In the center of our interest is the circadian timekeeping system of mammals and its entrainment. Mammals, similarly to all other living organisms, exhibit daily rhythms of biological variables at molecular, cellular, organismal, functional and behavioral levels. The rhythms exist even under nonperiodic conditions and are thus generated by an endogenous clock, i.e., pacemaker. Under nonperiodic conditions, the rhythms free-run with a period close to, but not equa1 to 24 hours. Therefore, the rhythms as well as the pacemaker, which controls them, are called circadian. Within the 24-h cycle, the rhythms are entrained mostly by the light-dark cycle, namely by the light period of the day, but other entraining agents may be also involved. The circadian pacemaker is located in the suprachiasmatic nuclei (SCN) of hypothalamus. According to biochemical, morphological and functional characteristics, the SCN may be divided into two parts, namely into the ventrolateral (VL) part, which receives photic input from the retina, and into the dorsomedial (DM) part.

The aim of the studies performed in the Department is to elucidate the dynamics and the mechanism of photic and non-photic entrainment of the circadian pacemaker itself and of overt rhythms controlled by the pacemaker. The research concentrates mostly on anima1 models, namely on the rat, but human studies are also performed as well.

Main results

A. Anima1 studies

The VL-SCN exhibits a rhythm in the light induction of immediate early genes, namely of c-fos mRNA and c-fos protein. We found that the interval enabling high c-fos photoinduction in the VL-SCN depends on previous day length, i.e., photoperiod, under which rats were maintained; the effect of the photoperiod is not mediated by the endogenous melatonin signal. When the photoperiod changes from a long "summer" one to a short "winter" one, the interval adjusts gradually to the change. In the DM-SCN, we have described a rhythm in the spontaneous c-fos immunoreactivity in darkness. Even the DM-SCN rhythm depends on the previous photoperiod and adjusts gradually to a change from long to short days. Also, the rhythm in mRNA for arginine vasopressin in the DM-SCN is photoperioddependent. Hence a functional state of both parts of the SCN is modulated by the photoperiod. The whole SCN may thus serve not just as a daily, but also as a seasonal clock. Under a regime with alternating periods of light and darkness, the mechanism of the morning c-fos increase which may indicate an increase in neuronal activity, is different in both parts of the SCN: whereas in the DM-SCN the increase is spontaneous, in the VL-SCN it is induced by light.

Rhythmicity of the VL- as well as of the DM-SCN may be reset by photic stimuli administered during the subjective night. Preliminary results indicate that the resetting may not proceed exactly in the same way in the two SCN parts. The magnitude of the phaseshifts depends on the photoperiodmodulated functional state of the SCN pacemacker. Interva1 enabling photic resetting of the circadian rhythmicity is also photoperiod dependent and resembles that enabling high c-fos photoinduction; both intervals may thus represent a rhythm of sensitivity to light. Also, nonphotic stimuli may reset the SCN rhythmicity. Melatonin administered in the late day resets instantaneously the evening rise in c-fos photoinduction. Melatonin administered in the late night suppresses the morning c-fos rise in the DM-SCN but it does not phaseshift the DM-SCN rhythmicity.

B. Human studies

Two overt rhythms controlled by the SCN, namely the salivary melatonin and cortisol rhythm, were used as markers of the human circadian rhythmicity. Exposure of human subjects to an actual natural 16 h summer photoperiod shortens the melatonin signal duration as compared with the winter pattern followed under a combined artificial and natural 16-h photoperiod. Summer days experienced from sunrise till sunset thus evoke a true longday response of the human circadian system. However, the compressed summer melatonin rhythm waveform extends rapidly, following a shortening of the photoperiod. Hence the natural summer photoperiod may mask a true waveform of circadian rhythms.

We reported earlier that a single exposure to bright light in the morning phase advanced the human circadian system within a day. Now we found that a fixed morning awakening coupled with lowintensity light can maintain the advance. The human circadian system may be, however, reset only by shifting of the sleep time under dim light at a time that may occur commonly in everyday life, without any previous bright light exposure. In collaboration with the lst Medical School in Prague we found changes of the circadian system of patients suffering from idiopathic hypersomnia as compared with control subjects.

Future prospects

Studies will continue to elucidate further the dynamics and mechanisms of photic, photoperiodic and non-photic entrainment of the mammalian circadian system. Attention will be concentrated on processes in the two SCN parts induced by environmental stimuli, on the time course of the processes and on the interaction between both SCN parts. Besides expression of immediate early genes, expression of core clock genes and clockcontrolled genes will be followed to understand the molecular basis of action of the entraining agents. In human studies, collaboration with the lst Medical School in revealing the function of the circadian system in sleep disorders will continue. Knowledge of the mechanisms of entrainment will allow us to propose a better therapy for circadian and sleep disorders as well as ways how to prevent them.

Publications