Factors regulating ethanol tolerance

Modulation of BK Channels by Ethanol

In alcohol-naïve systems, ethanol (<100mM) exposure of calcium-gated BK channels perturbs physiology and behavior. Brief (several minutes) ethanol exposure usually leads to increased BK current, which results from ethanol interaction with a pocket mapped to the BK channel-forming slo1 protein cytosolic tail domain. The importance of this region in ethanol-induced intoxication has been independently supported by an unbiased screen of Caenorhabditis elegans slo1 mutants. However, ethanol-induced BK activation is not universal as refractoriness and inhibition have been reported. The final effect depends on many factors, including intracellular calcium levels, slo1 isoform, BK beta subunit composition, posttranslational modification of BK proteins, channel lipid microenvironment, and type of ethanol administration. Studies in Drosophila melanogaster, C. elegans, and rodents show that protracted/repeated ethanol administration leads to tolerance to ethanol-induced modification of BK-driven physiology and behavior. Unveiling the mechanisms underlying tolerance is of major importance, as tolerance to ethanol has been proposed as predictor of risk for alcoholism.

Low ethanol sensitivity and increased ethanol consumption in mice lacking adenosine A2A receptors

We have shown previously that the severity of handling-induced convulsions during ethanol withdrawal was reduced in A2A receptor knock-out (A2AR-/-) mice. In the present report, we further characterize the role of adenosine A(2A) receptors in ethanol consumption and neurobiological responses to this drug of abuse. Male A2AR-/- mice showed increased consumption of solutions containing 6 and 20% (v/v) ethanol compared with wild-type (A2AR+/+) control mice; female A2AR-/- mice showed increased consumption of solutions containing 6 and 10% ethanol. This slightly higher ethanol consumption was also related to increased ethanol preference. In contrast, A2AR-/- mice showed normal consumption of solutions containing either sucrose or quinine. Relative to A2AR+/+ mice, A2AR-/- mice were found to be less sensitive to the sedative effect of 3.0 gm/kg ethanol, as measured by more rapid recovery from ethanol-induced loss of righting reflex, and to the hypothermic effects of 1.5, 3.0, and 4.0 gm/kg ethanol, although plasma ethanol levels did not differ significantly between the two genotypes. The selective adenosine A2A receptor antagonist ZM 241385 (4-(2-[7-amino-2-(2-furyl)[1,2,4]triazolo[2,3-a][1,3,5]triazin-5-ylamino]ethyl)phenol) (10-30 mg/kg) significantly attenuated ethanol-induced (4.0 gm/kg) hypothermia in CD1 mice. To assess whether ethanol administration would induce differential tolerance in A2AR-/- and wild-type mice, we administered ethanol (3.0 gm/kg) over 4 consecutive days and found no difference in the development of tolerance; however, female A2AR-/- mice showed a lower tolerance-acquisition rate. These data suggest that activating the A2A receptors may play a role in suppressing alcohol-drinking behavior and is associated with the sensitivity to the intoxicating effects of acute ethanol administration.

Chronic ethanol consumption induces tolerance to the spatial memory impairing effects of acute ethanol administration in rats

A large number of studies in rats have investigated the effects of acute and chronic ethanol administration on performance on many spatial learning and memory tasks. However, no study has addressed the problem of whether chronic ethanol consumption induces tolerance to acute ethanol-induced spatial memory deficits. In this study, we analyzed the behavioral effects of acute ethanol administration on spatial memory and locomotor activity in rats chronically intoxicated by ethanol. Male Sprague-Dawley rats were given as their only available liquid source a 10% (v/v) aqueous ethanol solution for 2 weeks before behavioral testing and during the 1-week behavioral testing period. They were treated intraperitoneally with 1.5 g/kg of ethanol 30 min before daily training in the Morris water maze, a spatial memory task sensitive to hippocampal damage. Our results demonstrate that learning and spatial memory of ethanol-consuming animals were not altered compared with control rats. Chronic ethanol consumption had no effect on spatial reference memory in terms of either the distance traveled to find the hidden platform during the acquisition phase of the experiment, or the time spent in the training quadrant during the retention trial. Acute ethanol administration impaired spatial memory in control rats and this impairment was reversed in chronic ethanol-consuming animals, revealing that chronic ethanol consumption did induce tolerance to the spatial memory deficits induced by acute ethanol injection, although plasma ethanol levels did not differ between the two groups. In contrast, chronic ethanol consumption did not induce tolerance to the acute ethanol-induced stimulatory locomotor activity measured in the same animals. Our results, therefore, indicate that chronic ethanol consumption induces tolerance to the cognitive impairing effects, but not to the locomotor stimulatory effects of acute ethanol administration in rats, suggesting that these two behavioral effects of ethanol do not share a common mechanism in the CNS.

Effect of epipregnanolone and pregnenolone sulfate on chronic tolerance to ethanol

The aim of the present study was to investigate the influence of neurosteroids on the development of tolerance to ethanol. Male Swiss mice were injected daily with the positive allosteric modulator of the gamma amino butyric acid-A (GABA(A)) receptor epipregnanolone (5beta-pregnan-3beta-ol-20-one; 0.15 mg/kg i.p.) or pregnenolone sulfate (5-pregnen-3beta-ol-20-one sulfate sodium; 0.08 mg/kg i.p.) - considered a negative allosteric modulator of this receptor and/or positive allosteric modulator of the N-methyl-D-aspartate (NMDA) receptor - 30 min before ethanol (2.5 g/kg i.p.). They were tested on the rota-rod apparatus, under continuous acceleration (1rpm/s), at 30, 60 and 90 min after ethanol injections for 5 days. The results showed that tolerance to the motor incoordinating effect of ethanol occurred on the fifth day of treatment when this effect was blocked by pretreatment with epipregnanolone. On the other hand, ethanol tolerance was enhanced by pretreatment with pregnenolone sulfate from the second to the fifth days of treatment. Taken together, our results suggest that neurosteroids can either stimulate or block the development of chronic tolerance to ethanol. Moreover, since neurosteroids can interact with GABA(A) or NMDA receptor systems, our results suggest the involvement of these systems in the actions of neurosteroids upon ethanol tolerance.

The biological, social and clinical bases of drug addiction: commentary and debate

This article summarizes the main discussions at a meeting on the biological, social and clinical bases of drug addiction focused on contemporary topics in drug dependence. Four main domains are surveyed, reflecting the structure of the meeting: psychological and pharmacological factors; neurobiological substrates; risk factors (including a consideration of vulnerability from an environmental and genetic perspective); and clinical treatment. Among the topics discussed were tolerance, sensitization, withdrawal, craving and relapse; mechanisms of reinforcing actions of drugs at the behavioural, cognitive and neural levels; the role of subjective factors in drug dependence; approaches to the behavioural and molecular genetics of drug dependence; the use of functional neuroimaging; pharmaceutical and psychosocial strategies for treatment; epidemiological and sociological aspects of drug dependence. The survey takes into account the considerable disagreements and controversies arising from the discussions, but also reaches a degree of consensus in certain areas.

Cross-tolerance to ethanol and gamma-hydroxybutyric acid

In the present study, the development of tolerance to the motor impairing effects of gamma-hydroxybutyric acid (GHBA) and ethanol was compared (Experiment 1). Rats were required to perform a motor coordination task daily shortly after ethanol (3.5 g/kg) and GHBA (1.0 g/kg) administration for 9 consecutive days. Tolerance to the motor impairing effects of ethanol and GHBA developed to a similar extent but with different patterns. On the tenth day, the presence of cross-tolerance to the motor impairing effects of GHBA and ethanol was assessed (Experiment 2). Administration of 1.0 g/kg GHBA produced a significantly lower impairment in ethanol-tolerant rats than in ethanol-naive rats. Similarly, administration of 3.5 g/kg ethanol induced a significantly lower impairment in GHBA-tolerant rats than in GHBA-naive rats. The presence of cross-tolerance between GHBA and ethanol is discussed in terms of common pathways of neuroadaptation to chronic GHBA and ethanol.

Absence of acute tolerance to ethanol hypnosis in F-344 and BN/BIRIJ rats

Male Fischer 344 (F-344) rats of three different ages (4, 13, and 25 months) and male Brown Norway (BN/BIRIJ) rats (4 months) were injected (IP) with a hypnotic dose of ethanol (3.0 g/kg, 10% w/v in saline). Half of the animals were decapitated and brains extracted immediately upon loss of the righting reflex (LRR), while the other half were allowed to reach recovery of the righting reflex (RRR), at which time brains were extracted. Brain alcohol analyses revealed significantly higher concentrations in young F-344 rats at both LRR and RRR, indicating less sensitivity of target tissue to ethanol hypnosis in the young of that strain. All age groups of the F-344 rats as well as the young BN/BIRIJ rats displayed lower brain concentrations of ethanol at RRR than at LRR, a finding opposite to that characteristic of acute tolerance. The relationship of LRR to RRR values did not differ among the age groups of F-344 rats. We conclude that the F-344 and BN/BIRIJ strains do not develop acute tolerance to ethanol hypnosis. The findings are consistent with the hypothesis that animals with low ethanol preferences, such as the F-344 and BN/BIRIJ strains, also have low capabilities to develop acute tolerance to ethanol hypnosis.

Neurobiology of alcohol abuse

Excessive consumption of beverage alcohol (ethanol) is a major health concern worldwide. Understanding the mechanisms by which ethanol affects neural functioning, after both acute and chronic exposure, has become a major goal in the study of alcoholism. With such an understanding, we should be able to institute more effective treatments and preventative measures for alcohol abuse problems. Recent studies have found, contrary to earlier assumptions, that ethanol has selective, dose-dependent effects on various neurotransmitter systems within the CNS. These effects are observed at all levels of analysis, from molecular to behavioral. This review by Herman Samson and Adron Harris covers these recent findings, with the intent of generating questions that will focus further research efforts.