Glutamate Decarboxylase Distribution in Discrete Motor Nuclei in the Cat Brain

Abstract

The distribution of activity of glutamate decarboxylase (GAD), the enzyme synthesizing GABA, was measured in the cat brain by microdissection of the structures from frozen slices and a radioisotopic assay for the enzyme. About 20 cerebral regions were chosen by study because of their role in sensorimotor integration. GAD presented an uneven distribution among these areas. Highest activities were found in the basal ganglia, particularly in the substantia nigra and in the globus pallidus and to a lesser extent in the cerebellum. Relatively low levels of the enzyme were found in the thalamus and in the cerebral motor cortex. Special detailed studies were made in the caudate nucleus, the substantia nigra and in the red nucleus for the purpose of defining the intranuclear distribution of their GABAergic innervation. There were only small differences in the rostro-caudal distribution of the enzyme in the head of the caudate nucleus but GAD activity was higher in the ventral than in the dorsal part of the structure. In the substantia nigra, GAD activity was high in both the medial and intermediate thirds of the structure. The GAD activity decreased from the caudal to the rostral part of the nucleus. GAD levels were lower in the caudal part of the red nucleus than in the rostral part. GABA apparently would be present as a putative neurotransmitter in many motor nuclei of the cat brain. In view of the general inhibitory action of this amino acid, this could be related to the presence of inhibitory responses widely distributed in these nuclei as identified by electrophysiological studies. The origin of these GABAergic innervations in many cases must be determined.