Analysis of spectral opponent mechanism in the optic lobe of the butterfly, Papilio xuthus

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Spectral opponent responses have been observed at the level of photoreceptors in butterflies. To explain its mechanism, I proposed a “histamine hypothesis”: the opponency may be attributed to the antagonistic interaction between photoreceptors via the activation of histaminergic channels.Immunolocalization on histamine-gated chloride channels in Papilio xuthus suggested that the lamina monopolar cells (LMCs) and photoreceptors express PxHCLA and PxHCLB, respectively. Using sharp microelectrode recording with current injections, the responses of both LMCs and spectrally opponent photoreceptors could be reversed when the membrane potential was close to the equilibrium potential of chloride ions.Intracellular recording in the lamina showed that LMCs are divided into non-spectrally opponent and spectrally opponent ones. The former includes three spectrally heterogeneous classes, which are ommatidial type dependent. It suggests that LMCs seem to integrate chromatic signals from the photoreceptors in the same ommatidium via PxHCLA. I also correlated LMC spectral types with morphological types. The findings of spectrally opponent LMCs and variable spectral classes provide a new insight into the role of LMCs in color signaling circuitry. In addition, several classes of spectrally opponent photoreceptors were recorded. The recorded opponent responses can be well explained by the ommatidial type-specific inter-photoreceptor synaptic connections via PxHCLB. The results presented in this project will be a springboard for future studies of early chromatic processing in insects.