Mania-like behavior induced by disruption of CLOCK

Abstract

Circadian rhythms and the genes that make up the molecular clock have long been implicated in bipolar disorder. Genetic evidence in bipolar patients suggests that the central transcriptional activator of molecular rhythms, CLOCK, may be particularly important. However, the exact role of this gene in the development of this disorder remains unclear. Here we show that mice carrying a mutation in the Clock gene display an overall behavioral profile that is strikingly similar to human mania, including hyperactivity, decreased sleep, lowered depression-like behavior, lower anxiety, and an increase in the reward value for cocaine, sucrose, and medial forebrain bundle stimulation. Chronic administration of the mood stabilizer lithium returns many of these behavioral responses to wild-type levels. In addition, the Clock mutant mice have an increase in dopaminergic activity in the ventral tegmental area, and their behavioral abnormalities are rescued by expressing a functional CLOCK protein via viral-mediated gene transfer specifically in the ventral tegmental area. These findings establish the Clock mutant mice as a previously unrecognized model of human mania and reveal an important role for CLOCK in the dopaminergic system in regulating behavior and mood.

Fig. 1.

Effect of Clock disruption on rewarding electrical stimulation of the MFB and sucrose preference. (a) Clock mice required a lower minimal current (mean ± SEM) to sustain reliable ICSS (∗, P < 0.01, Student's t test, n = 7–9 per group). (b) Cocaine decreased ICSS thresholds (mean ± SEM) at a lower dose in Clock mutant mice (5.0 mg/kg) than in controls (10 mg/kg). Furthermore, the threshold-lowering effects of 10 mg/kg of cocaine were significantly larger in Clock mutant mice. (c) Cocaine significantly increased ICSS rates (mean ± SEM) in Clock mutant mice. ∗, P < 0.01 for within-group comparisons with saline (SAL; 0.0 dose); ∗, P < 0.01 for between-genotype comparisons, Scheffé tests, n = 6–7 mice per group. (d) Clock mutant mice display a greater preference for 1% sucrose (Suc) than WT mice over several days of testing when given a choice between sucrose and water. ∗, P < 0.05, Student's t test, n = 8–10.

Fig. 2.

Effect of the Clock mutation in models of depression and anxiety. (a) Clock mutants (clk mut) spend less time immobile in the forced swim test (∗, P < 0.05, Student's t test, n = 5–9). (b) Clock mutants had fewer escape failures in the learned helplessness paradigm (∗, P < 0.05, Student's t test, n = 5–6). (c) Clock mutants spend more time in the center of an open field (∗, P < 0.05, Student's t test, n = 9–12). (d) Clock mutants enter the open arms of the elevated plus maze more frequently (∗, P < 0.05, Student's t test, n = 9–12). (e) Clock mutants have a shorter latency to approach and eat a cracker in the presence of aversive stimuli (gray line, wt; black line, clk mut; differences in the combined response to aversive stimuli are significant, P < 0.05, Student's t test, n = 9–10).

Fig. 3.

Effect of lithium treatment on the Clock mutants in behavioral measures. (a) LiCl (600 mg/liter, 10 days) leads to an increase in the time immobile of the Clock mutants in the forced swim test. LiCl treatment decreases the time spent by the Clock mutants in the left of an open field (b) and the number of entries into the open arms of the elevated plus maze (c). (In all tests, ∗, P < 0.05, Student's t test, n = 8.)

Fig. 4.

Effect of CLOCK viral expression in the VTA of Clock mutant mice. (a) Locomotor hyperactivity in response to novelty and time spent in the left of an open field (b) is reduced after 2 weeks of functional CLOCK overexpression in the VTA of Clock mutant mice. In a, total locomotor activity levels are significantly different (∗, P < 0.05, Student's t test, n = 9–12).