THE DISTRIBUTION OF GABA-CONTAINING PERIKARYA, FIBERS, AND TERMINALS IN THE FOREBRAIN AND MIDBRAIN OF PIGEONS, WITH PARTICULAR REFERENCE TOTHE BASAL GANGLIA AND ITS PROJECTION TARGETS

Immunohistochemical techniques were used to study the distributions of glutamic acid decarboxylase (GAD) and gamma-aminobutyric acid (GABA) in pigeon forebrain and midbrain to determine the organization of GABA ergic systems in these brain areas in birds. In the basal ganglia, num erous medium-sized neurons throughout the striatum were labeled for GA BA, while pallidal neurons, as well as a small population of large, as piny striatal neurons, labeled for GAD and GABA. GAD+ and GABA+ fibers and terminals were abundant throughout the basal ganglia, and GABAerg ic fibers were found in all extratelencephalic targets of the basal ga nglia. Most of these targets also contained numerous GABAergic neurons . In pallial regions, approximately 10-12% of the neurons were GABAerg ic. The outer rind of the pallium was more intensely labeled for GABAe rgic fibers than the core. The olfactory tubercle region, the ventral pallidum, and the hypothalamus were extremely densely labeled for GABA ergic fibers, while GABAergic neurons were unevenly distributed in the hypothalamus. GABAergic neurons and fibers were abundant in the dorsa lmost part of thalamus and the dorsal geniculate region, while GABAerg ic neurons and fibers were sparse (or lightly labeled) in the thalamic nuclei rotundus, triangularis, and ovoidalis. Further, GABAergic neur ons 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 p articularly abundant in the superficial tectal layers, the parvocellul ar isthmic nucleus, the inferior colliculus, the intercollicular regio n, the central gray, and the interpeduncular nucleus. These results su ggest that GABA plays a role as a neurotransmitter in nearly all fore- and midbrain regions of birds, and in many instances the observed dis tributions of GABAergic neurons and fibers closely resemble the patter ns seen in mammals, as well as in other vertebrates. (C) 1994 Wiley-Li ss, Inc.