Immunohistochemical studies in several mammalian species and in red‐eared turtles have shown that somatostatin (SS) and neuropeptide Y (NPY) co‐occur in a substantial proportion of the telencephalic neurons containing either. To explore further the possibility that telencephalic neurons co‐containing SS and NPY may be evolutionarily conserved among amniotes, we determined the distribution and co‐occurrence of SS and NPY in forebrain neurons in pigeons. Single‐label immunohistochemical studies revealed the presence of overlapping populations of SS + neurons and NPY+ neurons in most of the major subdivisions of the telencephalon. Double‐label immunofluorescence studies revealed that in subdivisions of the telencephalon that are comparable to mammalian cortex (i.e., those dorsal and lateral to the basal ganglia), the vast majority of NPY+ neurons were also SS+, whereas a major and regionally variable percentage of the SS+ neurons were not NPY+. In contrast, within the basal telencephalon (including the basal ganglia and several other structures) neurons labeled only for NPY or only SS were more abundant than those containing both neuropeptides. Outside the telencephalon, the only forebrain cell group containing neurons in which SS and NPY were co‐localized was in the lateral hypothalamus. A series of double‐ and triple‐label immunohistochemical studies was undertaken to determine the extent of co‐occurrence of SS and NPY in striatal neurons and the relationship of these neurons to striatal neurons containing other neuropeptides. In addition, immunohistochemical single‐ and double‐label techniques were employed in conjunction with retrograde‐labeling by fluorogold to determine the projections of SS+ and NPY+ striatal neurons. The results indicate that: (1) a population of striatal interneurons containing both SS and NPY exists in pigeons and constitutes approximately the same fraction of all striatal neurons as reported in mammals, (2) neurons containing NPY (but not SS) form a second, larger population of striatal interneurons, (3) neurons containing SS (but not NPY) form a third population of striatal interneurons that is approximately half as abundant as the NPY+ interneuron population, and (4) one‐third of the substance P‐containing striatonigral projection neurons also contain SS. The existence in pigeons of a major population of neurons containing both SS and NPY throughout the telencephalon, the existence of a population of neurons containing only SS in cortex‐equivalent parts of the telencephalon, and the existence of a population of interneurons containing only NPY in the striatum is consistent with findings in mammals and turtles. The existence of these peptide‐specific populations in members of each class of amniotes favors the view that these populations were present in the last common ancestor of amniotes and were conserved during amniote evolution. Thus, they most likely play fundamentally important roles in telencephalic circuits. However, the avian striatum contains a substantial population of SS+ interneurons and a population of SS+/SP+ striatonigral projection neurons. SS+ projection neurons have also been reported in the guinea pig, but not in other mammals. Thus, the presence of some types of SS‐containing striatal neurons may vary among species. Copyright © 1990 Wiley‐Liss, Inc.
