A double-label analysis demonstrating the partial coexistence of tyrosine hydroxylase and GABA in retinal neurons of ichthyophis kohtaoensis (Amphibia; gymnophiona).

Ichthyophis kohtaoensis is a limbless amphibian species with a subterranean mode of life and a predominantly olfactorily guided orientation. The only visually guided behavior seems to be negative phototaxis. As these animals possess extremely small eyes (only 540 microm in diameter in adults), functional investigations of the retina by electrophysiological single cell recordings have so far failed. Therefore, immunohistochemical transmitter studies constitute a starting point for a functional investigation of the caecilian retina. Previous immunohistochemical examinations have revealed immunoreactivity for gamma-aminobutyric acid (GABA), serotonin, dopamine and tyrosine hydroxylase (TH). The double-labeling experiments of the present study show that some TH-immunoreactive cells also express GABA-like immunoreactivity and some GABAergic neurons also label for TH immunocytochemistry, revealing a partial coexistence of TH and GABA in caecilian retinal neurons. On the one hand, these results contrast with previous reports, stating that in amphibians GABA-immunoreactive cells constitute a separate population from TH-positive neurons and colocalization is restricted to higher vertebrates. On the other hand, the findings indicate that a functional system which is under no strong selective pressure obviously has a long evolutionary persistence irrespective of its need for use.