Single unit analysis of the hippocampal projections to the septum in the cat

The purpose of this study was to map the hippocampal efferent projections to the septum and to determine the synaptic organization of the hippocampal-septal system in the cat. Single unit responses were recorded in the septum with tungsten microelectrodes following electrical stimulation of the dorsal or ventral hippocampus in the anesthetized cat. Dorsal hippocampal stimulation produced excitatory unit driving at short latencies in the medial septum while ventral hippocampal stimulation produced short latency excitation in the lateral septum. The excitatory phase was invariably followed by a period of inhibition. Inhibition without a prior excitatory phase was seen in widespread regions of the septum upon either dorsal or ventral hippocampal stimulation. It was concluded that efferents from the dorsal and ventral hippocampus terminate in a topographic manner in the medial and lateral septum, respectively, and have excitatory effects upon these neurons as well. An interneuronal inhibitory network capable of influencing large regions of the septum was suggested from the data.