Free-running period of circadian rhythms is shorter in rats with a genetically upregulated central cholinergic system.

The free-running circadian rhythm of drinking activity was monitored in constant darkness in a genetic line of rats, the Flinders Sensitive Line (FSL), which show increased muscarinic receptors in several brain regions. Compared to control rats, the Flinders Resistant Line (FRL), that do not show increased muscarinic receptors, the free-running period of drinking activity was shorter in the FSL rats (FSL period = 24.02 +/- 0.01 vs FRL period = 24.12 +/- 0.02; p < .001). In an attempt to determine whether other rhythms were similarly affected, we simultaneously monitored drinking activity, gross motor activity, and core temperature in free-running constant darkness conditions. The results from three FSL and FRL rats showed that the circadian periods of all three rhythms were shorter in FSL rats. These findings indicate that an animal model with an upregulated central cholinergic system demonstrates an accelerated circadian pacemaker.