FREQUENCY-DEPENDENT INFORMATION-FLOW FROM THE ENTORHINAL CORTEX TO THE HIPPOCAMPUS

Storage and retrieval of information in the hippocampus is dependent o n information transfer from the entorhinal cortex (EC). We studied how the separate pathways from layer II and III of the EC to the hippocam pus are selected for information transfer during repetitive synaptic s timulation. Intracellular recordings were made from EC layer II and II I projection cells in horizontal combined EC-hippocampal slices. Synap tic responses to stimulation of deep layers or the lateral EC with sti mulus intensities similar to 70% of that required to elicit an action potential were analyzed during short trains of repetitive stimulation. The threshold intensities for induction of action potentials were in layer II cells 8.2 +/- 3.8 (SE) V, significantly larger than 4.4 +/- 1 .5 V in type 1, and 5.2 +/- 3.3 V in type 2 layer III cells, respectiv ely. During repetitive subthreshold stimulation with frequencies below 5 Hz the pathway from the EC layer II remained quiet and was preferen tially activated with stimulation frequencies above 5 Hz. In contrast the EC layer III cells responded preferentially to low stimulus freque ncies (<10 Hz) and became strongly inhibited when synaptically stimula ted with frequencies above 10 Hz. Interestingly during stimulus freque ncies between 5 and 10 Hz the likelihood that both layer II and III ce lls fire was large. Thus a frequency switch operates in the entrohinal cortex regulating output of layer II and III cells to the hippocampus . We suggest that such frequency dependent regulation of information f low presents a new principle of neuronal information processing.