THE JOURNAL OF COMPARATIVE NEUROLOGY 339209-250 (1994) The Distribution of GABA-Containing Perikarya, Fibers, and Terminals in the Forebrain and Midbrain of Pigeons, With Particular Reference to the Basal Ganglia and Its Projection Targets C. LEO VEENMAN zyxwvut AND ANTON REINER Department of Anatomy and Neurobiology, The University of Tennessee-Memphis, The Health Science Center, Memphis, Tennessee 38163 ABSTRACT Immunohistochemical techniques were used to study the distributions of glutamic acid decarboxylase (GAD) and y-aminobutyric acid (GABA) in pigeon forebrain and midbrain to determine the organization of GABAergic systems in these brain areas in birds. In the basal ganglia, numerous medium-sized neurons throughout the striatum were labeled for GABA, while pallidal neurons, as well as a small population of large, aspiny striatal neurons, labeled for GAD and GABA. GAD+ and GABA+ fibers and terminals were abundant throughout the basal ganglia, and GABAergic fibers were found in all extratelencephalic targets of the basal ganglia. Most of these targets also contained numerous GABAergic neurons. In pallial regions, approximately 10-12% of the neurons were GABAergic. The outer rind of the pallium was more intensely labeled for GABAergic fibers than the core. The olfactory tubercle region, the ventral pallidum, and the hypothalamus were extremely densely labeled for GABAergic fibers, while GABAergic neurons were unevenly distributed in the hypothalamus. GABAergic neurons and fibers were abundant in the dorsalmost part of thalamus and the dorsal geniculate region, while GABAergic neurons and fibers were sparse (or lightly labeled) in the thalamic nuclei rotundus, triangularis, and ovoidalis. Further, GABAergic neurons were abundant in the superficial tectal layers, the magnocellular isthmic nucleus, the inferior colliculus, the intercollicular region, the central gray, and the reticular formation. GABAergic fibers were particularly abundant in the superficial tectal layers, the parvocellular isthmic nucleus, the inferior colliculus, the intercol- licular region, the central gray, and the interpeduncular nucleus. These results suggest that GABA plays a role as a neurotransmitter in nearly all fore- and midbrain regions of birds, and in many instances the observed distributions of GABAergic neurons and fibers closely resemble the patterns seen in mammals, as well as in other vertebrates. zyxw o 1994 Wiley-Liss, Inc. Key words: immunohistochemistry, glutamic acid decarboxylase, neocortex, optic tectum, striatum We have used immunohistochemical techniques to deter- mine the distribution of GABAergic neurons and fibers in the pigeon forebrain and midbrain. The organization of the central nervous system of pigeons is among the most extensively studied in birds, particularly with respect to the connections and neurotransmitters of the basal ganglia (Karten and Dubbeldam, '73; Brauth et al., '78; Kitt and Brauth, '81, '82, '86a,b; Reiner et al., '83, '84a; Hall et al., '84; Reiner, '86a,b; Reiner and Carraway, '87; Reiner and Anderson, '90). These studies have revealed that birds possess a basal ganglia with the same major constituent cell groups (striatum and pallidum) and neuronal populations (as defined by connections and neuropeptide content) as the basal ganglia in mammals (Reiner et al., '84a; Reiner and Anderson, '90). In addition to these structural similarities, the basal ganglia of birds and mammals also share many commonalities in their role in movement control (Rieke, '80, '81; Goodman, '89). One of the hallmarks of the mammalian basal ganglia is that all or nearly all pallidal and striatal projection neurons utilize y-aminobutyric acid (GABA) as a neurotransmitter, Accepted June 28,1993. Address reprint requests to Dr. C. Leo Veenman, Dept. of Anatomy and Neurobiology, University of Tennessee-Memphis, 875 Monroe Ave., Mem- phis, TN 38163. zyxwvu O 1994 WILEY-LISS, INC.