Immunohistochemical techniques were used to determine the distribution of enkephalinlike immunoreactivity in the telencephalon of chicken. The densest accumulation of enkephalinergic neurons and fibers was observed within the paleostriatal complex, the avian equivalent of the mammalian basal ganglia. Numerous small enkephalinergic neurons were observed in both lobus parolfactorius (LPO) and the paleostriatum augmentatum (PA), the two components of the small-celled portion of the paleostriatal complex. The enkephalinergic neurons of LPO-PA appeared to give rise to a dense plexus of enkephalinergic fibers within the large-celled zone of the paleostriatal complex, the paleostriatum primitivum (PP). The distribution of enkephalin within the avian paleostriatal complex, when compared to the distribution of enkephalin within the mammalian basal ganglia, supports previous proposals that PP is comparable to the mammalian globus pallidus and that PA-LPO are comparable to the caudate-putamen (Karten and Dubbeldam, '73; Kitt and Brauth, '81; Parent and Olivier, '70; Reiner et al., '83). Observations on the development of enkephalinlike immunoreactivity within the chicken paleostriatal complex also support the suggestion that the major component nuclei of the avian paleostriatal complex have correspondents within the mammalian basal ganglia. Enkephalinlike immunoreactivity was also observed within cell bodies and fibers in other portions of the avian telencephalon. Within the ventrolateral telencephalon, the nucleus accumbens, nucleus of the diagonal band, and tuberculum olfactorium contained enkephalinergic cell bodies and fibers while only enkephalinergic fibers were observed in the portion of the avian telencephalon that has been termed the ventral paleostriatum (Kitt and Brauth, '81; Reiner et al., '83). Within the medial wall of the telencephalon, enkephalinergic fibers were observed in the lateral septal nucleus, while enkephalinergic cell bodies and fibers were observed in the parahippocampal area. Little enkephalinlike immunoreactivity was observed dorsal to the paleostriatal complex except in the hyperstriatum dorsale. Within the hyperstriatum dorsale, a band of enkephalinergic neurons appeared to give rise to an overlying parallel band of dense enkephalinergic fibers. The distribution of enkephalinlike immunoreactivity within the avian telencephalon thus shows remarkable similarity to that seen in the mammalian telencephalon. The largest accumulation of enkephalinlike immunoreactivity within the telencephalon of both vertebrate classes appears to be found within the ventrolateral wall of the telencephalon, including the basal ganglia.(ABSTRACT TRUNCATED AT 400 WORDS)
