A DiI-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

Abstract.

The pineal organ of elasmobranchs is an elongated photoreceptive organ. In order to investigate the afferent and efferent connections of the pineal organ of two elasmobranchs, the skate (Raja montagui) and the dogfish (Scyliorhinus canicula), a fluorescent carbocyanine (DiI) was applied to the pineal organ of paraformaldehyde-fixed brains. This application strongly labeled the pineal tract, which formed extensive bilateral projections. In both species, the pinealofugal fibers coursed to the dorsomedial thalamus, the medial pretectal area, the posterior tubercle, and the medial mesencephalic tegmentum and branched profusely in these areas. Application of DiI to the pineal organ also labeled occasional perikarya in the dorsomedial thalamus, posterior commissural region, posterior tubercle, and mesencephalic tegmentum. A comparison of these results with those of immunocytochemical analyses of the dogfish brain with an anti-salmon gonadotropin-releasing hormone (sGnRH) antiserum revealed a close topographical relation between the pineal projections and the midbrain sGnRH-immunoreactive (ir) nucleus, the only structure in the dogfish brain that contained sGnRHir neurons. This and the widespread distribution of sGnRHir fibers in the brain suggest that the midbrain sGnRHir nucleus is a part of the secondary pineal pathways and may be involved in light-mediated pineal regulation of brain function. Although GnRH distribution has not been studied in the skate, a midbrain GnRHir nucleus has been identified in three other elasmobranchs, including a skate relative. The probable existence of direct pineal projections to the GnRHir midbrain nucleus in elasmobranchs and other anamniotes is discussed.