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

Abstract

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 µm 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 γ-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.

The eye of the caecilian Rhinatrema bivittatum (Amphibia: Gymnophiona: Rhinatrematidae)

“…Starting from their first mention in the scientific literature as Serpens Caecilia Ceylonica (Seba ) and as reflected in their English vernacular derived from the Latin (caecus = blind) and the German name ‘Blindwuhlen’, caecilians have often been interpreted as having non‐functional, rudimentary visual systems associated with their mostly fossorial habits (Noble ; Walls ; Storch and Welsh ; Himstedt ). Although caecilian eyes are very small compared with those of most other amphibians, both absolutely and in relation to head and body size (Himstedt ), several more recent works have provided evidence from anatomy, behaviour, physiology, neurology and molecular biology that whereas caecilian eyes are reduced in many respects, they remain functional (Wake , ; Himstedt ; Dünker ,b, ; Mohun et al . ).…”

Fine structure of the visual dorsolateral anterior thalamic nucleus of the pigeon (Columba livia): A hodological and GABA‐immunocytochemical study

“…However, it should be pointed out that the catecholaminergic terminals described in different regions of the mammalian and avian brain (for review, see Papadopolous and Parnavelas, 1990;Karle et al, 1996;Metzger et al, 2002) show several cytological features in common with the DCgT2 terminals of the DLLv of the pigeon; both contain large dense-core vesicles and many small, round, synaptic vesicles throughout the axoplasm of their profiles. It has also been shown that in various structures of the central nervous system of a number of different species, the catecholaminergic neurons can also colocalize GABA (see Kosaka et al, 1987;Dunker, 1998;Diaz-Rios et al, 2002); catecholaminergic fibers have also been observed under the light microscope in the DLA of the pigeon (Gü ntü rkü n and Reiner et al, 1994). It is thus possible that at least one of the categories of DCgT terminals, while exhibiting GABA-immunoreactivity, are in fact the terminals of these fibers.…”