Afferent and efferent connections of the parapineal organ in lampreys: A tract tracing and immunocytochemical study

The neural connections of the parapineal organ of two species of lampreys w ere studied with the fluorescent dye 1,1'-dioctadecyl-3,3,3',3'-tetramethyl indocarbocyanine perchlorate (DiI) and with immunocytochemistry. The lampre y parapineal organ consists of a vesicle and a ganglion that are connected to the left habenula. Labeling experiments included the application of DiI to the parapineal organ, left and right fasciculus retroflexus, left habenu la, and the left pretectal region. Afferent parapineal fibers run in the le ft fasciculus retroflexus to the interpeduncular nucleus. The parapineal fi bers of this fascicle arose from parapineal ganglion cells, whereas DiI app lication to the left habenula labeled both neurons of this ganglion and bip olar cells in the parapineal vesicle. Efferent neurons were observed in the left habenula, and bilaterally in the subhippocampal nucleus and the dorsa l pretectum. Labeling with DiI also revealed a hippocampal projection. Immunocytochemical study of the parapineal vesicle revealed serotonin-immun oreactive cells in both species of lamprey, as well as substance P-immunore active (SP-ir) cells in sea lamprey and choline acetyltransferase-immunorea ctive (ChAT-ir) cells in the river lamprey. The SP-ir cells and ChAT-ir cel ls formed a rich neuropil in the parapineal ganglion. Calretinin-ir cells w ere numerous in the ganglion. Neuropeptide Y-immunoreactive and gamma-amino butyric acid-immunoreactive efferent fibers were observed in the parapineal organ. Neuropeptide Y-immunoreactive fibers originate in the subhippocampa l nucleus, whereas gamma-aminobutyric acid-immunoreactive fibers might also arise in the pretectal nucleus. A few galanin-ir fibers were observed. The se results indicate that the parapineal connections are completely differen t from those of the pineal organ. The possible homology between parapineal organs of vertebrates is discussed. (C) 1999 Wiley-Liss, Inc.