Fig. 10. White's proposed involvement in the ebony–tan histamine shuttle. Histamine (HA) is released from the photoreceptor terminal (R1–R6) into the synaptic cleft (1), where it activates postsynaptic histamine receptors on the dendrites of a lamina monopolar cell (LMC) target. Its action is terminated by glial uptake (2), which our evidence indicates may be partly white dependent. Histamine in the epithelial glia (EG) is then conjugated to β-alanine (β-A) to form carcinine (CA), regulated by Ebony, and the carcinine then extruded from the glia by an unknown transporter (3). Carcinine is next taken up from the cleft by the transporter Inebriated (4) (Gavin et al., 2007), into the R1–R6 terminal, where it is hydrolyzed under the action of Tan to liberate histamine and β-alanine. No clear evidence exists for a direct histamine reuptake mechanism at the R1–R6 terminal membrane (5).