Electrophysiological analysis of the hippocampal-septal projections: II. Functional characteristics

Summary 1. The septum and dorsal hippocampus were exposed in rats by the ablation of the overlying cortex and corpus callosum. The hippocampal efferent system, fornix and fimbria, were stimulated with either single or paired shocks and field potentials, extra- and intra-cellular unitary potentials were recorded in the lateral septal nucleus. 2. The double-shock pairing within the same hippocampal system resulted in facilitation of the monosynaptic test field response. 3. Pairing across systems resulted in long-term suppressions of the mono-synaptic test field response. 4. IPSPs and lack of convergence of excitatory input are responsible for the long-term suppression. 5. It is suggested that the IPSPs are mediated via interneurons, and that some of which at least are activated recurrently. 6. Hippocampal stimulation suppresses the amygdaloid evoked field response for very long periods, often beyond 1000 msec.This study was supported by U.S. Public Health Service Grants RR 5384 and NB 00405.