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The Journal of Neuroscience, February 15, 1999, 19(4):1217-1225

Uptake of Precursor and Synthesis of Transmitter in a Histaminergic Photoreceptor

Jennifer R. Morgan¹, Kelley A. Gebhardt², and Ann E. Stuart²

¹ Department of Neurobiology, Duke University Medical Center, Durham, North Carolina 27710, and ² Department of Cell and Molecular Physiology, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina, 27599-7545

As a first step in understanding how the supply of the neurotransmitter histamine is maintained in a photoreceptor, we followed the uptake and metabolism of the immediate precursor of histamine, histidine. [³H]Histidine taken up into photoreceptors and glia was detected using autoradiography, and synthesis of [³H]histamine from [³H]histidine was assayed with thin-layer chromatography. Photoreceptors from barnacles were pulsed (15 min) with [³H]histidine (0.2-200 µM), then maintained in normal saline for up to 24 hr. Autoradiography showed that photoreceptor somata, axons, and presynaptic arbors were labeled, but only weakly, like (nonhistaminergic) ganglion cells. Label instead was concentrated over surrounding glia. Stimulating preparations with light did not increase photoreceptor labeling. Grain counts from photoreceptor axons showed uptake of [³H]histidine into these neurons by a Na⁺-dependent mechanism with a K_m of ~50 µM. Over 24 hr only 1% of the [³H]histidine taken up by preparations was converted to [³H]histamine either in the dark or in the light. Injections of [³H]histidine directly into photoreceptors established that synthesis takes place within the photoreceptors and confirmed that stimulation with light did not measurably affect the rate of conversion of [³H]histidine to [³H]histamine. These results suggest that de novo synthesis of transmitter is unlikely to be as important as its reuptake in maintaining neurotransmitter supply in these photoreceptor terminals. In support of this conclusion, photoreceptors accumulated more label when transmitter release was stimulated with high K⁺ and histamine uptake was antagonized with chlorpromazine.

Key words: histamine; neurotransmitter synthesis; histidine uptake; photoreceptor; arthropod; barnacle

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Copyright © 1999 Society for Neuroscience  0270-6474/99/1941217-09$05.00/0

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