Pineal projections in the zebrafish (Danio rerio): overlap with retinal and cerebellar projections.

The pineal organ in fishes is a photoreceptive organ with dual outputs, neuroendocrine and neural. The neural projections of the zebrafish pineal were experimentally studied by means of tract-tracing with carbocyanine dyes (1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI), 3,3'-dioctadecyloxacarbocyanine perchlorate (DiO)). Double-labeling experiments were also performed in order to investigate the degree of overlapping of pineal, retinal or cerebellar projections in zebrafish. The pineal organ sends efferent fibers bilaterally to the rostral hypothalamus, thalamus, pretectum, posterior tubercle and the mesencephalic tegmentum. A few pinealofugal fibers could also be traced to the optic tectum. Most of the targets of the zebrafish pineal also receive retinal and/or cerebellar afferents, indicating a high degree of overlap with these projections. Since most of the targets of the pineal projections of zebrafish appear to be premotor and precerebellar centers, the neural output of the pineal organ is probably, because of its photoreceptive and circadian function, involved in photic and circadian modulation of these centers.