A comparative analysis of glial and neuronal markers in the retina of fish: variable character of horizontal cells.

The immunohistochemical localizations of the enzymes glutamine synthetase, carbonic anhydrase-C, and the intermediate filament protein GFA were examined for potential neuroglial specificity in the retinas of several types of fish. Both glutamine synthetase and carbonic anhydrase-C appear to be characteristic markers for retinal Müller cells. However, the horizontal neurons of most fish examined also possess high levels of carbonic anhydrase. Furthermore, GFA, the characteristic marker for fibrous astroglia in higher vertebrates, was found specifically localized in the horizontal neurons of several teleost fish. The identity of the GFA antigens was qualified by immunochemical as well as cytological examinations. Furthermore, specific antisera to other intermediate filament proteins, including neurofilaments, validated and contrasted with the observations made with antisera to GFA. The presence of GFA in horizontal neurons of fish is widespread but not typical of all fish. These observations indicate an evolutionary constancy of retinal Müller glial cells. However, these results again focus attention on whether horizontal cells are truly neurons or rather represent an intermediate cell type that may prove useful in studying the evolution, ontogeny, and functional significance of the neuronal-glial phenotypic dichotomy.