Immunolocalization suggests a role of the histamine-gated chloride channel PxHCLB in spectral opponent processing in butterfly photoreceptors.

Spectrally opponent responses, i.e. wavelength-dependent inversions of response polarity, have been observed at the level of photoreceptors in butterflies. Since inter-photoreceptor connections have been found in the butterfly Papilio xuthus, and histamine is the only neurotransmitter so far identified in insect photoreceptors, we hypothesize that histaminergic sign-inverting synapses exist in the lamina between different spectral receptors as a mechanism for spectral opponency as in the medulla of Drosophila. Here, we localized two histamine-gated chloride channels, PxHCLA (Drosophila Ort homolog) and PxHCLB (Drosophila HisCl1 homolog), in the visual system of Papilio xuthus by using specific antisera. The anti-PxHCLA labeling was associated with the membrane of non-photoreceptor cells that are postsynaptic to photoreceptors, while the anti-PxHCLB labeling overlapped with photoreceptor axons, indicating that PxHCLB is expressed at inter-photoreceptor synapses: PxHCLB is likely involved in producing spectral opponency at the first visual synapses. Color processing in Papilio may appear earlier than previously hypothesized in insect visual systems. This article is protected by copyright. All rights reserved.