Neurophysiological mechanisms modifying afferent hypothalamo-hippocampal conduction

Abstract

Previous studies have demonstrated the projection of oligosynaptic sensory afferents to the posterolateral hypothalamus. The purpose of the present investigation was to study the possible extention of this pathway from the hypothalamus to the hippocampus, as well as the effects of midbrain reticular formation stimulation on afferent conduction to the hippocampus. Stimulation of the sciatic nerve or the medial lemniscus evoked short-latency potentials in the posterolateral hypothalamus, similar to those which have been described in the specific projection nuclei of the thalamus. The stimulation of numerous points in this hypothalamic area, and in more rostral hypothalamic and preoptic regions, evoked potentials in the dorsal hippocampus which had the properties of responses propagating in a unidirectional oligosynaptic pathway. Inhibition of the afferent input into the hypothalamus resulted from stimulation of the hippocampus and the midbrain reticular formation, while the latter facilitated the conduction from the hypothalamus to the hippocampus. These results of reticular formation stimulation correspond to its previously described influences on distal and proximal segments of classical sensory pathways. The physiological significance of the reciprocal connections between the hypothalamus and the hippocampus are discussed. It is suggested that the afferent pathway to the hypothalamus and the hippocampus may play an important role in the regulation of autonomic and behavioral neurophysiological mechanisms.