Distribution of GABA-like immunoreactivity in the pigeon brain.

The distribution of GABA-like immunoreactivity in glutaraldehyde-fixed pigeon brains was studied by means of a monoclonal antibody. GABA-like immunoreactivity was observed in neuronal perikarya of different sizes as well as in neuropil and in certain fiber tracts. Certain staining patterns indicated the existence of several GABAergic projection systems in the pigeon brain. Indeed, a high density of immunostained perikarya and a low density of labeled terminal-like elements was the prominent pattern in the nuclei subpretectalis and posteroventralis, while an absence of perikaryal GABA-like immunoreactivity and accumulations of immunoreactive dots were observed in the isthmo-optic nucleus, amongst others. In the optic tectum, stained cell bodies with radially oriented processes in layer IIi (10) and with horizontally oriented processes in layer IId (5) were seen and were reminiscent of autoradiographic labeling patterns obtained previously following tectal injection of tritiated GABA. In the cerebellum, GABA-like immunoreactivity involved all types of neurons with the exception of granule cells. Purkinje cells showed regionally different intensities of immunostaining. In addition, in folium X no stained basket-like elements were observed. Although there is no evidence as yet about the function of GABA in most of the structures, the present results indicate an important role for this neurotransmitter in the pigeon brain.