Neuropeptide Y-immunoreactive neurons in the striatum of cat and monkey: morphological characteristics, intrinsic organization and co-localization with somatostatin.

A detailed study of the distribution of neuropeptide Y (NPY) in the striatum of squirrel monkey (Saimiri sciureus) and cat was undertaken by means of indirect immunofluorescence and peroxidase-antiperoxidase (PAP) methods. In monkey, the NPY-immunoreactivity is homogeneously distributed along the entire extent of the caudate nucleus (CD) and putamen (PUT), while in cat marked heterogeneities are noted. In the CD of cat, the NPY-immunoreactive fibers and cell bodies are concentrated in numerous patches of various sizes, which can be readily distinguished from zones of poor NPY-immunostaining. In the CD and PUT of squirrel monkey the NPY-positive neurons are either triangular, fusiform or globular, with long and smooth dendrites branching infrequently. The numerical density of NPY-immunoreactive cell bodies is greater in the CD than in the PUT, and it increases markedly along the rostrocaudal extent of the striatum. In the rostral CD and PUT the densities are 23 cells/mm2 and 14 cells/mm2, respectively, whereas the values for caudal CD and PUT are 35 cells/mm2 and 20 cells/mm2, respectively. Quantitative measurements reveal that these NPY-immunoreactive cells belong to a single subset of striatal neurons having a maximum diameter of 19.2 +/- 0.1 micron and a cross-sectional area of 145.5 +/- 0.6 micron2 (mean +/- S.E.M.; n = 1238 CD cells and 1169 PUT cells). Furthermore, experiments combining the use of lectin-conjugated HRP as retrograde tracer with PAP immunohistochemical method demonstrate that striatal NPY-immunoreactive neurons in squirrel monkey and cat do not project outside the striatum. Finally, co-localization studies in monkey reveal that the vast majority of striatal NPY-positive neurons also contains somatostatin. These results show that the NPY-immunoreactive neurons in mammalian striatum form a subpopulation of medium-sized interneurons containing somatostatin.