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Quantitative receptor autoradiography was used to investigate the distribution of high-affinity GABA receptors (GABAA) in left and right hemispheres of the brains of 3-week-old chicks. The receptors were labelled with the potent GABA agonist [3H]muscimol. High levels of [3H]muscimol labelling were found throughout the fore-, mid-, and hindbrain, though considerable variation was found in different regions. In the telencephalon the highest concentration of specific binding was found in the hyperstriatum ventrale followed by the neostriatum, and then the lobus parolfactorius of the paleostriatal complex, whilst in the diencephalon highest levels of labelling were present in the infundibulum. In the midbrain distinct lamination was observed in the high levels of [3H]muscimol binding in the optic tectum and in the hind brain the highest density of labelling occurred in the granular layers of the cerebellum. Levels of labelling were generally low in the brainstem regions. The distribution of [3H]muscimol binding in the optic tectum and in the hind brain the highest density of labelling occurred in the granular layers of the cerebellum. Levels of labelling were generally low in the brainstem regions. The distribution of [3H]muscimol binding sites is in good agreement with our previous work on the distribution of GABA-immunoreactivity in the chick brain.

The distribution of [3H]naloxone binding sites was investigated in the brains of one-day-old domestic chicks using quantitative receptor autoradiography. Among the forebrain regions, the greatest levels of opiate binding were found in hyperstriatum dorsale, hyperstriatum ventrale, hyperstriatum intercalatum supremum and neostriatum. Intermediate levels were found throughout the paleostriatal regions, septum, thalamus, archistriatum, hyperstriatum accessorium and area parahippocampalis whilst in hippocampus and ectostriatum the density of [3H]naloxone binding sites was low. In the hindbrain, high levels of opiate binding were found in optic tectum whereas in cerebellum the density of binding sites was barely above background. The greatest densities of opiate binding appear to coincide with regions found to be involved in sensory processing and memory storage.