A Dil-tracing study of the neural connections of the pineal organ in two elasmobranchs (Scyliorhinus canicula and Raja montagui) suggests a pineal projection to the midbrain GnRH-immunoreactive nucleus

The pineal organ of elasmobranchs is an elongated photoreceptive organ. In order to investigate the afferent and efferent connections of the pineal or gan of two elasmobranchs, the skate (Raja montagui) and the dogfish (Scylio rhinus canicula), a fluorescent carbocyanine (DiI) was applied to the pinea l organ of paraformaldehyde-fixed brains. This application strongly labeled the pineal tract, which formed extensive bilateral projections. In both sp ecies, the pinealofugal fibers coursed to the dorsomedial thalamus, the med ial pretectal area, the posterior tubercle. and the medial mesencephalic te gmentum and branched profusely in these areas. Application of DiI to the pi neal organ also labeled occasional perikarya in the dorsomedial thalamus, p osterior commissural region, posterior tubercle, and mesencephalic tegmentu m. A comparison of these results with those of immunocytochemical analyses of the dogfish brain with an anti-salmon gonadotropin-releasing hormone (sG nRH) antiserum revealed a close topographical relation between the pineal p rojections and the midbrain sGnRH-immunoreactive (ir) nucleus, the only str ucture in the dogfish brain that contained sGnRHir neurons. This and the wi despread distribution of sGnRHir fibers in the brain suggest that the midbr ain sGnRHir nucleus is a part of the secondary pineal pathways and may be i nvolved in light-mediated pineal regulation of brain function. Although GnR H distribution has not been studied in the skate, a midbrain GnRHir nucleus has been identified in three other elasmobranchs, including a skate relati ve. The probable existence of direct pineal projections to the GnRHir midbr ain nucleus in elasmobranchs and other anamniotes is discussed.