A comparison of GABA- and GAD-like immunoreactivity within the area postrema of the rat and cat.

The immunohistochemical localization of gamma-aminobutyric acid (GABA) was compared to that of glutamate decarboxylase (GAD) in the rat and cat area postrema with the aid of a polyclonal antibody produced in rabbits directed against GABA. In both the rat and cat, dense and very dense accumulations of GABA-like immunoreactive (GABA-LI) varicosities were present throughout the area postrema. GABA-LI cell bodies were present in both species and were evenly distributed throughout the area postrema's extent. However, the rat area postrema contained more GABA-LI cell bodies and varicosities than the cat area postrema. In the cat area postrema, a range of cell sizes were immunostained with the GABA antibody. The GAD antibody, however, failed to reveal cell bodies in the area postrema of the cat, thus indicating that the GABA polyclonal antibody may be a better indicator of GABA-containing somata. Although the mechanism of action of GABA in the area postrema is not understood, it is possible that GABA may play a role in the different functions of the area postrema in emetic and nonemetic species.