Tachykinin-related peptide and GABA-mediated presynaptic inhibition of crayfish photoreceptors

Off-axis illumination elicits lateral inhibition at the primary visual syna pse in crustacea and insects. The evidence suggests that the inhibitory act ion is presynaptic (i.e., on the photoreceptor terminal) and that the amacr ine neurons of the lamina ganglionaris (the first synaptic layer) may be pa rt of the inhibitory pathway. The neurotransmitters and the synaptic mechan isms are unknown. We show by immunocytochemistry that GABA and a tachykinin -related peptide (TRP) are localized in the amacrine neurons of the crayfis h lamina ganglionaris. Indirect evidence suggests that GABA and TRP may be colocalized in these neurons. The extensive processes of the amacrine neuro ns occupy lamina layers containing the terminals of photoreceptors. Applica tion 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 ligh t-elicited current) by similar to 40% and potentiation of the action of GAB A by similar to 100%. The mechanisms of TRP action in crayfish are not know n, but a plausible pathway is a TRP-dependent elevation of intracellular Ca 2+ that reduces photoreceptor sensitivity in arthropods. Although the mecha nisms are not established, the results indicate that in crayfish photorecep tors TRP displays actions on two time scales and can exert profound modulat ory control over cell function.