Daily rhythms in cells of the fly's optic lobe: taking time out from the circadian clock.

Considerable progress has recently been reported in locating the cellular basis and molecular mechanisms of the circadian clock in the fruitfly, Drosophila melanogaster. To advance beyond the clock, towards the outputs that lie between the clock itself and the circadian rhythms in behaviour that it regulates, will present new challenges. This is because most behaviours are generated by complex neuronal circuits, which are themselves difficult to unravel. Recently described anatomical changes in the optic lobe of the related housefly, Musca domestica, exhibit 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 pigment-dispersing hormone, also expresses in Drosophila the product of the period (per) gene, the most widely studied of the so-called clock genes that are essential for the correct expression of circadian rhythmicity. The second, called LBO5HT, is immunoreactive to 5-HT, a widely invoked transmitter system in insect circadian rhythms. The identification of these elements, and a widening cascade of events which their actions apparently trigger, opens up new opportunities to examine old problems in the regulation of circadian rhythms in the nervous system.