DAILY RHYTHMS IN CELLS OF THE FLYS OPTIC LOBE - TAKING TIME OUT FROM THE CIRCADIAN CLOCK

Considerable progress has recently been reported in locating the cellu lar basis and molecular mechanisms of the circadian clock in the fruit fly, Drosophila melanogaster. To advance beyond the clock, towards the outputs that lie between the clock itself and the circadian rhythms i n behaviour that it regulates, will present new challenges. This is be cause most behaviours are generated by complex neuronal circuits,which are themselves difficult to unravel. Recently described anatomical ch anges in the optic lobe of the related housefly, Musca domestica, exhi bit a circadian rhythm that is, by contrast, relatively easy to assay. This rhythm is apparently controlled by at least two sets of diffuse modulatory neurones. One of these, immunoreactive to the peptide pigme nt-dispersing hormone,also expresses in Drosophila the product of the period (per) gene, the most widely studied of the so-called clock gene s that are essential for the correct expression of circadian rhythmici ty. The second, called LBO5HT is immunoreactive to 5-HT, a widely invo ked transmitter system in insect circadian rhythms. The identification of these elements, and a widening cascade of events which their actio ns apparently trigger, opens up new opportunities to examine old probl ems in the regulation of circadian rhythms in the nervous system.