Neuronal and glial localization of homocysteate-like immunoreactivity in the rat retina.

To study the distribution of L-homocysteate in the rat retina, specific polyclonal and monoclonal anti-homocysteate antibodies have been used in combination with a highly sensitive postembedding method for light microscopic immunocytochemistry. In central and peripheral retina, the most strongly immunoreactive cell bodies lay in the inner nuclear layer. They represented about 17% of the total neuronal cell population of the layer and were identified as bipolar cells (19-20% of cells in the outer half of the inner nuclear layer) and amacrine cells (15% of cells in the inner half of the inner nuclear layer). A third cell type showing heavy homocysteate-like immunoreactivity was identified as Müller glial cells. Characteristically, their descending processes formed three immunoreactive bands in the inner plexiform layer. Furthermore, the outer and inner limiting membranes as well as glia around and between ganglion cell axons and in the vicinity of blood vessels were labelled intensely. Photoreceptors and their terminals, and ganglion cells, were not immunostained. These findings indicate the presence of homocysteate in some bipolar and amacrine cells of the inner nuclear layer and support a role for this sulphur-containing excitatory amino acid as a neurotransmitter candidate in the retina.