SELECTIVE, ACTIVITY-DEPENDENT UPTAKE OF HISTAMINE INTO AN ARTHROPOD PHOTORECEPTOR

The synapses made by many arthropod photoreceptors are disinhibitory a nd use histamine as their transmitter. Because decreases and not incre ases in the cleft concentration of transmitter constitute the importan t event at these synapses, a transporter to clear the cleft of histami ne would seem particularly crucial to signal transfer, We report here that H-3-histamine is taken up selectively into barnacle photoreceptor s by a Na+-dependent mechanism, presumably a transporter, Using light microscopic autoradiography, we observe heavy label over axons and pre synaptic terminals of these neurons when they are stimulated during up take, The radioactivity taken up was identified as H-3-histamine by th in layer chromatography; no metabolites were detected, even after 5 hr , Radiolabeled 5-hydroxytryptamine and GABA are not taken up by the ph otoreceptor. H-3-histamine uptake into photoreceptors is decreased mar kedly by an excess of unlabeled histamine and by chlorpromazine and ph enoxybenzamine. Unexpectedly for uptake dependent on the Na+ gradient, photoreceptor terminals label more intensely in the light (when depol arized) than in the dark (when hyperpolarized), Glia label more strong ly than photoreceptors in dark-incubated preparations, The presence of presynaptic uptake strengthens the evidence that histamine is the neu rotransmitter of arthropod photoreceptors and provides a mechanism by which this synapse could recycle transmitter, control its steady-state cleft concentration, and clear it from the cleft in response to decre ases in its release from the photoreceptors.