Hippocampal potentials evoked by stimulation of olfactory basal forebrain and lateral septum in the rat

Abstract

Electrophysiological characteristics of olfactory-hippocampal relations were examined because recent anatomical studies have described a substantial olfactory input to the hippocampus via the entorhinal cortex. Potentials evoked in the dorsal hippocampus of anesthetized rats by stimulation of the prepyriform cortex, pyriform cortex, diagonal band, lateral olfactory tract, anterior commissure, olfactory tubercle and anterior olfactory nucleus had similar characteristics, although latencies differed. For example, latencies were twice as long after stimulation of the obliquely oriented portion of the diagonal band than after stimulation of the prepyriform cortex. A relatively low-amplitude, initially negative wave was recorded in the subiculum, CA1 and CA2, and a relatively high-amplitude, initially positive wave was recorded in CA4 and the dentate gyrus. In CA3 negative potentials were observed at dorsal recording sites and positive potentials were recorded at more ventral sites. Peak latencies were usually two to four msec shorter for the negative than for the positive wave. Laminar distributions of responses evoked in the hippocampus by stimulation of the prepyriform cortex and diagonal band were evaluated by driving eight electrodes mounted on one carrier through the brain and were found to be strikingly similar. Maximal amplitudes of the negative wave were recorded at the level of stratum moleculare of CA1 and the subiculum, and peak amplitudes of the positive wave were associated with the hilus of the dentae gyrus. Transition from negative to positive waveforms occurred approximately at the hippocampal fissure. Although the negative and positive waves were usually elicited together, they also were separable in that only negative waves were recorded along some tracks and only positive waves along others. Also, various stimulation sites in the prepyriform cortex elicited stable high-amplitude positive waves accompanied by negative waves of varying amplitude. It is suggested that branches of the perforant path are involved in generation of the two waves and that activity in a number of olfactory structures may influence the hippocampus, probably via the perforant pathway. Thus, hippocampal potentials following prepyriform or diagnonal band stimulation were not abolished by transection of the fornix-fimbria. Dorsolateral septal stimulation evoked hippocampal responses with characteristics and distribution distinctly different from those evoked by stimulation of olfactory areas. The findings suggest that lateral septal stimulation may activate the hippocampus antidromically.