Distributions of GABAA, GABAB, and benzodiazepine receptors in the forebrain and midbrain of pigeons.

Autoradiographic and immunohistochemical methods were used to study the distributions of GABAA, GABAB and benzodiazepine (BDZ) receptors in the pigeon fore- and midbrain. GABAA, GABAB and BDZ binding sites were found to be abundant although heterogeneously distributed in the telencephalon. The primary sensory areas of the pallium of the avian telencephalon (Wulst and dorsal ventricular ridge) tended to be low in all three binding sites, while the surrounding second order belt regions of the pallium were typically high in all three. Finally, the outermost rind of the pallium (termed the pallium externum by us), which surrounds the belt regions and projects to the striatum of the basal ganglia, was intermediate in all three GABAergic receptors types. Although both GABAA and benzodiazepine receptors were abundant within the basal ganglia, GABAA binding sites were densest in the striatum and BDZ binding sites were densest in the pallidum. Among the brainstem regions receiving GABAergic basal ganglia input, the anterior and posterior nuclei of the ansa lenticularis showed very low levels of all three receptors, while the lateral spiriform nucleus and the ventral tegmental area/substantia nigra complex contained moderate abundance of the three binding sites. The dorsalmost part of the dorsal thalamus (containing nonspecific nuclei) was rich in all three binding sites, while the more ventral part of the dorsal thalamus (containing specific sensory nuclei), the ventral thalamus and the hypothalamus were poor in all three binding sites. The pretectum was also generally poor in all three, although some nuclei displayed higher levels of one or more binding sites. The optic tectum, inferior colliculus, and central gray were rich in all three sites, while among the isthmic nuclei, the parvicellular isthmic nucleus was conspicuously rich in BDZ sites. The results show a strong correlation of the regional abundance of GABA binding sites with previously described distributions of GABAergic fibers and terminals in the avian forebrain and midbrain. The regional distribution of these binding sites is also remarkably similar to that in mammals, indicating a conservative evolution of forebrain and midbrain GABA systems among amniotes.