Neurons to Sustain Histamine Recycling 2 3 4

19 20 21 Running title: Carcinine transport in histamine recycling 22 23 24 25 Abstract 28 Synaptic transmission from Drosophila photoreceptors to lamina neurons requires 29 recycling of histamine neurotransmitter. Synaptic histamine is cleared by uptake into glia and 30 conversion into carcinine, which functions as transport metabolite. How carcinine is transported 31 from glia to photoreceptor neurons remains unclear. In a targeted RNAi screen for genes 32 involved in this pathway we identified carT, which encodes a member of the SLC22A transporter 33 family. CarT expression in photoreceptors is necessary and sufficient for fly vision and behavior. 34 In the lamina of carT flies, carcinine accumulates. Wild-type levels are restored by 35 photoreceptor-specific expression of CarT, and endogenous tagging suggests CarT localizes to 36 synaptic endings. Heterologous expression of CarT in S2 cells is sufficient for carcinine uptake, 37 demonstrating the ability of CarT to utilize carcinine as a transport substrate. Together, our 38 results demonstrate that CarT transports the histamine metabolite carcinine into photoreceptor 39 neurons, thus contributing an essential step in the histamine-carcinine cycle. 40