Progress towards finding genes for alcoholism in mice

Oral operant ethanol self-administration in the absence of explicit cues, food restriction, water restriction and ethanol fading in C57BL/6J mice

RATIONALE

Mouse models of ethanol (EtOH) self-administration are useful to identify genetic and biological underpinnings of alcohol use disorder.

OBJECTIVES

These experiments developed a novel method of oral operant EtOH self-administration in mice without explicitly paired cues, food/water restriction, or EtOH fading.

METHODS

Following magazine and lever training for 0.2 % saccharin (SAC), mice underwent nine weekly overnight sessions with lever pressing maintained by dipper presentation of 0, 3, 10, or 15 % EtOH in SAC or water vehicle. Ad libitum water was available from a bottle.

RESULTS

Water vehicle mice ingested most fluid from the water bottle in contrast to SAC vehicle mice, which despite lever pressing demands, drank most of their fluid from the liquid dipper. Although EtOH in SAC vehicle mice showed concentration-dependent increases of g/kg EtOH intake, lever pressing decreased with increasing EtOH concentration and did not exceed that of SAC vehicle alone at any EtOH concentration. Mice reinforced with EtOH in water ingested less EtOH than mice reinforced with EtOH in SAC. EtOH in water mice, however, showed concentration-dependent increases in g/kg EtOH intake and lever presses. Fifteen percent EtOH in water mice showed significantly greater levels of lever pressing than water vehicle mice and a significant escalation of responding across weeks of exposure. Naltrexone pretreatment reduced EtOH self-administration and intake in these mice without altering responding in the vehicle control condition during the first hour of the session.

CONCLUSIONS

SAC facilitated EtOH intake but prevented observation of EtOH reinforcement. Water vehicle unmasked EtOH's reinforcing effects.

Mouse inbred strain differences in ethanol drinking to intoxication

Recently, we described a simple procedure, Drinking in the Dark (DID), in which C57BL/6J mice self-administer ethanol to a blood ethanol concentration (BEC) above 1 mg/ml. The test consists of replacing the water with 20% ethanol in the home cage for 4 h early during the dark phase of the light/dark cycle. Three experiments were conducted to explore this high ethanol drinking model further. In experiment 1, a microanalysis of C57BL/6J behavior showed that the pattern of ethanol drinking was different from routine water intake. In experiment 2, drinking impaired performance of C57BL/6J on the accelerating rotarod and balance beam. In experiment 3, 12 inbred strains were screened to estimate genetic influences on DID and correlations with other traits. Large, reliable differences in intake and BEC were detected among the strains, with C57BL/6J showing the highest values. Strain means were positively correlated with intake and BEC in the standard (24 h) and a limited (4 h) two-bottle ethanol vs. water test, but BECs reached higher levels for DID. Strain mean correlations with other traits in the Mouse Phenome Project database supported previously reported genetic relationships of high ethanol drinking with low chronic ethanol withdrawal severity and low ethanol-conditioned taste aversion. We extend these findings by showing that the correlation estimates remain relatively unchanged even after correcting for phylogenetic relatedness among the strains, thus relaxing the assumption that the strain means are statistically independent. We discuss applications of the model for finding genes that predispose pharmacologically significant drinking in mice.

Patterns of brain activation associated with contextual conditioning to methamphetamine in mice

Classical conditioning is thought to play a key role in addiction. The authors used c-Fos immunohistochemistry to demonstrate a conditioned physiological response to methamphetamine (meth) in mice. Male outbred mice were placed into an environment where they had previously experienced 2 mg/kg meth or saline. The meth-paired mice displayed increased c-Fos in several brain regions, including the nucleus accumbens, prefrontal cortex, orbitofrontal cortex, basolateral amygdala, and bed nucleus of the stria terminalis. No conditioned locomotor activity was observed, but individual activity levels strongly correlated with c-Fos in many regions. A batch effect among immunohistochemical assays was demonstrated. Results implicate specific brain regions in classical conditioning to meth and demonstrate the importance of considering locomotor activity and batch in a c-Fos study.

Evaluation of a simple model of ethanol drinking to intoxication in C57BL/6J mice

Because of intrinsic differences between humans and mice, no single mouse model can represent all features of a complex human trait such as alcoholism. It is therefore necessary to develop partial models. One important feature is drinking to the point where blood ethanol concentration (BEC) reaches levels that have measurable affects on physiology and/or behavior (>1.0 mg ethanol/ml blood). Most models currently in use examine relative oral self-administration from a bottle containing alcohol versus one containing water (two-bottle preference drinking), or oral operant self-administration. In these procedures, it is not clear when or if the animals drink to pharmacologically significant levels because the drinking is episodic and often occurs over a 24-h period. The aim of this study was to identify the optimal parameters and evaluate the reliability of a very simple procedure, taking advantage of a mouse genotype (C57BL/6J) that is known to drink large quantities of ethanol. We exchanged for the water bottle a solution containing ethanol in tap water for a limited period, early in the dark cycle, in the home cage. Mice regularly drank sufficient ethanol to achieve BEC>1.0 mg ethanol/ml blood. The concentration of ethanol offered (10%, 20% or 30%) did not affect consumption in g ethanol/kg body weight. The highest average BEC ( approximately 1.6 mg/ml) occurred when the water-to-ethanol switch occurred 3 h into the dark cycle, and when the ethanol was offered for 4 rather than 2 h. Ethanol consumption was consistent within individual mice, and reliably predicted BEC after the period of ethanol access. C57BL/6J mice from three sources provided equivalent data, while DBA/2J mice drank much less than C57BL/6J in this test. We discuss advantages of the model for high-throughput screening assays where the goal is to find other genotypes of mice that drink excessively, or to screen drugs for their efficacy in blocking excessive drinking.