Mice with a mutation in the Clock gene (ClockΔ19) exhibit increased preference for stimulant rewards and sucrose. They also have an increase in dopaminergic activity in the ventral tegmental area (VTA) and a general increase in glutamatergic tone which might underlie these behaviors.
However, it is unclear if their phenotype would extend to a very different class of drug (ethanol) and if so, whether these systems might be involved in their response.
Continuous access voluntary ethanol intake was evaluated in ClockΔ19 mutants and WT mice. We found that ClockΔ19 mice exhibited significantly increased ethanol intake in a two bottle choice paradigm. Interestingly, this effect was more robust in female mice. Moreover, chronic ethanol experience resulted in a long lasting decrease in VTA Clock expression.
To determine the importance of VTA Clock expression in ethanol intake, we knocked-down Clock expression in the VTA of WT mice via RNA interference. We found that reducing Clock expression in the VTA resulted in significantly increased ethanol intake similar to the ClockΔ19 mice. Interestingly, we also discovered that ClockΔ19 mice exhibit significantly augmented responses to the sedative effects of ethanol and ketamine, but not pentobarbital.
However, their drinking behavior was not affected by acamprosate, an FDA approved drug for the treatment of alcoholism, suggesting that their increased glutamatergic tone might underlie the increased sensitivity to the sedative/hypnotic properties of ethanol but not the rewarding properties of ethanol.
Taken together, we have identified a significant role for Clock in the VTA as a negative regulator of ethanol intake and implicate the VTA dopamine system in this response.
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