Immunocytochemical studies of GABAergic neurons in rat basal ganglia and their relations to other neuronal systems.

GABAergic neurons were localized in the rat basal ganglia by glutamate decarboxylase (GAD) immunohistochemistry. In the striatum (caudato-putamen, accumbens nucleus) a medium density of GAD-positive terminals was observed; a small number of medium-to-large size neurons and the vast majority of medium-size neurons were GAD immunoreactive. In addition, opioid peptide-like immunoreactivity was colocalized in a subclass of GAD-positive medium-size striatal neurons. The pallido-nigral system (GP, VP, EP, SNR) displayed a high density of GAD-positive axon terminals which synapsed upon dendrites and nerve cell bodies. The majority of pallido-nigral neurons also were GAD-immunoreactive. In contrast, the substantia nigra pars compacta and the subthalamic nucleus contained only few GAD-immunoreactive neurons.