CIRCADIAN-RHYTHMS IN SCREENING PIGMENT AND INVAGINATING ORGANELLES INPHOTORECEPTOR TERMINALS OF THE HOUSEFLYS FIRST OPTIC NEUROPIL

Screening pigment granules occur in the synaptic terminals of photorec eptors in the fly's (Musca domestica, L.) compound eye. The granules r esemble ommochrome granules in the overlying photoreceptor cell body. There are also two types of invagination into receptor terminals: capi tate projections (from glial cells) and invaginations from neighboring receptor terminals. The number of profiles of these organelles in the first optic neuropile, the lamina, have been counted using single-sec tion quantitative electron microscopic methods. Pigment granules are c oncentrated proximally in the terminal, toward the brain. The numbers change, increasing during the night (Ih after lights off) up to values more than twice the number Ih after lights on, apparently by longitud inal migration of granules from the cell body into the terminal. Flies entrained to day/night conditions and then held under constant darkne ss continue to exhibit changes in the numbers of profiles. Even though overall there were 80-90% fewer granule profiles than under day/night conditions, the numbers attained a peak many times higher at the end of the subjective day. Thus, the changes are endogenous, showing circa dian rhythmicity. Although their significance is unknown, these change s parallel previously described circadian rhythms in the receptor term inals and their lamina monopolar-cell targets. The invaginations from receptor terminals were more numerous under day/night conditions than under constant darkness, and cycled in constant darkness, peaking at t he end of subjective night. Capitate projections, by contrast, failed to change significantly under the experimental conditions analyzed, a lack of responsiveness they share with photoreceptor tetrad synapses. (C) 1997 John Wiley & Sons, Inc.