NEUROTRANSMITTERS REGULATE RHYTHMIC SIZE CHANGES AMONGST CELLS IN THEFLYS OPTIC LOBE

Axon calibre in monopolar cells L1 and L2 of the fly's lamina can chan ge dynamically. Swelling by day, L2 exhibits a daily rhythm of changin g size apparently mediated by wide-field LBO5HT and PDH cells. L1/L2 a xon profiles were measured planimetrically in the housefly, Musca dome stica, from 1 mu m cross sections. Four hours after injecting 80-100 n l of 1.25 x 10(-4) M 5-HT into the optic lobe, L1's axon swelled but L 2's did not, whereas 2.2 x 10(-5) M of PDH enlarged both axons. Simila r to 5-HT, 1.63 x 10(-4) M histamine (the photoreceptor transmitter) e nlarged L1 but not L2, mimicking light exposure, while 1.7 x 10(-4) M glutamate and 1.94 x 10(-4) M GABA both decreased L1 and L2. 2.5 x 10( -4) M of 5,7-dihydroxytryptamine decreased L2 and, somewhat, L1, an ef fect attributable to the loss of LBO5HT neurites. Twenty four hours af ter cutting LBO5HT and PDH commissural pathways, L1 and L2 both shrank . Apparently, L2's size depends on either LBO5HT or sufficient 5-HT, a nd L1 and L2 have different response ranges to 5-HT. Responses to PDH imply that daytime PDH release drives a circadian rhythm, enlarging L1 and L2.