Endogenous clocks need to be adjusted by Zeitgeber stimuli to correct external time to fullfill their biological function. Photic Zeitgebers such as the daily changes of environmental illumination have proven to be the most powerful ones among a variety of different periodic events, like temperature and social encounters. Under lab conditions simple on/off light regimes have almost exclusively been used to investigate the underlying mechanisms of entrainment. However, in nature, photic zeitgebers are quite different in several regards: e.g., light intensity and color of light gradually changes at a typical rate during dusk and dawn. There is no "light on/off" in nature that carries information about the time of the day. Any change of light (and there is many) not caused by the earth's rotation is "noise" in the photic circadian zeitgeber.

Reports on the effectiveness of gradual light changes as zeitgebers go back even to the 50s (e.g. Aschoff and Meyer-Lohmann (1954) Z. Tierpsychol. 12, 254-265) and gained new interest by Terman's work on human melatonin rhythm (Terman et al. (1990) 2^nd Meeting Soc.Res.Biol.Rhythms, Abstr. 30).

In the scorpion Androctonus australis we have studied the reaction of various circadian rhythm parameters to zeitgeber regimes with different time courses of light intensity during the transition between light (L) and dark (D) phases and compared the results from light on/off regimes to steplike and to gradual changes of light intensity. The gradual change of about 4 log units in 30 minutes is the closest simulation of dusk and dawn we used in this study.

In summary we can state so far that simulated dusk indeed causes "best" synchronization of the circadian ERG-rhythm, locomotor rhythm (Fig. a,b), and moment of surface appearance (Fig. c). "Best" in this regard means: the phase relation between zeitgeber and the circadian rhythm is more precise, the range of entrainment is larger, the momentary variation of tau is smaller, less activity occurs during subjective day, the pattern of locomotion is more uniform, the time of surface appearance is more precise. Dawn, in contrast to dusk, seems to have no or only a weak zeitgeber effect.

The scorpion seems to be a good model system to investigate which time cues are relevant in the daily changes of environmental light (slope, absolute intensity, color etc.), and which receptors, receptor combinations or neuronal wiring patterns can achieve this perception of time, which might be rather a network problem than a simple phase shift of an oscillator cell.

* present address: Universitaet Ulm, Gravitationsphysiologie, Albert-Einstein-Allee 11, 89081 Ulm, Germany

Taken from: Goettingen Neurobiology Report 1996, Georg Thieme Verlag TB 31