Hepatic aldehyde and alcohol dehydrogenases in alcohol-preferring and alcohol-avoiding rat lines

FACTORS CONTRIBUTING TO THE ESCALATION OF ALCOHOL CONSUMPTION

Understanding factors that contribute to the escalation of alcohol consumption is key to understanding how an individual transitions from non/social drinking to AUD and to providing better treatment. In this review, we discuss how the way ethanol is consumed as well as individual and environmental factors contribute to the escalation of ethanol consumption from intermittent low levels to consistently high levels. Moreover, we discuss how these factors are modelled in animals. It is clear a vast array of complex, interacting factors influence changes in alcohol consumption. Some of these factors act early in the acquisition of ethanol consumption and initial escalation, while others contribute to escalation of ethanol consumption at a later stage and are involved in the development of alcohol dependence. There is considerable need for more studies examining escalation associated with the formation of dependence and other hallmark features of AUD, especially studies examining mechanisms, as it is of considerable relevance to understanding and treating AUD.

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.

Predictors of ethanol consumption in adult Sprague-Dawley rats: relation to hypothalamic peptides that stimulate ethanol intake

To investigate mechanisms in outbred animals that increase the propensity to consume ethanol, it is important to identify and characterize these animals before or at early stages in their exposure to ethanol. In the present study, different measures were examined in adult Sprague-Dawley rats to determine whether they can predict long-term propensity to overconsume ethanol. Before consuming 9% ethanol with a two-bottle choice paradigm, rats were examined with the commonly used behavioral measures of novelty-induced locomotor activity and anxiety, as assessed during 15 min in an open-field activity chamber. Two additional measures, intake of a low 2% ethanol concentration or circulating triglyceride (TG) levels after a meal, were also examined with respect to their ability to predict chronic 9% ethanol consumption. The results revealed significant positive correlations across individual rats between the amount of 9% ethanol ultimately consumed and three of these different measures, with high scores for activity, 2% ethanol intake, and TGs identifying rats that consume 150% more ethanol than rats with low scores. Measurements of hypothalamic peptides that stimulate ethanol intake suggest that they contribute early to the greater ethanol consumption predicted by these high scores. Rats with high 2% ethanol intake or high TGs, two measures found to be closely related, had significantly elevated expression of enkephalin (ENK) and galanin (GAL) in the hypothalamic paraventricular nucleus (PVN) but no change in neuropeptide Y (NPY) in the arcuate nucleus (ARC). This is in contrast to rats with high activity scores, which in addition to elevated PVN ENK expression showed enhanced NPY in the ARC but no change in GAL. Elevated ENK is a common characteristic related to all three predictors of chronic ethanol intake, whereas the other peptides differentiate these predictors, with GAL enhanced with high 2% ethanol intake and TG measures but NPY related to activity.

Alcohol dehydrogenase (ADH) isoenzymes and aldehyde dehydrogenase (ALDH) activity in the sera of patients with liver cancer

The principal enzymes catalyzing the conversion of ethanol to acetate are alcohol dehydrogenase (ADH) and aldehyde dehydrogenase (ALDH). The activities of these enzymes are elevated in the serum during the course of alcoholism or cirrhosis. In previous investigations we have found elevated levels of ADH, ALDH, and class I ADH activity in liver cancer cells. It can suggest that these changes may be reflected by enzyme activity in the serum. In this work, the activity of ADH isoenzymes, and ALDH in the sera of patients with liver cancer was measured. Serum samples were taken from 64 patients (28 drinkers, 36 nondrinkers), with liver cancer. 25 patients had primary and 39 metastatic liver tumors. Total ADH activity was measured by photometric method with p-nitrosodimethylaniline (NDMA) as a substrate and ALDH activity by the fluorimetric method with 6-methoxy-2-naphtaldehyde as a substrate. For the measurement of the activity of class I and II isoenzymes we employed the fluorimetric methods, with class-specific fluorogenic substrates. The activity of class III ADH was measured by the photometric method with formaldehyde and class IV with m-nitrobenzaldehyde as a substrate. A statistically significant increase of class I ADH isoenzymes was found in the sera of cancer patients. The median activity of this class isoenzyme in the total cancer group increased about 51% (2.94 mU/L) in the comparison to the control level (1.43 mU/L). The activity of the class I ADH isoenzyme was significantly higher in the sera of patients with metastatic tumors than with primary cancers. The activity of this class in the sera of drinkers and group of moderate drinkers was significantly higher in comparison to the control group and higher in the sera of heavy drinkers when compared with moderate drinking patients. The total ADH activity was significantly higher (44%) among patients with cancer than healthy ones. The activity of class I ADH isoenzymes was elevated only in the serum of patients with metastatic liver cancer. This increase of activity seems to be caused by the enzyme released from liver cancer cells and primary tumors originating in other organs.

Cannabinoid-serotonin interactions in alcohol-preferring vs. alcohol-avoiding mice

Because cannabinoid and serotonin (5-HT) systems have been proposed to play an important role in drug craving, we investigated whether cannabinoid 1 (CB1) and 5-HT(1A) receptor ligands could affect voluntary alcohol intake in two mouse strains, C57BL/6 J and DBA/2 J, with marked differences in native alcohol preference. When offered progressively (3-10% ethanol) in drinking water, in a free-choice procedure, alcohol intake was markedly lower (approximately 70%) in DBA/2 J than in C57BL/6 J mice. In DBA/2 J mice, chronic treatment with the cannabinoid receptor agonist WIN 55,212-2 increased alcohol intake. WIN 55,212-2 effect was prevented by concomitant, chronic CB1 receptor blockade by rimonabant or chronic 5-HT(1A) receptor stimulation by 8-hydroxy-2-(di-n-propylamino)-tetralin, which, on their own, did not affect alcohol intake. In C57BL/6 J mice, chronic treatment with WIN 55,212-2 had no effect but chronic CB1 receptor blockade or chronic 5-HT(1A) receptor stimulation significantly decreased alcohol intake. Parallel autoradiographic investigations showed that chronic treatment with WIN 55,212-2 significantly decreased 5-HT(1A)-mediated [35S]guanosine triphosphate-gamma-S binding in the hippocampus of both mouse strains. Conversely, chronic rimonabant increased this binding in C57BL/6 J mice. These results show that cannabinoid neurotransmission can exert a permissive control on alcohol intake, possibly through CB1-5-HT(1A) interactions. However, the differences between C57BL/6 J and DBA/2 J mice indicate that such modulations of alcohol intake are under genetic control.

The role of acetaldehyde in the neurobehavioral effects of ethanol: A comprehensive review of animal studies

Acetaldehyde has long been suggested to be involved in a number of ethanol's pharmacological and behavioral effects, such as its reinforcing, aversive, sedative, amnesic and stimulant properties. However, the role of acetaldehyde in ethanol's effects has been an extremely controversial topic during the past two decades. Opinions ranged from those virtually denying any role for acetaldehyde in ethanol's effects to those who claimed that alcoholism is in fact "acetaldehydism". Considering the possible key role of acetaldehyde in alcohol addiction, it is critical to clarify the respective functions of acetaldehyde and ethanol molecules in the pharmacological and behavioral effects of alcohol consumption. In the present paper, we review the animal studies reporting evidence that acetaldehyde is involved in the pharmacological and behavioral effects of ethanol. A number of studies demonstrated that acetaldehyde administration induces a range of behavioral effects. Other pharmacological studies indicated that acetaldehyde might be critically involved in several effects of ethanol consumption, including its reinforcing consequences. However, conflicting evidence has also been published. Furthermore, it remains to be shown whether pharmacologically relevant concentrations of acetaldehyde are achieved in the brain after alcohol consumption in order to induce significant effects. Finally, we review current evidence about the central mechanisms of action of acetaldehyde.

Genetic polymorphism in ethanol metabolism: acetaldehyde contribution to alcohol abuse and alcoholism

Acetaldehyde, the first product of ethanol metabolism, has been speculated to be involved in many pharmacological and behavioral effects of ethanol. In particular, acetaldehyde has been suggested to contribute to alcohol abuse and alcoholism. In the present paper, we review current data on the role of acetaldehyde and ethanol metabolism in alcohol consumption and abuse. Ethanol metabolism involves several enzymes. Whereas alcohol dehydrogenase metabolizes the bulk of ethanol within the liver, other enzymes, such as cytochrome P4502E1 and catalase, also contributes to the production of acetaldehyde from ethanol oxidation. In turn, acetaldehyde is metabolized by the enzyme aldehyde dehydrogenase. In animal studies, acetaldehyde is mainly reinforcing particularly when injected directly into the brain. In humans, genetic polymorphisms of the enzymes alcohol dehydrogenase and aldehyde dehydrogenase are also associated with alcohol drinking habits and the incidence of alcohol abuse. From these human genetic studies, it has been concluded that blood acetaldehyde accumulation induces unpleasant effects that prevent further alcohol drinking. It is therefore speculated that acetaldehyde exerts opposite hedonic effects depending on the localization of its accumulation. In the periphery, acetaldehyde is primarily aversive, whereas brain acetaldehyde is mainly reinforcing. However, the peripheral effects of acetaldehyde might also be dependent upon its peak blood concentrations and its rate of accumulation, with a narrow range of blood acetaldehyde concentrations being reinforcing.

Development of an Experimental System for Evaluation of Stress Effect on Ethyl Alcohol Metabolism Using Radiorespirometric Analysis in Rat

We attempted to develop an experimental system for evaluation of stress effect on ethyl alcohol metabolism by the radiorespirometric analysis using [(14)C]ethyl alcohol. Each rat was immobilized in an adjustable restraint device for 3 h. The excretion of (14)CO(2) in the breath was significantly decreased by restraint stress. The liver alcohol dehydrogenase activity was also decreased by restraint stress. We think that radiorespirometric analysis may be useful for the evaluation of stress effect on real metabolism of ethyl alcohol in the liver.

Acetaldehyde-reinforcing effects: differences in low-alcohol-drinking (UChA) and high-alcohol-drinking (UChB) rats

It has been suggested that acetaldehyde has a biphasic effect on voluntary alcohol consumption. At low brain concentration, it might exert reinforcing effects, whereas high acetaldehyde levels would be predominantly aversive. The objective of the current study was to compare the effect of an intraperitoneal dose of acetaldehyde (50 mg/kg) in high-alcohol-drinking (UChB) and low-alcohol-drinking (UChA) rat lines, which differ in the activity of the brain mitochondrial class 2 aldehyde dehydrogenase (ALDH2) as a consequence of differences in their ALDH2 genotypes. A classical place-conditioning procedure was used to determine the reinforcing or aversive (or both) effects of acetaldehyde in ethanol-naive UChB and UChA rats. Environmental cues were paired with an intraperitoneal 50-mg/kg injection of acetaldehyde. On 10 consecutive days, each rat received one place conditioning per day; the acetaldehyde-pairing was alternated with saline-pairing. Results showed that conditioning with the 50-mg/kg dose of acetaldehyde induced place preference in UChB rats and place aversion in UChA rats. In a second experiment, UChB and UChA rats, pretested for ethanol preference, were injected with one 50-mg/kg dose of acetaldehyde or saline and tested for their voluntary ethanol consumption during 4 weeks. Results showed that the acetaldehyde dose induced a persistent and long-lasting enhancement of ethanol intake in UChB rats, but not in UChA rats. These results, together with the finding that after administration of a 50-mg/kg dose of acetaldehyde cerebral venous blood acetaldehyde levels in UChA rats were consistently higher than levels in UChB rats, support the suggestion that differential acetaldehyde levels, differential brain ALDH2 activity, or both were responsible for the different effects of acetaldehyde in the two rat lines.

Comparative analysis of hepatic ethanol metabolism in Fawn-Hooded and Wistar-Kyoto rats

Results of a number of studies have supported the suggestion that a correlation exists between voluntary ethanol consumption and enhanced ethanol metabolism in some (but not all) rodent strains. However, as yet, the capacity for alcohol-preferring Fawn-Hooded (FH) rats to metabolize ethanol has not been investigated. Hence, the aim of the current study was to compare the activities of the major hepatic enzymes involved in ethanol metabolism--cytosolic alcohol dehydrogenase (ADH) and mitochondrial aldehyde dehydrogenase (ALDH)--in the FH rat and its alcohol-nonpreferring counterpart, the Wistar-Kyoto (WKY) rat. In addition, the effect of chronic (5 weeks in vivo) ethanol pretreatment on the activity of these enzymes was investigated. Alcohol-naive FH rats were found to have significantly higher ADH activity (+61%) and no significant change in ALDH activity when compared with findings for WKY rats. In addition, chronic ethanol self-administration produced a small increase in ADH activity (+14%) in WKY rats only. Taken as a whole, these findings are the first to demonstrate an increased in vitro hepatic ethanol metabolism in alcohol-preferring FH rats and further demonstrate an association between hepatic ethanol metabolism and voluntary ethanol self-administration in rodents.

The role of acetaldehyde in the actions of alcohol (update 2000)

BACKGROUND

Recent advances in the field of acetaldehyde (AcH) research have raised the need for a comprehensive review on the role of AcH in the actions of alcohol. This update is an attempt to summarize the available AcH research.

METHODS

The descriptive part of this article covers not only recent research but also the development of the field. Special emphasis is placed on mechanistic analyses, new hypotheses, and conclusions.

RESULTS

Elevated AcH during alcohol intoxication causes alcohol sensitivity, which involves vasodilation associated with increased skin temperature, subjective feelings of hotness and facial flushing, increased heart and respiration rate, lowered blood pressure, sensation of dry mouth or throat associated with bronchoconstriction and allergy reactions, nausea and headache, and also reinforcing reactions like euphoria. These effects seem to involve catecholamine, opiate peptide, prostaglandin, histamine, and/or kinin mechanisms. The contribution of AcH to the pathological consequences of chronic alcohol intake is well established for different forms of cancer in the digestive tract and the upper airways. AcH seems to play a role in the etiology of liver cirrhosis. AcH may have a role in other pathological developments, which include brain damage, cardiomyopathy, pancreatitis, and fetal alcohol syndrome. AcH creates both unpleasant aversive reactions that protect against excessive alcohol drinking and euphoric sensations that may reinforce alcohol drinking. The protective effect of AcH may be used in future treatments that involve gene therapy with or without liver transplantation.

CONCLUSIONS

AcH plays a role in most of the actions of alcohol. The individual variability in these AcH-mediated actions will depend on the genetic polymorphism, not only for the alcohol and AcH-metabolizing enzymes but also for the target sites for AcH actions. The subtle balance between aversive and reinforcing, protecting and promoting factors will determine the overall behavioral and pathological developments.

The influence of co-exposure to dimethyldithiocarbamate on butadiene metabolism

Treatment of rats and mice with a single oral dose of dimethyldithiocarbamate (DMDTC; 250 mg/kg) had a marked effect on hepatic CYP2E1 and aldehyde dehydrogenase activities, measured in vitro, for up to 24 h after dosing. The same treatment did not affect CYP2A6, glutathione S-transferase, epoxide hydrolase, alcohol dehydrogenase activities or hepatic glutathione levels. As a consequence of the loss of CYP2E1 activity, butadiene metabolism in liver fractions from DMDTC treated rats and mice was markedly reduced, as was the metabolism of the mono-epoxide to the di-epoxide in mouse liver. The conversion of the mono-epoxide to the diol by epoxide hydrolases was not affected by DMDTC treatment. Urinary excretion of radioactivity, following dosing with DMDTC and exposure to 200 ppm C-14 butadiene for 6 h, was markedly reduced in rats, but increased in mice. The profiles of urinary metabolites were qualitatively similar from mice exposed to butadiene to those exposed after dosing with DMDTC. In the rat, pre-dosing with DMDTC resulted in the formation of three additional urinary metabolites following exposure to butadiene. Overall, DMDTC appears to impact qualitatively and quantitatively on the metabolism of butadiene. The nature and full significance of these changes has yet to be characterised.

The role of acetaldehyde in the actions of alcohol (update 2000)

BACKGROUND

Recent advances in the field of acetaldehyde (AcH) research have raised the need for a comprehensive review on the role of AcH in the actions of alcohol. This update is an attempt to summarize the available AcH research.

METHODS

The descriptive part of this article covers not only recent research but also the development of the field. Special emphasis is placed on mechanistic analyses, new hypotheses, and conclusions.

RESULTS

Elevated AcH during alcohol intoxication causes alcohol sensitivity, which involves vasodilation associated with increased skin temperature, subjective feelings of hotness and facial flushing, increased heart and respiration rate, lowered blood pressure, sensation of dry mouth or throat associated with bronchoconstriction and allergy reactions, nausea and headache, and also reinforcing reactions like euphoria. These effects seem to involve catecholamine, opiate peptide, prostaglandin, histamine, and/or kinin mechanisms. The contribution of AcH to the pathological consequences of chronic alcohol intake is well established for different forms of cancer in the digestive tract and the upper airways. AcH seems to play a role in the etiology of liver cirrhosis. AcH may have a role in other pathological developments, which include brain damage, cardiomyopathy, pancreatitis, and fetal alcohol syndrome. AcH creates both unpleasant aversive reactions that protect against excessive alcohol drinking and euphoric sensations that may reinforce alcohol drinking. The protective effect of AcH may be used in future treatments that involve gene therapy with or without liver transplantation.

CONCLUSIONS

AcH plays a role in most of the actions of alcohol. The individual variability in these AcH-mediated actions will depend on the genetic polymorphism, not only for the alcohol and AcH-metabolizing enzymes but also for the target sites for AcH actions. The subtle balance between aversive and reinforcing, protecting and promoting factors will determine the overall behavioral and pathological developments.

Conditioned stimulus preference after acetaldehyde but not ethanol injections

Acetaldehyde, the first ethanol metabolite, has been suggested to mediate some of the behavioral effects of ethanol and particularly its reinforcing properties, although this later hypothesis remains extremely controversial. While several studies demonstrated the reinforcing effects of brain acetaldehyde, blood acetaldehyde accumulation is believed to be primarily aversive. In the present study, a conditioned reinforcement procedure has been used to investigate the reinforcing and/or aversive effects of intraperitoneal injections of both acetaldehyde and ethanol in Wistar rats. An olfactory stimulus was paired with daily injections of either ethanol (0, 0.25, 0.5, 1 and 2 g/kg) or acetaldehyde (0, 10, 20, 100 and 150 mg/kg). After eight conditioning sessions, all rats were tested for their stimulus preference or aversion. The results show that conditioning with small, 0.25 and 0.5 g/kg, ethanol doses induced neither preference nor aversion for the olfactory cue. In contrast, higher ethanol doses (1.0 and 2.0 g/kg) resulted in significant stimulus aversions. Acetaldehyde conditioning led to a biphasic stimulus preference, with a maximal preference around 20 mg/kg acetaldehyde. No evidence of aversive effects was found with increasing doses of acetaldehyde, even with concentrations close to the lethal limit. The present study clearly shows that systemic acetaldehyde injections induced significant stimulus preferences. This suggests that acetaldehyde may be, at least in part, responsible for the reinforcing effects of alcohol intake.

Polymorphism of alcohol-metabolizing genes affects drinking behavior and alcoholic liver disease in Japanese men

Alcohol is known to be mainly metabolized in the liver by alcohol dehydrogenase 2 (ADH2) and aldehyde dehydrogenase 2 (ALDH2), and cytochrome P-450IIEI. The purpose of this study was to clarify the role of polymorphism of these ethanol-metabolizing enzymes in drinking behavior and the progression of alcoholic liver disease among Japanese men. Polymorphism of the ADH2, ALDH2, and P-45IIEI genes were determined by polymerase chain reaction, followed by restriction fragment-length polymorphism analysis in 189 normal Japanese men and 26 male patients with alcoholic liver disease. Drinking behavior was estimated by self-assessment according to DSM-III-R criteria. Facial flushing was reported in 91 subjects heterozygous for ALDH2*1/*2 and in two subjects homozygous for ALDH2*2/*2, but was not found in 96 subjects homozygous for ALDH2*1/*1. In contrast, polymorphism of ADH2 and P-450IIEI did not differ between flushers and nonflushers. Although the flushers only drank a small amount of alcohol (< 20 g of ethanol/day), the nonflushers were divided into a group of moderate drinkers (20 to 80 g/day; n = 54) and a group of heavy drinkers (> 80 g/day; n = 42). A high preponderance of heterozygosity for the ADH2*1/*2 genes (20/42; 60%) and a high frequency of the ADH2*1 allele were found in heavy drinkers, compared with moderate drinkers. However, cytochrome P-45IIEI gene polymorphism was similar among the moderate and heavy drinkers. Not only a high frequency of the ALDH2*1 and ADH2*1 alleles, but also a high frequency of the P-450IIEI c2 allele was found in the patients with alcoholic liver disease. From these results, the drinking behavior of Japanese men is strongly influenced by the ALDH2*1 allele, and the level of alcohol intake is affected by the ADH2*1 allele, but not by cytochrome P-45IIEI. However, progression to alcoholic liver disease among heavy drinkers may be affected by the cytochrome P-450IIEI c2 allele.

Voluntary alcohol drinking and acetaldehyde metabolism in F2 hybrid crosses of AA and ANA rat lines

Alcohol-preferring AA and alcohol-avoiding ANA rat lines differ in their acetaldehyde metabolism and this has been suggested to be one reason for their different ethanol drinking behavior. To study whether acetaldehyde accumulation is indeed associated with alcohol drinking behavior and to evaluate which enzymatic differences previously observed in these rat lines are of importance in this regard, we produced an F2 generation from them. ADH and ALDH activities, and ALDH patterns were then assessed from these hybrids and correlated with their voluntary ethanol drinking and blood acetaldehyde concentrations measured during ethanol metabolism. A significant negative correlation between voluntary ethanol intake and blood acetaldehyde concentration was observed in F2 females drinking less than 17% of the total fluid as ethanol. In F2 males, hepatic microsomal high Km ALDH activities correlated negatively with blood acetaldehyde concentrations, indicating that low activity of this isoenzyme in ANA rats could be at least in part responsible for the accumulation of acetaldehyde in their blood. Finally, F2 rats that possessed the cytosolic ALDH isoenzyme pattern most frequently found in the AA rat line drank significantly more ethanol than the animals with typical ANA pattern, suggesting that this polymorphism might also be relevant in the regulation of voluntary ethanol drinking although it is probably not associated with acetaldehyde metabolism.

Role of acetaldehyde in the actions of ethanol on the brain--a review

Over the last 30 years, acetaldehyde has been postulated to mediate various actions of ethanol on the brain. Experiments have studied ethanol consumption after acetaldehyde infusions into the brain, in rodents with high or low activities of hepatic and brain ethanol-metabolizing enzymes, and after treatment with drugs that alter the metabolism of acetaldehyde after ethanol ingestion. Evidence that acetaldehyde is involved in the actions of ethanol has been inconsistent because of the lack of knowledge of the brain acetaldehyde concentrations required to exert their effects, the lack of correlation between the activities of ethanol-metabolizing enzymes across strains of rodents and ethanol consumption, and the lack of specificity of drugs altering acetaldehyde metabolism. The formation of significant amounts of acetaldehyde the brain in vivo after ethanol ingestion and by what mechanism has not been clearly established, although catalase is a promising candidate. Future research needs to directly demonstrate in brain the formation of acetaldehyde in vivo, determine the concentrations in brain areas involved in ethanol consumption, and evaluate the possible actions of drugs other than an ability to block acetaldehyde metabolism.