Identification of putative neurotransmitters in the lizard parietal eye

The retina of the parietal eye of lizards is a simple vertebrate retina that may be useful in studies of basic principles of information processing in visual systems. The chemical substances which mediate the cell-to-cell communication of this eye are not known. As a step in attempting to understand the cellular basis for visual information processing the authors have studied the ability of the parietal eye to synthesize conventional neurotransmitter substances. The eyes were incubated in radiolabeled precursors, then an extract of the tissue was subjected to high-voltage paper electrophoresis. The results of this study indicate that gamma-amino butyric acid (GABA) was synthesized from glutamic acid, acetylcholine (ACh) was synthesized from choline and serotonin (5-HT) was synthesized from tryptophan. Endogenous 5-HT was localized at sites outside the sensory epithelium by immunohistochemical means. Autoradiography following an incubation with 3H-GABA revealed uptake by lens cells and probably both glia and neurons. The authors conclude that GABA and ACh may be involved in the processing of visual information within the parietal eye but that 5-HT probably is not.