Frequency Dependence of the Spatial Pattern of Synchronized Activity in Rat Hippocampus in vivo and in vitro

The Spatiotemporal activity evoked in the hippocampal CA1 by contralateral CA3 stimulation was investigated in vivo. Spatial activity was localized and the spatial size of the activity depended on stimulus frequency. LTP induced by tetanic stimulation of contralateral CA3 just emphasized spatiotemporal activity and did not influence the spatial size of the activity. in vitro, Spatiotemporal activity evoked by periodic stimulation of stratum moleculare in DG was recorded from the stratum pyramidale in CA3. Spatial activity was localized and depended on stimulus frequency as well as in vivo. Responses of the hippocampal CA1 and CA3 was highly dependent on the stimulus frequency in vivo and in vitro both, and 20-30 Hz stimulation seemed to be optimal to drive hippocampus. However, enhanced activity caused by such stimulation was stably localized and did not spread across the hippocampus resulting in epileptic activity. The size of the localized activity was consistent with the lamella hypothesis. 1.Introduction The brain consists of many neurons and has three dimensional structure. These neurons, which construct complicated neural networks, process information. Rhythmic brain activity which appears to result from behavior of animals is often observed [1,2]. The rhythmic activity reflects synchronization of neuronal activity [3,4]; larger amplitude and lower frequency of rhythmic activity indicate better synchronization. Synchronized epileptiform bursts in rat transversal hippocampal slices and field potential of somatosensory cortex of anesthetized rat cause phase lockings and chaotic responses to periodic stimulation [5,6]. The rat hippocampus has tri-synaptic pathways perpendicular to the longitudinal axis of the hippocampus. Therefore, Andersen and his coworkers proposed the lamella hypothesis [7]. It has been supposed that the lamellas (about 2 mm in thickness) would be functional units and the trisynaptic pathways are important for information processing in the hippocampus. However, activity of intranetworks in the dentate gyrus (DG), CA3 and CA1 would also be important, besides trisynaptic pathways. It is interesting to see how spatiotemporal activity is evoked in the hippocampus by periodical input. In this paper, anesthetized rats were used, and spatiotemporal activity evoked in the hippocampal CA1 by contralateral CA3 stimulation was