Pineal neural connections with the brain in two teleosts, the crucian carp and the European eel.

The neural connections between the pineal organ and the brain were investigated in two teleost species: the crucian carp and the European eel. Horseradish peroxidase (HRP) was administered into the pineal parenchyma in vivo or in vitro. After 12-96 hours, the brains were fixed, and HRP was reacted in frozen sections with either diaminobenzidine (DAB) or Hanker-Yate's reagent. The HRP had entered the pineal neurons, and had labeled their axonal processes (the pineal tract) in a Golgi-like manner. In both species, HRP-labeled axons were observed in the habenular nuclei, the pretectal area, the dorsal and ventral thalamus, the dorsal periventricular tegmentum, and in the posterior periventricular hypothalamus. In the eel, single axons were also observed in the anterior hypothalamus and in the brain stem. Thin, varicose (preterminal) axons were observed in great numbers in the pretectal area and dorsal thalamus in both species, and in small numbers in the habenular nuclei and posterior periventricular hypothalamus. For comparison, the retinofugal projections were visualized after intraocular HRP-injection. A close association of retinal and pineal terminal fields were noted in the pretectal area and the dorsal thalamus, whereas other diencephalic retinal and pineal recipient areas seem more segregated.