l-cysteine Prevents Ethanol-Induced Stimulation of Mesolimbic Dopamine Transmission

Effects of N-acetylcysteine treatment on ethanol's rewarding properties and dopaminergic alterations in mesocorticolimbic and nigrostriatal pathways

Recent reports have shown that N-acetylcysteine (N-AC) has beneficial effects in the treatment of cocaine and nicotine abuse. Considering the similar neurobiologic mechanisms involved in the development of addiction to different drugs, N-AC treatment could be useful in the treatment of ethanol abuse. The rewarding properties of the drugs of abuse plays an important role in the development of addiction and can be studied using the conditioned place preference (CPP) paradigm. Thus, to study the effects of N-AC treatment in the rewarding effects of ethanol, we investigated the effects of N-AC administration in the ethanol-induced CPP and neurochemical alterations within the mesocorticolimbic and the nigrostriatal dopaminergic pathways. Adult male Swiss mice were pretreated with N-AC (60 or 120 mg/kg intraperitoneal) and tested for the development, expression, or extinction of the ethanol-induced CPP. Another cohort of animals received N-AC (60 or 120 mg/kg intraperitoneal) 2-h before an acute administration of ethanol and had their brains removed for dopamine and its metabolites quantification in the mesocorticolimbic and nigrostriatal pathways. Pretreatment with N-AC (120 mg/kg) blocked the development of ethanol-induced CPP. On the other hand, N-AC at both doses did not alter the expression nor the extinction of ethanol-induced CPP. N-AC increased 3,4-dihydroxyphenylacetic acid content in the medial prefrontal cortex and dopaminergic turnover within the substantia nigra. Besides that, there was an increase in dopamine content in the nucleus accumbens of ethanol-treated animals. In summary, N-AC treatment blocked the development of ethanol CPP, without altering ethanol effects on dopaminergic neurotransmission.

The Role of Amino Acids in Neurotransmission and Fluorescent Tools for Their Detection

Neurotransmission between neurons, which can occur over the span of a few milliseconds, relies on the controlled release of small molecule neurotransmitters, many of which are amino acids. Fluorescence imaging provides the necessary speed to follow these events and has emerged as a powerful technique for investigating neurotransmission. In this review, we highlight some of the roles of the 20 canonical amino acids, GABA and β-alanine in neurotransmission. We also discuss available fluorescence-based probes for amino acids that have been shown to be compatible for live cell imaging, namely those based on synthetic dyes, nanostructures (quantum dots and nanotubes), and genetically encoded components. We aim to provide tool developers with information that may guide future engineering efforts and tool users with information regarding existing indicators to facilitate studies of amino acid dynamics.

N-acetylcysteine treatment blocks the development of ethanol-induced behavioural sensitization and related ΔFosB alterations

Ethanol addiction is a serious public health problem that still needs more effective pharmacological treatment. A key factor in the development and maintenance of this disease is the advent of neuroadaptations in the mesocorticolimbic brain pathway upon chronic ethanol abuse. In general, these neuroadaptations are maladaptive and affect numerous neurotransmitter systems and intracellular molecules. One of these molecules is ΔFosB, a transcription factor that is altered after chronic drug use. Behavioural sensitization is a useful model for the study of the neuroadaptations related to addiction. Recent works have shown a role for the imbalance of glutamatergic neurotransmission in the symptoms found in addicted people. In this sense, the treatment with N-acetylcysteine, a l-cysteine prodrug that acts by restoring extrasynaptic concentrations of glutamate through the activation of cystine-glutamate antiporter, has shown promising results in the treatment of addiction. Thus, an animal model of behavioural sensitization was used to evaluate the effects of N-acetylcysteine treatment in the behavioural and molecular alterations induced by chronic ethanol administration. Swiss mice were subject to 13 days of daily ethanol administration to induce behavioural sensitization. Two hours before each ethanol administration and locomotor activity evaluation, the animals received intraperitoneally N-acetylcysteine injections. Immediately after the last test session, their brains were removed for ΔFosB and cystine-glutamate antiporter quantification. It was found that N-acetylcysteine treatment blocked ethanol-induced behavioural sensitization, the increase of ΔFosB content in the prefrontal cortex, and its reduction in the nucleus accumbens. The results suggest a possible use of N-acetylcysteine in ethanol-related disorders.

Acetaldehyde sequestration by D-penicillamine prevents ethanol relapse-like drinking in rats: evidence from an operant self-administration paradigm

RATIONALE

Previous experiments in our laboratory have shown that D-penicillamine (DP) (acetaldehyde sequestering agent) is able to block the increase in ethanol consumption observed after a period of imposed deprivation (the so-called alcohol deprivation effect (ADE)), using a non-operant paradigm in Wistar rats.

OBJECTIVES

This study is aimed at investigating the robustness and reproducibility of our previous data using an operant paradigm, which is considered to be a valid and reliable model of human drug consumption, and the ADE, probably the most often used measure of ethanol relapse-drinking behaviour in rats.

METHODS

Male Wistar rats with a limited (30-min sessions), intermittent and extended background of ethanol operant self-administration were used. In order to evaluate the efficacy of several DP doses (6.25, 12.5 and 25 mg/kg i.p.) in preventing alcohol relapse, we set up a protocol based on the ADE. In a separate experiment, the effect of DP on spontaneous motor activity of rats was also tested.

RESULTS

A significant ADE was observed in animals treated with saline. DP treatment blocked the increase in ethanol responses following the imposed abstinence period. The higher dose suppressed the ADE and provoked a significant reduction in ethanol consumption with respect to the baseline conditions. Basal motor activity was not altered after DP treatment.

CONCLUSION

Our positive results with DP, using two different paradigms that evaluate relapse of ethanol drinking, will help to increase the positive predictive value of pre-clinical experiments and offer a solid base to inspire human studies with DP.

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.

Induction of brain cytochrome P450 2E1 boosts the locomotor-stimulating effects of ethanol in mice

In the central nervous system ethanol (EtOH) is metabolized into acetaldehyde by different enzymes. Brain catalase accounts for 60% of the total production of EtOH-derived acetaldehyde, whereas cerebral cytochrome P450 2E1 (CYP 2E1) produces 20% of this metabolite. Acetaldehyde formed by the activity of central catalase has been implicated in some of the neurobehavioral properties of EtOH, yet the contribution of CYP 2E1 to the pharmacological actions of this drug has not been investigated. Here we assessed the possible participation of CYP 2E1 in the behavioral effects of EtOH. Thus, we induced CYP 2E1 activity and expression by exposing mice to chronic acetone intake (1% v/v for 10 days) and examined its consequences on the stimulating and uncoordinating effects of EtOH (0-3.2 g/kg) injected intraperitoneally. Our data showed that 24 h after withdrawal of acetone brain expression and activity of CYP 2E1 was induced. Furthermore, the locomotion produced by EtOH was boosted over the same interval of time. Locomotor stimulation produced by amphetamine or tert-butanol was unchanged by previous treatment with acetone. EtOH-induced motor impairment as evaluated in a Rota-Rod apparatus was unaffected by the preceding exposure to acetone. These results indicate that cerebral CYP 2E1 activity could contribute to the locomotor-stimulating effects of EtOH, and therefore we suggest that centrally produced acetaldehyde might be a possible mediator of some EtOH-induced pharmacological effects.

Reduction in central H2O2 levels prevents voluntary ethanol intake in mice: a role for the brain catalase-H2O2 system in alcohol binge drinking

BACKGROUND

Hydrogen peroxide (H2 O2 ) is the cosubstrate used by the enzyme catalase to form Compound I (the catalase-H2 O2 system), which is the major pathway for the conversion of ethanol (EtOH) into acetaldehyde in the brain. This centrally formed acetaldehyde has been shown to be involved in some of the psychopharmacological effects induced by EtOH in rodents, including voluntary alcohol intake. It has been observed that different levels of this enzyme in the central nervous system (CNS) result in variations in the amount of EtOH consumed. This has been interpreted to mean that the brain catalase-H2 O2 system, by determining EtOH metabolism, mediates alcohol self-administration. To date, however, the role of H2 O2 in voluntary EtOH drinking has not been investigated.

METHODS

In the present study, we explored the consequence of a reduction in cerebral H2 O2 levels in volitional EtOH ingestion. With this end in mind, we injected mice of the C57BL/6J strain intraperitoneally with the H2 O2 scavengers alpha-lipoic acid (LA; 0 to 50 mg/kg) or ebselen (Ebs; 0 to 25 mg/kg) 15 or 60 minutes, respectively, prior to offering them an EtOH (10%) solution following a drinking-in-the-dark procedure. The same procedure was followed to assess the selectivity of these compounds in altering EtOH intake by presenting mice with a (0.1%) solution of saccharin. In addition, we indirectly tested the ability of LA and Ebs to reduce brain H2 O2 availability.

RESULTS

The results showed that both LA and Ebs dose-dependently reduced voluntary EtOH intake, without altering saccharin consumption. Moreover, we demonstrated that these treatments decreased the central H2 O2 levels available to catalase.

CONCLUSIONS

Therefore, we propose that the amount of H2 O2 present in the CNS, by determining brain acetaldehyde formation by the catalase-H2 O2 system, could be a factor that determines an animal's propensity to consume EtOH.

Efficacy of D-penicillamine, a sequestering acetaldehyde agent, in the prevention of alcohol relapse-like drinking in rats

RATIONALE

Nowadays, very few approved anti-relapse treatments for alcoholism exist, and their overall efficacy can be considered moderate. An exciting rationale drug development opportunity for the treatment of chronic alcoholism is the use of acetaldehyde sequestering agents. Although these compounds are able to attenuate or prevent most of the behavioral and neurochemical effects of ethanol, the efficacy of acetaldehyde sequestration, by using agents such as D-penicillamine (DP), in relapse prevention has not been studied yet.

OBJECTIVES

The aim of this study was to analyze the effects of DP treatment on the alcohol deprivation effect (ADE) in long-term ethanol-experienced rats as a model of relapse behavior and measure drug plasma and brain levels during treatment.

METHODS

Rats were subcutaneously implanted with mini-osmotic pumps delivering 0, 0.25, or 1 mg/h of DP during 1 week. The efficacy to prevent ADE was determined. DP plasma and brain levels achieved during its subcutaneous administration were measured. In a second experiment, animals received bilateral infusions of 0 or 1.5 μg/h of DP directly into pVTA, and the appearance of ADE was evaluated.

RESULTS

One milligram per hour, but not 0.25 mg/h, DP infusion prevented ADE and reduced the total ethanol preference in animals. DP plasma concentrations associated with ADE suppression were around 3-4 μg/ml, and brain DP levels in these conditions were about 2-3 % of those found in plasma. Intra-pVTA DP administration also suppressed ADE.

CONCLUSION

DP is able to prevent alcohol-relapse-like drinking in rats suggesting that this drug may be a useful new tool in the management of relapse in alcohol-dependent patients.

Involvement of the endogenous opioid system in the psychopharmacological actions of ethanol: the role of acetaldehyde

Significant evidence implicates the endogenous opioid system (EOS) (opioid peptides and receptors) in the mechanisms underlying the psychopharmacological effects of ethanol. Ethanol modulates opioidergic signaling and function at different levels, including biosynthesis, release, and degradation of opioid peptides, as well as binding of endogenous ligands to opioid receptors. The role of β-endorphin and µ-opioid receptors (OR) have been suggested to be of particular importance in mediating some of the behavioral effects of ethanol, including psychomotor stimulation and sensitization, consumption and conditioned place preference (CPP). Ethanol increases the release of β-endorphin from the hypothalamic arcuate nucleus (NArc), which can modulate activity of other neurotransmitter systems such as mesolimbic dopamine (DA). The precise mechanism by which ethanol induces a release of β-endorphin, thereby inducing behavioral responses, remains to be elucidated. The present review summarizes accumulative data suggesting that the first metabolite of ethanol, the psychoactive compound acetaldehyde, could participate in such mechanism. Two lines of research involving acetaldehyde are reviewed: (1) implications of the formation of acetaldehyde in brain areas such as the NArc, with high expression of ethanol metabolizing enzymes and presence of cell bodies of endorphinic neurons and (2) the formation of condensation products between DA and acetaldehyde such as salsolinol, which exerts its actions via OR.

Ethanol-derived acetaldehyde: pleasure and pain of alcohol mechanism of action

Acetaldehyde (ACD), the first metabolite of ethanol (EtOH), has been implicated in several actions of alcohol, including its reinforcing effects. Previously considered an aversive compound, ACD was useful in alcoholic’s pharmacological treatment aimed at discouraging alcohol drinking. However, it has recently been shown that EtOH-derived ACD is necessary for EtOH-induced place preference and self-administration, thereby suggesting a possible involvement of ACD in EtOH motivational properties. In addition, EtOH-stimulating properties on DA neurons are prevented by pharmacological blockade of local catalase H₂O₂ system, the main metabolic step for biotransformation of EtOH into ACD within the central nervous system. It was further shown that pretreatment with thiol compounds, like L-Cysteine or D-Penicillamine, reduced EtOH and ACD-induced motivational effects, in fact preventing self-administration of both EtOH and ACD, thus suggesting a possible role for ACD as a biomarker useful in evaluating potential innovative treatments of alcohol abuse. These findings suggest a key role of ACD in the EtOH reinforcing effects. In the present paper we review the role of EtOH-derived ACD in the reinforcing effects of EtOH and the possibility that ACD may serve as a therapeutically targetable biomarker in the search for novel treatments in alcohol abuse and alcoholism.

Behavioral and biochemical evidence of the role of acetaldehyde in the motivational effects of ethanol

Since Chevens' report, in the early 50's that his patients under treatment with the aldehyde dehydrogenase inhibitor, antabuse, could experience beneficial effects when drinking small volumes of alcoholic beverages, the role of acetaldehyde (ACD) in the effects of ethanol has been thoroughly investigated on pre-clinical grounds. Thus, after more than 25 years of intense research, a large number of studies have been published on the motivational properties of ACD itself as well as on the role that ethanol-derived ACD plays in the effects of ethanol. Accordingly, in particular with respect to the motivational properties of ethanol, these studies were developed following two main strategies: on one hand, were aimed to challenge the suggestion that also ACD may exert motivational properties on its own, while, on the other, with the aid of enzymatic manipulations or ACD inactivation, were aimed to test the hypothesis that ethanol-derived ACD might have a role in ethanol motivational effects. Furthermore, recent evidence significantly contributed to highlight, as possible mechanisms of action of ACD, its ability to commit either dopaminergic and opioidergic transmission as well as to activate the Extracellular signal Regulated Kinase cascade transduction pathway in reward-related brain structures. In conclusion, and despite the observation that ACD seems also to have inherited the elusive nature of its parent compound, the behavioral and biochemical evidence reviewed points to ACD as a neuroactive molecule able, on its own and as ethanol metabolite, to exert motivational effects.

c-Fos immunoreactivity in prefrontal, basal ganglia and limbic areas of the rat brain after central and peripheral administration of ethanol and its metabolite acetaldehyde

Considerable evidence indicates that the metabolite of ethanol (EtOH), acetaldehyde, is biologically active. Acetaldehyde can be formed from EtOH peripherally mainly by alcohol dehydrogenase (ADH), and also centrally by catalase. EtOH and acetaldehyde show differences in their behavioral effects depending upon the route of administration. In terms of their effects on motor activity and motivated behaviors, when administered peripherally acetaldehyde tends to be more potent than EtOH but shows very similar potency administered centrally. Since dopamine (DA) rich areas have an important role in regulating both motor activity and motivation, the present studies were undertaken to compare the effects of central (intraventricular, ICV) and peripheral (intraperitoneal, IP) administration of EtOH and acetaldehyde on a cellular marker of brain activity, c-Fos immunoreactivity, in DA innervated areas. Male Sprague-Dawley rats received an IP injection of vehicle, EtOH (0.5 or 2.5 g/kg) or acetaldehyde (0.1 or 0.5 g/kg) or an ICV injection of vehicle, EtOH or acetaldehyde (2.8 or 14.0 μmoles). IP administration of EtOH minimally induced c-Fos in some regions of the prefrontal cortex and basal ganglia, mainly at the low dose (0.5 g/kg), while IP acetaldehyde induced c-Fos in virtually all the structures studied at both doses. Acetaldehyde administered centrally increased c-Fos in all areas studied, a pattern that was very similar to EtOH. Thus, IP administered acetaldehyde was more efficacious than EtOH at inducing c-Fos expression. However, the general pattern of c-Fos induction promoted by ICV EtOH and acetaldehyde was similar. These results are consistent with the pattern observed in behavioral studies in which both substances produced the same magnitude of effect when injected centrally, and produced differences in potency after peripheral administration.

Effect of (L)-cysteine on acetaldehyde self-administration

Acetaldehyde (ACD), the first metabolite of ethanol, has been implicated in several behavioural actions of alcohol, including its reinforcing effects. Recently, we reported that l-cysteine, a sequestrating agent of ACD, reduced oral ethanol self-administration and that ACD was orally self-administered. This study examined the effects of l-cysteine pre-treatment during the acquisition and maintenance phases of ACD (0.2%) self-administration as well as on the deprivation effect after ACD extinction and on a progressive ratio (PR) schedule of reinforcement. In a separate PR schedule of reinforcement, the effect of l-cysteine was assessed on the break-point produced by ethanol (10%). Furthermore, we tested the effect of l-cysteine on saccharin (0.2%) reinforcement. Wistar rats were trained to self-administer ACD by nose poking on a fixed ratio (FR1) schedule in 30-min daily sessions. Responses on an active nose-poke caused delivery of ACD solution, whereas responses on an inactive nose-poke had no consequences. l-cysteine reduced the acquisition (40 mg/kg), the maintenance and the deprivation effect (100 mg/kg) of ACD self-administration. Furthermore, at the same dose, l-cysteine (120 mg/kg) decreased both ACD and ethanol break point. In addition, l-cysteine was unable to suppress the different responses for saccharin, suggesting that its effect did not relate to an unspecific decrease in a general motivational state. Compared to saline, l-cysteine did not modify responses on inactive nose-pokes, suggesting an absence of a non-specific behavioural activation. Taken together, these results could support the hypotheses that ACD possesses reinforcing properties and l-cysteine reduces motivation to self-administer ACD.

Effects of L-cysteine on reinstatement of ethanol-seeking behavior and on reinstatement-elicited extracellular signal-regulated kinase phosphorylation in the rat nucleus accumbens shell

BACKGROUND

Alcoholism is a neuroadaptive disorder, and the understanding of the mechanisms of the high rates of relapse, which characterize it, represents one of the most demanding challenges in alcoholism and addiction research. The extracellular signal-regulated kinase (ERK) is an intracellular kinase, critical for neuroplasticity in the adult brain that is suggested to play a fundamental role in the molecular mechanisms underlying drug addiction and relapse. We previously observed that a nonessential amino acid, L-cysteine, significantly decreases oral ethanol (EtOH) self-administration, reinstatement of EtOH-drinking behavior, and EtOH self-administration break point.

METHODS

Here, we tested whether L-cysteine can affect the ability of EtOH priming to induce reinstatement of EtOH-seeking behavior. In addition, we determined the ability of EtOH priming to induce ERK phosphorylation as well as the ability of L-cysteine to affect reinstatement-elicited ERK activation. To these purposes, Wistar rats were trained to nose-poke for a 10% v/v EtOH solution. After stable drug-taking behavior was obtained, nose-poking for EtOH was extinguished, and reinstatement of drug seeking, as well as reinstatement-elicited pERK, was determined after an oral, noncontingent, priming of EtOH (0.08 g/kg). Rats were pretreated with either saline or L-cysteine (80 to 120 mg/kg) 30 minutes before testing for reinstatement.

RESULTS

The findings of this study confirm that the noncontingent delivery of a nonpharmacologically active dose of EtOH to rats, whose previous self-administration behavior had been extinguished, results in significant reinstatement into EtOH-seeking behavior. In addition, the results indicate that reinstatement selectively activates ERK phosphorylation in the shell of the nucleus accumbens (Acb) and that pretreatment with L-cysteine reduces either reinstatement of EtOH seeking and reinstatement-elicited pERK in the AcbSh.

CONCLUSIONS

Altogether, these results indicate that L-cysteine could be an effective pharmacological agent for the prevention of behavioral and molecular correlates of EtOH-primed reinstatement of EtOH seeking and that the shell of the Acb represents a critical neural substrate for priming-elicited reinstatement mechanisms involving ERK phosphorylation.

Effect of opioid receptor blockade on acetaldehyde self-administration and ERK phosphorylation in the rat nucleus accumbens

We have previously shown that acetaldehyde (ACD), the first metabolite of ethanol, regulates its motivational properties and possesses reinforcing effects by itself. A large and still growing body of evidence indicates that the endogenous opioidergic system plays a critical role in the motivational effects of ethanol and suggests a role for extracellular signal-regulated kinase (ERK) in these effects of both ethanol and ACD. The present study was undertaken to examine if opioid-mediated mechanisms are involved in the reinforcing properties of ACD and in ACD-elicited ERK activation. To this end, Wistar rats were trained to orally self-administer ACD (0.2%) by nose poking. Responses on active nose poke caused delivery of ACD solution, whereas responses on inactive nose poke had no consequences. The effect of pretreatment with a nonselective opioid receptor antagonist, naltrexone (NTX), was evaluated during (1) maintenance of ACD self-administration, (2) deprivation effect after ACD extinction, and (3) ACD self-administration under a progressive-ratio schedule of reinforcement. Additionally, we tested the effect of NTX on saccharin (0.05%) reinforcement, as assessed by oral self-administration, and on ACD-elicited ERK phosphorylation in the nucleus accumbens (Acb), as assessed by immunohistochemistry. Finally, we examined the effect of a μ(1)-selective opioid receptor antagonist, naloxonazine (NLZ), on the maintenance phase of ACD and saccharin self-administration. The results indicate that NTX (0.4-0.8mg/kg) reduced the maintenance, the deprivation effect, and the break points of ACD self-administration without suppressing saccharin self-administration. Moreover, NTX decreased ACD-elicited ERK activation in the Acb shell and core. NLZ (10-15mg/kg) reduced the maintenance phase of ACD self-administration without interfering with saccharin self-administration, whereas both NTX and NLZ failed to modify responses on inactive nose poke indicating the lack of a nonspecific behavioral activation. Overall, these results indicate that the opioid system is implicated in the reinforcing properties of ACD and suggest an involvement of ERK. The finding that NTX and NLZ reduce ACD but not saccharin self-administration indicates that these effects are specific to ACD.