NEURAL CONNECTIONS OF THE PINEAL ORGAN IN THE PRIMITIVE BONY FISH ACIPENSER BAERI - A CARBOCYANINE DYE TRACT-TRACING STUDY

The pineal organ of the sturgeon (Acipenser baeri; Chondrostei) consis ts of an elongated terminal vesicle and a long stalk that runs to the region of the posterior commissure. In order to investigate the neural connections of the pineal organ of the sturgeon, a fluorescent indoca rbocyanine (DiI) was applied to the pineal stalk of paraformaldehyde-f ixed brains. This application strongly labeled the pineal tracts (righ t and left), which form an extensive bilateral projection to the brain . The fibers of these tracts could be followed to their different targ ets in the brain, namely the preoptic region, the rostral habenulae, t he medial subhabenular and retrohabenular (post- and subcommissural) r egion, the medial thalamus, the dorsal hypothalamus, the pretectal are a, and medial and dorsolateral tegmental mesencephalic regions (inters titial nucleus of Cajal, oculomotor nucleus, mesencephalic reticular a rea, nucleus profundus mesencephali, and central gray). A few fibers r each the interpeduncular nucleus and the rostral optic tectum. Pinealo fugal fibers branched profusely, and some of them crossed the midline ventrally in the optic chiasma, the postoptic commissure, the conspicu ous preinfundibular commissure or, occasionally, the mesencephalic teg mentum. In addition to pinealofugal fibers, this type of DiI applicati on labeled a few cells, here interpreted as pinealopetal neurons, in t he posthabenular-dorsomedial thalamic region and the central gray of t he mesencephalon. The wide distribution of pinealofugal projections in sturgeon is reminiscent of that in the lamprey. Comparison of the pre sent findings with those of other authors in teleosts, amphibians, and reptiles also suggests that there has been a reduction in the number of pineal projections in these lines. J. Comp. Neurol. 398:151-161, 19 98. (C) 1998 Wiley-Liss, Inc.