter.

Raphe serotonin projections dynamically regulate feeding behavior through targeting inhibitory circuits from rostral zona incerta to paraventricular thalamus

OBJECTIVE

Rostral zona incerta (ZIR) evokes feeding by sending GABA transmission to paraventricular thalamus (PVT). Although central serotonin (5-HT) signaling is known to play critical roles in the regulation of food intake and eating disorders, it remains unknown whether raphe 5-HT neurons functionally innervate ZIR-PVT neural pathway for feeding control. Here, we sought to reveal how raphe 5-HT signaling regulates both ZIR and PVT for feeding control.

METHODS

We used retrograde neural tracers to map 5-HT projections in Sert-Cre mice and slice electrophysiology to examine the mechanism by which 5-HT modulates ZIR GABA neurons. We also used optogenetics to test the effects of raphe-ZIR and raphe-PVT 5-HT projections on feeding motivation and food intake in mice regularly fed, 24 h fasted, and with intermittent high-fat high-sugar (HFHS) diet. In addition, we applied RNAscope in situ hybridization to identify 5-HT receptor subtype mRNA in ZIR.

RESULTS

We show raphe 5-HT neurons sent projections to both ZIR and PVT with partial collateral axons. Photostimulation of 5-HT projections inhibited ZIR but excited PVT neurons to decrease motivated food consumption. However, both acute food deprivation and intermittent HFHS diet downregulated 5-HT inhibition on ZIR GABA neurons, abolishing the inhibitory regulation of raphe-ZIR 5-HT projections on feeding motivation and food intake. Furthermore, we found high-level 5-HT1a and 5-HT2c as well as low-level 5-HT7 mRNA expression in ZIR. Intermittent HFHS diet increased 5-HT7 but not 5-HT1a or 5-HT2c mRNA levels in the ZIR.

CONCLUSIONS

Our results reveal that raphe-ZIR 5-HT projections dynamically regulate ZIR GABA neurons for feeding control, supporting that a dynamic fluctuation of ZIR 5-HT inhibition authorizes daily food intake but a sustained change of ZIR 5-HT signaling leads to overeating induced by HFHS diet.

Prolonged ethanol exposure modulates constitutive internalization and recycling of 5-HT1A receptors

Alcohol exposure alters the signaling of the serotoninergic system, which is involved in alcohol consumption, reward, and dependence. In particular, dysregulation of serotonin receptor type 1A (5-HT1AR) is associated with alcohol intake and withdrawal-induced anxiety-like behavior in rodents. However, how ethanol regulates 5-HT1AR activity and cell surface availability remains elusive. Using neuroblastoma 2a cells stably expressing human 5-HT1ARs tagged with hemagglutinin at the N-terminus, we found that prolonged ethanol exposure (18 h) reduced the basal surface levels of 5-HT1ARs in a concentration-dependent manner. This reduction is attributed to both enhanced receptor internalization and attenuated receptor recycling. Moreover, constitutive 5-HT1AR internalization in ethanol naïve cells was blocked by concanavalin A (ConA) but not nystatin, suggesting clathrin-dependent 5-HT1AR internalization. In contrast, constitutive 5-HT1AR internalization in ethanol-treated cells was blocked by nystatin but not by ConA, indicating that constitutive 5-HT1AR internalization switched from a clathrin- to a caveolin-dependent pathway. Dynasore, an inhibitor of dynamin, blocked 5-HT1AR internalization in both vehicle- and ethanol-treated cells. Furthermore, ethanol exposure enhanced the activity of dynamin I via dephosphorylation and reduced myosin Va levels, which may contribute to increased internalization and reduced recycling of 5-HT1ARs, respectively. Our findings suggest that prolonged ethanol exposure not only alters the endocytic trafficking of 5-HT1ARs but also the mechanism by which constitutive 5-HT1AR internalization occurs.

The Roles of Serotonin in Neuropsychiatric Disorders

The serotonergic system extends throughout the central nervous system (CNS) and the gastrointestinal (GI) tract. In the CNS, serotonin (5-HT, 5-hydroxytryptamine) modulates a broad spectrum of functions, including mood, cognition, anxiety, learning, memory, reward processing, and sleep. These processes are mediated through 5-HT binding to 5-HT receptors (5-HTRs), are classified into seven distinct groups. Deficits in the serotonergic system can result in various pathological conditions, particularly depression, schizophrenia, mood disorders, and autism. In this review, we outlined the complexity of serotonergic modulation of physiologic and pathologic processes. Moreover, we provided experimental and clinical evidence of 5-HT's involvement in neuropsychiatric disorders and discussed the molecular mechanisms that underlie these illnesses and contribute to the new therapies.

Acute ethanol exposure reduces serotonin receptor 1A internalization by increasing ubiquitination and degradation of β-arrestin2

Acute alcohol exposure alters the trafficking and function of many G-protein-coupled receptors (GPCRs) that are associated with aberrant behavioral responses to alcohol. However, the molecular mechanisms underlying alcohol-induced changes in GPCR function remain unclear. β-Arrestin is a key player involved in the regulation of GPCR internalization and thus controls the magnitude and duration of GPCR signaling. Although β-arrestin levels are influenced by various drugs of abuse, the effect of alcohol exposure on β-arrestin expression and β-arrestin-mediated GPCR trafficking is poorly understood. Here, we found that acute ethanol exposure increases β-arrestin2 degradation via its increased ubiquitination in neuroblastoma-2a (N2A) cells and rat prefrontal cortex (PFC). β-Arrestin2 ubiquitination was likely mediated by the E3 ligase MDM2 homolog (MDM2), indicated by an increased coupling between β-arrestin2 and MDM2 in response to acute ethanol exposure in both N2A cells and rat PFC homogenates. Importantly, ethanol-induced β-arrestin2 reduction was reversed by siRNA-mediated MDM2 knockdown or proteasome inhibition in N2A cells, suggesting β-arrestin2 degradation is mediated by MDM2 through the proteasomal pathway. Using serotonin 5-HT1A receptors (5-HT1ARs) as a model receptor system, we found that ethanol dose-dependently inhibits 5-HT1AR internalization and that MDM2 knockdown reverses this effect. Moreover, ethanol both reduced β-arrestin2 levels and delayed agonist-induced β-arrestin2 recruitment to the membrane. We conclude that β-arrestin2 dysregulation by ethanol impairs 5-HT1AR trafficking. Our findings reveal a critical molecular mechanism underlying ethanol-induced alterations in GPCR internalization and implicate β-arrestin as a potential player mediating behavioral responses to acute alcohol exposure.

Serotonergic Neuroplasticity in Alcohol Addiction

Alcohol addiction is a debilitating disorder producing maladaptive changes in the brain, leading drinkers to become more sensitive to stress and anxiety. These changes are key factors contributing to alcohol craving and maintaining a persistent vulnerability to relapse.

Serotonin (5-Hydroxytryptamine, 5-HT) is a monoamine neurotransmitter widely expressed in the central nervous system where it plays an important role in the regulation of mood. The serotonin system has been extensively implicated in the regulation of stress and anxiety, as well as the reinforcing properties of all of the major classes of drugs of abuse, including alcohol. Dysregulation within the 5-HT system has been postulated to underlie the negative mood states associated with alcohol use disorders.

This review will describe the serotonergic (5-HTergic) neuroplastic changes observed in animal models throughout the alcohol addiction cycle, from prenatal to adulthood exposure. The first section will focus on alcohol-induced 5-HTergic neuroadaptations in offspring prenatally exposed to alcohol and the consequences on the regulation of stress/anxiety. The second section will compare alterations in 5-HT signalling induced by acute or chronic alcohol exposure during adulthood and following alcohol withdrawal, highlighting the impact on the regulation of stress/anxiety signalling pathways. The third section will outline 5-HTergic neuroadaptations observed in various genetically-selected ethanol preferring rat lines. Finally, we will discuss the pharmacological manipulation of the 5-HTergic system on ethanol- and anxiety/stress-related behaviours demonstrated by clinical trials, with an emphasis on current and potential treatments.

MAOA expression predicts vulnerability for alcohol use

The role of the monoamines dopamine (DA) and serotonin (5HT) and the monoamine-metabolizing enzyme monoamine oxidase A (MAOA) have been repeatedly implicated in studies of alcohol use and dependence. Genetic investigations of MAOA have yielded conflicting associations between a common polymorphism (MAOA-LPR) and risk for alcohol abuse. The present study provides direct comparison of tissue-specific MAOA expression and the level of alcohol consumption. We analyzed rhesus macaque MAOA (rhMAOA) expression in blood from males before and after 12 months of alcohol self-administration. In addition, nucleus accumbens core (NAc core) and cerebrospinal fluid (CSF) were collected from alcohol access and control (no alcohol access) subjects at the 12-month time point for comparison. The rhMAOA expression level in the blood of alcohol-naive subjects was negatively correlated with subsequent alcohol consumption level. The mRNA expression was independent of rhMAOA-LPR genotype and global promoter methylation. After 12 months of alcohol use, blood rhMAOA expression had decreased in an alcohol dose-dependent manner. Also after 12 months, rhMAOA expression in the NAc core was significantly lower in the heavy drinkers, as compared with control subjects. The CSF measured higher levels of DA and lower DOPAC/DA ratios among the heavy drinkers at the same time point. These results provide novel evidence that blood MAOA expression predicts alcohol consumption and that heavy alcohol use is linked to low MAOA expression in both the blood and NAc core. Together, the findings suggest a mechanistic link between dampened MAOA expression, elevated DA and alcohol abuse.

Lipids and Oxidative Stress Associated with Ethanol-Induced Neurological Damage

The excessive intake of alcohol is a serious public health problem, especially given the severe damage provoked by chronic or prenatal exposure to alcohol that affects many physiological processes, such as memory, motor function, and cognitive abilities. This damage is related to the ethanol oxidation in the brain. The metabolism of ethanol to acetaldehyde and then to acetate is associated with the production of reactive oxygen species that accentuate the oxidative state of cells. This metabolism of ethanol can induce the oxidation of the fatty acids in phospholipids, and the bioactive aldehydes produced are known to be associated with neurotoxicity and neurodegeneration. As such, here we will review the role of lipids in the neuronal damage induced by ethanol-related oxidative stress and the role that lipids play in the related compensatory or defense mechanisms.

Serotonergic Systems in the Pathophysiology of Ethanol Dependence: Relevance to Clinical Alcoholism

Alcoholism is a progressive brain disorder that is marked by increased sensitivity to the positive and negative reinforcing properties of ethanol, compulsive and habitual use despite negative consequences, and chronic relapse to alcohol drinking despite repeated attempts to reduce intake or abstain from alcohol. Emerging evidence from preclinical and clinical studies implicates serotonin (5-hydroxytryptamine; 5-HT) systems in the pathophysiology of alcohol dependence, suggesting that drugs targeting 5-HT systems may have utility in the treatment of alcohol use disorders. In this Review, we discuss the role of 5-HT systems in alcohol dependence with a focus on 5-HT interactions with neural circuits that govern all three stages of the addiction cycle. We attempt to clarify how 5-HT influences circuit function at these different stages with the goal of identifying neural targets for pharmacological treatment of this debilitating disorder.

The effects of chronic alcohol self-administration on serotonin-1A receptor binding in nonhuman primates

BACKGROUND

Previous studies have found interrelationships between the serotonin system and alcohol self-administration. The goal of this work was to directly observe in vivo effects of chronic ethanol self-administration on serotonin 5-HT1A receptor binding with [(18)F]mefway PET neuroimaging in rhesus monkeys. Subjects were first imaged alcohol-naïve and again during chronic ethanol self-administration to quantify changes in 5-HT1A receptor binding.

METHODS

Fourteen rhesus monkey subjects (10.7-12.8 years) underwent baseline [(18)F]mefway PET scans prior to alcohol exposure. Subjects then drank gradually increasing ethanol doses over four months as an induction period, immediately followed by at least nine months ad libidum ethanol access. A post [(18)F]mefway PET scan was acquired during the final three months of ad libidum ethanol self-administration. 5-HT1A receptor binding was assayed with binding potential (BPND) using the cerebellum as a reference region. Changes in 5-HT1A binding during chronic ethanol self-administration were examined. Relationships of binding metrics with daily ethanol self-administration were also assessed.

RESULTS

Widespread increases in 5-HT1A binding were observed during chronic ethanol self-administration, independent of the amount of ethanol consumed. A positive correlation between 5-HT1A binding in the raphe nuclei and average daily ethanol self-administration was also observed, indicating that baseline 5-HT1A binding in this region predicted drinking levels.

CONCLUSIONS

The increase in 5-HT1A binding levels during chronic ethanol self-administration demonstrates an important modulation of the serotonin system due to chronic alcohol exposure. Furthermore, the correlation between 5-HT1A binding in the raphe nuclei and daily ethanol self-administration indicates a relationship between the serotonin system and alcohol self-administration.

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.