Ganglion cells in the turtle retina contain the neuropeptide LANT-6

This study investigated the presence of the neurotensin-related hexapeptide, LANT-6, in retinal ganglion cells and their central projections in the turtle Pseudemys scripta elegans. Immunocytochemical techniques demonstrated that many of the cells in the ganglion cell layer of the turtle retina could be labeled with an antiserum specific for LANT-6. Radioimmunoassay and chromatographic analysis confirmed the presence of LANT-6-related peptides in retina, as well as brain. Several molecular forms of LANT-6 were observed, some larger than LANT-6. Characterization of the cells labeled in the ganglion cell layer in terms of their cell body size and their dendritic arborization patterns revealed that at least 6 specific LANT-6-positive cell types were present in the ganglion cell layer. Morphologically, the LANT-6-positive cells strongly resembled turtle ganglion cells, as previously described. In addition, two other lines of evidence supported this interpretation. First, double-label studies were performed in which retinal ganglion cells projecting to the tectum were retrogradely labeled by HRP injected into the tectum (using a cobalt chloride color-modified DAB reaction product) and immunocytochemically labeled with DAB using the antiserum against LANT-6. These double-label studies revealed that many of the LANT-6-positive cells in the ganglion cell layer in the portion of the retina labeled retrogradely by the HRP injection did project to the tectum. Within the retrogradely labeled portion of the retina, LANT-6-positive cells that were not labeled retrogradely, as well as neurons labeled retrogradely that did not contain LANT-6 were also observed. Second, the central projections of LANT-6-positive cells of the ganglion cell layer were examined by studying the effects of monocular enucleation on the distribution of LANT-6-positive fibers in the central projection targets of the turtle retina. Two to 8 weeks after enucleation, a substantial reduction in LANT-6-positive fibers was observed in all retinal target areas contralateral to the enucleated eye. Radioimmunoassay and chromatographic studies confirmed the presence of LANT-6-related peptides in the turtle brain and corroborated the reduction of LANT-6 observed in the contralateral tectum following monocular enucleation. Previous studies have demonstrated that LANT-6-related material is present in cells of the ganglion cell layer in a variety of vertebrates. The present results indicate that LANT-6 is in ganglion cells and that it may play a role in neurotransmission between retinal ganglion cells and their central target areas.