Neurotransmitters regulate rhythmic size changes amongst cells in the fly's optic lobe

Axon calibre in monopolar cells L1 and L2 of the fly's lamina can change dynamically. Swelling by day, L2 exhibits a daily rhythm of changing size apparntly mediated by wide-field LBO5HT and PDH cells. L1/L2 axon profiles were measured planimetrically in the housefly, Musca domestica, from 1 m cross sections. Four hours after injecting 80–100 nl of 1.25 × 10^–4 M 5-HT into the optic lobe, L1's axon swelled but L2's did not, whereas 2.2 × 10^–5 M of PDH enlarged both axons. Similar to 5-HT, 1.63 × 10^–4 M histamine (the photoreceptor transmitter) enlarged L1 but not L2, mimicking light exposure, while 1.7 × 10^–4 M glutamate and 1.94 × 10^–4 M GABA both decreased L1 and L2. 2.5 × 10^–4 M of 5,7-dihydroxytryptamine decreased L2 and, somewhat, L1, an effect attributable to the loss of LBO5HT neurites. Twenty four hours after cutting LBO5HT and PDH commissural pathways, L1 and L2 both shrank. Apparently, L2's size depends on either LBO5HT or sufficient 5-HT, and 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.Abbreviations ERG electroretinogram - GABA -aminobutyric acid - PDH pigment-dispersing hormone - PDF pigment-dispersing factor - 5-HT 5-hydroxytryptamine - EM electron microscopy(ic)