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 were studied with the fluorescent dye 1,1'-dioctadecyl-3,3,3'3'-tetramethylindocarbocyanine perchlorate (DiI) and with immunocytochemistry. The lamprey 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 habenula, and the left pretectal region. Afferent parapineal fibers run in the left fasciculus retroflexus to the interpeduncular nucleus. The parapineal fibers of this fascicle arose from parapineal ganglion cells, whereas DiI application to the left habenula labeled both neurons of this ganglion and bipolar cells in the parapineal vesicle. Efferent neurons were observed in the left habenula, and bilaterally in the subhippocampal nucleus and the dorsal pretectum. Labeling with DiI also revealed a hippocampal projection. Immunocytochemical study of the parapineal vesicle revealed serotonin-immunoreactive cells in both species of lamprey, as well as substance P-immunoreactive (SP-ir) cells in sea lamprey and choline acetyltransferase-immunoreactive (ChAT-ir) cells in the river lamprey. The SP-ir cells and ChAT-ir cells formed a rich neuropil in the parapineal ganglion. Calretinin-ir cells were numerous in the ganglion. Neuropeptide Y-immunoreactive and gamma-aminobutyric acid-immunoreactive efferent fibers were observed in the parapineal organ. Neuropeptide Y-immunoreactive fibers originate in the subhippocampal nucleus, whereas gamma-aminobutyric acid-immunoreactive fibers might also arise in the pretectal nucleus. A few galanin-ir fibers were observed. These results indicate that the parapineal connections are completely different from those of the pineal organ. The possible homology between parapineal organs of vertebrates is discussed.