Geniculo-geniculate projection of enkephalin and neuropeptide Y containing neurons in the intergeniculate leaflet of the thalamus in the rat.

Geniculo-geniculate projections of immunoreactive neurons for gamma-aminobutyric acid (GABA), leucine-enkephalin (ENK), neuropeptide Y (NPY) and substance P (SP) in the intergeniculate leaflet (IGL) and ventral lateral geniculate nucleus (VLG) of the rat were examined by using a combination of retrograde tracing method and immunocytochemistry. After injection of the fast blue (FB) dye into the IGL and VLG, many ENK immunoreactive neurons, and some NPY immunoreactive neurons were labelled by FB dye in the contralateral IGL. However, GABA and SP immunoreactive neurons in the IGL and VLG were not labelled retrogradely by FB dye. A unilateral electrical lesion of the IGL and VLG caused a reduction of the ENK and NPY immunoreactive fibres in the contralateral IGL and VLG. These findings suggested that ENK is one major component of the neuroactive substances in the geniculo-geniculate projection, and NPY also contributes partly to this projection. Furthermore, the unilateral destruction of the IGL and VLG showed a marked loss of the NPY immunoreactive fibres and a slight loss of the ENK immunoreactive fibres in the bilateral suprachiasmatic nuclei.