AFFERENT AND EFFERENT CONNECTIONS OF THE HABENULA IN THE RAINBOW-TROUT (ONCORHYNCHUS-MYKISS) - AN INDOCARBOCYANINE DYE (DII) STUDY

The habenula is a conserved structure in the brain of vertebrates. Wit h the aim of further understanding of the evolution of the habenular s ystem in vertebrates, we studied the afferent and efferent connections of the habenula of the rainbow trout. Experiments included applicatio n of the carbocyanine dye -dioctadecyl-3,3,3',3'-tetramethylindocarboc yanine perchlorate (DiI) into the habenula, telencephalon, pineal orga n, posterior tubercle, and interpeduncular nucleus (IPN). The results obtained reveal a consistent pattern of habenular connections. Most af ferents originate from three nuclei, one extending from the preoptic r egion to the rostral thalamus (the entopeduncular nucleus), the second located in the region of the hypothalamus-posterior tubercle and cons isting of large bipolar cells (tuberculohabenular nucleus), and the th ird in the preoptic region (preoptic nucleus). A few large neurons of the locus coeruleus appeared to be labeled in some cases. The trout ha benula also receives pineal and parapineal projections. Small labeled glial cells were observed in the thalamus around the fasciculus retrof lexus and, sometimes, around the IPN. The most conspicuous efferents c oursed in the fasciculus retroflexus to the IPN, the isthmal raphe, an d the central gray. The existence of olfactohabenular or habenulotelen cephalic projections is discussed. (C) 1996 Wiley-Liss, Inc.