The colour vision of butterflies is crucial for the proper identification of foraging flowers, for the successful identification of mates and predators, and ultimately for the suitable oviposition sites. We hypothesized that photoreceptor activity is plastic and adjustable by intrinsic neuromodulator release. Using electrophysiological intracellular measurements, we first determined the set of photoreceptors that make up the mosaic in the retina of trichromatic and tetrachromatic butterfly species. We attempted to influence colour vision through the application of agonists and antagonists of the octopaminergic and histaminergic systems. The effect of octopamine and its antagonist epinastine on the receptor signal was evaluated through changes in the degree of direct synaptic opponency between photoreceptors. This is due to the action of inhibitory interphotoreceptor connections in the lamina and medulla of butterflies. While octopamine had no effect on spectral sensitivity, epinastine-treated cells had increased inhibition of the opponent unit. The effect on opponency was even greater when histaminergic chloride channel blockers were applied. By successful application of octopamine antagonists, we have shown that colour vision in butterflies is regulated by neuromodulators at the earliest stage, and by successful blockade of histaminergic channels, we have developed a method for analytically studying colour opponency between photoreceptors.
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