• Energy Research

Acetaldehyde (ACD) has been postulated to mediate some of the neurobehavioral effects of ethanol (EtOH). In this study we sought to evaluate whether the stimulatory effects of EtOH on mesolimbic dopamine (DA) transmission are affected by the administration of ACD-sequestering agent D-penicillamine (Dp). To this end we studied the effect of EtOH and ACD in the rat mesoaccumbens pathway by in vivo microdialysis in the nucleus accumbens shell (NAccs), and by single cell extracellular recordings from antidromically identified mesoaccumbens DA neurons in the ventral tegmental area (VTA). Both EtOH (1g/kg) and ACD (20mg/kg) administration increased DA levels in the NAccs and increased the activity of mesoaccumbens DA neurons. Pretreatment with Dp (50mg/kg i.p. 1h before drug challenge) prevented both EtOH- and ACD-induced stimulation of the DA mesolimbic system without affecting morphine stimulatory actions. These observations add further support to the notion that EtOH-derived ACD stimulates the mesolimbic DA system and is essential in EtOH-induced stimulation of the DA mesoaccumbens system. We conclude that modulation of ACD bioavailability may influence the addictive profile of EtOH by decreasing its psychotropic effects and possibly leading the way to new pharmacological treatments of alcoholism.

BackgroundAlcoholism 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.MethodsHere, 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.ResultsThe 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.ConclusionsAltogether, 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.

Acetaldehyde (ACD), the first metabolite of ethanol (EtOH) appears to be involved in many of the psychoactive effects of its parent compound, including EtOH-induced activation of the mesolimbic dopamine (DA) system, thereby suggesting that ACD may participate in EtOH motivational properties. l-cysteine (Lcys), a thiol compound sequestering ACD, is able to prevent the behavioral effect of EtOH and ACD. Here we show that the stimulatory effect of both EtOH and ACD on the mesolimbic DA system is prevented by Lcys pretreatment.Male Wistar rats were implanted with a microdialysis probe in the nucleus accumbens shell (NAccs), and pretreated intraperitoneally with Lcys (30 mg/kg) before intragastric administration of EtOH (1 g/kg) or ACD (20 mg/kg) or before intraperitoneal administration of morphine (2.5 mg/kg). Pretreatment with Lcys prevented both EtOH and ACD-induced DA release in the NAccs without influencing morphine-induced DA release, suggesting that Lcys specifically affects EtOH-induced DA release possibly through ACD sequestering.Our results underscore the role of ACD on EtOH-induced stimulation of DA mesoaccumbens system and support the notion that thiol compounds such as Lcys, by modulating EtOH-derived ACD bioavailability, would blunt EtOH rewarding properties.

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.

Ethyl alcohol (EtOH), the main psychoactive ingredient of alcoholic drinks, is widely considered responsible for alcohol abuse and alcoholism through its positive motivational properties, which depend, at least partially, on the activation of the mesolimbic dopaminergic system. On the other hand, acetaldehyde (ACD), EtOH's first metabolite, has been classically considered aversive and useful in the pharmacologic therapy of alcoholics. Here we show that EtOH‐derived ACD is necessary for EtOH‐induced place preference, a preclinical test with high predictive validity for reward liability. We also found that ACD is essential for EtOH‐increased microdialysate dopamine (DA) levels in the nucleus accumbens (NAcc), and that this effect is mimicked by ACD administration to the intraventral tegmental area (VTA). Furthermore,in vitro, ACD enhances VTA DA neuronal firing. Coherently, EtOH‐stimulating properties on DA neurons are prevented by pharmacologic blockade of local catalase: the main metabolic step for biotransformation of EtOH into ACD in the central nervous system. These results providein vivoandin vitroevidence for a key role of ACD in EtOH motivational properties and its activation of the mesolimbic DA system. Additionally, these observations suggest that ACD, by increasing VTA DA neuronal activity, would oppose its well‐known peripherally originating aversive properties. These findings could help in devising new effective pharmacologic therapies in alcoholism.

Background:  Acetaldehyde (ACD), the first metabolite of ethanol (EtOH), is produced peripherally by gastric and hepatic alcohol dehydrogenase (ADH) and centrally by brain catalase. In spite of the aversive properties classically ascribed to ACD, it has recently been suggested that ACD might mediate some of the motivational effects of EtOH. Accordingly, the relative role of ACD in the positive motivational properties of EtOH ingested is increasingly becoming the matter of debate. Thus, we studied the ability of intragastrically administered EtOH, ACD and EtOH‐derived ACD to induce conditioned place preference (cpp) in rats.Methods:  Wistar rats were pretreated intraperitoneally with saline, the peripheral competitive inhibitor of ADH, 4‐methylpyrazole (4‐MP, 22.5, 45 or 67.5 mg/kg) or with the selective ACD‐sequestrating agent, d‐penicillamine (DP, 25 or 50 mg/kg), before the intragastric administration of saline, EtOH (0.5, 1 or 2 g/kg) or ACD (10, 20, or 40 mg/kg). The specificity of 4‐MP and DP effects was addressed using morphine‐induced cpp (2.5 mg/kg).Results:  Both, EtOH and ACD dose‐dependently induced cpp; further, while EtOH‐induced cpp was prevented by the administration of 4‐MP and by DP, ACD‐induced cpp was unaltered by 4‐MP administration and prevented by DP. Both pretreatments did not interfere with morphine‐induced cpp indicating that 4‐MP and DP specifically modulate the motivational properties of EtOH and ACD.Conclusion:  The ability of 4‐MP and DP to decrease EtOH‐induced cpp suggests that a reduction of ACD levels is crucial in depriving EtOH from its motivational properties as indexed by the cpp procedure. In addition, this conclusion is supported by the inefficacy of 4‐MP in preventing ACD‐induced cpp, and by its blockade observed after administration of the selective ACD sequestrating agent DP. The present results underscore the role of EtOH‐derived ACD in EtOH‐induced motivational properties as well as its abuse liability.

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.