Rising taurine and ethanol concentrations in nucleus accumbens interact to produce dopamine release after ethanol administration

Acute taurine reduced alcohol intake and preference in alcohol-experienced, but not in alcohol-näive rats by central mechanisms

Taurine is a non-essential β sulfonated amino acid involved in a plethora of biological functions in the mammalian central nervous system. Taurine is easily accessible in energy drinks for human consumption. Previous preclinical and clinical reports suggest that acute systemic administration of taurine could inhibit some of the behavioral and metabolic effects of alcohol use disorder. Overall, both in rodent and human studies, acute taurine administration reduced voluntary alcohol intake. This study aimed to assess the pharmacological effects of taurine (intracerebroventricular; i.c.v.) on ethanol intake/preference of rats either control (i.e., alcohol naïve) or forced ethanol intake (since juvenile age with a chronic intermittent access model). In addition, to explore anxiety-like behavior (through defensive burying behavior test) as pharmacological control of taurine. We found that acute (i.c.v.) taurine reduced alcohol consumption, i.e., taurine significantly decreased both alcohol intake and preference in adult male Wistar rats. Moreover, taurine elicits an anxiolytic-like effect in all administered groups independently of previous alcohol exposure.

Alcohol and the dopamine system

The mesolimbic dopamine pathway plays a major role in drug reinforcement and is likely involved also in the development of drug addiction. Ethanol, like most addictive drugs, acutely activates the mesolimbic dopamine system and releases dopamine, and ethanol-associated stimuli also appear to trigger dopamine release. In addition, chronic exposure to ethanol reduces the baseline function of the mesolimbic dopamine system. The molecular mechanisms underlying ethanol´s interaction with this system remain, however, to be unveiled. Here research on the actions of ethanol in the mesolimbic dopamine system, focusing on the involvement of cystein-loop ligand-gated ion channels, opiate receptors, gastric peptides and acetaldehyde is briefly reviewed. In summary, a great complexity as regards ethanol´s mechanism(s) of action along the mesolimbic dopamine system has been revealed. Consequently, several new targets and possibilities for pharmacotherapies for alcohol use disorder have emerged.

Acamprosate reduces ethanol intake in the rat by a combined action of different drug components

Alcohol misuse accounts for a sizeable proportion of the global burden of disease, and Campral® (acamprosate; calcium-bis-(N-acetylhomotaurinate)) is widely used as relapse prevention therapy. The mechanism underlying its effect has in some studies been attributed to the calcium moiety and not to the N-acetylhomotaurine part of the compound. We recently suggested that the dopamine elevating effect of acamprosate is mediated both by N-acetylhomotaurine and calcium in a glycine receptor dependent manner. Here we aimed to explore, by means of in vivo microdialysis, if our previous study using local administration was functionally relevant and if systemic administration of the sodium salt of N-acetylhomotaurine (sodium acamprosate; 200 mg/kg, i.p.) enhanced the effects of calcium chloride (CaCl2; 73.5 mg/kg, i.p.) on nucleus accumbens (nAc) dopamine and/or taurine levels in male Wistar rats. In addition, we investigated the impact of regular acamprosate and the combination of CaCl2 and N-acetylhomotaurine on the alcohol deprivation effect (ADE). Finally, we assessed if N-acetylhomotaurine potentiates the ethanol-intake reducing effect of CaCl2 in a two-bottle choice voluntary ethanol consumption model followed by an ADE paradigm. Systemic administration of regular acamprosate, sodium acamprosate and CaCl2 all trended to increase nAc dopamine whereas the combination of CaCl2 and sodium acamprosate produced a significant increase. Sodium acamprosate elevated extracellular taurine levels without additional effects of CaCl2. Ethanol intake was significantly reduced by systemic administration of CaCl2 without additional effects of the combination of CaCl2 and sodium acamprosate. Both acamprosate and CaCl2 combined with sodium acamprosate blocked the ADE following acute treatment. The data presented suggest that CaCl2 and N-acetylhomotaurine act in concert on a neurochemical level, but calcium appears to have the predominant effect on ethanol intake.

Energy drinks at adolescence: Awareness or unawareness?

Energy drinks (EDs) are beverages similar to soft drinks, characterized by high caffeine concentrations with additional ingredients like taurine and vitamins, marketed for boosting energy, reducing tiredness, increasing concentration, and for their ergogenic effect. The majority of consumers are children, adolescents, and young athletes. Although EDs companies claim about the ergogenic and remineralizing properties of their products, there is a serious lack of evidence at preclinical as well as clinical level to validate their benefits. The regular intake and long-term consequences of these caffeinated drinks are not well documented, especially the possible negative effects in adolescents whose brain is still developing. EDs combined with alcohol are also gaining popularity among adolescents and different publications indicate that this combined consumption might increase the risk to develop an alcohol use disorder, as well as produce serious adverse cardiovascular effects. There is an increasing need to disseminate knowledge on EDs damage on health, so that adolescents can be aware about the potential harmful outcomes of consuming these drinks.

The implications of alcohol mixed with energy drinks from medical and socio-legal standpoints

Co-ingestion of energy drinks and alcohol has long been in practice and has been poorly regulated despite a growing body of literature of their potential negative health impacts. Co-ingestion of energy drinks with alcohol has multiple counter-active effects such as reduction of body sway, fatigue and sedative effects induced by alcohol, along with increased subjective feeling of alertness, which may lead to increased binge-drinking, intoxication, decreased perception of intoxication, dehydration, and alcohol poisoning. Adding energy drinks to alcohol may also have synergistic effects in causing alcohol dependency and addiction. The association between caffeine, a common active ingredient in energy drinks, and alcohol is relatively well defined, however association with other active ingredients such as taurine, niacin, and pyridoxine, is less understood, pointing to a gap in our knowledge regarding this practice. Nonetheless, the current associations between AMED (Alcohol Mixed with Energy Drinks) and risky behavior secondary to intoxication and cases of alcohol poisoning have led various national governing bodies to regulate this practice. This review highlights the potential effects of AMED on human physiology based on what is known from human and animal models, and sheds light on specific biochemical interactions between alcohol and active ingredients found in energy drinks; Caffeine, Taurine, and Glucuronolactone. The review also touches on the regulation of this practice around the world, and the impact it has on its users, and points researchers to gaps in our knowledge on the interactions between alcohol and EDs and the full extent of their effects.

OUP accepted manuscript

STUDY QUESTION

Are taurine and its transporter TAUT associated with spermiogenesis and early embryo development?

SUMMARY ANSWER

Morphologically abnormal spermatozoa increased after local functional interference by intratesticular injection, and taurine depletion significantly reduced the normal embryo numbers in vivo and blastocyst formation rate in vitro.

WHAT IS KNOWN ALREADY

Taurine is one of the most abundant amino acids in the male reproductive system and it has been demonstrated that taurine can efficiently improve spermatogenic function in rat models of testicular injury. However, limited information is known about the role of taurine and its transporter TAUT in spermatogenesis and early embryo development.

STUDY DESIGN, SIZE, DURATION

Clinical characteristics from 110 couples who have experienced recurrent pregnancy loss (RPL) were collected from December 2014 to March 2018. According to whether a fetal heartbeat was seen in the previous pregnancy under ultrasonic monitoring, patients with RPL were divided into two groups: an RPL without heartbeat (pregnancy with no fetal heartbeat, ROH) group, and an RPL with heartbeat (one or more pregnancies with fetal heartbeat, RWH) group. Semen samples (21 ROH and 20 RWH) were finally used for metabolomic analysis. Furthermore, semen samples were obtained from 30 patients with teratozoospermia (normal sperm morphology <4%) seeking evaluation for infertility and 25 age-matched control subjects with normal semen quality for western blotting. Animal experiments were performed in CD-1/ICR mice.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Metabolomics was performed to determine the metabolic changes between the ROH and RWH groups. Sperm proteins from patients with teratozoospermia and healthy controls were extracted for detecting TAUT expression using western blot analysis. Immunofluorescence was used to characterize the localization of TAUT in the testis and ejaculated spermatozoa. Functional analysis in mice was performed by intratesticular injection of siRNAs or antagonist (β-alanine) and 5% β-alanine was provided in drinking water to 3-week-old male mice for 5 weeks with the aim of depleting taurine. Murine epididymal spermatozoa were stained with hematoxylin and eosin for morphological assessment. IVF and mating tests were performed in mice for assessing fertility.

MAIN RESULTS AND THE ROLE OF CHANCE

Metabolomic analysis demonstrated that the taurine content was lower in spermatozoa but higher in seminal plasma from the ROH than the RWH group. TAUT expression was lower in spermatozoa from patients with teratozoospermia than controls. Immunofluorescence showed that TAUT was localized to the manchette in mouse elongated spermatids functional analysis showed that morphologically abnormal spermatozoa increased after interference, and this defect increased after supplementation with 5% β-alanine but was improved by 5% taurine supplementation. Supplementation with 5% β-alanine significantly reduced the normal embryo number in the mouse uterus as well as blastocyst formation rate in vitro.

LARGE SCALE DATA

N/A

LIMITATIONS, REASONS FOR CAUTION

The sample size was low and larger cohorts are needed to confirm the positive effect of taurine on human sperm quality. A comprehensive safety examination should be performed to evaluate whether taurine is a possible treatment for teratozoospermia. Furthermore, the specific molecular mechanism of TAUT involvement in spermiogenesis remains to be clarified.

WIDER IMPLICATIONS OF THE FINDINGS

The study provides new insights into the role of taurine and its transporter TAUT in male reproduction and embryo development. The results also indicate that TAUT is a promising molecular candidate for the assessment of sperm quality, which may contribute to the diagnosis and treatment for teratozoospermia.

STUDY FUNDING/COMPETING INTEREST(S)

This work was supported by grants from the National Natural Science Foundation of China (no. 81774075, 31900605, 81971451), Jiangsu Science and Technology Program Grant (BK20190654) and Maternal and child health scientific research of Jiangsu Province (F202121). The authors declare no competing financial interests.

Taurine modulates behavioral effects of intermittent ethanol exposure without changing brain monoamine oxidase activity in zebrafish: Attenuation of shoal- and anxiety-like responses, and abolishment of memory acquisition deficit

Prolonged alcohol consumption has been considered as an important risk factor for various diseases. Chronic ethanol (EtOH) intake is associated with deleterious effects on brain functions culminating in robust behavioral changes. Notably, drugs available to treat the effects of EtOH have low therapeutic efficacy so far. Taurine (TAU) appears as a promising neuroprotective molecule due to its pleiotropic action in the brain. Here, we investigated whether TAU plays a beneficial role in different behavioral domains of zebrafish submitted to an intermittent EtOH exposure model, specially focusing on social behavior, anxiety-like responses, and memory. Moreover, since monoamines play a role in EtOH-mediated responses, we also evaluated the influence of both TAU and EtOH exposures on brain monoamine oxidase (Z-MAO) activity. Fish were exposed to non-chlorinated water or 1% EtOH for 8 consecutive days (20 min per day). From the 5th day until the end of the experimental period (8th day), animals were kept in the absence or presence of TAU (42, 150, or 400 mg/L) 1 h per day immediately after EtOH exposure. Behavioral measurements started 24 h after the last EtOH exposure. We observed that TAU showed modest attenuating effects on shoaling behavior and anxiety-like responses, while 42 and 150 mg/L TAU abolished the memory acquisition deficit in the inhibitory avoidance task. Biochemical analysis revealed that TAU did not modulate EtOH-induced increase on brain Z-MAO activity. Collectively, our novel data show a potential beneficial effect of TAU in an intermittent EtOH exposure model in zebrafish. Moreover, these findings foster the growing utility of this aquatic species to investigate the neurobehavioral basis of EtOH- and TAU-mediated responses in vertebrates.

Alcohol mixed with energy drinks: what about taurine?

RATIONALE

Since energy drinks (EDs) were marketed to the general public as recreational and soft drinks, mixing these with alcohol has become a popular practice, especially in the younger population. Alcohol mixed with EDs (AmEDs) is a particularly alarming combination, given the evidence that consistently associate these drinks with increased risk behaviours and greater alcohol consumption. Caffeine and taurine are commonly found in EDs. In contrast to caffeine, the studies on taurine psychoactive properties and how this amino acid influences ethanol intake alone or in combination with caffeine are not so numerous.

OBJECTIVES

We summarised relevant and available data on the studies focusing on taurine as a psychoactive agent and its influence on ethanol (EtOH)-induced behaviours. Given the increased risk that represents mixing alcohol with energy drinks, we put emphasis on the research exploring the impact of these combinations on motivated behaviour towards EtOH consumption.

RESULTS

The research on taurine properties on motivated behaviour towards EtOH consumption is limited, and mostly all done in combination with caffeine or other molecules. This makes it difficult to elucidate the effect of this amino acid when combined with alcohol.

CONCLUSIONS

Incomplete understanding of the properties and effects of AmEDs is unavoidable until more studies are performed on the influence of taurine on motivation to consume alcohol. Taurine should be further explored, particularly in regard to its potential beneficial applications, motivational properties and synergies with other psychoactive ingredients (i.e. caffeine).

Taurine enhances voluntary alcohol intake and promotes anxiolytic-like behaviors in rats

Taurine is an amino acid usually added to energy drinks. In rodents, acute taurine administration decreases voluntary alcohol intake, and subchronic administration restores different behavioral features impaired by alcohol withdrawal. In the present study, we evaluated the effects of chronic taurine treatment on voluntary alcohol consumption and changes in behavioral parameters in rats. Adult male Wistar rats were divided into two groups and were allowed to choose from two bottles containing 20% alcohol or 0.08% saccharin (vehicle solution), or two bottles containing vehicle, 24 h per day, for 5 weeks. After 3 weeks, rats received 100 mg/kg taurine (TAU) or saline (SAL) intraperitoneally once a day for 2 weeks, and daily alcohol consumption was monitored. On days 22 and 33, rats were tested in the open-field, and on day 34, they were exposed to the light/dark task (LDT). Our results show for the first time that chronic taurine treatment enhanced voluntary alcohol intake and preference in rats, and that these changes were accompanied by an anxiolytic-like phenotype in alcohol-treated rats, possibly due to its synergistic effect with alcohol on the dopaminergic and GABAergic systems.

Different dopamine tone in ethanol high- and low-consuming Wistar rats

Excessive alcohol use causes considerable morbidity and mortality worldwide. Changes in the mesolimbic dopamine system have been postulated as a neurobiological underpinning of excessive alcohol consumption, and recent research also suggests that the amino acid taurine plays a central role in ethanol-induced dopamine elevation. The aim of this study was to further outline the role of dopamine and taurine in regulating alcohol consumption. In this study, a choice between ethanol (20%) and water was administered to Wistar rats in an intermittent manner (three times/week) for seven consecutive weeks. In vivo microdialysis was used to explore baseline levels as well as ethanol-induced increases of extracellular dopamine and taurine, in the nucleus accumbens (nAc) of Wistar rats voluntarily consuming large or small amounts of ethanol. Basal levels of taurine were also measured in cerebrospinal fluid (CSF) and serum in a subset of rats. Ethanol-induced increases in nAc dopamine and taurine did not differ between alcohol-consuming and naïve rats. However, when categorized based on ethanol intake, rats consuming larger amounts of ethanol exhibited a lower dopamine tone in the nucleus accumbens and responded to ethanol with a slower elevation of extracellular taurine levels, as compared with low-consuming animals. Basal levels of taurine in nAc, CSF, or serum did not differ between ethanol high- and low-consuming rats. Our data support previous studies claiming an association between low endogenous dopamine levels and excessive alcohol intake.

Taurine Suppression of Central Amygdala GABAergic Inhibitory Signaling via Glycine Receptors Is Disrupted in Alcohol Dependence

BACKGROUND

Alcohol use disorder (AUD) increases brain stress systems while suppressing reward system functioning. One expression of stress system recruitment is elevated GABAergic activity in the central amygdala (CeA), which is involved in the excessive drinking seen with AUD. The sulfonic amino acid taurine, a glycine receptor partial agonist, modulates GABAergic activity in the rewarding effects of alcohol. Despite taurine abundance in the amygdala, its role in the dysregulation of GABAergic activity associated with AUD has not been studied. Thus, here, we evaluated the effects of taurine on locally stimulated GABAergic neurotransmission in the CeA of naïve- and alcohol-dependent rats.

METHODS

We recorded intracellularly from CeA neurons of naïve- and alcohol-dependent rats, quantifying locally evoked GABA_A receptor-mediated inhibitory postsynaptic potentials (eIPSP). We examined the effects of taurine and alcohol on CeA eIPSP to characterize potential alcohol dependence-induced changes in the effects of taurine.

RESULTS

We found that taurine decreased amplitudes of eIPSP in CeA neurons of naïve rats, without affecting the acute alcohol-induced facilitation of GABAergic responses. In CeA neurons from dependent rats, taurine no longer had an effect on eIPSP, but now blocked the ethanol (EtOH)-induced increase in eIPSP amplitude normally seen. Additionally, preapplication of the glycine receptor-specific antagonist strychnine blocked the EtOH-induced increase in eIPSP amplitude in neurons from naïve rats.

CONCLUSIONS

These data suggest taurine may act to oppose the effects of acute alcohol via the glycine receptor in the CeA of naïve rats, and this modulatory system is altered in the CeA of dependent rats.

Sub-chronic taurine administration induces behavioral sensitization but does not influence ethanol-induced dopamine release in the nucleus accumbens

Preclinical studies have shown that the amino acid taurine is of importance for the dopamine elevating properties of ethanol. Taurine intake has escalated over the last decade due to increased consumption of taurine-containing energy drinks and dietary supplements. Whether long-term intake of large amounts of taurine induces adaptations affecting ethanol-induced dopamine elevation is not clear. Thus the aim of the present studies was to explore the impact of repeated administration of large amounts of taurine on ethanol-induced behavior and dopamine neurotransmission. Repeated daily systemic administration of taurine increased taurine-induced locomotor activity and rearing. Acute administration of taurine and ethanol in naïve animals produced an additive effect on extracellular taurine but no alteration of the ethanol-induced dopamine elevation, as measured by in vivo microdialysis. Sub-chronic administration of taurine did not modify the taurine- or dopamine-elevating properties of ethanol. Daily taurine treatment also failed to change the mRNA expression of the taurine transporter and GABA_A- and glycine-receptor subunits, as measured by qPCR in nucleus accumbens tissue. We conclude that systemic administration of taurine may have long lasting central effects, here displayed as behavioral sensitization. However, repeated daily exposure to taurine does not appear to influence the dopamine elevating properties of ethanol.

Energy drink constituents (caffeine and taurine) selectively potentiate ethanol-induced locomotion in mice

Mixing alcohol with energy drinks has emerged as a popular trend over the last decade. However, epidemiological studies have found this consumption to be associated with increased hazards, such as binge drinking, increased alcohol-related harm and risk of developing alcohol use disorder. The mechanisms underlying these effects are not clear, but much attention has been attributed to caffeine. However, taurine, another common ingredient in energy drinks, has also been associated with the dopamine elevating properties of ethanol, and may in this respect contribute to the increased liability associated with the mixture of alcohol and energy drinks. In the present study we measured locomotor activity, a phenomenon previously linked to the dopamine activating and reinforcing properties of the drug, following acute systemic administration with caffeine (1, 5, 15, 30 mg/kg), taurine (30, 60, 300, 600 mg/kg) and ethanol (1.75, 2.5, 3.25 g/kg), alone or in combination. We found that ethanol and caffeine, but not taurine, increased locomotion compared to vehicle. In addition, when combined with ethanol, caffeine, but not taurine, increased the locomotor stimulatory effect of ethanol. Furthermore, the combination of caffeine and taurine were able to further enhance the ethanol-induced locomotor response. Eleven days of intermittent caffeine exposure produced a sensitized response to the caffeine-induced locomotion, but did not alter the additive effect produced by the combination of caffeine and taurine on ethanol-induced locomotion. Based on the present study we suggest that the combination of caffeine and taurine, at a specific dose range, enhances the locomotor stimulatory properties of ethanol, a phenomenon previously linked to the reinforcing properties of the drug.

Gut-brain axis and addictive disorders: A review with focus on alcohol and drugs of abuse

Due to the limited efficacy of existing medications for addictive disorders including alcohol use disorder (AUD), the need for additional medications is substantial. Potential new medications for addiction can be identified through investigation of the neurochemical substrates mediating the ability of drugs of abuse such as alcohol to activate the mesolimbic dopamine system. Interestingly, recent studies implicate neuropeptides of the gut-brain axis as modulators of reward and addiction processes. The present review therefore summarizes the current studies investigating the ability of the gut-brain peptides ghrelin, glucagon-like peptide-1 (GLP-1), amylin and neuromedin U (NMU) to modulate alcohol- and drug-related behaviors in rodents and humans. Extensive literature demonstrates that ghrelin, the only known orexigenic neuropeptide to date, enhances reward as well as the intake of alcohol, and other drugs of abuse, while ghrelin receptor antagonism has the opposite effects. On the other hand, the anorexigenic peptides GLP-1, amylin and NMU independently inhibits reward from alcohol and drugs of abuse in rodents. Collectively, these rodent and human studies imply that central ghrelin, GLP-1, amylin and NMU signaling may contribute to addiction processes. Therefore, the need for randomized clinical trials investigating the effects of agents targeting these aforementioned systems on drug/alcohol use is substantial.

Minor Adaptations of Ethanol-Induced Release of Taurine Following Chronic Ethanol Intake in the Rat

Alcohol dependence is a puzzling brain disorder causing enormous suffering and financial costs world-wide. One of the few common denominators of all addictive drugs is activation of the mesolimbic dopamine system resulting in increased dopamine levels in the nucleus accumbens. In order to understand the development of addiction and find new efficient treatment strategies we need to understand how addictive drugs increase dopamine following acute and chronic administration of drugs. In the search for mechanisms underlying ethanol's ability to increase dopamine in the nucleus accumbens we have found taurine to be of major importance, although the complete picture remains to be disclosed. The aim of the present study was to explore whether chronic voluntary ethanol intake influences the ethanol-induced elevation of taurine. By means of in vivo micro-dialysis we found that voluntary intake of large amounts of ethanol for 12 weeks only had a modest influence on ethanol-induced elevations of taurine in the rat.

Ethanol-Induced Taurine Elevation in the Rat Dorsal Striatum

In the search for the primary mechanism underlying the dopamine elevating properties of ethanol we have established that raised levels of taurine in the nucleus accumbens (nAc) is pivotal. In the nAc, the release of taurine appears to be connected to osmoregulation, and neither taurine nor dopamine is increased if ethanol is administered in a hypertonic saline solution. However, even though the nAc is important for drug-reinforcement, manifestation of addiction has been postulated to recruit the more dorsal parts of the striatum (DS). How ethanol influences dopamine and taurine in the DS and their role in addiction is thus far poorly understood. By means of in vivo microdialysis in freely moving rats we concomitantly monitored extracellular levels of dopamine and taurine in the DS following administration of ethanol diluted either in an isotonic or hypertonic saline solution. In a different set of rats, placed in a voluntary ethanol consumption paradigm (intermittent access to 20% ethanol for 2 months), taurine and dopamine were monitored following an acute injection of ethanol. We found that neither administration of ethanol diluted in a hypertonic saline solution, nor 2 months of moderate ethanol consumption, influence the ethanol-induced increase of taurine in the DS. We propose that there may be regional differences in the relationship between taurine, dopamine and ethanol in the nAc and in the DS. It remains to be determined if this subregion-specificity is important for the transition from recreational drug use to a compulsive habit.

Acamprosate's ethanol intake-reducing effect is associated with its ability to increase dopamine

Previous studies indicate that the anti-craving substance acamprosate modulates nucleus accumbens (nAc) dopamine levels via a dopamine-controlling nAc-VTA-nAc neurocircuitry. It was demonstrated that glycine receptors in the nAc are involved both in the dopamine-elevating effect and the ethanol intake-reducing effect of the drug. Here we wanted to explore the interaction of ethanol and acamprosate on nAc dopamine and investigate whether dopaminergic transmission may be related to the ethanol intake-reducing effects. In three separate studies we investigated nAc extracellular dopamine levels by means of in vivo microdialysis after administration of acamprosate and ethanol in 1) naïve rats, 2) rats pre-treated with acamprosate for two days or 3) ethanol medium- and high-preferring rats receiving ten days of acamprosate pre-treatment. In the first two studies, acamprosate elevated dopamine and simultaneously prevented ethanol from further increasing dopamine output. In the third study, long-term acamprosate pre-treatment produced a loss of the ethanol intake-reducing as well as the dopamine-elevating effects of acamprosate, and the dopamine elevating property of ethanol was restored. We suggest that acamprosate may partly substitute for the dopamine-elevating effect of ethanol but once tolerance develops to this effect, the ability to decrease ethanol intake is lost.

GLP-1 signaling and alcohol-mediated behaviors; preclinical and clinical evidence

Alcohol addiction, affecting approximately four percent of the population, contributes significantly to the global burden of diseases and is a substantial cost to the society. The neurochemical mechanisms regulating alcohol mediated behaviors is complex and in more recent years a new physiological role of the gut-brain peptides, traditionally known to regulate appetite and food intake, have been suggested. Indeed, regulators of alcohol-mediated behaviors. One of these gut-brain peptides is the annorexigenic peptide glucagon-like peptide-1 (GLP-1), Preclinical studies show that GLP-1 receptor activation, either by GLP-1 or analogues, attenuate the ability of alcohol to activate the mesolimbic dopamine system as well as decrease alcohol consumption and operant self-administration. In further support for the endogenous GLP-1 system in addiction processes are the experimental data showing that a GLP-1 receptor antagonist increases alcohol intake. Moreover, GLP-1 receptor agonists prevent the ability of other addictive drugs to activate the mesolimbic dopamine system. The number of clinical studies is limited, but show i) that genetic variation in the GLP-1 receptor gene is associated with alcohol addiction as well as increased alcohol infusion in humans, ii) that plasma levels of GLP-1 are associated with the subjective experience of cocaine and iii) that a GLP-1 receptor agonist reduces alcohol intake in patients with type-2 diabetes mellitus. These experimental and clinical studies raises the concern that clinically available GLP-1 receptor agonists deserves to be tested as potential treatments of patients with addictive disorders including alcohol addiction. This article is part of the Special Issue entitled 'Metabolic Impairment as Risk Factors for Neurodegenerative Disorders.'

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.

Single Channel Analysis of Isoflurane and Ethanol Enhancement of Taurine-Activated Glycine Receptors

The amino acid taurine is an endogenous ligand acting on glycine receptors (GlyRs), which is released by astrocytes in many brain regions, such as the nucleus accumbens and prefrontal cortex. Taurine is a partial agonist with an efficacy significantly lower than that of glycine. Allosteric modulators such as ethanol and isoflurane produce leftward shifts of glycine concentration-response curves but have no effects at saturating glycine concentrations. In contrast, in whole-cell electrophysiology studies these modulators increase the effects of saturating taurine concentrations. A number of possible mechanisms may explain these enhancing effects, including modulator effects on conductance, channel open times, or channel closed times. We used outside-out patch-clamp single channel electrophysiology to investigate the mechanism of action of 200 mM ethanol and 0.55 mM isoflurane in enhancing the effects of a saturating concentration of taurine. Neither modulator enhanced taurine-mediated conductance. Isoflurane increased the probability of channel opening. Isoflurane also increased the lifetimes of the two shortest open dwell times while both agents decreased the likelihood of occurrence of the longest-lived intracluster channel-closing events. The mechanism of enhancement of GlyR functioning by these modulators is dependent on the efficacy of the agonist activating the receptor and the concentration of agonist tested.

The Glycine Receptor-A Functionally Important Primary Brain Target of Ethanol

Identification of ethanol's (EtOH) primary molecular brain targets and determination of their functional role is an ongoing, important quest. Pentameric ligand-gated ion channels, that is, the nicotinic acetylcholine receptor, the γ-aminobutyric acid type A receptor, the 5-hydroxytryptamine₃ , and the glycine receptor (GlyR), are such targets. Here, aspects of the structure and function of these receptors and EtOH's interaction with them are briefly reviewed, with special emphasis on the GlyR and the importance of this receptor and its ligands for EtOH pharmacology. It is suggested that GlyRs are involved in (i) the dopamine-activating effect of EtOH, (ii) regulating EtOH intake, and (iii) the relapse preventing effect of acamprosate. Exploration of the GlyR subtypes involved and efforts to develop subtype specific agonists or antagonists may offer new pharmacotherapies for alcohol use disorders.

New Implications for the Melanocortin System in Alcohol Drinking Behavior in Adolescents: The Glial Dysfunction Hypothesis

Alcohol dependence causes physical, social, and moral harms and currently represents an important public health concern. According to the World Health Organization (WHO), alcoholism is the third leading cause of death worldwide, after tobacco consumption and hypertension. Recent epidemiologic studies have shown a growing trend in alcohol abuse among adolescents, characterized by the consumption of large doses of alcohol over a short time period. Since brain development is an ongoing process during adolescence, short- and long-term brain damage associated with drinking behavior could lead to serious consequences for health and wellbeing. Accumulating evidence indicates that alcohol impairs the function of different components of the melanocortin system, a major player involved in the consolidation of addictive behaviors during adolescence and adulthood. Here, we hypothesize the possible implications of melanocortins and glial cells in the onset and progression of alcohol addiction. In particular, we propose that alcohol-induced decrease in α-MSH levels may trigger a cascade of glial inflammatory pathways that culminate in altered gliotransmission in the ventral tegmental area and nucleus accumbens (NAc). The latter might potentiate dopaminergic drive in the NAc, contributing to increase the vulnerability to alcohol dependence and addiction in the adolescence and adulthood.

Alcohol and basal ganglia circuitry: Animal models

Brain circuits that include the cortex and basal ganglia make up the bulk of the forebrain, and influence behaviors related to almost all aspects of affective, cognitive and sensorimotor functions. The learning of new actions as well as association of existing action repertoires with environmental events are key functions of this circuitry. Unfortunately, the cortico-basal ganglia circuitry is also the target for all drugs of abuse, including alcohol. This makes the circuitry susceptible to the actions of chronic alcohol exposure that impairs circuit function in ways that contribute to cognitive dysfunction and drug use disorders. In the present review, we describe the connectivity and functions of the associative, limbic and sensorimotor cortico-basal ganglia circuits. We then review the effects of acute and chronic alcohol exposure on circuit function. Finally, we review studies examining the roles of the different circuits and circuit elements in alcohol use and abuse. We attempt to synthesize information from a variety of studies in laboratory animals and humans to generate hypotheses about how the three circuits interact with each other and with the other brain circuits during exposure to alcohol and during the development of alcohol use disorders. This article is part of the Special Issue entitled "Alcoholism".

Disentangling the Role of Astrocytes in Alcohol Use Disorder

Several laboratories recently identified that astrocytes are critical regulators of addiction machinery. It is now known that astrocyte pathology is a common feature of ethanol (EtOH) exposure in both humans and animal models, as even brief EtOH exposure is sufficient to elicit long-lasting perturbations in astrocyte gene expression, activity, and proliferation. Astrocytes were also recently shown to modulate the motivational properties of EtOH and other strongly reinforcing stimuli. Given the role of astrocytes in regulating glutamate homeostasis, a crucial component of alcohol use disorder (AUD), astrocytes might be an important target for the development of next-generation alcoholism treatments. This review will outline some of the more prominent features displayed by astrocytes, how these properties are influenced by acute and long-term EtOH exposure, and future directions that may help to disentangle astrocytic from neuronal functions in the etiology of AUD.

Binge-like ingestion of a combination of an energy drink and alcohol leads to cognitive deficits and motivational changes

The combination of alcohol with an energy drink (ED) is believed to contribute to risky alcohol-drinking behaviors, such as binge drinking. However, the long-term effects on cognition and reward function that are caused by the repeated binge-like ingestion of alcohol and EDs are still poorly known. The present study examined the effects of a history of repeated exposure to alcohol and/or an ED on short-term memory and alcohol-seeking behavior. Male Wistar rats were given daily intragastric administration of alcohol (3.4g/kg) combined or not with an ED (10.71ml/kg) for 6 consecutive days. The rats were tested for locomotion 15min after the first intragastric treatment. Short-term memory was assessed in the novel object recognition and social discrimination tests 2-3days after the last intragastric administration. The rewarding effect of alcohol was tested 1-3weeks following the last intragastric administration in a conditioned place preference paradigm. The acute binge-like ingestion of alcohol decreased locomotor activity, whereas the combination of alcohol and an ED increased locomotion in the first minutes of assessment. Alcohol exposure produced cognitive deficits in both the object recognition and social discrimination tests, and adding the ED to the alcohol solution did not modify these effects. The combination of alcohol and the ED increased alcohol-induced conditioned place preference. Thus, a history of binge-like alcohol exposure combined with the ED caused subsequent cognitive deficits and increased alcohol seeking behavior, and such behavioral effects might contribute to the progression to alcohol abuse disorders.

A comprehensive review of the effects of mixing caffeinated energy drinks with alcohol

BACKGROUND

In response to concern about whether mixing caffeinated energy drinks with alcohol (AED) increases alcohol consumption and related harm, and the role of industry in this debate, we conducted a comprehensive review of the research evidence on the effects of AED and documented industry involvement in this research.

METHOD

A systematic review of 6 databases. Studies must have examined the effect of consuming alcohol with energy drinks (ED) or caffeine on alcohol-related outcomes.

RESULTS

62 studies were identified; 29 were experiments, 9 had industry ties (8 with Red Bull GmbH). Young adults who consumed AED drank more alcohol and experienced more alcohol-related harm than other drinkers. There was insufficient evidence to conclude that AED led to increased alcohol consumption or altered the nature of alcohol-related harm. However, AED consumers reported that AED increased stimulation and alertness, offset fatigue from drinking, and facilitated drinking. Experimental research also found that combining ED or caffeine with alcohol increased stimulation and alertness, offset alcohol-related fatigue and increased the desire to keep drinking. It did not change BAC, perceived intoxication, perceived impairment and it did not reverse alcohol-induced impairment on simple psychomotor tasks. Combining ED/caffeine with alcohol reduced alcohol-induced impairment on some but not all aspects of complex tasks. Although few in number, studies with industry ties presented contrary evidence.

CONCLUSION

A growing body of evidence suggests that AED may facilitate drinking and related harms via its effects on intoxication but a causal link needs to be confirmed. The influence of industry involvement in this area of research needs to be monitored.

Alterations in ethanol-induced accumbal transmission after acute and long-term zinc depletion

Alcoholism is subject to extensive research, but the role of changes in metabolism caused by alcohol consumption has been poorly investigated. Zinc (Zn(2+) ) deficiency is a common metabolic aberration among alcoholics and Zn(2+) influences the function of ligand-gated ion channels, known pharmacological targets of ethanol (EtOH). Here, we investigate whether manipulation of extracellular levels of Zn(2+) modulates EtOH-induced increases of dopamine (DA) output, as measured by in vivo microdialysis in the rat, and whether voluntary EtOH consumption is altered by Zn(2+) deficiency. Our findings show that the Zn(2+) -chelating agent tricine slowly raises DA levels when perfused in the nucleus accumbens (nAc), whereas the more potent Zn(2+) chelator TPEN reduces DA levels. We also show that pre-treatment with either tricine or TPEN blocks the EtOH-induced DA elevation. Chronic Zn(2+) deficiency induced by a Zn(2+) -free diet did not affect EtOH consumption, but excitatory transmission, assessed by striatal field-potential recordings in the nAc shell, was significantly modulated both by Zn(2+) -free diet and by EtOH consumption, as compared with the EtOH naïve controls. The present study indicates that Zn(2+) influences EtOH's interaction with the brain reward system, possibly by interfering with glycine receptor and GABAA receptor function. This also implies that Zn(2+) deficiency among alcoholics may be important to correct in order to normalize important aspects of brain function.

Increase in nucleus accumbens dopamine levels following local ethanol administration is not mediated by acetaldehyde

AIMS

Ethanol (EtOH) activates the mesolimbic dopamine system and increases dopamine levels in the nucleus accumbens (nAc), which is believed to underlie the rewarding effects of alcohol. Accumulating evidence now implicates that acetaldehyde, the first metabolite of EtOH, may play an important role in mediating some of the rewarding properties of its parent compound. The objective of this study was to investigate if the increase in accumbal dopamine output observed when administering EtOH locally in the nAc by reversed microdialysis is mediated by acetaldehyde.

METHODS

Acetaldehyde (1, 10, 100 or 200 µM) or EtOH (300 mM) was administered via reversed microdialysis in the nAc of male Wistar rats. In a separate experiment, animals were administered EtOH (300 mM) in the nAc, following pre-treatment with the acetaldehyde-sequestering agent d-penicillamine (50 mg/kg injected intraperitoneally 60 min before drug challenge). Microdialysates from the nAc were collected every 20 min and dopamine content was quantified using high-performance liquid chromatography.

RESULTS

Acetaldehyde administered in the nAc did not influence accumbal dopamine levels at any of the concentrations applied, whereas EtOH induced a significant increase in accumbal dopamine. The dopamine-elevating properties of EtOH were not attenuated by pre-treatment with d-penicillamine.

CONCLUSION

The current results show that EtOH administered in the nAc induces an elevation in accumbal dopamine levels, which is not mimicked by acetaldehyde alone, nor is it influenced by acetaldehyde sequestering. This would suggest that the increase in accumbal dopamine following nAc EtOH administration is not mediated by acetaldehyde.

Locus of onset and subpopulation specificity of in vivo ethanol effect in the reciprocal ventral tegmental area-nucleus accumbens circuit

Elevated levels of dopamine in the nucleus accumbens (nAc) as a consequence of increased activation of dopaminergic neurons in the VTA are associated with the reinforcing properties of ethanol consumption, but whether the initiation of drug-response is connected to a direct activation of dopaminergic cell bodies in the VTA region or involves GABAergic neurons in VTA and/or the nAc is unclear. To this end, neuronal firing rate was recorded simultaneously in the VTA and nAc of awake and freely-moving C57BL6/J mice receiving an intraperitoneal (i.p.) injection of ethanol (0.75, 2.0, or 3.5g/kg) or saline. Recorded units were classified based on electrophysiological properties and the pharmacological response to the dopamine D2 receptor agonist quinpirole into putative dopaminergic (DA) neurons and fast-spiking or slow-spiking putative GABAergic neurons. Our data show that ethanol acutely decreases the firing frequency of GABAergic units in both the VTA and nAc in a dose-dependent manner, and enhances the firing rate of DA neurons. In order to define the onset of ethanol-induced rate changes normalized population vectors describing the collective firing rate of classes of neurons over time were generated and compared with saline-treatment. Population vectors of DA neurons in the VTA and GABAergic units in the nAc showed a significant deviation from the saline condition within 40s following ethanol-administration (2.0g/kg), while inhibition of GABAergic units in the VTA had a slower onset. In conclusion, the data presented here suggests that EtOH exerts a direct effect on DA firing frequency, but that decreased firing frequency of inhibitory neurons in VTA and nAc contributes to the dopamine-elevating properties of ethanol.

The involvement of accumbal glycine receptors in the dopamine-elevating effects of addictive drugs

The ability of drugs of abuse to increase mesolimbic levels of dopamine is a characteristic associated with their rewarding effects. Exactly how these effects are produced by different substances is not as well characterised. Our previous work in rats has demonstrated that accumbal glycine receptors (GlyRs) are involved in mediating the dopamine-activating effects of ethanol, and in modulating ethanol intake. In this study the investigation of GlyR involvement was extended to include several different drugs of abuse. By using microdialysis and electrophysiology we compared effects of addictive drugs, with and without the GlyR antagonist strychnine, on dopamine levels and neurotransmission in nucleus accumbens. The dopamine-increasing effect of systemic ethanol and the drug-induced change in neurotransmission in vitro, as measured by microdialysis and field potential recordings, were dependent on GlyRs in nAc. Accumbal GlyRs were also involved in the actions of tetrahydrocannabinol and nicotine, but not in those of cocaine or morphine. These data indicate that accumbal GlyRs play a key role in ethanol-induced dopamine activation and contribute also to that of cannabinoids and nicotine.

Physiological concentrations of zinc reduce taurine-activated GlyR responses to drugs of abuse

Taurine is an endogenous ligand acting on glycine receptors in many brain regions, including the hippocampus, prefrontal cortex, and nucleus accumbens (nAcc). These areas also contain low concentrations of zinc, which is known to potentiate glycine receptor responses. Despite an increasing awareness of the role of the glycine receptor in the rewarding properties of drugs of abuse, the possible interactions of these compounds with zinc has not been thoroughly addressed. Two-electrode voltage-clamp electrophysiological experiments were performed on α1, α2 α1β and α2β glycine receptors expressed in Xenopus laevis oocytes. The effects of zinc alone, and zinc in combination with other positive modulators on the glycine receptor, were investigated when activated by the full agonist glycine versus the partial agonist taurine. Low concentrations of zinc enhanced responses of maximally-effective concentrations of taurine but not glycine. Likewise, chelation of zinc from buffers decreased responses of taurine- but not glycine-mediated currents. Potentiating concentrations of zinc decreased ethanol, isoflurane, and toluene enhancement of maximal taurine currents with no effects on maximal glycine currents. Our findings suggest that the concurrence of high concentrations of taurine and low concentrations of zinc attenuate the effects of additional modulators on the glycine receptor, and that these conditions are more representative of in vivo functioning than effects seen when these modulators are applied in isolation.

Region-specific depression of striatal activity in Wistar rat by modest ethanol consumption over a ten-month period

The nucleus accumbens (nAc) is the primary target for the mesolimbic dopamine system and a key brain region for the reinforcing effects displayed by drugs of abuse, including ethanol. During the transition from recreational to compulsive consumption of reinforcing drugs, however, the dorsal striatum seems to be recruited. Understanding how synaptic activity is altered in a sub-region specific manner in the striatum during the course of long-term drug consumption thus could be essential for understanding the long-lasting changes produced by addictive substances, including ethanol. Here we evaluated synaptic activity in the dorsolateral striatum (DLS) and ventral striatum (nucleus accumbens, nAc) of single-housed Wistar rats consuming water, or water and ethanol, for up to 10 months. Even though ethanol intake was moderate, it was sufficient to decrease input/output function in response to stimulation intensity in the DLS, while recorded population spike (PS) amplitudes in the nAc were unaffected. Striatal disinhibition induced by the GABAA receptor antagonist bicuculline had a slower onset in rats that had consumed ethanol for 2 months, and was significantly depressed in slices from rats that had consumed ethanol for 4 months. Bicuculline-induced disinhibition in the nAc, on the other hand, was not significantly altered by long-term ethanol intake. Changes in PS amplitude induced by taurine or the glycine receptor antagonist strychnine were not significantly altered by ethanol in any brain region. Even though input/output function was not significantly affected by age, there was a significant decline in antagonist-induced disinhibition in brain slices from aged rats. The data presented here suggest that even modest consumption of ethanol is sufficient to alter neurotransmission in the striatum, while synaptic activity appears to be relatively well-preserved in the nAc during the course of long-term ethanol consumption.

Ethanol-induced alterations of amino acids measured by in vivo microdialysis in rats: a meta-analysis

PURPOSE

In recent years in vivo microdialysis has become an important method in research studies investigating the alterations of neurotransmitters in the extracellular fluid of the brain. Based on the major involvement of glutamate and γ-aminobutyric acid (GABA) in mediating a variety of alcohol effects in the mammalian brain, numerous microdialysis studies have focused on the dynamical behavior of these systems in response to alcohol.

METHODS

Here we performed multiple meta-analyses on published datasets from the rat brain: (i) we studied basal extracellular concentrations of glutamate and GABA in brain regions that belong to a neurocircuitry involved in neuropsychiatric diseases, especially in alcoholism (Noori et al., Addict Biol 17:827-864, 2012); (ii) we examined the effect of acute ethanol administration on glutamate and GABA levels within this network and (iii) we studied alcohol withdrawal-induced alterations in glutamate and GABA levels within this neurocircuitry.

RESULTS

For extraction of basal concentrations of these neurotransmitters, datasets of 6932 rats were analyzed and the absolute basal glutamate and GABA levels were estimated for 18 different brain sites. In response to different doses of acute ethanol administration, datasets of 529 rats were analyzed and a non-linear dose response (glutamate and GABA release) relationship was observed in several brain sites. Specifically, glutamate in the nucleus accumbens shows a decreasing logarithmic dose response curve. Finally, regression analysis of 11 published reports employing brain microdialysis experiments in 104 alcohol-dependent rats reveals very consistent augmented extracellular glutamate and GABA levels in various brain sites that correlate with the intensity of the withdrawal response were identified.

CONCLUSIONS

In summary, our results provide standardized basal values for future experimental and in silico studies on neurotransmitter release in the rat brain and may be helpful to understand the effect of ethanol on neurotransmitter release. Furthermore, this study illustrates the benefit of meta-analyses using the generalization of a wide range of preclinical data.

Neurocircuitry for modeling drug effects

The identification and functional understanding of the neurocircuitry that mediates alcohol and drug effects that are relevant for the development of addictive behavior is a fundamental challenge in addiction research. Here we introduce an assumption-free construction of a neurocircuitry that mediates acute and chronic drug effects on neurotransmitter dynamics that is solely based on rodent neuroanatomy. Two types of data were considered for constructing the neurocircuitry: (1) information on the cytoarchitecture and neurochemical connectivity of each brain region of interest obtained from different neuroanatomical techniques; (2) information on the functional relevance of each region of interest with respect to alcohol and drug effects. We used mathematical data mining and hierarchical clustering methods to achieve the highest standards in the preprocessing of these data. Using this approach, a dynamical network of high molecular and spatial resolution containing 19 brain regions and seven neurotransmitter systems was obtained. Further graph theoretical analysis suggests that the neurocircuitry is connected and cannot be separated into further components. Our analysis also reveals the existence of a principal core subcircuit comprised of nine brain regions: the prefrontal cortex, insular cortex, nucleus accumbens, hypothalamus, amygdala, thalamus, substantia nigra, ventral tegmental area and raphe nuclei. Finally, by means of algebraic criteria for synchronizability of the neurocircuitry, the suitability for in silico modeling of acute and chronic drug effects is indicated. Indeed, we introduced as an example a dynamical system for modeling the effects of acute ethanol administration in rats and obtained an increase in dopamine release in the nucleus accumbens-a hallmark of drug reinforcement-to an extent similar to that seen in numerous microdialysis studies. We conclude that the present neurocircuitry provides a structural and dynamical framework for large-scale mathematical models and will help to predict chronic drug effects on brain function.

Behavioral effects of taurine pretreatment in zebrafish acutely exposed to ethanol

Taurine (TAU) is an amino sulfonic acid that plays protective roles against neurochemical impairments induced by ethanol (EtOH). Mounting evidence shows the applicability of zebrafish for evaluating locomotor parameters and anxiety-like behavioral phenotypes after EtOH exposure in a large scale manner. In this study, we assess the effects of TAU pretreatment on the behavior of zebrafish in the open tank after acute 1% EtOH (v/v) exposure (20 and 60 min of duration) and on brain alcohol contents. The exposure for 20 min exerted significant anxiolytic effects, which were prevented by 42, 150, and 400 mg/L TAU. Conversely, the 60-min condition induced depressant/sedative effects, in which the changes on vertical activity were associated to modifications on the exploratory profile. Although all TAU concentrations kept locomotor parameters at basal levels, 150 mg/L TAU, did not prevent the impairment on vertical activity of EtOH[60]. Despite the higher brain EtOH content detected in the 60-min exposure, 42, 150, and 400 mg/L TAU attenuated the increase of alcohol content in EtOH[60] group. In conclusion, our data suggest that both protocols of acute EtOH exposure induce significant changes in the spatio-temporal behavior of zebrafish and that TAU may exert a preventive role by antagonizing the effects induced by EtOH possibly due to its neuromodulatory role and also by decreasing brain EtOH levels. The hormetic dose-response of TAU on vertical exploration suggests a complex interaction between TAU and EtOH in the central nervous system.

¹H NMR-based metabonomics in brain nucleus accumbens and striatum following repeated cocaine treatment in rats

Studies have shown a few cerebral metabolites modified by cocaine in brain regions; however, endogenous metabolic profiling has been lacking. Ex vivo (1)H NMR (hydrogen-1 nuclear magnetic resonance) spectroscopy-based metabonomic approach coupled with partial least squares was applied to investigate the changes of cerebral metabolites in nucleus accumbens (NAc) and striatum of rats subjected to cocaine treatment. Our results showed that both single and repeated cocaine treatment can induce significant changes in a couple of cerebral metabolites. The increase of neurotransmitters glutamate and gamma-amino butyric acid (GABA) were observed in NAc and striatum from the rats repeatedly treated with cocaine. Creatine and taurine increased in NAc whereas taurine increased and creatine decreased in striatum after repeated cocaine treatment. Elevation of N-acetylaspartate in NAc and striatum and decrease of lactate in striatum were observed, which may reflect the mitochondria dysregulation caused by cocaine; moreover, alterations of choline, phosphocholine and glycerol in NAc and striatum could be related to membrane disruption. Moreover, groups of rats with and without conditioned place preference (CPP) apparatus are presenting difference in metabolites. Collectively, our results provide the first evidence of metabonomic profiling of NAc and striatum in response to cocaine, exhibiting a regionally-specific alteration patterns. We find that repeated cocaine administration leads to significant metabolite alterations, which are involved in neurotransmitter disturbance, oxidative stress, mitochondria dysregulation and membrane disruption in brain.

Positive allosteric modulators differentially affect full versus partial agonist activation of the glycine receptor

Taurine acts as a partial agonist at the glycine receptor (GlyR) in some brain regions such as the hippocampus, striatum, and nucleus accumbens. Ethanol, volatile anesthetics, and inhaled drugs of abuse are all known positive allosteric modulators of GlyRs, but their effects on taurine-activated GlyRs remain poorly understood, especially their effects on the high concentrations of taurine likely to be found after synaptic release. Two-electrode voltage-clamp electrophysiology in Xenopus laevis oocytes was used to compare the enhancing effects of ethanol, anesthetics, and inhalants on human homomeric α1-GlyR activated by saturating concentrations of glycine versus taurine. Allosteric modulators had negligible effects on glycine-activated GlyR while potentiating taurine-activated currents. In addition, inhaled anesthetics markedly enhanced desensitization rates of taurine- but not glycine-activated receptors. Our findings suggest that ethanol, volatile anesthetics, and inhalants differentially affect the time courses of synaptic events at GlyR, depending on whether the receptor is activated by a full or partial agonist.

Behavioral characterization of knockin mice with mutations M287L and Q266I in the glycine receptor α1 subunit

We used behavioral pharmacology to characterize heterozygous knockin mice with mutations (Q266I or M287L) in the α1 subunit of the glycine receptor (GlyR) (J Pharmacol Exp Ther 340:304-316, 2012). These mutations were designed to reduce (M287L) or eliminate (Q266I) ethanol potentiation of GlyR function. We asked which behavioral effects of ethanol would be reduced more in the Q266I mutant than the M287L and found rotarod ataxia to be the behavior that fulfilled this criterion. Compared with controls, the mutant mice also differed in ethanol consumption, ethanol-stimulated startle response, signs of acute physical dependence, and duration of loss of righting response produced by ethanol, butanol, ketamine, pentobarbital, and flurazepam. Some of these behavioral changes were mimicked in wild-type mice by acute injections of low, subconvulsive doses of strychnine. Both mutants showed increased acoustic startle response and increased sensitivity to strychnine seizures. Thus, in addition to reducing ethanol action on the GlyRs, these mutations reduced glycinergic inhibition, which may also alter sensitivity to GABAergic drugs.

Subregion-Specific Modulation of Excitatory Input and Dopaminergic Output in the Striatum by Tonically Activated Glycine and GABA(A) Receptors

The flow of cortical information through the basal ganglia is a complex spatiotemporal pattern of increased and decreased firing. The striatum is the biggest input nucleus to the basal ganglia and the aim of this study was to assess the role of inhibitory GABA_A and glycine receptors in regulating synaptic activity in the dorsolateral striatum (DLS) and ventral striatum (nucleus accumbens, nAc). Local field potential recordings from coronal brain slices of juvenile and adult Wistar rats showed that GABA_A receptors and strychnine-sensitive glycine receptors are tonically activated and inhibit excitatory input to the DLS and to the nAc. Strychnine-induced disinhibition of glutamatergic transmission was insensitive to the muscarinic receptor inhibitor scopolamine (10 μM), inhibited by the nicotinic acetylcholine receptor antagonist mecamylamine (10 μM) and blocked by GABA_A receptor inhibitors, suggesting that tonically activated glycine receptors depress excitatory input to the striatum through modulation of cholinergic and GABAergic neurotransmission. As an end-product example of striatal GABAergic output in vivo we measured dopamine release in the DLS and nAc by microdialysis in the awake and freely moving rat. Reversed dialysis of bicuculline (50 μM in perfusate) only increased extrasynaptic dopamine levels in the nAc, while strychnine administered locally (200 μM in perfusate) decreased dopamine output by 60% in both the DLS and nAc. Our data suggest that GABA_A and glycine receptors are tonically activated and modulate striatal transmission in a partially subregion-specific manner.

The mGluR5 antagonist MPEP elevates accumbal dopamine and glycine levels; interaction with strychnine-sensitive glycine receptors

Studies have indicated that the metabotropic glutamate receptor 5 (mGluR5) antagonist 6-methyl-2-(phenylethynyl)-pyridine (MPEP) decreases ethanol self-administration, and the same receptor type was also suggested to be involved in the mechanism of action of the anti-craving substance acamprosate. Our previous research suggested that glycine receptors (GlyRs) in the nucleus accumbens (nAc) play a major part in mediating the dopamine-elevating properties of ethanol and are highly involved in the ethanol intake-reducing effect of acamprosate. The aim of this study was to examine if modulation of nAc dopamine via mGluR5 antagonism or GlyR agonism is a linked or separated phenomena. The extracellular levels of dopamine as well as of the GlyR ligands, glycine, taurine and β-alanine were measured in the nAc by means of microdialysis after local perfusion of MPEP (100 or 500 µM) with or without pre-treatment with strychnine. MPEP increased dopamine levels, an effect that was blocked by pre-treatment with strychnine. In addition, the higher MPEP concentration increased glycine output, whereas no alterations of taurine or β-alanine were observed. These results indicate a relationship between the glutamatergic and glycinergic transmitter systems in regulating dopamine output, possibly via alteration of extracellular glycine levels. Taken together with our previous data demonstrating the importance of accumbal GlyRs both in ethanol-induced elevation of nAc dopamine and in ethanol consumption, it is plausible that the effects of MPEP treatment, on dopamine output and on ethanol intake, may be mediated via interaction with the same neuronal circuitry that previously has been demonstrated for ethanol, taurine and acamprosate.