Distribution of gamma aminobutyric acid containing neurons and terminals in the septal area. An immunohistochemical study using antibodies to glutamic acid decarboxylase in the rat brain.

The distribution of gamma aminobutyric acid (GABA)-containing neurons and nerve terminals was examined in the rat septal area by using specific antibodies to the enzyme glutamic acid decarboxylase (GAD) in combination with the avidin-biotin immunoperoxidase method. Whereas only a few GAD positive neurons were present in the septum of normal rats, the septal area of rats treated with colchicine, an inhibitor of fast axonal transport, showed numerous GAD-immunoreactive neurons. These neurons were evenly filled with GAD-immunoreactive material throughout the cytoplasm of the soma and proximal parts of the dendrites. Although GAD-positive neurons were present in most parts of the septal area, their density differed greatly in the different septal subnuclei. Both the diagonal band of Broca (vertical and horizontal parts) and the lateral septum were rich in GAD positive cell bodies, whereas the medial septal nucleus and the intermediate parts of the lateral septum contained relatively few. Within the lateral septum itself a larger number of labeled cell bodies was present in its ventral subdivision. The anterior hippocampal rudiment (taenia tecta) contained numerous GAD-positive neurons, while the septal component of the island of Calleja (insula magna) was devoid of them. GAD-immuno-positive neurons found within the septum ranged from small (15 microns) to large (30-35 micron). They were round or multipolar in the diagonal band, medium-sized multipolar in the lateral septum, and pyramidal, round or fusiform in the anterior hippocampal rudiment. GAD-immunoreactive nerve terminals are present in most subdivisions of the septal nuclei, with the exception of myelinated fiber tracts, and throughout all rostrocaudal levels of the septum. However, the density of the innervation is not the same within all individual nuclei. The lateral septum (dorsal and ventral parts) contained high density innervation but the diagonal band of Broca had a lower density of GAD-positive terminals. The lateral border of the islands of Calleja was rich in thick GAD-positive processes that appeared to be continuous with GAD-immunoreactive processes of the substantia inominata. The inner portion of the molecular layer adjacent to the granule cells of the anterior hippocampal rudiment contained a rich GAD-positive terminal field.