Guitarfish possess ipsilateral as well as contralateral retinofugal projections.

Autoradiographic analysis of the primary retinal projections in the thornback guitarfish reveals both contralateral and ipsilateral projections to diencephalic, pretectal, and tegmental nuclei and the optic tectum. A total of 12 retino-recipient cell groups receive ipsilateral as well as contralateral inputs. To date, all studies using Fink-Heimer or autoradiographic methods in elasmobranch fishes have revealed only contralateral retinofugal projections, except for an ipsilateral projection to the rostral preoptic area [2-4, 7, 8]. Ebbesson and Meyer [2] recently reported that the retinal projections of a guitarfish, Rhinobatos, appear completely crossed, like those of sharks. However, this study employed only the Fink-Heimer method, which is not as sensitive as the more recently developed autoradiographic method, particularly in elasmobranch fishes. As part of an ongoing analysis of the visual system of the thornback guitarfish, Platyrhinoidis triseriata, we examined the primary retinal projections using the autoradiographic method and here report that in the thornback there are 12 distinct retinal targets, and each of these neural populations receives ipsilateral as well as contralateral retinal inputs. Six adult thornback guitarfish received intraocular injections of 50-100/~Ci of [3H]proline (50/~Ci/#l) under MS222 anesthesia and survived 4, 6, 8 or l0 days at 14-18°C before being reanesthetized and perfused with 0.7% saline followed by AFA. The brains were dehydrated, embedded in paraffin and sectioned in the transverse plane. Sections were coated with NTB-3 emulsion and exposed for 30-40 days at 7°C before development in Dektol and staining with cresyl violet. The majority (perhaps 90%) of the labeled optic nerve fibers decussate in the optic chiasm (Fig. 1A). As the fibers emerge from the chiasm a dense field of terminals occurs in the suprachiasmatic nucleus of the hypothalamus (Fig. 1B, C) whose cells are easily recognized extending ventrally into the caudal half of the chiasm to form paired tongue-like extensions of the preoptic area.