Neuropeptide Y immunoreactivity in the hamster geniculo-suprachiasmatic tract

The distributions of neuropeptide Y (NPY) and avian pancreatic polypeptide (APP) immunoreactivity were examined in the suprachiasmatic nucleus and the geniculate area of male golden hamster brains. In some cases, colchicine was injected intraventricularly to aid in visualization of immunoreactive cell bodies. A group of hamsters were given bilateral or unilateral radiofrequency lesions of the geniculate area and neuropeptide Y immunoreactivity was examined in the suprachiasmatic nucleus after survival times varying between 8 and 300 days. Another group of hamsters received unilateral intraocular injections of anterograde tracers and the overlap of NPY-immunoreactive cells in the geniculate area and labeled retinal afferents was assessed. It was found that NPY- and APP-immunoreactive fibers formed a dense plexus in the ventro-lateral suprachiasmatic nucleus. NPY-immunoreactive cell bodies were observed in the intergeniculate leaflet as well as in the external lamina of the anterior portion of the ventral lateral geniculate nucleus. Unilateral lesions of the geniculate produced a relative reduction in neuropeptide Y immunoreactivity in the ipsilateral suprachiasmatic nucleus whereas bilateral lesions produced a reduction of neuropeptide Y immunoreactivity in both suprachiasmatic nuclei. All NPY-immunoreactive cells in the intergeniculate leaflet were overlapped by bilateral retinal afferents. In the ventral lateral geniculate nucleus, all NPY-immunoreactive cells were overlapped by contralateral retinal afferents; however, not all such cells were in areas receiving ipsilateral retinal afferents. These results indicate that the hamster geniculo-suprachiasmatic tract originates in part from NPY-immunoreactive cell bodies and that these cells lie in areas receiving direct retinal afferents.