Neurotransmitters alter the numbers of synapses and organelles in photoreceptor terminals in the lamina of the housefly, Musca domestica

Various organelles in the lamina terminals of housefly photoreceptors exhib it daily rhythms having a circadian basis. These include changes in the num bers of photoreceptor tetrad and L2 feedback synapses, and longitudinal mov ements of screening pigment. Circadian information has previously been sugg ested to spread from the clock to the lamina via widefield cells expressing either 5-hydroxytryptamine or pigment-dispersing hormone-like immunoreacti vity. We examined the action of these neuromodulators, and other candidate neurotransmitters, 4h after injecting either the transmitter of a control i nto the medulla. We counted electron microscope profiles of organelles that normally exhibit circadian changes, and two types of invagination into pho toreceptor terminals, capitate projections and inter-receptor invaginations . No single substance mediated the changes observed. Injected pigment-dispe rsing hormone peptide decreased the number of pigment granules, implicating this peptide in screening pigment migration, but produced no changes in sy napse-related organelles. alpha-Aminobutyric acid exclusively decreased the number of L2 feedback synapses. Responses to other transmitters were speci fic, and often large, but generally not statistically significant. Histamin e, for example, may decrease the number of tetrads, possibly by direct auto regulation. The results suggest that there is likely to be more than one ef fector in the circadian pathways to the lamina.