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The Journal of Neuroscience, March 1, 2000, 20(5):1780-1790

Tachykinin-Related Peptide and GABA-Mediated Presynaptic Inhibition of Crayfish Photoreceptors

Raymon M. Glantz¹, Clyde S. Miller¹, and Dick R. Nässel²

¹ Department of Biochemistry and Cell Biology, Rice University, Houston, Texas 77251, and ² Department of Zoology, Stockholm University, Stockholm, Sweden

Off-axis illumination elicits lateral inhibition at the primary visual synapse in crustacea and insects. The evidence suggests that the inhibitory action is presynaptic (i.e., on the photoreceptor terminal) and that the amacrine neurons of the lamina ganglionaris (the first synaptic layer) may be part of the inhibitory pathway. The neurotransmitters and the synaptic mechanisms are unknown. We show by immunocytochemistry that GABA and a tachykinin-related peptide (TRP) are localized in the amacrine neurons of the crayfish lamina ganglionaris. Indirect evidence suggests that GABA and TRP may be colocalized in these neurons. The extensive processes of the amacrine neurons occupy lamina layers containing the terminals of photoreceptors. Application of exogenous GABA and TRP to photoreceptor terminals produces a short-latency, dose-dependent hyperpolarization with a decay time constant on the order of a few seconds. TRP also exhibits actions that evolve over several minutes. These include a reduction of the receptor potential (and the light-elicited current) by ~40% and potentiation of the action of GABA by ~100%. The mechanisms of TRP action in crayfish are not known, but a plausible pathway is a TRP-dependent elevation of intracellular Ca²⁺ that reduces photoreceptor sensitivity in arthropods. Although the mechanisms are not established, the results indicate that in crayfish photoreceptors TRP displays actions on two time scales and can exert profound modulatory control over cell function.

Key words: tachykinin; photoreceptor; lateral inhibition; GABA; visual transduction; presynaptic inhibition; crayfish; invertebrate

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Copyright © 2000 Society for Neuroscience  0270-6474/00/2051780-11$05.00/0

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