The organization of the pretectal nuclei in the trout: a revision based on experimental holodogical studies.

The neuronal tracer DiI was applied to different brain centers of the rainbow trout in order to study the connections of pretectal nuclei. Our results showed that some pretectal nuclei receive a direct projection from the contralateral retina: the parvocellular superficial pretectal nucleus, the central pretectal nucleus, the intermediate pretectal nucleus and the ventral accessory optic nucleus. In turn, the central pretectal, the intermediate pretectal and the ventral accessory optic nuclei, together with the paracommissural nucleus, project to the cerebellum and the torus longitudinalis. The magnocellular superficial pretectal nucleus does not receive retinal projections, but receives ipsilateral projections from the optic tectum and the mesencephalic tegmentum. In turn, it projects to the ipsilateral oculomotor nucleus and lateral nucleus of the valvula. The posterior pretectal nucleus and the parvocellular superficial pretectal nucleus receive afferents from the ipsilateral nucleus isthmi. The posterior pretectal nucleus projects to the inferior hypothalamic lobe. Our results reveal a conspicuous projection from the ipsilateral parvocellular superficial pretectal nucleus to the contralateral one and also to the contralateral posterior prectectal nucleus, not reported in previous experimental studies of teleosts. Pretectal centers appear to integrate visual/optic-related centers mainly with the hypothalamus and the cerebellum. The organization of the trout pretectum was compared with the pretectal organization patterns proposed in various teleosts.