Differentiation of brain stem loci which affect hippocampal and neocortical electrical activity.

Abstract Systematic mapping by high frequency stimulation (100 Hz) of the brain stem of cats maintained under a mixture of nitrous oxide and oxygen has revealed five somewhat delimited and dispersed sites where stimulation caused hippocampal theta rhythm (synchrony) and two where a contrasting pattern of desynchronization was elicited. The theta-producing sites were found in nucleus reticularis pontis oralis, nucleus locus coeruleus, the giant cell region of the pontine tegmental field, the midbrain tegmentum, and in the ventrolateral portion of the periaqueductal gray substance. The two desynchronizing sites were localized in nucleus reticularis pointis caudalis and in the nuclei of the raphe. The synchronizing sites differed with respect to the frequency of the theta rhythm induced in the hippocampus. Usually, when the hippocampus displayed a synchronized theta rhythm the electrical activity of the neocortex was desynchronized and vice versa, however, there were exceptions in which either the anterior or posterior neocortex displayed a thetalike rhythm while the hippocampus was in either a synchronized or desynchronized state. These findings have been discussed in terms of possible multiple systems of brain stem influence upon hippocampus and neocortex and it is suggested that such ascending systems may be responsible for differential states of activation and arousal.