Forebrain inhibitory mechanisms: Cortical synchronization induced by basal forebrain stimulation

Abstract Cortical synchronization is of interest because of its similarity to the spontaneously occurring cortical spindle bursts associated with sleep and other states of central nervous system suppression. Although the diffuse thalamic projection system is primarily associated with this cortical response, other subcortical regions demonstrate involvement with the visceral, somatic, and endocrine correlates to these states. The preoptic region of the hypothalamus is of particular interest in this regard, and the present experiment attempted to determine whether a cortically directed influence from this region also exits. Stimulation and recording experiments were performed in the brains of acutely prepared, immobilized adult cats. Stimulating electrodes were lowered into anterior hypothalamic and basal forebrain regions. Electroencephalographic activity from several cortical areas was monitored during subcortical stimulation. An immediate and sustained cortical synchronization was observed upon stimulation of a specific preoptic-basal forebrain zone, which has been named the basal forebrain synchronizing area. A second focus inducing cortical synchronization was observed in the medial and basal amygdaloid complex. Stimulation interactions of the basal forebrain synchronizing area with the brainstem reticular activating system were assessed in other experiments, and the prevailing cortical influence appeared to depend on a factor of order and intensity of stimulation. It was also noted that basal forebrain induced cortical synchronization is completely abolished by the intravenous administration of 15 mg per kg of Nembutal. We believe that the basal forebrain synchronizing area is significant, in that it provides a stable anatomical starting point for the further exploration of forebrain suppressor mechanisms.