Distribution of somatostatin immunoreactivity in the forebrain of the squirrel monkey: Basal ganglia and amygdala

Abstract

The distribution of somatostatin immunoreactivity in the basal ganglia and amygdala of the squirrel monkey ( Saimiri sciureus ) was studied with specific polyclonal antibodies directed against somatostatin-28 and somatostatin-28(1–12). Both antibodies gave similar results with regard to the distribution of somatostatin-immunoreactive neuronal profiles. A moderately dense and highly heterogeneous network of somatostatin-positive fibers was observed throughout the striatum. A dorsoventral gradient of increasing immunoreactivity was noted in the striatum and the caudate nucleus was found to stain generally less intensely than the putamen. The immunoreactive fibers within the striatum were mostly thin and varicose and formed patches corresponding to the striosomes, as visualized on adjacent sections immunostained for calbindin. Although some somatostatin cell bodies rimmed the striosomes, most of the positive cells were rather uniformly scattered in the striatum. These medium-sized cells were significantly smaller in the caudate nucleus (93 μm 2, S.D. = 26 μm 2) than in the putamen (122 μm 2,S.D. = 39 μm 2), but their density was significantly higher in the caudate nucleus (29.7 cells/mm 2,S.D. = 8.8cells/mm 2) than in the putamen (20.5 cells/mm 2,S.D. = 7.0cells/mm 2). The nucleus accumbens stained moderately and positive cell bodies were evenly dispersed throughout this structure. In contrast, the olfactory tubercle displayed a heavily stained neuropil but positive neurons were encountered only in its polymorph layer. In the sublenticular region, dense fiber plexuses appeared in register with nonreactive cell clusters of the nucleus basalis of Meynert and of the nucleus of the anterior commissure. More caudally, a dense bundle of positive fibers was observed at the level of the ansa lenticularis, the inferior thalamic peduncle, and the adjoining bed nucleus of the stria terminalis. Several fibers contributing to this bundle were of the woolly type. Woolly fibers also coursed in the substantia innominata between the ventral aspect of the globus pallidus and the optic tract, and ascended in the internal medullary lamina separating the internal and external segments of the globus pallidus. Somatostatin-immunoreactive cell bodies were uniformly scattered throughout the substantia innominata. The various nuclei of the amygdala showed a wide range of immunoreactivity. The central nucleus was lightly reactive, whereas the intercalated masses displayed a moderate staining. A dorsoventral gradient of immunostaining was noted in the ventrolateral portion of the amygdala, the lateral nucleus being moderately to densely stained and the basal nucleus very lightly to lightly immunoreactive. The ventromedial portion of the amygdala, especially the periamygdaloid cortex and the accessory basal nucleus, were among the most intensely stained areas of the basal forebrain. These results reveal that the somatostatin immunoreactivity in the basal forebrain of the squirrel monkey is distributed according to a pattern that displays both similarities and differences with that disclosed in nonprimate species.