Direct Synaptic Contacts of Medial Septal Efferents With Somatostatin Neurons in the Immunoreactive Rat Hippocampus

YAMANO, M. AND P. G. M. LUITEN. Direct synaptic contacts of medial septal efferents with somatostatin immunoreactive neurons in the rat hippocampus. BRAIN RES BULL 22(6) 993-1001, 1989.-Anterogradely labeled projections from the medial septum to hippocampal somatostatin immunoreactive (SOM-I) neurons were studied with double-label imrnunocytochemistry under light (LM) and electron microscopic (EM) conditions. Medial septal projections were identified after injecting the anterograde tracer Phaseolus vulgaris leucoagglutinin (PHA-L) followed by immunohistochemical visualization of PHA-L presynaptic terminal labeling and concurrent immunocytochemical staining of SOM in hippocampal target cell bodies. This double-label procedure yielded blue-black nickel enhanced DAB stained, PHA-L-immunoreactive terminals on light brown SOM-i neurons that were investigated by correlative LM and BM observations. PHA-L-labeled terminal contacts with often basket-like appearance were. localized with highest densities on soma and proximal dendrites of SOM-i neurons in stratum oriens of Ammon’s horn and hilus of dentate gyrus, and some minor projections to stratum pyramidale and radiatum. Most double-labeled contacts could be identified as symmetric type synapses equally divided over soma and proximal dendrites of several forms of SOMd neurons. These data indicate monosynaptic regulation of the hippocampal intrinsic SOM system by septal input, which probably represents a peptidergic subpopulation of the hippocampal GABAergic system.