Histamine is a monoamine neurotransmitter synthesized in the cytosol from L-histidine at presynaptic terminals. In photoreceptor cells, histamine is loaded and stored in synaptic vesicles. Once released into the synaptic cleft, histamine can be cleared and recycled by the lamina glia, where it is converted to carcinine. Carcinine is then transported into photoreceptor cells, where it is hydrolyzed back to histamine. Histamine signals through gated chloride channels regulating various processes including visual transmission, the wake-sleep cycle, thermosensing and courtship. (Adapted from FBrf0259379.)