Comparison between hypothalamic, hippocampal and septal multiple unit activity and basal corticotropic function in...

Multiple unit activity (MUA) was obtained from various forebrain regions in unanesthetized, unrestrained resting pigeons throughout the whole photoperiod and compared with plasma corticosterone levels. The pattern of electrical activity recorded from the adrenocorticotropic area of the hypothalamus showed diurnal variations which paralleled the plasma corticosterone fluctuations during the 24 h photoperiod. Both parameters were low in the late afternoon and the evening and high in the early morning. Hypothalamic activation slightly preceded the peak o corticosteronemia. Conversely, in hippocampal (H) and septal (S) regions, the peak of MUA occurred in phase opposition with respect to the hypothalamic peak, and there was a marked decrease of firing rates at the moment when adrenocorticotropic activation was initiated.