Enzymatic production of acetaldehyde from ethanol in rat brain tissue

Time-course concentration of ethanol, acetaldehyde and acetate in rat brain dialysate following alcohol self-administration

The unclear mechanisms of ethanol metabolism in the brain highlight the need for a deeper understanding of its metabolic pathways. This study used in vivo microdialysis to simultaneously sample ethanol and its metabolites, acetaldehyde and acetate, in the rat striatum following self-administration of ethanol, emphasizing the natural oral exposure route. To enhance the self-administration, rats underwent two-bottle-choice and limited access training. Dialysate samples, collected every 10 minutes for 2.5 hours, were analyzed using gas chromatography with flame ionization detection (GC-FID). The measured time courses of dialysate concentrations of ethanol, acetaldehyde, and acetate provided insights into dynamics of ethanol metabolism. Notably, in a subject with low ethanol consumption (0.29 g/kg), the concentration of acetaldehyde remained below the limit of detection throughout the experiment. However, the acetate concentration was clearly increased after ethanol consumption in this subject and was comparable to that of other rats with higher ethanol consumption. Compared with focusing only on peak values in the time-courses of concentrations of ethanol and its metabolites, calculating areas under curves provided better models of the relationships between ethanol intake and individual ethanol metabolites, as indicated by the r-square values for the linear regressions. This approach of using the area under the curve accounts for both the amplitude and duration of the concentration profiles, reducing the impact of variations in individual drinking patterns. In vivo microdialysis enables concurrent sampling of brain metabolites during oral ethanol administration, contributing insights into metabolite dynamics. To our knowledge, this paper is the first to report measurement of all three analytes in the brain following self-administration of ethanol. Future studies will explore regional variations and dynamics post-ethanol dependence, further advancing our understanding of ethanol metabolism in the brain.

Learning experiences comprising central ethanol exposure in rat neonates: Impact upon respiratory plasticity and the activity of brain catalase

Fetal ethanol exposure represents a risk factor for sudden infant death syndrome, and the respiratory effects of fetal ethanol exposure promote hypoxic ischemic consequences. This study analyzes central ethanol's effects upon breathing plasticity during an ontogenetic stage equivalent to the human third gestational trimester. Ethanol's unconditioned breathing effects and their intervention in learning processes were examined. Since central ethanol is primarily metabolized via the catalase system, we also examined the effects of early history with the drug upon this system. During postnatal days 3, 5, and 7 (PDs 3-7), pups were intracisternally administered with vehicle or ethanol (300 mg%). They were tested in a plethysmograph scented or not scented with ethanol odor. The state of intoxication attenuated the onset of apneas, a phenomenon that is suggestive of ethanol's anxiolytic effects given the state of arousal caused by the novel environment and the stress of ethanol administration. At PD9, pups were evaluated when sober under sequential air conditions (initial-normoxia, hypoxia, and recovery-normoxia), with or without the presence of ethanol odor. Initial apneic episodes increased when ethanol intoxication was previously associated with the odor. Pups then ingested ethanol, and brain catalase activity was determined. Pre-exposure to ethanol intoxication paired with the odor of the drug resulted in heightened enzymatic activity. Central ethanol exposure appears to exert antianxiety effects that attenuate apneic disruptions. However, during withdrawal, the cues associated with such effects elicit an opposite reaction. The activity of the catalase system was also dependent upon learning processes that involved the association of environmental stimuli and ethanol intoxication.

Brain ethanol-metabolizing enzymes are differentially expressed in lead-exposed animals after voluntary ethanol consumption: Pharmacological approaches

Developmentally-lead (Pb)-exposed rats showed an enhanced vulnerability to the stimulating and motivational effects of ethanol (EtOH). This is accompanied by differential activity of the brain EtOH-metabolizing enzymes catalase (CAT) and mitochondrial aldehyde dehydrogenase (ALDH2). Based on the theory that brain acetaldehyde accumulation is associated with the reinforcing properties of EtOH, this study sought to determine brain CAT and ALDH2 expression in limbic areas of control and Pb-exposed animals after voluntary EtOH intake. Thirty-five-day-old rats perinatally exposed to 220 ppm Pb were offered with water or increasing EtOH solutions (2-10% v/v) during 28 days until postnatal day (PND) 63. Once intake was stable, the animals were administered: 1) saline (SAL; test days 21-24 or 21-28, as corresponds), or 2) a CAT inhibitor: 3-amine 1, 2, 4-triazole (AT; 250 mg/kg intraperitoneally [i.p.], 5 h before the last eight EtOH intake sessions -test days 21-24 and 25-28), or 3) a CAT booster: 3-nitropropionic acid (3NPA; 20 mg/kg subcutaneously [s.c.], 45 min before the last four EtOH intake sessions -test days 25-28). Two additional groups were centrally-administered cyanamide (CY, an ALDH2 inhibitor, 0.3 mg i.c.v. immediately before the last four EtOH sessions, test days 25-28) or its corresponding vehicle (VEH). Lead exposure increased EtOH intake, an effect potentiated in both groups by 3NPA or CY pretreatments and reduced by AT, albeit selectivity in the Pb group. Catalase abundance in limbic areas parallels these observations in the Pb group, showing higher CAT expression in all areas after EtOH consumption respect to the controls, an effect prevented by AT administration. In contrast, ALDH2 expression was reduced in the Pb animals after EtOH intake, with CY potentiating this effect in all brain areas under study. Based on these results and on previous evidences, we suggest that Pb exposure promotes acetaldehyde accumulation in limbic regions, providing some insights into the mechanism of action that underlies the vulnerability to the excessive EtOH consumption reported in these animals.

Effects of Acute Ethanol Administration on Brain Oxidative Status: The Role of Acetaldehyde

BACKGROUND

Ethanol (EtOH), one of the most widely consumed substances of abuse, can induce brain damage and neurodegeneration. EtOH is centrally metabolized into acetaldehyde, which has been shown to be responsible for some of the neurophysiological and cellular effects of EtOH. Although some of the consequences of chronic EtOH administration on cell oxidative status have been described, the mechanisms by which acute EtOH administration affects the brain's cellular oxidative status and the role of acetaldehyde remain to be elucidated in detail.

METHODS

Swiss CD-I mice were pretreated with the acetaldehyde-sequestering agent d-penicillamine (DP; 75 mg/kg, i.p.) or the antioxidant lipoic acid (LA; 50 mg/kg, i.p.) 30 minutes before EtOH (2.5 g/kg, i.p.) administration. Animals were sacrificed 30 minutes after EtOH injection. Glutathione peroxidase (GPx) mRNA levels; GPx and glutathione reductase (GR) enzymatic activities; reduced glutathione (GSH), glutathione disulfide (GSSG), glutamate, g-L-glutamyl-L-cysteine (Glut-Cys), and malondialdehyde (MDA) concentrations; and protein carbonyl group (CG) content were determined in whole-brain samples.

RESULTS

Acute EtOH administration enhanced GPx activity and the GSH/GSSG ratio, while it decreased GR activity and GSSG concentration. Pretreatment with DP or LA only prevented GPx activity changes induced by EtOH.

CONCLUSIONS

Altogether, these results show the capacity of a single dose of EtOH to unbalance cellular oxidative homeostasis.

Salsolinol: an Unintelligible and Double-Faced Molecule-Lessons Learned from In Vivo and In Vitro Experiments

Salsolinol (1-methyl-6,7-dihydroxy-1,2,3,4-tetrahydroisoquinoline) is a tetrahydroisoquinoline derivative whose presence in humans was first detected in the urine of Parkinsonian patients on l-DOPA (l-dihydroxyphenylalanine) medication. Thus far, multiple hypotheses regarding its physiological/pathophysiological roles have been proposed, especially related to Parkinson’s disease or alcohol addiction. The aim of this review was to outline studies related to salsolinol, with special focus on in vivo and in vitro experimental models. To begin with, the chemical structure of salsolinol together with its biochemical implications and the role in neurotransmission are discussed. Numerous experimental studies are summarized in tables and the most relevant ones are stressed. Finally, the ability of salsolinol to cross the blood–brain barrier and its possible double-faced neurobiological potential are reviewed.

Is catalase involved in the effects of systemic and pVTA administration of 4-methylpyrazole on ethanol self-administration?

The oxidative metabolism of ethanol into acetaldehyde involves several enzymes, including alcohol dehydrogenase (ADH) and catalase-hydrogen peroxide (H₂O₂). In this regard, while it is well known that 4-methylpyrazole (4-MP) acts by inhibiting ADH in the liver, little attention has been placed on its ability to interfere with fatty acid oxidation-mediated generation of H₂O₂, a mechanism that may indirectly affect catalase whose enzymatic activity requires H₂O₂. The aim of our investigation was twofold: 1) to evaluate the effect of systemic (i.p. [intraperitoneal]) and local (into the posterior ventral tegmental area, pVTA) administration of 4-MP on oral ethanol self-administration, and 2) to assess ex vivo whether or not systemic 4-MP affects liver and brain H₂O₂ availability. The results show that systemic 4-MP reduced ethanol but not acetaldehyde or saccharin self-administration, and decreased the ethanol deprivation effect. Moreover, local intra-pVTA administration of 4-MP reduced ethanol but not saccharin self-administration. In addition, although unable to affect basal catalase activity, systemic administration of 4-MP decreased H₂O₂ availability both in liver and in brain. Overall, these results indicate that 4-MP interferes with ethanol self-administration and suggest that its behavioral effects could be due to a decline in catalase-H₂O₂ system activity as a result of a reduction of H₂O₂ availability, thus highlighting the role of central catalase-mediated metabolism of ethanol and further supporting the key role of acetaldehyde in the reinforcing properties of ethanol.

Thirst sensation and oral dryness following alcohol intake

Substantial acute and chronic intakes of alcohol or ethanol (EtOH) severely influence oral sensations, such as thirst and oral dryness (dry mouth, xerostomia). Thirst sensation and oral dryness are primarily caused by the activation of neurons in brain regions, including the circumventricular organs and hypothalamus, which are referred to as the dipsogenic center, and by a decrease in salivary secretion, respectively. The sensation of thirst experienced after heavy-alcohol drinking is widely regarded as a consequence of EtOH-induced diuresis; however, EtOH in high doses induces anti-diuresis. Recently, it has been proposed that the ethanol metabolite acetaldehyde induces thirst via two distinct processes in the central nervous system from EtOH-induced diuresis, based on the results of animal experiments. The present review describes new insights regarding the induction mechanism of thirst sensation and oral dryness after drinking alcohol.

On the Accuracy of In Vivo Ethanol and Acetaldehyde Monitoring, a Key Tile in the Puzzle of Acetaldehyde as a Neuroactive Agent

Over the last 20 years researchers have explored the postulated role of acetaldehyde (ACD) as a mediator of some of the actions of ethanol (EtOH) in the central nervous system (CNS). However, efforts have been hampered mainly by the difficulty of directly measuring in vivo EtOH and ACD levels in the CNS and thus, our knowledge is based on indirect evidences. Although technically challenging, the development of reliable methods for in vivo measurement of ACD and EtOH is of paramount importance to solve the “puzzle of acetaldehyde as a neuroactive agent.” In this short review we discuss the recent advances on brain EtOH pharmacokinetic and state-of-the-art available techniques that could be used for in vivo detect EtOH and ACD both non-invasively (magnetic resonance spectroscopy), and invasively (microdialysis and biosensors). Among the different in vivo sampling techniques described, particular emphasis is paid to the field of enzyme-based amperometric biosensors. Biosensors have gained much attention in recent years for their ability to online monitor biological signals in vivo, and several micro- and nano-structured devices have been successfully used for in vivo studies. Owing to their high temporal and spatial resolution, biosensors could provide the adequate technology for studying in vivo EtOH pharmacokinetic.

Brain Acetaldehyde Exposure Impacts upon Neonatal Respiratory Plasticity and Ethanol-Related Learning in Rodents

Prior studies indicate that neonates are very sensitive to ethanol's positive reinforcing effects and to its depressant effects upon breathing. Acetaldehyde (ACD) appears to play a major role in terms of modulating early reinforcing effects of the drug. Yet, there is no pre-existing literature relative to the incidence of this metabolite upon respiratory plasticity. The present study analyzed physiological and behavioral effects of early central administrations of ethanol, acetaldehyde or vehicle. Respiration rates (breaths/min) were registered at post-natal days (PDs) 2 and 4 (post-administration time: 5, 60, or 120 min). At PD5, all pups were placed in a context (plethysmograph) where they had previously experienced the effects of central administrations and breathing patterns were recorded. Following this test, pups were evaluated using and operant conditioning procedure where ethanol or saccharin served as positive reinforcers. Body temperatures were also registered prior to drug administrations as well as at the beginning and the end of each specific evaluation. Across days, breathing responses were high at the beginning of the evaluation session and progressively declined as a function of the passage of time. At PDs 2 and 4, shortly after central administration (5 min), ACD exerted a significant depression upon respiration frequencies. At PD5, non-intoxicated pups with a prior history of ACD central administrations, exhibited a marked increase in respiratory frequencies; a result that probably indicates a conditioned compensatory response. When operant testing procedures were conducted, prior ethanol or ACD central administrations were found to reduce the reinforcing effects of ethanol. This was not the case when saccharin was employed as a reinforcer. As a whole, the results indicate a significant role of central ACD upon respiratory plasticity of the neonate and upon ethanol's reinforcing effects; phenomena that affect the physiological integrity of the immature organism and its subsequent affinity for ethanol operationalized through self-administration procedures.

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.

Dual motor responses elicited by ethanol in the posterior VTA: Consequences of the blockade of μ-opioid receptors

A recent hypothesis, based on electrophysiological and behavioural findings, suggests that ethanol simultaneously exerts opposed effects on the activity of dopamine (DA) neurons in the ventral tegmental area (VTA) through two parallel mechanisms, one promoting and the other reducing the GABA release onto VTA DA neurons. In this sense, the activating effects are mediated by salsolinol, a metabolite of ethanol, acting on the μ-opioid receptors (MORs) located in VTA GABA neurons. The inhibitory effects are, however, triggered by the non-metabolized fraction of ethanol which would cause the GABAA receptors-mediated inhibition of VTA DA neurons. Since both trends tend to offset each other, only the use of appropriate pharmacological tools allows analysis of this phenomenon in depth. Herein, we present new behavioural findings supporting this hypothesis. Motor activity was evaluated in rats after intra-VTA administration of ethanol 35 nmol, an apparently ineffective dose, 24 h after the irreversible blockade of MORs in the VTA with β-FNA. Our results showed that this pre-treatment turned the initially ineffective ethanol dose into a depressant one, confirming that the activating effect of ethanol can be selectively suppressed without affecting the depressant effects mediated by the non-biotransformed fraction of ethanol.

The ethanol metabolite acetaldehyde induces water and salt intake via two distinct pathways in the central nervous system of rats

The sensation of thirst experienced after heavy alcohol drinking is widely regarded as a consequence of ethanol (EtOH)-induced diuresis, but EtOH in high doses actually induces anti-diuresis. The present study was designed to investigate the introduction mechanism of water and salt intake after heavy alcohol drinking, focusing on action of acetaldehyde, a metabolite of EtOH and a toxic substance, using rats. The aldehyde dehydrogenase (ALDH) inhibitor cyanamide was used to mimic the effect of prolonged acetaldehyde exposure because acetaldehyde is quickly degraded by ALDH. Systemic administration of a high-dose of EtOH at 2.5 g/kg induced water and salt intake with anti-diuresis. Cyanamide enhanced the fluid intake following EtOH and acetaldehyde administration. Systemic administration of acetaldehyde with cyanamide suppressed blood pressure and increased plasma renin activity. Blockade of central angiotensin receptor AT1R suppressed the acetaldehyde-induced fluid intake and c-Fos expression in the circumventricular organs (CVOs), which form part of dipsogenic mechanism in the brain. In addition, central administration of acetaldehyde together with cyanamide selectively induced water but not salt intake without changes in blood pressure. In electrophysiological recordings from slice preparations, acetaldehyde specifically excited angiotensin-sensitive neurons in the CVO. These results suggest that acetaldehyde evokes the thirst sensation following heavy alcohol drinking, by two distinct and previously unsuspected mechanisms, independent of diuresis. First acetaldehyde indirectly activates AT1R in the dipsogenic centers via the peripheral renin-angiotensin system following the depressor response and induces both water and salt intake. Secondly acetaldehyde directly activates neurons in the dipsogenic centers and induces only water intake.

Metabolic methanol: molecular pathways and physiological roles

Methanol has been historically considered an exogenous product that leads only to pathological changes in the human body when consumed. However, in normal, healthy individuals, methanol and its short-lived oxidized product, formaldehyde, are naturally occurring compounds whose functions and origins have received limited attention. There are several sources of human physiological methanol. Fruits, vegetables, and alcoholic beverages are likely the main sources of exogenous methanol in the healthy human body. Metabolic methanol may occur as a result of fermentation by gut bacteria and metabolic processes involving S-adenosyl methionine. Regardless of its source, low levels of methanol in the body are maintained by physiological and metabolic clearance mechanisms. Although human blood contains small amounts of methanol and formaldehyde, the content of these molecules increases sharply after receiving even methanol-free ethanol, indicating an endogenous source of the metabolic methanol present at low levels in the blood regulated by a cluster of genes. Recent studies of the pathogenesis of neurological disorders indicate metabolic formaldehyde as a putative causative agent. The detection of increased formaldehyde content in the blood of both neurological patients and the elderly indicates the important role of genetic and biochemical mechanisms of maintaining low levels of methanol and formaldehyde.

The "first hit" toward alcohol reinforcement: role of ethanol metabolites

This review analyzes literature that describes the behavioral effects of 2 metabolites of ethanol (EtOH): acetaldehyde and salsolinol (a condensation product of acetaldehyde and dopamine) generated in the brain. These metabolites are self-administered into specific brain areas by animals, showing strong reinforcing effects. A wealth of evidence shows that EtOH, a drug consumed to attain millimolar concentrations, generates brain metabolites that are reinforcing at micromolar and nanomolar concentrations. Salsolinol administration leads to marked increases in voluntary EtOH intake, an effect inhibited by mu-opioid receptor blockers. In animals that have ingested EtOH chronically, the maintenance of alcohol intake is no longer influenced by EtOH metabolites, as intake is taken over by other brain systems. However, after EtOH withdrawal brain acetaldehyde has a major role in promoting binge-like drinking in the condition known as the "alcohol deprivation effect"; a condition seen in animals that have ingested alcohol chronically, are deprived of EtOH for extended periods, and are allowed EtOH re-access. The review also analyzes the behavioral effects of acetate, a metabolite that enters the brain and is responsible for motor incoordination at low doses of EtOH. Also discussed are the paradoxical effects of systemic acetaldehyde. Overall, evidence strongly suggests that brain-generated EtOH metabolites play a major role in the early ("first-hit") development of alcohol reinforcement and in the generation of relapse-like drinking.

Key role of salsolinol in ethanol actions on dopamine neuronal activity of the posterior ventral tegmental area

Ethanol excites dopamine (DA) neurons in the posterior ventral tegmental area (pVTA). This effect is responsible for ethanol's motivational properties and may contribute to alcoholism. Evidence indicates that catalase-mediated conversion of ethanol into acetaldehyde in pVTA plays a critical role in this effect. Acetaldehyde, in the presence of DA, condensates with it to generate salsolinol. Salsolinol, when administered in pVTA, excites pVTA DA cells, elicits DA transmission in nucleus accumbens and sustains its self-administration in pVTA. Here we show, by using ex vivo electrophysiology, that ethanol and acetaldehyde, but not salsolinol, failed to stimulate pVTA DA cell activity in mice administered α-methyl-p-tyrosine, a DA biosynthesis inhibitor that reduces somatodendritic DA release. This effect was specific for ethanol and acetaldehyde since morphine, similarly to salsolinol, was able to excite pVTA DA cells in α-methyl-p-tyrosine-treated mice. However, when DA was bath applied in slices from α-methyl-p-tyrosine-treated mice, ethanol-induced excitation of pVTA DA neurons was restored. This effect requires ethanol oxidation into acetaldehyde given that, when H2 O2 -catalase system was impaired by either 3-amino-1,2,4-triazole or in vivo administration of α-lipoic acid, ethanol did not enhance DA cell activity. Finally, high performance liquid chromatography-tandem mass spectrometry analysis of bath medium detected salsolinol only after co-application of ethanol and DA in α-methyl-p-tyrosine-treated mice. These results demonstrate the relationship between ethanol and salsolinol effects on pVTA DA neurons, help to untangle the mechanism(s) of action of ethanol in this area and contribute to an exciting research avenue prosperous of theoretical and practical consequences.

Ethanol-induced oxidant stress modulates hepatic autophagy and proteasome activity

In this review, we describe research findings on the effects of alcohol exposure on two major catabolic systems in liver cells: the ubiquitin-proteasome system (UPS) and autophagy. These hydrolytic systems are not unique to liver cells; they exist in all eukaryotic tissues and cells. However, because the liver is the principal site of ethanol metabolism, it sustains the greatest damage from heavy drinking. Thus, the focus of this review is to specifically describe how ethanol oxidation modulates the activities of the UPS and autophagy and the mechanisms by which these changes contribute to the pathogenesis of alcohol-induced liver injury. Here, we describe the history and the importance of cellular hydrolytic systems, followed by a description of each catabolic pathway and the differential modulation of each by ethanol exposure. Overall, the evidence for an involvement of these catabolic systems in the pathogenesis of alcoholic liver disease is quite strong. It underscores their importance, not only as effective means of cellular recycling and eventual energy generation, but also as essential components of cellular defense.

Salsolinol, free of isosalsolinol, exerts ethanol-like motivational/sensitization effects leading to increases in ethanol intake

Salsolinol is formed non-enzymatically when ethanol-derived acetaldehyde binds to dopamine, yielding 2 distinct products, i.e., salsolinol and isosalsolinol. Early animal studies, revealing that salsolinol promotes alcohol consumption and recent evidence that animals will readily self-administer salsolinol into the posterior ventral tegmental area (p-VTA) together with the finding that salsolinol is able to induce conditioned place preference and to increase locomotor activity, have outlined a role of salsolinol in the behavioral and neurobiological actions of ethanol. Until recently, the only commercially available salsolinol was a mixture containing 85% salsolinol and 10-15% isosalsolinol. The possibility thus exists that either salsolinol or isosalsolinol explains the reinforcing properties of ethanol. We report here that a newly available salsolinol is free of isosalsolinol. Thus, salsolinol, free of isosalsolinol, was injected intracerebrally (30 pmol/0.2 μL, into the ventral tegmental area [VTA]) or intraperitoneally (i.p.) (10 mg/kg) to naïve rats bred as alcohol drinkers to study salsolinol's motivational effects and its role on voluntary ethanol intake. Salsolinol produced conditioned place preference and increased locomotor activity, whether injected intra-VTA or intraperitoneally. Following systemic (i.p.) administration of 10 mg/kg salsolinol, this molecule was detected in vivo by microdialysis of neostriatum, reaching an estimated concentration of 100 nM in the dialyzate. These results indicate that systemically administered salsolinol is able to cross the blood-brain barrier (BBB). Repeated administration of salsolinol sensitized rats to the locomotor activity and led to increases in voluntary ethanol consumption, which was prevented by intra-VTA pretreatment with naltrexone.

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.

Alcohol-medical drug interactions

Concomitant use of alcohol and medications may lead to potentially serious medical conditions. Increasing prescription medication abuse in today's society necessitates a deeper understanding of the mechanisms involved in alcohol-medication interactions in order to help prevent adverse events. Interactions of medications with alcohol result in altered bioavailability of the medication or alcohol (pharmacokinetic interactions) or modification of the effects at receptor or ion channel sites to alter behavioral or physical outcome (pharmacodynamic interactions). The nature of pharmacokinetic or pharmacodynamic interactions involved in alcohol-medication interactions may differ between acute and chronic alcohol use and be influenced by race, gender, or environmental or genetic factors. This review focuses on the mechanisms underlying pharmacokinetic and pharmacodynamic interactions between alcohol and medications and provides examples for such interactions from replicated research studies. In conclusion, further translational research is needed to address several gaps in our current knowledge of alcohol-medication interactions, including those under various pathologic conditions.

Modulation of ethanol-induced conditioned place preference in mice by 3-amino-1,2,4-triazole and D-penicillamine depends on ethanol dose and number of conditioning trials

Previous studies have shown that both 3-amino-1,2,4-triazole (AT), which inhibits metabolism of ethanol (EtOH) to acetaldehyde by inhibiting catalase, and D-penicillamine (D-P), an acetaldehyde-sequestering agent, modulate EtOH-conditioned place preference (CPP) in male albino Swiss (IOPS Orl) mice. These studies followed a reference-dose-like procedure, which involves comparing cues that have both been paired with EtOH. However, the role of EtOH-derived acetaldehyde has not been examined using a standard CPP method, and efficacy of these treatments could be different under the two circumstances. In the present investigation, we manipulated the strength of CPP across five separate studies and evaluated the effect of D-P and AT on EtOH-induced CPP following a standard unbiased CPP procedure. Mice received pairings with vehicle-saline injections with one cue and, alternatively, with AT- and D-P-EtOH with another cue. Our studies indicate that AT and D-P only disrupt CPP induced by EtOH in mice when the number of conditioning sessions and the dose of EtOH are low. These findings suggest that acquisition of EtOH-induced CPP may depend on the levels of acetaldehyde available during memory acquisition and the strength of the memory. Therefore, we propose that, at least when the memory processes are labile, brain acetaldehyde could participate in the formation of Pavlovian learning elicited by EtOH.

Locally-generated Acetaldehyde Contributes to the Effects of Ethanol on Neurosteroids and LTP in the Hippocampus

BACKGROUND

As severe alcohol intoxication impairs memory function, a high concentration of ethanol (60 mM) acutely inhibits long-term potentiation (LTP), a cellular model of learning and memory, in rat hippocampal slices. Neurosteroids are involved in this LTP inhibition. We recently reported that the inhibitory effects of 60 mM ethanol are blocked by 4-methylpyrazole (4MP), an inhibitor of alcohol dehydrogenase, suggesting that acetaldehyde locally generated within the hippocampus participates in LTP inhibition.

AIM

We investigated whether acetaldehyde generated by ethanol metabolism contributes to neurosteroidogenesis and LTP inhibition.

RESULTS

Like 60 mM ethanol, we found that exogenous acetaldehyde enhanced neurosteroid immunostaining in CA1 pyramidal neurons, and that augmented neurosteroid immunostaining by high ethanol alone was blocked by 4MP but not by inhibitors of other ethanol metabolism pathways. The inhibitory effects of 60 mM ethanol on LTP were mimicked by a lower concentration of ethanol (20 mM) plus acetaldehyde (60 μM), although neither agent alone was effective at these concentrations, suggesting that 60 mM ethanol inhibits LTP via multiple actions, one of which involves acetaldehyde and the other of which requires only 20 mM ethanol. The effects of ethanol and acetaldehyde on neurosteroid staining and LTP were overcome by inhibition of neurosteroid synthesis and by blockade of N-methyl-D-aspartate receptors (NMDARs).

CONCLUSION

These observations indicate that acetaldehyde generated by local ethanol metabolism within the hippocampus serves as a signal for neurosteroid synthesis in pyramidal neurons, and participates in the synaptic dysfunction associated with severe alcohol intoxication.

Oxidation of ethanol in the rat brain and effects associated with chronic ethanol exposure

It has been reported that chronic and acute alcohol exposure decreases cerebral glucose metabolism and increases acetate oxidation. However, it remains unknown how much ethanol the living brain can oxidize directly and whether such a process would be affected by alcohol exposure. The questions have implications for reward, oxidative damage, and long-term adaptation to drinking. One group of adult male Sprague-Dawley rats was treated with ethanol vapor and the other given room air. After 3 wk the rats received i.v. [2-(13)C]ethanol and [1, 2-(13)C2]acetate for 2 h, and then the brain was fixed, removed, and divided into neocortex and subcortical tissues for measurement of (13)C isotopic labeling of glutamate and glutamine by magnetic resonance spectroscopy. Ethanol oxidation was seen to occur both in the cortex and the subcortex. In ethanol-naïve rats, cortical oxidation of ethanol occurred at rates of 0.017 ± 0.002 µmol/min/g in astroglia and 0.014 ± 0.003 µmol/min/g in neurons, and chronic alcohol exposure increased the astroglial ethanol oxidation to 0.028 ± 0.002 µmol/min/g (P = 0.001) with an insignificant effect on neuronal ethanol oxidation. Compared with published rates of overall oxidative metabolism in astroglia and neurons, ethanol provided 12.3 ± 1.4% of cortical astroglial oxidation in ethanol-naïve rats and 20.2 ± 1.5% in ethanol-treated rats. For cortical astroglia and neurons combined, the ethanol oxidation for naïve and treated rats was 3.2 ± 0.3% and 3.8 ± 0.2% of total oxidation, respectively. (13)C labeling from subcortical oxidation of ethanol was similar to that seen in cortex but was not affected by chronic ethanol exposure.

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.

Gene specific modifications unravel ethanol and acetaldehyde actions

Ethanol is metabolized into acetaldehyde mainly by the action of alcohol dehydrogenase in the liver, while mainly by the action of catalase in the brain. Aldehyde dehydrogenase-2 metabolizes acetaldehyde into acetate in both organs. Gene specific modifications reviewed here show that an increased liver generation of acetaldehyde (by transduction of a gene coding for a high-activity liver alcohol dehydrogenase ADH1^*B2) leads to increased blood acetaldehyde levels and aversion to ethanol in animals. Similarly aversive is an increased acetaldehyde level resulting from the inhibition of liver aldehyde dehydrogenase-2 (ALDH2) synthesis (by an antisense coding gene against aldh2 mRNA). The situation is diametrically different when acetaldehyde is generated in the brain. When the brain ventral tegmental area (VTA) is endowed with an increased ability to generate acetaldehyde (by transfection of liver rADH) the reinforcing effects of ethanol are increased, while a highly specific inhibition of catalase synthesis (by transduction of a shRNA anti catalase mRNA) virtually abolishes the reinforcing effects of ethanol as seen by a complete abolition of ethanol intake in rats bred for generations as high ethanol drinkers. Data shows two divergent effects of increases in acetaldehyde generation: aversive in the periphery but reinforcing in the brain.

Alpha-lipoic acid reduces ethanol self-administration in rats

BACKGROUND

The main system of central ethanol (EtOH) oxidation is mediated by the enzyme catalase. By reacting with H2 O2 , brain catalase forms compound I (the catalase-H2 O2 system), which is able to oxidize EtOH to acetaldehyde (ACD) in the brain. We have previously shown that ACD regulates EtOH motivational properties and possesses reinforcing effects by itself. In this study, we investigate the effects of alpha-lipoic acid (ALA), a scavenging agent for H2 O2 , on oral EtOH self-administration.

METHODS

To this end, we trained Wistar rats to orally self-administer EtOH (10%) by nose poking. The effect of intraperitoneal pretreatment with ALA was evaluated during (i) maintenance of EtOH self-administration, (ii) EtOH self-administration under a progressive ratio (PR) schedule of reinforcement, and (iii) oral EtOH priming to induce reinstatement of EtOH seeking behavior. Moreover, we tested the effect of ALA on saccharin (0.05%) reinforcement, as assessed by oral self-administration.

RESULTS

The results indicate that ALA dose-dependently reduced the maintenance, the break point of EtOH self-administration under a PR and the reinstatement of EtOH seeking behavior without suppressing saccharin self-administration.

CONCLUSIONS

These results support that ALA may have a potential use in alcoholism treatment.

Acetaldehyde involvement in ethanol's postabsortive effects during early ontogeny

Clinical and biomedical studies sustains the notion that early ontogeny is a vulnerable window to the impact of alcohol. Experiences with the drug during these stages increase latter disposition to prefer, use or abuse ethanol. This period of enhanced sensitivity to ethanol is accompanied by a high rate of activity in the central catalase system, which metabolizes ethanol in the brain. Acetaldehyde (ACD), the first oxidation product of ethanol, has been found to share many neurobehavioral effects with the drug. Cumulative evidence supports this notion in models employing adults. Nevertheless very few studies have been conducted to analyze the role of ACD in ethanol postabsorptive effects, in newborns or infant rats. In this work we review recent experimental literature that syndicates ACD as a mediator agent of reinforcing aspects of ethanol, during early ontogenetic stages. We also show a meta-analytical correlational approach that proposes how differences in the activity of brain catalase across ontogeny, could be modulating patterns of ethanol consumption.

Acetaldehyde reinforcement and motor reactivity in newborns with or without a prenatal history of alcohol exposure

Animal models have shown that early ontogeny seems to be a period of enhanced affinity to ethanol. Interestingly, the catalase system that transforms ethanol (EtOH) into acetaldehyde (ACD) in the brain, is more active in the perinatal rat compared to adults. ACD has been found to share EtOH's behavioral effects. The general purpose of the present study was to assess ACD motivational and motor effects in newborn rats as a function of prenatal exposure to EtOH. Experiment 1 evaluated if ACD (0.35 μmol) or EtOH (0.02 μmol) supported appetitive conditioning in newborn pups prenatally exposed to EtOH. Experiment 2 tested if prenatal alcohol exposure modulated neonatal susceptibility to ACD's motor effects (ACD dose: 0, 0.35 and 0.52 μmol). Experiment 1 showed that EtOH and ACD supported appetitive conditioning independently of prenatal treatments. In Experiment 2, latency to display motor activity was altered only in neonates prenatally treated with water and challenged with the highest ACD dose. Prenatal EtOH experience results in tolerance to ACD's motor activity effects. These results show early susceptibility to ACD's appetitive effects and attenuation of motor effects as a function of prenatal history with EtOH, within a stage in development where brain ACD production seems higher than later in life.

Acquisition and reconditioning of ethanol-induced conditioned place preference in mice is blocked by the H₂O₂ scavenger alpha lipoic acid

RATIONALE

Hydrogen peroxide (H2O2) is the co-substrate used by catalase to metabolize ethanol to acetaldehyde in the brain. This centrally formed acetaldehyde has been involved in several ethanol-related behaviors.

OBJECTIVES

The present research evaluated the effect of the H2O2 scavenger, alpha lipoic acid (LA), on the acquisition and reconditioning of ethanol-induced conditioned place preference (CPP).

METHODS

Mice received pairings of a distinctive floor stimulus (CS+) associated with intraperitoneal injections of ethanol (2.5 g/kg). On alternate days, animals received pairings of a different floor stimulus (CS-) associated with saline injections. A different group of animals received pairings with the (CS-) associated with saline injections, and on alternate days they received LA (100 mg/kg) injected 30 min prior to ethanol (2.5 g/kg) administration paired with the (CS+). A preference test assessed the effect of LA on the acquisition of ethanol-induced CPP. A similar procedure was followed to study the effect of LA on the acquisition of cocaine- and morphine-induced CPP. A separate experiment evaluated the effect of LA on the reconditioning of ethanol-induced CPP. In addition, we investigated the consequence of LA administration on central H2O2 levels.

RESULTS

LA selectively blocked the acquisition of ethanol-induced CPP. Moreover, this compound impaired the reconditioning of ethanol-induced CPP. Additionally, we found that LA diminished H2O2 levels in the brain.

CONCLUSIONS

These data suggest that a decline in H2O2 availability by LA might impede the formation of brain ethanol-derived acetaldehyde by catalase, which results in an impairment of the rewarding properties of ethanol.

The role of acetaldehyde in ethanol reinforcement assessed by Pavlovian conditioning in newborn rats

RATIONALE

Animal studies indicate that central acetaldehyde, dependent on catalase metabolism of ethanol (EtOH), modulates ethanol reinforcement. Brain catalase activity and acetaldehyde (ACD) production are significantly higher in rat pups compare d with adults. Interestingly, infant rats show high EtOH affinity for alcohol consumption and are particularly sensitive to the drug's reinforcing effects.

OBJECTIVES

We tested whether central ACD is necessary and sufficient to induce appetitive conditioning in newborn rats through the artificial nipple technique.

METHODS

Vehicle, EtOH (100 mg%), and acetaldehyde (0.35 μmol) were administered into the cisterna magna (1 μl). Half of the animals also received a central administration of 75 μg (experiment 1) or 40 μg of D-penicillamine (experiment 2). Afterwards, pups were exposed to an olfactory cue (conditioned stimulus). One hour later, neonates were tested with an artificial nipple in the presence of the conditioned cue. Nipple attachment duration, mean grasp duration, and number of nipple disengagements served as dependent variables.

RESULTS

Positive responses to the scented nipple occurred in neonates conditioned with EtOH or ACD (experiments 1 and 2). In experiment 1, there were indications that D-penicillamine weakened the reinforcing effects of EtOH and ACD. In experiment 2, D-penicillamine (40 μg) significantly inhibited appetitive conditioned responses dependent upon EtOH or ACD.

CONCLUSIONS

Appetitive conditioning was observed when employing either central EtOH or ACD as unconditioned stimuli. Central abduction of ACD inhibited conditioned appetitive responsiveness to the surrogate nipple. Central ACD is involved in the determination or modulation of EtOH's motivational properties during early stages in development.

Locally-generated acetaldehyde is involved in ethanol-mediated LTP inhibition in the hippocampus

Consistent with the ability of severe alcohol intoxication to impair memory, high concentrations of ethanol (60mM) acutely inhibit long-term potentiation (LTP) in the CA1 region of rat hippocampal slices. To account for this, we hypothesized that local metabolism to acetaldehyde may contribute to the effects of high ethanol on synaptic function. However, sodium azide, a catalase inhibitor, and allyl sulfide, an inhibitor of cytochrome P450 2E1 (CYP2E1), failed to overcome LTP inhibition by 60mM ethanol. In contrast, LTP was successfully induced in the presence of ethanol plus 4-methylpyrazole (4MP), an inhibitor of alcohol dehydrogenase, suggesting that local metabolism via alcohol dehydrogenase contributes to synaptic effects. Furthermore, exogenously administered acetaldehyde overcame the effects of 4MP on LTP inhibition mediated by ethanol. These observations indicate that acetaldehyde generated by local metabolism within the hippocampus participates in the synaptic dysfunction associated with severe alcohol intoxication.

The role of CYP2E1 in alcohol metabolism and sensitivity in the central nervous system

Ethanol consumption has effects on the central nervous system (CNS), manifesting as motor incoordination, sleep induction (hypnosis), anxiety, amnesia, and the reinforcement or aversion of alcohol consumption. Acetaldehyde (the direct metabolite of ethanol oxidation) contributes to many aspects of the behavioral effects of ethanol. Given acetaldehyde cannot pass through the blood brain barrier, its concentration in the CNS is primarily determined by local production from ethanol. Catalase and cytochrome P450 2E1 (CYP2E1) represent the major enzymes in the CNS that catalyze ethanol oxidation. CYP2E1 is expressed abundantly within the microsomes of certain brain cells and is localized to particular brain regions. This chapter focuses on the discussion of CYP2E1 in ethanol metabolism in the CNS, covering topics including how it is regulated, where it is expressed and how it influences sensitivity to ethanol in the brain.

The H2O2 scavenger ebselen decreases ethanol-induced locomotor stimulation in mice

BACKGROUND

In the brain, the enzyme catalase by reacting with H(2)O(2) forms Compound I (catalase-H(2)O(2) system), which is the main system of central ethanol metabolism to acetaldehyde. Previous research has demonstrated that acetaldehyde derived from central-ethanol metabolism mediates some of the psychopharmacological effects produced by ethanol. Manipulations that modulate central catalase activity or sequester acetaldehyde after ethanol administration modify the stimulant effects induced by ethanol in mice. However, the role of H(2)O(2) in the behavioral effects caused by ethanol has not been clearly addressed. The present study investigated the effects of ebselen, an H(2)O(2) scavenger, on ethanol-induced locomotion.

METHODS

Swiss RjOrl mice were pre-treated with ebselen (0-50mg/kg) intraperitoneally (IP) prior to administration of ethanol (0-3.75g/kg; IP). In another experiment, animals were pre-treated with ebselen (0 or 25mg/kg; IP) before caffeine (15mg/kg; IP), amphetamine (2mg/kg; IP) or cocaine (10mg/kg; IP) administration. Following these treatments, animals were placed in an open field to measure their locomotor activity. Additionally, we evaluated the effect of ebselen on the H(2)O(2)-mediated inactivation of brain catalase activity by 3-amino-1,2,4-triazole (AT).

RESULTS

Ebselen selectively prevented ethanol-induced locomotor stimulation without altering the baseline activity or the locomotor stimulating effects caused by caffeine, amphetamine and cocaine. Ebselen reduced the ability of AT to inhibit brain catalase activity.

CONCLUSIONS

Taken together, these data suggest that a decline in H(2)O(2) levels might result in a reduction of the ethanol locomotor-stimulating effects, indicating a possible role for H(2)O(2) in some of the psychopharmacological effects produced by ethanol.

α-Lipoic acid, a scavenging agent for H₂O₂, reduces ethanol-stimulated locomotion in mice

RATIONALE

The main system of central ethanol oxidation is mediated by the enzyme catalase. By reacting with H(2)O(2), brain catalase forms compound I (the catalase-H(2)O(2) system), which is able to oxidize ethanol to acetaldehyde in the brain. Previous studies have demonstrated that pharmacological manipulations of brain catalase activity modulate the stimulant effects of ethanol in mice. However, the role of H(2)O(2) in the behavioral effects of ethanol has not yet been clearly addressed.

OBJECTIVES

In the present study, we investigated the effects of alpha-lipoic acid (LA), a scavenging agent for H(2)O(2), on ethanol-induced locomotor stimulation.

METHODS

CD-1 mice were pretreated with LA [0-100 mg/kg, intraperitoneally (IP)] 0-60 min prior to administration of ethanol (0-3.75 g/kg, IP). In another experiment, animals were pretreated with LA (0, 25, or 50 mg/kg, IP) 30 min before cocaine (10 mg/kg, IP), amphetamine (2 mg/kg, IP), or caffeine (25 mg/kg, IP). After these treatments the animals were placed in an open-field chamber and their locomotor activity was measured for 20 min.

RESULTS

LA 25, 50, and 100 mg/kg IP prevented ethanol-induced locomotor stimulation. LA did not affect the locomotor-stimulating effects of cocaine, amphetamine, and caffeine. Additionally, we demonstrated that LA prevents the inactivation of brain catalase by 3-amino-1,2,4-triazole, thus indicating that H(2)O(2) levels are reduced by LA.

CONCLUSIONS

These data support the idea that a decrease in cerebral H(2)O(2) production by LA administration inhibits ethanol-stimulated locomotion. This study suggests that the brain catalase-H(2)O(2) system, and by implication centrally formed acetaldehyde, plays a key role in the psychopharmacological effects of ethanol.

In vivo detection of intermediate metabolic products of [1-(13) C]ethanol in the brain using (13) C MRS

In this study, in vivo (13) C MRS was used to investigate the labeling of brain metabolites after intravenous administration of [1-(13) C]ethanol. After [1-(13) C]ethanol had been administered systemically to rats, (13) C labels were detected in glutamate, glutamine and aspartate in the carboxylic and amide carbon spectral region. (13) C-labeled bicarbonate HCO 3- (161.0 ppm) was also detected. Saturating acetaldehyde C1 at 207.0 ppm was found to have no effect on the ethanol C1 (57.7 ppm) signal intensity after extensive signal averaging, providing direct in vivo evidence that direct metabolism of alcohol by brain tissue is minimal. To compare the labeling of brain metabolites by ethanol with labeling by glucose, in vivo time course data were acquired during intravenous co-infusion of [1-(13) C]ethanol and [(13) C(6) ]-D-glucose. In contrast with labeling by [(13) C(6) ]-D-glucose, which produced doublets of carboxylic/amide carbons with a J coupling constant of 51 Hz, the simultaneously detected glutamate and glutamine singlets were labeled by [1-(13) C]ethanol. As (13) C labels originating from ethanol enter the brain after being converted into [1-(13) C]acetate in the liver, and the direct metabolism of ethanol by brain tissue is negligible, it is suggested that orally or intragastrically administered (13) C-labeled ethanol may be used to study brain metabolism and glutamatergic neurotransmission in investigations involving alcohol administration. In vivo (13) C MRS of rat brain following intragastric administration of (13) C-labeled ethanol is demonstrated.

Acetaldehyde elicits ERK phosphorylation in the rat nucleus accumbens and extended amygdala

Recent advances suggest that acetaldehyde mediates some of the neurobiological properties of ethanol. In a recent study, we have shown that ethanol elicits the phosphorylation of extracellular signal-regulated kinase (pERK) in the nucleus accumbens and extended amygdala, via a dopamine D(1) receptor-mediated mechanism. The aim of this study was to determine whether acetaldehyde and ethanol-derived acetaldehyde elicit the activation of ERK in the nucleus accumbens and extended amygdala. The effects of acetaldehyde (10 and 20 mg/kg) and ethanol (1 g/kg), administered to rats intragastrically, were assessed by pERK peroxidase immunohistochemistry. To establish the role of ethanol-derived acetaldehyde, the alcohol dehydrogenase inhibitor, 4-methylpyrazole (90 mg/kg), and the acetaldehyde-sequestering agent, D-penicillamine (50 mg/kg), were administered before ethanol. Acetaldehyde increased pERK immunoreactivity in the nucleus accumbens and extended amygdala. Inhibition of ethanol metabolism and sequestration of newly synthesized acetaldehyde completely prevented ERK activation by ethanol. In addition, to establish the role of D(1) receptors stimulation in acetaldehyde-elicited ERK phosphorylation, we studied the effect of the D(1) receptor antagonist, SCH 39166. Pretreatment with the D(1) receptor antagonist (50 μg/kg) fully prevented acetaldehyde-elicited ERK activation. Overall, these results indicate that ethanol activates ERK by means of its metabolic conversion into acetaldehyde and strengthen the view that acetaldehyde is a centrally acting compound with a pharmacological profile similar to ethanol.

Ethanol as a prodrug: brain metabolism of ethanol mediates its reinforcing effects

BACKGROUND

 While the molecular entity responsible for the rewarding effects of virtually all drugs of abuse is known, that for ethanol remains uncertain. Some lines of evidence suggest that the rewarding effects of alcohol are mediated not by ethanol per se but by acetaldehyde generated by catalase in the brain. However, the lack of specific inhibitors of catalase has not allowed strong conclusions to be drawn about its role on the rewarding properties of ethanol. The present studies determined the effect on voluntary alcohol consumption of two gene vectors, one designed to inhibit catalase synthesis and one designed to synthesize alcohol dehydrogenase (ADH), to respectively inhibit or increase brain acetaldehyde synthesis.

METHODS

 The lentiviral vectors, which incorporate the genes they carry into the cell genome, were (i) one encoding a shRNA anticatalase synthesis and (ii) one encoding alcohol dehydrogenase (rADH1). These were stereotaxically microinjected into the brain ventral tegmental area (VTA) of Wistar-derived rats bred for generations for their high alcohol preference (UChB), which were allowed access to an ethanol solution and water.

RESULTS

 Microinjection into the VTA of the lentiviral vector encoding the anticatalase shRNA virtually abolished (-94% p < 0.001) the voluntary consumption of alcohol by the rats. Conversely, injection into the VTA of the lentiviral vector coding for ADH greatly stimulated (2 to 3 fold p < 0.001) their voluntary ethanol consumption.

CONCLUSIONS

The study strongly suggests that to generate reward and reinforcement, ethanol must be metabolized into acetaldehyde in the brain. Data suggest novel targets for interventions aimed at reducing chronic alcohol intake.

Ethanol as a prodrug: brain metabolism of ethanol mediates its reinforcing effects--a commentary

BACKGROUND

This commentary discusses a study by Karahanian and colleagues (2011) on the role of central nervous system acetaldehyde in the reinforcing effects of ethanol. The goal is to emphasize the importance of the study and to discuss future directions.

RESULTS

This important paper solidifies the idea that the levels of acetaldehyde in the central nervous system have profound effects in mediating the reinforcing actions of ethanol. This is accomplished by manipulating the brain levels of acetaldehyde produced from ethanol by the injection of lentivirus containing either an anti-catalase shRNA construct or a rat liver alcohol dehydrogenase into the central nervous system and observing the effects on alcohol preference by high ethanol-consuming rats. A factor not directly considered is that acetaldehyde is further metabolized to acetate, which also has some behavioral actions.

CONCLUSIONS

The efficacy of lentivirus injections of enzyme inhibitors or enzymes themselves to alter a behavioral response to ethanol is clearly demonstrated here. The many other actions of ethanol that are postulated to be a result of the production of acetaldehyde in the brain remain to be investigated by similar techniques. Possible "therapeutic avenues to reduce chronic alcohol use" are envisioned.

Ethanol and acetaldehyde disturb TNF-alpha and IL-6 production in cultured astrocytes

Ethanol disturbs astroglial growth and differentiation and causes functional alterations. Furthermore, many signalling molecules produced by astrocytes contribute to these processes. The aim of the present study was to investigate the influence of ethanol and its primary metabolite, acetaldehyde, on TNF-alpha and IL-6 production in a rat cortical astrocyte primary culture. We are the first to report that both ethanol and acetaldehyde can modulate TNF-alpha and IL-6 secretion from cultured astrocytes. Long-term exposure (7 days) to ethanol and acetaldehyde was more toxic than an acute (24 hours) exposure. However, both compounds showed a biphasic, hormestic effect on the IL-6 secretion after the acute as well as the long-term exposure, and the maximum stimulation was reached for 50-mM ethanol and 1-mM acetaldehyde after 7-day exposure. In contrast, both compounds reduced the TNF-alpha secretion, where the effect was concentration-dependent. The catalase inhibitor 2-amino-1,2,4 triazole significantly reduced the ethanol toxicity in the cultured astrocytes after the acute as well as the long-term exposure. In conclusion, both ethanol and acetaldehyde affect the production of IL-6 and TNF-alpha in cultured astrocytes. The effect depends on the concentration of the compounds and the duration of the exposure. Acetaldehyde is a more potent toxin than ethanol, and ethanol's toxicity in the brain is at least partially due to its primary metabolite, acetaldehyde.

Comparison of ethanol and acetaldehyde toxicity in rat astrocytes in primary culture

This study compared the effects of toxicity of ethanol and its first metabolite acetaldehyde in rat astrocytes through cell viability and cell proliferation. The cells were treated with different concentrations of ethanol in the presence or absence of a catalase inhibitor 2-amino-1,2,4 triazole (AMT) or with different concentrations of acetaldehyde. Cell viability was assessed using the trypan blue test. Cell proliferation was assessed after 24 hours and after seven days of exposure to either ethanol or acetaldehyde.We showed that both ethanol and acetaldehyde decreased cell viability in a dose-dependent manner. In proliferation studies, after seven days of exposure to either ethanol or acetaldehyde, we observed a significant dose-dependent decrease in cell number. The protein content study showed biphasic dose-response curves, after 24 hours and seven days of exposure to either ethanol or acetaldehyde. Co-incubation in the presence of AMT significantly reduced the inhibitory effect of ethanol on cell proliferation.We concluded that long-term exposure of astrocytes to ethanol is more toxic than acute exposure. Acetaldehyde is a much more potent toxin than ethanol, and at least a part of ethanol toxicity is due to ethanol's first metabolite acetaldehyde.

Ethanol modulates corticotropin releasing hormone release from the rat hypothalamus: does acetaldehyde play a role?

BACKGROUND AND METHODS

Ethanol (EtOH) activates hypothalamic-pituitary-adrenal (HPA) axis, resulting in adrenocorticotropin hormone, glucocorticoid release, and in modifications of the response of the axis to other stressors. The initial site of EtOH action within the HPA system seems to be the hypothalamus. Thus, to determine the mechanisms responsible for these effects, we investigated: (i) whether EtOH was able to release corticotrophic releasing hormone (CRH) from incubated hypothalamic explants; (ii) whether acetaldehyde (ACD), its first metabolite formed in the brain by catalase activity, might play a role in EtOH activity. To this aim, rat hypothalamic explants were incubated with: (i) medium containing EtOH at 32.6 x 10(3) microM; (ii) different concentration of ACD (1, 3, 10, and 30 microM); (iii) EtOH plus 3amino-1,2,4-triazole (3AT, 32 x 10(3) microM) an inhibitor of cerebral catalase; (iv) ACD plus D-penicillamine (DP, 50.3 x 10(3) microM) an ACD-trapping agent. CRH levels were evaluated by a radioimmunoassay.

RESULTS

Incubation with EtOH induced a 7-fold increase in CRH secretion, with respect to basal levels; ACD was able to stimulate CRH release in a dose-dependent manner; the inhibition of cerebral catalase by 3AT blocked EtOH-induced CRH outflow; the inactivation of ACD by DP reverted the ACD-stimulating effect on CRH secretion.

CONCLUSIONS

These data show that both EtOH and acetaldehyde are able to increase hypothalamic CRH release from the rat hypothalamus and that acetaldehyde itself appears to be the mediator of EtOH activity.

Putative role of brain acetaldehyde in ethanol addiction

The putative contribution of brain acetaldehyde (AcH) to ethanol (EtOH) tolerance and dependence (addiction) is reviewed. Although the role of AcH in EtOH addiction has been controversial, there are data showing a relationship. AcH can be formed in the brain tissues through the peroxidatic activity of catalase and by oxidation via other oxidizing enzymes such as cytochrome P-4502E1. Significant formation of AcH occurs in vitro in brain tissue at concentrations of EtOH that can be achieved by voluntary consumption of EtOH by rodents. AcH itself possesses reinforcing properties, which suggests that some of the behavioral pharmacological effects attributed to EtOH may be a result of the formation of AcH, and supports the involvement of AcH in EtOH addiction. Modulation of aldehyde dehydrogenase (ALDH) and brain catalase activity can change EtOH-related addictive behaviors presumably by changing AcH levels. Moreover, some condensation reaction products of AcH may promote some actions of EtOH and its consumption. On the basis of the findings, it can be concluded that AcH may mediate some of the CNS actions of EtOH including tolerance and dependence, although further exploration the involvement of AcH in EtOH addiction is warranted.

Ethanol injected into the hypothalamic arcuate nucleus induces behavioral stimulation in rats: an effect prevented by catalase inhibition and naltrexone

It is suggested that some of the behavioral effects of ethanol, including its psychomotor properties, are mediated by beta-endorphin and opioid receptors. Ethanol-induced increases in the release of hypothalamic beta-endorphin depend on the catalasemic conversion of ethanol to acetaldehyde. Here, we evaluated the locomotor activity in rats microinjected with ethanol directly into the hypothalamic arcuate nucleus (ArcN), the main site of beta-endorphin synthesis in the brain and a region with high levels of catalase expression. Intra-ArcN ethanol-induced changes in motor activity were also investigated in rats pretreated with the opioid receptor antagonist, naltrexone (0-2 mg/kg) or the catalase inhibitor 3-amino-1,2,4-triazole (AT; 0-1 g/kg). We found that ethanol microinjections of 64 or 128, but not 256 microg, produced locomotor stimulation. Intra-ArcN ethanol (128 microg)-induced activation was prevented by naltrexone and AT, whereas these compounds did not affect spontaneous activity. The present results support earlier evidence indicating that the ArcN and the beta-endorphinic neurons of this nucleus are necessary for ethanol to induce stimulation. In addition, our data suggest that brain structures that, as the ArcN, are rich in catalase may support the formation of ethanol-derived pharmacologically relevant concentrations of acetaldehyde and, thus be of particular importance for the behavioral effects of ethanol.

Reduction in the anxiolytic effects of ethanol by centrally formed acetaldehyde: the role of catalase inhibitors and acetaldehyde-sequestering agents

RATIONALE

Considerable evidence indicates that brain ethanol metabolism mediated by catalase is involved in modulating some of the behavioral and physiological effects of this drug, which suggests that the first metabolite of ethanol, acetaldehyde, may have central actions. Previous results have shown that acetaldehyde administered into the lateral ventricles produced anxiolysis in a novel open arena in rats.

OBJECTIVES

The present studies investigate the effects of centrally formed acetaldehyde on ethanol-induced anxiolysis.

MATERIALS AND METHODS

The effects of the catalase inhibitor sodium azide (SA; 0 or 10 mg/kg, IP) on ethanol-induced anxiolysis (0.0, 0.5, or 1.0 g/kg, IP) were evaluated in CD1 mice in two anxiety paradigms, the elevated plus maze and the dark/light box. Additional studies assessed the effect of the noncompetitive catalase inhibitor 3-amino-1,2,4-triazole (AT; 0.5 g/kg, IP) and the acetaldehyde inactivation agent D: -penicillamine (50 mg/kg, IP) on the plus maze.

RESULTS

SA reduced the anxiolytic effects of ethanol on several parameters evaluated in the elevated plus maze and in the dark/light box. In the plus maze, AT completely blocked and D-penicillamine significantly reduced the anxiolytic properties of ethanol.

CONCLUSIONS

Thus, when cerebral metabolism of ethanol into acetaldehyde is blocked by catalase inhibitors, or acetaldehyde is inactivated, there is a suppressive effect on the anxiolytic actions of ethanol. These data provide further support for the idea that centrally formed or administered acetaldehyde can contribute to some of the psychopharmacological actions of ethanol, including its anxiolytic properties.