NEUROBIOLOGY OF THE LACERTILIAN PARIETAL EYE SYSTEM

The lacertilian parietal eye is a highly organized photoreceptive organ found on the dorsal midline of the head. Developmentally, it is related to the pineal gland but structurally, it resembles the retina of the eye. In contrast to the retina, the parietal eye has a reduced number of components. The photoreceptors synapse directly onto the ganglion cells and there is no convincing evidence of interneurons between these cell types. Retrograde transport studies have revealed two populations of ganglion cells, one in the ganglion cell layer and the other a population of <<displaced>> ganglion cells located in the photoreceptor layer. The pars dorsolatcralis of the left medial habenular nucleus is specialized to receive the projection of the parietal eye ganglion cells. A few centrifugal neurons project to the parietal eye and affect its photoresponsiveness. These centrifugal neurons are stimulated differentially by norepinephrine and serotonin. Several neurotransmitter systems are thought to exist in the parietal eye. Radiolabeled precursor studies have shown that acetylcholine, gamma-aminobutyric acid and serotonin are synthesized. Substance P has been localized in neuronal processes of the plexiform layer and in the left medial habenular nucleus. The cells that synthesize these neurotransmitters are not yet identified. Three-dimensional reconstruction from serial electron micrographs shows that the normal and displaced ganglion cells have different patterns of synaptic input. The patterns of synaptic input provide the ultrastructural substrate for the information processing performed by the parietal eye. It is known that this simple retina extracts information about the intensity and wavelength of light impinging upon it, codes the information, and then sends it to the brain. This coded information is used by the brain to coordinate the animal's physiology with fluctuations in environmental conditions.