Influences of hypothalamic stimulation upon septal and hippocampal electrical activity in the cat

Abstract

Spontaneous patterns of hippocampal EEG and septal cell activity were studied in immobilized cats, and the influences of high frequency stimulation of medial hypothalamus (MH) and lateral hypothalamus (LH) were determined. Septal cells were divided into 3 classes on the basis of their discharge patterns: (1), rhythmic bursting (2), non-rhythmic bursting and (3), non-bursting, and the relationship of these discharge patterns to hippocampal theta rhythm was analyzed. Rhythmic bursting cells displayed close frequency and phase relations to hippocampal theta rhythm and were located chiefly in the diagonal band of Broca. Cells of the other two categories were found both within and outside of the diagonal band region. High frequency stimulation of medial hypothalamus evoked theta rhythm in hippocampus and bursts of repetitive firing at theta frequency from rhythmic bursting septal cells, whereas lateral hypothalamic stimulation desynchronized hippocampal EEG activity and disrupted rhythmic bursting patterns in the septum. The majority (59%) of rhythmic bursting cells ceased bursting and fired continuously at an increased rate during high frequency stimulation of lateral hypothalamus, while other (33%) rhythmic bursting cells ceased bursting and were either completely inhibited or fired at a markedly reduced rate. Of the rhythmic bursting cells tested with high frequency stimulation of lateral hypothalamus 73% responded with short latencies (1.7–4.5 msec). No rhythmic bursting cells were found which responded to medial hypothalamic stimulation at fixed latencies. Brain stem-hypothalamic pathways which mediate these differential effects upon septum and hippocampus are discussed.