Journal of Neuroscience, Vol 7, 48-54, Copyright © 1987 by Society for Neuroscience
An ultrastructural study of GABA-immunoreactive neurons and terminals in the septum of the rat
B Onteniente, M Geffard, G Campistron and A Calas
The fine structure and types of contact made by GABAergic elements in the
septal nuclei were studied at the electronmicroscopic level by means of
peroxidase immunocytochemistry, using anti-GABA antibodies. Observations
were made on normal and colchicine-injected rats. GABA- immunoreactivity
was distributed within somata, dendrites, axonal varicosities and
terminals, and myelinated axons. The peroxidase reaction product was
diffuse in the cytoplasm; cytoplasmic organelles were generally devoid of
immunoreactivity, while showing a strong reaction on the outer surface of
their membrane. GABA-immunoreactive (GABA-I) neurons were small (10 microns
on average) to medium (20 microns) in size, with round or multipolar cell
bodies. Additionally, labeled large (30 microns) cells were observed within
the myelinated fibers of the medial septal nucleus after intraseptal
administration of colchicine. No difference in the ultrastructural features
and distribution of the immunoreactivity of the 2 kinds of cell was
noticed, except for a higher number of synaptic contacts on large neurons
of the medial septum. GABA-I cells of the medial and lateral nuclei
received synapses on their soma and dendrites, made by both immunonegative
and GABA-I terminals. Nonimmunoreactive boutons contacting GABA-I cell
bodies were of 2 types: those containing small, clear synaptic vesicles and
those that additionally contained large dense vesicles. Synaptic vesicles
of GABA-I boutons were rarely labeled internally, but showed varying
electron densities. Synapses made by GABA-I boutons on GABA-I or unlabeled
somata and dendrites were always of symmetrical type. Synapses made by
non-GABA-I boutons on GABA-I cells were either symmetrical or asymmetrical.