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

Dissecting operant alcohol self-administration using saccharin-fading procedure

BACKGROUND

Although alcohol use disorder is a complex human pathology, the use of animal models represents an opportunity to study some aspects of this pathology. One of the most used paradigms to study the voluntary alcohol consumption in rodents is operant self-administration (OSA).

AIMS

In order to facilitate the performance of this paradigm, we aim to describe some critical steps of OSA under a saccharin-fading procedure.

MATERIAL & METHODS

We used 40 male Wistar rats to study the process of acquiring the operant response through a saccharin-fading procedure under a fixed ratio (FR1) schedule of reinforcement. Next, we analyze the alcohol introduction and concentration increase, the effect of an alcohol deprivation, and the analogy between this paradigm with the Drinking in the Dark-Multiple Scheduled Access paradigm.

RESULTS

During alcohol concentration increase, animals reduced their lever presses in accordance with the increase in alcohol concentration. On the contrary, the consumption measured in g·kg^-1 BW showed a great stability. The lever presses pattern within operant session changes with the introduction of different alcohol concentrations: at higher alcohol concentrations, animals tended to accumulate most of their presses in the initial period of the session.

DISCUSSION

We show the utility of fading with low concentrations of saccharin and the evolution of the operant response through the different concentrations of alcohol.

CONCLUSION

Taken together, our results aimed to dissect the acquisition and maintenance of OSA behavior as well as other related variables, to facilitate the understanding and performance of this paradigm.

Behavioral consequences of the downstream products of ethanol metabolism involved in alcohol use disorder

Research concerning Alcohol Use Disorder (AUD) has previously focused primarily on either the behavioral or chemical consequences experienced following ethanol intake, but these areas of research have rarely been considered in tandem. Compared with other drugs of abuse, ethanol has been shown to have a unique metabolic pathway once it enters the body, which leads to the formation of downstream metabolites which can go on to form biologically active products. These metabolites can mediate a variety of behavioral responses that are commonly observed with AUD, such as ethanol intake, reinforcement, and vulnerability to relapse. The following review considers the preclinical and chemical research implicating these downstream products in AUD and proposes a chemobehavioral model of AUD.

Effects of Cocaine Exposure on Astrocytic Glutamate Transporters and Relapse-Like Ethanol-Drinking Behavior in Male Alcohol-Preferring Rats

AIM

Glutamate has been considered as neurotransmitter that is critical in triggering relapse to drugs of abuse, including ethanol and cocaine. Extracellular glutamate concentrations are tightly regulated by several mechanisms, including reuptake through glutamate transporters. Glutamate transporter type 1 (GLT-1) is responsible for clearing the majority of extracellular glutamate. The astrocytic cystine/glutamate antiporter (xCT) regulates also glutamate homeostasis. In this study, we investigated the effects of cocaine exposure and ampicillin/sulbactam (AMP/SUL), a β-lactam antibiotic known to upregulate GLT-1 and xCT, on relapse-like ethanol intake and the expression of astrocytic glutamate transporters in mesocorticolimbic brain regions.

METHODS

Male alcohol-preferring (P) rats had free access to ethanol for 5 weeks. On Week 6, rats were exposed to either cocaine (20 mg/kg, i.p.) or saline for 12 consecutive days. Ethanol bottles were then removed for 7 days; during the last 5 days, either AMP/SUL (100 or 200 mg/kg, i.p.) or saline was administered to the P rats. Ethanol bottles were reintroduced, and ethanol intake was measured for 4 days.

RESULTS

Cocaine exposure induced an alcohol deprivation effect (ADE), which was associated in part by a decrease in the expression of GLT-1 and xCT in the nucleus accumbens (NAc) core. AMP/SUL (100 mg/kg, i.p.) attenuated the ADE, while AMP/SUL (200 mg/kg, i.p.) reduced ethanol intake during 4 days of ethanol re-exposure and upregulated GLT-1 and xCT expression in the NAc core, NAc shell and dorsomedial prefrontal cortex (dmPFC).

CONCLUSION

This study suggests that these astrocytic glutamate transporters might be considered as potential targets for the treatment of polysubstance abuse.

Pre-Clinical Studies with D-Penicillamine as a Novel Pharmacological Strategy to Treat Alcoholism: Updated Evidences

Ethanol, as other drugs of abuse, is able to activate the ventral tegmental area dopamine (VTA-DA) neurons leading to positively motivational alcohol-seeking behavior and use, and, ultimately to ethanol addiction. In the last decades, the involvement of brain-derived acetaldehyde (ACD) in the ethanol actions in the mesolimbic pathway has been widely demonstrated. Consistent published results have provided a mechanistic support to the use of ACD inactivating agents to block the motivational and reinforcing properties of ethanol. Hence, in the last years, several pre-clinical studies have been performed in order to analyze the effects of the sequestering ACD agents in the prevention of ethanol relapse-like drinking behavior as well as in chronic alcohol consumption. In this sense, one of the most explored interventions has been the administration of D-Penicillamine (DP). These pre-clinical studies, that we critically summarize in this article, are considered a critical step for the potential development of a novel pharmacotherapeutic strategy for alcohol addiction treatment that could improve the outcomes of current ones. Thus, on one hand, several experimental findings provide the rationale for using DP as a novel therapeutic intervention alone and/or in combination to prevent relapse into alcohol seeking and consumption. On the other hand, its effectiveness in reducing voluntary ethanol consumption in long-term experienced animals still remains unclear. Finally, this drug offers the additional advantage that has already been approved for use in humans, hence it could be easily implemented as a new therapeutic intervention for relapse prevention in alcoholism.

The Role of Acetaldehyde in the Increased Acceptance of Ethanol after Prenatal Ethanol Exposure

Recent studies show that acetaldehyde, the first metabolite in the oxidation of ethanol, can be responsible for both, the appetitive and the aversive effects produced by ethanol intoxication. More specifically, it has been hypothesized that acetaldehyde produced in the periphery by the liver is responsible for the aversive effects of ethanol, while the appetitive effects relate to the acetaldehyde produced centrally through the catalase system. On the other hand, from studies in our and other laboratories, it is known that ethanol exposure during the last gestational days (GD) consistently enhances the postnatal acceptance of ethanol when measured during early ontogeny in the rat. This increased liking of ethanol is a conditioned appetitive response acquired by the fetus by the association of ethanol’s flavor and an appetitive reinforcer. Although this reinforcer has not yet been fully identified, one possibility points to acetaldehyde produced centrally in the fetus as a likely candidate. This hypothesis is supported by data showing that very early in the rat’s ontogeny brain catalases are functional, while the liver’s enzymatic system is still immature. In this study, rat dams were administered on GD 17–20 with water or ethanol, together with an acetaldehyde-sequestering agent (D-penicillamine). The offspring’s responses to ethanol was then assessed at different postnatal stages with procedures adequate for each developmental stage: on day 1, using the “odor crawling locomotion test” to measure ethanol’s odor attractiveness; on day 5, in an operant conditioning procedure with ethanol as the reinforcer; and on day 14 in an ethanol intake test. Results show that the absence of acetaldehyde during prenatal ethanol exposure impeded the observation of the increased acceptance of ethanol at any age. This seems to confirm the crucial role of acetaldehyde as a reinforcer in the appetitive learning occurring during prenatal ethanol exposure.

From Ethanol to Salsolinol: Role of Ethanol Metabolites in the Effects of Ethanol

In spite of the global reputation of ethanol as the psychopharmacologically active ingredient of alcoholic drinks, the neurobiological basis of the central effects of ethanol still presents some dark sides due to a number of unanswered questions related to both its precise mechanism of action and its metabolism. Accordingly, ethanol represents the interesting example of a compound whose actions cannot be explained as simply due to the involvement of a single receptor/neurotransmitter, a scenario further complicated by the robust evidence that two main metabolites, acetaldehyde and salsolinol, exert many effects similar to those of their parent compound. The present review recapitulates, in a perspective manner, the major and most recent advances that in the last decades boosted a significant growth in the understanding on the role of ethanol metabolism, in particular, in the neurobiological basis of its central effects.