Peripheral Administration of Ethanol Results in a Correlated Increase in Dopamine and Serotonin Within the Posterior Ventral Tegmental Area

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.

Inhibitory and excitatory alcohol-seeking cues distinct roles in behavior, neurochemistry, and mesolimbic pathway in alcohol preferring (P) rats

Cues associated with alcohol use can readily enhance self-reported cravings for alcohol, which increases the likelihood of reusing alcohol. Understanding the neuronal mechanisms involved in alcohol-seeking behavior is important for developing strategies to treat alcohol use disorder. In all experiments, adult female alcohol-preferring (P) rats were exposed to three conditioned odor cues; CS+ associated with EtOH self-administration, CS- associated with the absence of EtOH (extinction training), and a CS0, a neutral stimulus. The data indicated that presentation of an excitatory conditioned cue (CS+) can enhance EtOH- seeking while the CS- can inhibit EtOH-seeking under multiple test conditions. Presentation of the CS+ activates a subpopulation of dopamine neurons within the interfascicular nucleus of the posterior ventral tegmental area (posterior VTA) and basolateral amygdala (BLA). Pharmacological inactivation of the BLA with GABA agonists inhibits the ability of the CS+ to enhance EtOH-seeking but does not alter context-induced EtOH-seeking or the ability of the CS- to inhibit EtOH-seeking. Presentation of the conditioned odor cues in a non-drug-paired environment indicated that presentation of the CS+ increased dopamine levels in the BLA. In contrast, presentation of the CS- decreased both glutamate and dopamine levels in the BLA. Further analysis revealed that presentation of a CS+ EtOH-associated conditioned cue activates GABA interneurons but not glutamate projection neurons. Overall, the data indicate that excitatory and inhibitory conditioned cues can contrarily alter EtOH-seeking behaviors and that different neurocircuitries are mediating these distinct cues in critical brain regions. Pharmacotherapeutics for craving should inhibit the CS+ and enhance the CS- neurocircuits.

Synaptic effects of ethanol on striatal circuitry: therapeutic implications for dystonia

Alcohol consumption affects motor behavior and motor control. Both acute and chronic alcohol abuse have been extensively investigated; however, the therapeutic efficacy of alcohol on some movement disorders, such as myoclonus-dystonia or essential tremor, still does not have a plausible mechanistic explanation. Yet, there are surprisingly few systematic trials with known GABAergic drugs mimicking the effect of alcohol on neurotransmission. In this brief survey, we aim to summarize the effects of EtOH on striatal function, providing an overview of its cellular and synaptic actions in a 'circuit-centered' view. In addition, we will review both experimental and clinical evidence, in the attempt to provide a plausible mechanistic explanation for alcohol-responsive movement disorders, with particular emphasis on dystonia. Different hypotheses emerge, which may provide a rationale for the utilization of drugs that mimic alcohol effects, predicting potential drug repositioning.

The enduring behavioral and neurobiological effects of a flavor cue paired with alcohol drinking during adolescence on the incentive properties of the flavor cue in adulthood in female alcohol-preferring (P) rats

BACKGROUND

Alcohol use disorders (AUDs) affect 15 million people nationwide, 4% of which are adolescents (ages 12-17) and adolescents who binge drink significantly increase their likelihood of suffering from an AUD in adulthood. Research shows that cues (i.e. flavors) paired with alcohol (EtOH) produce significant cue-induced alcohol craving and contribute to relapse in adolescent and adult populations. However, there is a lack of research focused on how cues that accompany EtOH drinking during adolescence, affect EtOH craving later in life. The current study sought to examine the sex- and developmental-dependent effects of adolescent exposure to flavor cues associated with EtOH on operant-lick behavior and cue-induced dopamine (DA) levels within the nucleus accumbens shell (AcbSh; reward structure) in adulthood.

METHODS

Adolescent alcohol-preferring (P) rats were randomly assigned to one of 4 groups and received 24 hr. access to three bottles on their home cage: Paired: 0.1% blueberry flavor extract (BB) + 15% v/v EtOH and 2 water bottles; Unpaired: 0.1% BB, 15% v/v EtOH, and water; 15% EtOH alone, and 2 water bottles; BB alone and 2 water bottles. Home cage fluid consumption was measured for 2-weeks. On the third week bottles were removed and all animals underwent 9 days of operant training using an operant sipper paradigm. This consisted of two sipper spouts connected to the computer by a lickometer, which registered tongue contacts with the sipper tube (Paired: BB+EtOH or water; Unpaired BB or EtOH; EtOH alone: EtOH or water; BB alone: BB or water). When the fixed ratio (FR) requirement for number of licks/tongue contacts was met, a liquid delivery solenoid dispensed 0.05 ml of fluid into the sipper tube. Following the final operant session all rats remained in their home-cage for approximately 40 days until adulthood at which point they were returned to the operant chambers and tested for appetitive and consummatory behavior in response to the flavor cue (all rats: BB or water; NO EtOH). Two weeks after the final operant session all rats underwent microdialysis testing to examine cue-induced DA levels in the AcbSh.

RESULTS

Data indicated that animals in the paired group exhibited a significantly greater level of licking at the BB sipper and a significantly greater level of DA release in response to the flavor cue compared to the other groups.

CONCLUSIONS

Overall, the data suggest that cues paired with EtOH during adolescence may produce persistent changes to the behavioral and neurobiological mechanisms that contribute to an increased risk of developing an AUD later in life.

Sex- and OGG1-dependent reversal of in utero ethanol-initiated changes in postnatal behaviour by neonatal treatment with the histone deacetylase inhibitor trichostatin A (TSA) in oxoguanine glycosylase 1 (Ogg1) knockout mice

Oxoguanine glycosylase 1 (OGG1) is both a DNA repair enzyme and an epigenetic modifier. We assessed behavioural abnormalities in OGG1-deficient progeny exposed once in utero to a low dose of ethanol (EtOH) and treated postnatally with a global histone deacetylase inhibitor, trichostatin A (TSA). The goal of this study was to determine if neurodevelopmental disorders initiated in the fetal brain by in utero exposure to EtOH could be mitigated by postnatal treatment with TSA. EtOH and TSA alone improved preference for novel location (short-term, 90 min) and novel object (long-term, 24 h) sex- and OGG1-dependently. Combined EtOH/TSA treatment reversed these effects in the short-term novel location test sex- and OGG1-dependently. In females but not males, the incidence of high shredders of nesting material was not altered by either TSA or EtOH alone, but was reduced by combined EtOH/TSA treatment in +/+ progeny. Similar but non-significant effects were observed in Ogg1 -/- females. Accelerated rotarod performance was enhanced by both EtOH and TSA alone in only male Ogg1 +/+ but not -/- progeny, and was not altered by combined EtOH/TSA exposure. The OGG1-dependent effects of EtOH and TSA particularly on novel location and the incidence of high shredders, and the reversal of EtOH effects on these parameters by combined EtOH/TSA treatment, suggests both xenobiotics may alter behaviour via a mechanism involving OGG1 acting as an epigenetic modifier, in addition to repairing DNA damage. These preliminary results suggest that the postnatal use of more selective epigenetic modifying agents may constitute a novel strategy for mitigating some components of ROS-initiated neurodevelopmental disorders.

Changes in Brain Dopamine Extracellular Concentration after Ethanol Administration; Rat Microdialysis Studies

AIMS

The purpose of this review is to evaluate microdialysis studies where alterations in the dopaminergic system have been evaluated after different intoxication states, in animals showing preference or not for alcohol, as well as during alcohol withdrawal.

METHODS

Ethanol administration induces varying alterations in dopamine microdialysate concentrations, thereby modulating the functional output of the dopaminergic system.

RESULTS

Administration of low doses of ethanol, intraperitoneally, intravenously, orally or directly into the nucleus accumbens, NAc, increases mesolimbic dopamine, transmission, as shown by increases in dopamine content. Chronic alcohol administration to rats, which show alcohol-dependent behaviour, induced little change in basal dopamine microdialysis content. In contrast, reduced basal dopamine content occurred after ethanol withdrawal, which might be the stimulus to induce alcohol cravings and consumption. Intermittent alcohol consumption did not identify any consistent changes in dopamine transmission. Animals which have been selectively or genetically bred for alcohol preference did not show consistent changes in basal dopamine content although, exhibited a significant ethanol-evoked dopamine response by comparison to non-preference animals.

CONCLUSIONS

Microdialysis has provided valuable information about ethanol-evoked dopamine release in the different animal models of alcohol abuse. Acute ethanol administration increases dopamine transmission in the rat NAc whereas chronic ethanol consumption shows variable results which might reflect whether the rat is prior to or experiencing ethanol withdrawal. Ethanol withdrawal significantly decreases the extracellular dopamine content. Such changes in dopamine surges will contribute to both drug dependence, e.g. susceptibility to drug withdrawal, and addiction, by compromising the ability to react to normal dopamine fluctuations.

Concomitant taurine exposure counteracts ethanol-induced changes in locomotor and anxiety-like responses in zebrafish

Taurine (TAU) is a β-amino sulfonic acid with pleiotropic roles in the brain, including the neuromodulatory activity via GABAergic and glycinergic agonism. This molecule is found at high concentrations in energy drinks and is often mixed with alcohol in beverages. Although TAU has a neuroprotective role in the brain, the putative risks of mixing TAU and EtOH are not fully understood. Here, we investigated whether TAU modulates locomotor and anxiety-like behavior in adult zebrafish by using the novel tank and light-dark tests following acute EtOH exposure at anxiogenic and anxiolytic concentrations. Zebrafish were individually exposed to water (control), TAU (42, 150, and 400 mg/L), and EtOH (0.25% (v/v) and 1% (v/v)) both independently and cotreated for 1 h. EtOH 0.25% and TAU produced U-shape anxiolytic-like behavior in the light-dark test, TAU 42 and 400 positively modulated EtOH effects, and TAU 150 exerted a protective effect. All TAU concentrations counteracted EtOH 1%-induced locomotion impairment, as well as the anxiogenic-like behavior. Finally, all TAU concentrations when given independently or cotreated with EtOH 0.25% and 1% decreased the risk assessment of the lit compartment. Principal component analyses revealed that exploration and anxiety-like responses were the main behaviors that contribute to the effects of TAU and EtOH. Overall, we demonstrate that TAU differently modulates EtOH-induced anxiolytic- and anxiogenic-like behaviors depending on the concentration, suggesting a complex mechanism underlying TAU and EtOH interactions.

Early postnatal allopregnanolone levels alteration and adult behavioral disruption in rats: Implication for drug abuse

Several studies have highlighted the role that early postnatal levels of allopregnanolone play in the development of the CNS and adult behavior. Changes in allopregnanolone levels related to stress have been observed during early postnatal periods, and perinatal stress has been linked to neuropsychiatric disorders. The alteration of early postnatal allopregnanolone levels in the first weeks of life has been proven to affect adult behaviors, such as anxiety-related behaviors and the processing of sensory inputs. This review focuses on the first studies about the possible relationship between the early postnatal allopregnanolone levels and the vulnerability to abuse of drugs such as alcohol in adulthood, given that (1) changes in neonatal allopregnanolone levels affect novelty exploration and novelty seeking has been linked to vulnerability to drug abuse; (2) early postnatal administration of progesterone, the main allopregnanolone precursor, affects the maturation of dopaminergic meso-striatal systems, which have been related to novelty seeking and drug abuse; and (3) alcohol consumption increases plasma and brain allopregnanolone levels in animals and humans. Manipulating neonatal allopregnanolone by administering finasteride, an inhibitor of the 5α-reductase enzyme that participates in allopregnanolone synthesis, increases alcohol consumption and decreases the locomotor stimulant effects of low alcohol doses. At a molecular level, finasteride decreases dopamine and serotonin in ventral striatum and dopamine release in nucleus accumbens. Preliminary results suggest that serotonin 5HT3 receptors could also be affected. Although an in-depth study is necessary, evidence suggests that there is a relation between early postnatal allopregnanolone and vulnerability to drug use/abuse.

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Early postnatal AlloP levels alteration affects brain maturation and adult behavior.

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Early stress interacts to AlloP influencing neuropsychiatric disorders vulnerability.

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Fluctuations in neonatal AlloP levels play a role in alcohol abuse vulnerability.

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Neonatal finasteride induces novelty-seeking profile and increases ethanol intake.

Histamine, histamine H3 receptor, and alcohol use disorder

Alcohol use disorder is associated with several mental, physical, and social problems. Its treatment is difficult and often requires a combination of pharmacological and behavioural therapy. The brain histaminergic system, one of the wake-active systems that controls whole-brain activity, operates through three neuronal GPCRs. The histamine H₃ receptor (Hrh3), which is expressed in many brain areas involved in alcohol drinking and alcohol reward, can be targeted with a number of drugs developed initially for cognitive disorders and/or disorders related to sleep, wakefulness, and alertness. In all rodent alcohol drinking models tested so far, H₃ receptor antagonists have reduced alcohol drinking and alcohol-induced place preference and cue-induced alcohol reinstatement. Several H₃ receptor antagonists tested and found to be safe for humans could be subjected to clinical tests to treat alcohol use disorder. Preference should be given to short-acting drugs to avoid the sleep problems associated with the wake-maintaining effects of the drugs. LINKED ARTICLES: This article is part of a themed section on New Uses for 21st Century. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v177.3/issuetoc.

Ethanol-induced enhancement of inhibitory synaptic transmission in the rat spinal substantia gelatinosa

Recent studies have shown that ethanol produces a widespread modulation of neuronal activity in the central nervous system. It is not fully understood, however, how ethanol changes nociceptive transmission. We investigated acute effects of ethanol on synaptic transmission in the substantia gelatinosa (lamina II of the spinal dorsal horn) and mechanical responses in the spinal dorsal horn. In substantia gelatinosa neurons, bath application of ethanol at low concentration (10 mM) did not change the frequency and amplitude of spontaneous inhibitory postsynaptic currents. At medium to high concentrations (20–100 mM), however, ethanol elicited a barrage of large amplitude spontaneous inhibitory postsynaptic currents. In the presence of tetrodotoxin, such enhancement of spontaneous inhibitory postsynaptic currents was not detected. In addition, ethanol (20–100 mM) increased the frequency of spontaneous discharge of vesicular GABA transporter-Venus-labeled neurons and suppressed the mechanical nociceptive response in wide-dynamic range neurons in the spinal dorsal horn. The present results suggest that ethanol may reduce nociceptive information transfer in the spinal dorsal horn by enhancement of inhibitory GABAergic and glycinergic synaptic transmission.

Alcohol and the Brain: Neuronal Molecular Targets, Synapses, and Circuits

Ethanol is one of the most commonly abused drugs. Although environmental and genetic factors contribute to the etiology of alcohol use disorders, it is ethanol's actions in the brain that explain (1) acute ethanol-related behavioral changes, such as stimulant followed by depressant effects, and (2) chronic changes in behavior, including escalated use, tolerance, compulsive seeking, and dependence. Our knowledge of ethanol use and abuse thus relies on understanding its effects on the brain. Scientists have employed both bottom-up and top-down approaches, building from molecular targets to behavioral analyses and vice versa, respectively. This review highlights current progress in the field, focusing on recent and emerging molecular, cellular, and circuit effects of the drug that impact ethanol-related behaviors. The focus of the field is now on pinpointing which molecular effects in specific neurons within a brain region contribute to behavioral changes across the course of acute and chronic ethanol exposure.