The Role of Acetaldehyde in the Central Effects of Ethanol

The role of acetaldehyde in mediating effects of alcohol on expression of endogenous opioid system genes in a neuroblastoma cell line

Ethanol (EtOH) alters neural activity through interaction with various neurotransmitters and neuromodulators. The endogenous opioid system seems to play a key role in the activities of EtOH, since the opioid antagonist naltrexone (ReVia) attenuates craving. We have investigated the transcriptional regulation of opioid system genes in response to EtOH exposure for up to 96 h in human neuroblastoma SH-SY5Y cells using quantitative real-time polymerase chain reaction. We observed a significant decrease in the expression of opioid peptide precursors (proopiomelanocortin, proenkephalin, and prodynorphin) and of the kappa opioid receptor after 48 and 72 h of EtOH exposure (10 and 40 mM). These alterations were not present when the EtOH metabolism was blocked by 4-methylpyrazole. To evaluate whether the effects evoked by EtOH were possibly due to the first product of EtOH metabolism, cells were exposed to 0.4 mM acetaldehyde. We observed the same pattern of changes for prodynorphin, proenkephalin, and the kappa opioid receptor as after 72 h exposure to EtOH. These results contribute to our understanding of EtOH action at a cellular level and provide evidence of the role of acetaldehyde in mediating some of the EtOH-induced effects.

Pharmacokinetic and pharmacodynamic basis for partial protection against alcoholism in Asians, heterozygous for the variant ALDH2*2 gene allele

OBJECTIVES

It has been well documented that although homozygosity of the variant aldehyde dehydrogenase-2 (ALDH2) gene allele, ALDH2*2, in Asians almost fully protects against alcoholism, the heterozygosity only affords a partial protection to varying degrees. The full protection against alcoholism has been ascribed to the low-amount of alcohol hypersensitivity accompanied by a prolonged and large accumulation of acetaldehyde in blood (Peng et al. Pharmacogenetics 1999; 9:463-476). The physiological basis for the partial protection, however, remains unclear.

METHODS

To address this question, we recruited a total of 32 adult Han Chinese men, matched by age, body-mass index, and nutritional state from a population base of 404 men. The subjects were divided into 3 combinatorial genotypic groups of alcohol dehydrogenase (ADH) and ALDH, that is, ALDH2*1/*1-ADH1B*1/*1-ADH1C*1/*1 (n=8), ALDH2*1/*1-ADH1B*2/*2-ADH1C*1/*1 (n=8), and ALDH2*1/*2-ADH1B*2/*2-ADH1C*1/*1 (n=16). Following a moderate dose of ethanol (0.5 g/kg body weight), blood ethanol/acetaldehyde/acetate concentrations, cardiac and extracranial/intracranial arterial hemodynamic parameters, as well as self-rated subjective sensations, were measured for 130 min.

RESULTS

Heterozygotic ALDH2*1/*2 subjects were found to be strikingly responsive to the moderate amount of alcohol, as evidenced by the prominent cardiovascular effects as well as subjective perceptions of general discomfort for as long as 2 h following ingestion. The ADH1B polymorphisms did not appear to correlate with the pharmacokinetic and pharmacodynamic effects.

CONCLUSIONS

These results indicate that acetaldehyde, rather than ethanol or acetate, is primarily responsible for the observed alcohol sensitivity reactions and suggest that substantially lower accumulation of blood acetaldehyde in the heterozygotes significantly reduces the aversion reaction to low amounts of alcohol that permits other biological as well as environmental factors to facilitate drinking and the according risk for the disease.

Enzymatic mechanisms of ethanol oxidation in the brain

BACKGROUND

The exact enzymatic mechanisms of ethanol oxidation in the brain are still unclear. The catalase-mediated oxidation of ethanol was demonstrated in rat brain using incubation of brain homogenates with catalase inhibitors. The role of the alcohol dehydrogenase (ADH) or cytochrome P450-dependent system in this process is possible, but has not been confirmed. The objective of the study was to determine the contribution of the different enzymatic pathways to ethanol oxidation in brain homogenates from mice and rats.

METHODS

Three approaches were used to investigate the enzymatic mechanisms of ethanol oxidation in the brain of rats and mice: (1) preincubation of brain homogenates with inhibitors of the ethanol-metabolizing enzymes (catalase, CYP2E1, ADH, and ALDH); (2) utilization of mice with genetic deficiency in ethanol-metabolizing enzymes (catalase, CYP2E1, or both enzymes); and (3) determination of ethanol oxidation in brain subcellular fractions known to have differential activity of ethanol-metabolizing enzymes. The ethanol-derived acetaldehyde (AC) and acetate were determined in brain samples by gas chromatography.

RESULTS

The catalase inhibitors sodium azide (5 mM) and aminotriazole (5 mM) as well as CYP2E1 inhibitors diallyl sulfide (2 mM) and beta-phenethyl isothiocyanate (0.1 mM) lowered significantly the accumulation of the ethanol-derived AC and acetate in brain homogenates. The ADH inhibitor 4-methyl pyrazole (5 mM) significantly decreased the acetate but not the AC accumulation. Ethanol-derived AC accumulation in brain homogenates of acatalasemic mice was 47% of the control value, 91% in CYP2E1-null mice, and 24% in double mutants (with deficiency of both catalase and CYP2E1). The highest levels of ethanol oxidation were found in microsomal and peroxisomal subcellular brain fractions, where CYP2E1 and catalase are located, respectively.

CONCLUSIONS

Catalase is the key enzyme of ethanol oxidation in the brain of rodents: it may be responsible for about 60% of the process. CYP2E1 plays an important role in ethanol oxidation in the rodent brains. Alcohol dehydrogenase plays a minor role, if any, in this process. Aldehyde dehydrogenase plays the crucial role in the further oxidation of ethanol-derived AC in the brain homogenates.

The alcohol-preferring AA and alcohol-avoiding ANA rats: neurobiology of the regulation of alcohol drinking

The AA (alko, alcohol) and ANA (alko, non-alcohol) rat lines were among the earliest rodent lines produced by bidirectional selection for ethanol preference. The purpose of this review is to highlight the strategies for understanding the neurobiological factors underlying differential alcohol-drinking behavior in these lines. Most early work evaluated functioning of the major neurotransmitter systems implicated in drug reward in the lines. No consistent line differences were found in the dopaminergic system either under baseline conditions or after ethanol challenges. However, increased opioidergic tone in the ventral striatum and a deficiency in endocannabinoid signaling in the prefrontal cortex of AA rats may comprise mechanisms leading to increased ethanol consumption. Because complex behaviors, such as ethanol drinking, are not likely to be controlled by single factors, system-oriented molecular-profiling strategies have been used recently. Microarray based expression analysis of AA and ANA brains and novel data-mining strategies provide a system biological view that allows us to formulate a hypothesis on the mechanism underlying selection for ethanol preference. Two main factors appear active in the selection: a recruitment of signal transduction networks, including mitogen-activated protein kinases and calcium pathways and involving transcription factors such as Creb, Myc and Max, to mediate ethanol reinforcement and plasticity. The second factor acts on the mitochondrion and most likely provides metabolic flexibility for alternative substrate utilization in the presence of low amounts of ethanol.

Motor stimulant effects of ethanol injected into the substantia nigra pars reticulata: importance of catalase-mediated metabolism and the role of acetaldehyde

A series of experiments was conducted to investigate the locomotor effects of local injections of ethanol and the ethanol metabolite, acetaldehyde, into substantia nigra pars reticulata (SNr). Infusions of ethanol into SNr resulted in a dose-related increase in locomotor activity, with maximal effects at a dose of 1.4 micromol. Ethanol injected into a control site dorsal to substantia nigra failed to stimulate locomotion, and another inactive site was identified in brainstem areas posterior to substantia nigra. The locomotor effects of intranigral ethanol (1.4 micromol) were reduced by coadministration of 10 mg/kg sodium azide, a catalase inhibitor that acts to reduce the metabolism of ethanol into acetaldehyde in the brain. SNr infusions of acetaldehyde, which is the first metabolite of ethanol, also increased locomotion. Taken together, these results indicate that SNr is one of the sites at which ethanol and acetaldehyde may be acting to induce locomotor activity. These results are consistent with the hypothesis that acetaldehyde is a centrally active metabolite of ethanol, and provide further support for the idea that catalase activity is a critical step in the regulation of ethanol-induced motor activity. These studies have implications for understanding the brain mechanisms involved in mediating the ascending limb of the biphasic dose-response curve for the effect of ethanol on locomotor activity.

Ethanol-induced conditioned place preference, but not aversion, is blocked by treatment with D -penicillamine, an inactivation agent for acetaldehyde

RATIONALE

There is evidence to suggest that acetaldehyde is involved in the control of ethanol-seeking behavior and reward. D -penicillamine, a thiol amino acid, is a highly selective agent for the inactivation of acetaldehyde. Previous studies from our laboratory have demonstrated that D -penicillamine prevents both behavioral stimulation induced by ethanol and acetaldehyde-produced locomotor depression in mice.

OBJECTIVES

The contribution of ethanol-derived acetaldehyde to the affective effects of ethanol (preference and aversion) was assessed using an unbiased place conditioning design.

METHODS

Male mice received four pairings of a distinctive floor stimulus (CS+: GRID+ or HOLE+) with injections of saline and ethanol (2 g/kg) given before (preference) or after (aversion) the 5-min exposure to the place conditioning apparatus. A different floor stimulus (CS-: GRID- or HOLE-), associated with saline-saline injections on alternate days, was presented. For a different group of animals, the pairings with the CS+ were associated with saline and ethanol injections, but on alternate days, they received D -penicillamine (50 or 75 mg/kg) and ethanol injections paired with the CS-floor stimulus. A 60-min preference test was carried out 24 h after the last conditioning trial. A similar procedure was followed to test the effect of D -penicillamine on morphine (16 mg/kg) and cocaine-induced (20 mg/kg) conditioned place preference (CPP).

RESULTS

CPP and conditioned place aversion (CPA) were observed for ethanol, but D -penicillamine only blocked CPP. D -penicillamine, by itself, did not produce either rewarding or aversive effects. CPP observed for morphine and cocaine was unaffected by D -penicillamine pretreatment.

CONCLUSIONS

The results of the present study suggest that the selective inactivation of acetaldehyde blocked the rewarding, but not aversive, effects of ethanol and support the role of this ethanol metabolite in the affective properties of ethanol.