Splitting of locomotor circadian rhythmicity in hamsters is facilitated by pinealectomy.

The role of the pineal gland in the mammalian circadian system has not been well established, in contrast to a fair number of reports indicating pharmacological effects of melatonin in the circadian organization. In order to establish the effects of pinealectomy on the time course of splitting of circadian rhythmicity, the wheel running locomotor activity was continuously recorded in golden hamsters under light-dark conditions or constant light. The analysis of transients from the actograms shows that removal of the pineal gland induces a reduction in the latency and an increase in the duration of transients before the splitting occurs. The power spectral analysis from selected segments of the data shows that concomitant to the development of the splitting there is an increase in the power of ultradian components. In pinealectomized animals the changes in the power spectrum occurs at least 30 days before that in the control animals. These observations suggest that pineal gland could be involved in the coupling mechanism among the different oscillators of the rodent circadian system. Furthermore, since the light intensity used in this study is enough to completely suppress the melatonin synthesis from the pineal, the present results suggest that a signal from the pineal other than melatonin is involved in the process.