Circadian rhythms as ubiquitous biological clocks

circadian rhythms are of special interest because of both their prevalence among all eukaryotic organisms, unicellular or multicellular, and their immediate relevance to our human experience (Moore-Ede et al., 1982) the sleep-wake and nutrition cycles remind us every day of the fundamental, physiological role of circadian rhythms in allowing us to cope with our periodically changing environment. 

Among the most investigated neuronal pacemakers are those located in the eyes of the molluscs Aplysia and Bulla (Jacklet, 1977, 1989b  

Block Wallace, 1982). An important study (Michel et al, 1993) recently showed that circadian rhythmicity occurs at the level of a single neuron in Bulla. This study indicated that the circadian variation of electrical activity involves the periodic variation of potassium conductance. Thanks to these advances, the Bulla circadian pacemaker has become a most promising experimental model for unravelling the molecular bases of circadian rhythmicity. Circadian pacemakers have also been identified in insects such as the cockroach (Page, 1982). 

Another system that may contain a pacemaker is the avian pineal gland, which generates a circadian rhythm in melatonin the rhythm has also been observed in pineal cell cultures (see fig. 11.1) (Binkley, Riebman Reilly, 1978 Takahashi, Hamm Menaker, 1980 Takahashi et al., 1989). Melatonin is involved in the control of the sleep-wake cycle, and so analogues of the hormone are currently being developed for the prevention of jet lag. A circadian rhythm of activity has been demonstrated for the enzyme N-acetyltransferase, which catalyses an early step in melatonin synthesis (Binkley, 1983). 

The study of circadian rhythms has seen a clear acceleration in recent years owing to the combined insights provided by genetics and molecular biology (Feldman, 1982 Hall Rosbash, 1988 Takahashi, 1992, 1993 Dunlap, 1993 Takahashi et al, 1993 Young, 1993). Mutants of circadian rhythms were obtained in Drosophila as early as 1971 (see section 11.2). The use of circadian mutants, recently reviewed by Dunlap (1993), has also proved fruitful in Neurospora (see below) and rodents. Clock mutants have thus been obtained in the hamster (Ralph Menaker, 1988), and the mouse (Vitatema et al, 1994). 

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