Serotonergic neurons and processes in the adult and developing retina of Ichthyophis kohtaoensis (Amphibia; Gymnophiona)

Abstract

 Ichthyophis kohtaoensis is a tropical, limbless amphibian species with extremely small eyes (540 μm in adults). Adapted to a subterranean, burrowing mode of life, orientation and prey capture are predominantly guided by olfaction. The only visually guided behavior seems to be negative phototaxis. As electrophysiological single cell recordings have so far failed, immunohistochemical transmitter studies are a starting-point for a functional investigation of the visual system of this group of amphibians. In the present study, the organization and development of the serotonergic system have been examined in the retinae of embryonic, larval and adult I. kohtaoensis, using an antiserum against serotonin. Labeled somata are situated in the inner nuclear layer, presumably representing amacrine cells, and in the ganglion cell layer. However, some immunoreactive cells are located in the middle of the inner nuclear layer that send processes to both plexiform layers, probably representing bipolar cells. An additional type of immunoreactive soma, situated in the inner plexiform layer, have been found only in retinae of embryonic stages. Varicose serotonergic fibers form a diffuse plexus that covers the whole inner plexiform layer. Immunolabeled fibers can occasionally be demonstrated in the optic nerve head. During retinal development, the distribution and number of transmitter-expressing cells changes but no general reduction of the visual system is detectable. Adult stages still have a serotonergic system comparable to amphibians with a well-developed visually guided behavior indicating that the eyes of I. kohtaoensis¸ although being of minor importance in a subterranean habitat, retain all the elements of functioning sense organs.