Free running circadian rhythms of melatonin, luteinizing hormone, and cortisol in Syrian hamsters bearing the circadian tau mutation

The tau mutation of Syrian hamsters induces a robust reduction in the period of circadian activity rhythms, from 24 h (wild-type; tau++) to 22 h (heterozygote; tau(S+)) and 20 h (homozygous mutant, tau(SS)). Here, we examine the effect of this mutation on circadian rhythms of LH, melatonin, and cortisol in ovariectomized hamsters. Free running circadian rhythms were observed in all three hormones. In each genotype, endocrine rhythms were synchronized with concurrently assessed activity rhythms, suggesting a shared period around 20 h in tau(SS), 22 h in tau(S+), and 24 h in tau++. Phasing with respect to the activity rhythm was generally similar in tau++ and mutant genotypes. However, melatonin concentrations rose significantly earlier in tau(SS) than in tau++ animals. Explanted pineals from both genotypes exhibited a similar time course of response to norepinephrine administration, suggesting that the phase advance of melatonin production observed in tau(SS) in vivo is not a direct effect of the tau mutation within the pinealocyte. The demonstration of reduced period endocrine rhythms in the mutant genotypes extends previous behavioral studies and, together with recent work on rhythmicity in the isolated retina, suggests an ubiquitous influence of the tau mutation on the processes of circadian rhythm generation in this species.