Chronobiology of alcohol: studies in C57BL/6J and DBA/2J inbred mice

Abstract

Human alcoholics display dramatic disruptions of circadian rhythms that may contribute to the maintenance of excessive drinking, thus creating a vicious cycle. While clinical studies cannot establish direct causal mechanisms, recent animal experiments have revealed bidirectional interactions between circadian rhythms and ethanol intake, suggesting that the chronobiological disruptions seen in human alcoholics are mediated in part by alterations in circadian pacemaker function. The present study was designed to further explore these interactions using C57BL/6J (B6) and DBA/2J (D2) inbred mice, two widely employed strains differing in both circadian and alcohol-related phenotypes. Mice were maintained in running-wheel cages with or without free-choice access to ethanol and exposed to a variety of lighting regimens, including standard light-dark cycles, constant darkness, constant light, and a "shift-lag" schedule consisting of repeated light-dark phase shifts. Relative to the standard light-dark cycle, B6 mice showed reduced ethanol intake in both constant darkness and constant light, while D2 mice showed reduced ethanol intake only in constant darkness. In contrast, shift-lag lighting failed to affect ethanol intake in either strain. Access to ethanol altered daily activity patterns in both B6 and D2 mice, and increased activity levels in D2 mice, but had no effects on other circadian parameters. Thus, the overall pattern of results was broadly similar in both strains, and consistent with previous observations that chronic ethanol intake alters circadian activity patterns while environmental perturbation of circadian rhythms modulates voluntary ethanol intake. These results suggest that circadian-based interventions may prove useful in the management of alcohol use disorders.

Fig. 1

Double-plotted (48-hour span) raster-style circadian activity records for one representative animal from each strain (B6: C57BL/6J; D2: DBA/2J) and treatment group (Ethanol-treated vs. Water-only controls; Sequence 1 vs. Sequence 2). See text for details on experimental sequences. Shaded areas indicate times of light exposure.

Fig. 2

Ethanol intake, ethanol preference, and water intake in B6 and D2 mice during exposure to either a standard 12:12 light–dark cycle (LD) or to shift-lag lighting (see text for details of shift-lag procedure).

Fig. 3

Ethanol intake, ethanol preference, and water intake in B6 and D2 mice during exposure to a standard 12:12 light–dark cycle (LD) and to subsequent free-running conditions. LD–LL: standard LD cycle followed by constant light (LL); LD–DD: standard LD cycle followed by constant darkness (DD). *, LL or DD significantly different from preceding LD; #, LL significantly different from DD.

Fig. 4

Averaged daily activity waveforms under initial light–dark conditions in ethanol-exposed (EtOH, black symbols) and water-only control animals (white symbols). The black horizontal bars indicate the hours of darkness under the light–dark cycle. Activity levels in each 30-minute bin were normalized to each individual animal’s daily mean prior to averaging across animals in order to eliminate the effects of individual and/or strain differences in activity levels.

Fig. 5

Circadian period and spectral magnitude in ethanol-exposed (EtOH, gray bars) and water-only control animals (black bars) during exposure to the shift-lag lighting regimen.

Fig. 6

Circadian period and spectral magnitude in ethanol-exposed (EtOH, gray bars) and water-only control animals (black bars) during exposure to constant darkness (DD, top panels) or constant light (LL, bottom panels).

Fig. 7

Daily wheel-turns in ethanol-exposed (EtOH, gray bars) and water-only control animals (black bars) under the second exposure to the standard light–dark cycle (LD) and subsequent exposure to constant darkness (DD) or constant light (LL).