DAILY AND CIRCADIAN-RHYTHMS OF SYNAPTIC FREQUENCY IN THE 1ST VISUAL NEUROPIL OF THE HOUSEFLYS (MUSCA-DOMESTICA L) OPTIC LOBE

Photoreceptors of the fly's compound eye generally show no very obviou s daily or circadian rhythms, a lack which prompted us to examine whet her their function might be regulated not in the retina, but at the si te of transmission in the first visual neuropile, or lamina. Here, pho toreceptor terminals (R1-R6) are reciprocally interconnected with one class of lamina monopolar cell, L2: L2 receives input from R1-R6 at so -called tetrad synapses, and in turn is presynaptic to R1-R6 at feedba ck synapses. We have calculated the mean frequencies of these synaptic profiles in electron micrographs of single lamina sections. L2 feedba ck synapses were more numerous at night than during the day, whereas t he number of tetrads showed only small modulations between day and nig ht. These changes persisted amongst feedback synapses in flies held in constant darkness, and are thus circadian. In contrast to the slow mo dulations during a 24 h cycle, the number of L2 feedback synapses afte r 1 h light pulse in flies held in constant darkness showed no clear c hange, whereas it increased the number of tetrad profiles. These findi ngs support the occurrence of cyclical daily and circadian changes amo ngst the two lamina synaptic populations, with tetrads showing rather weak modulations in frequency, but more pronounced responses to the li ght pulse than feedback synapses.