Immunocytochemical characterization of the suprachiasmatic nucleus and the intergeniculate leaflet in the diurnal ground squirrel, Spermophilus lateralis.

The suprachiasmatic nucleus (SCN) and the intergeniculate leaflet (IGL) are retinorecipient structures that play important roles in the expression of circadian rhythmicity. We examined these two structures in a diurnal ground squirrel, Spermophilus lateralis, using immunohistochemical techniques, and cholera toxin-bound horseradish peroxidase. A number of immunoreactive substances are distributed within the ground squirrel SCN in a pattern similar to that reported in many other mammals. These include vasopressin, vasoactive intestinal polypeptide, serotonin, neuropeptide Y (NPY), and glial fibrillary acidic protein. The squirrel SCN differs from that of most other species examined to date in two respects. First, a dense cluster of cells containing immunoreactive L-enkephalin (L-ENK-IR) is observed in the center of the SCN. Second, there is a contralateral, but no ipsilateral, projection from the retina to the SCN. In the lateral geniculate region there is a substantial region that contains NPY-immunoreactive cells and receives a bilateral retinal projection. This region is assumed to be homologous with the IGL described in other mammals. Cells containing L-ENK-IR are distributed throughout the LGN in groups that overlap, but which have a distinctly different distribution than the more extensive groups of NPY-IR cells.