Analysis of the actions of nucleus reuniens and the entorhinal cortex on EEG and evoked population behavior of the hippocampus.

Although the hippocampal theta rhythm is thought to be linked to memory processes, its mechanism of action is unknown. Furthermore, the hippocampus forms strong connections with a functionally similar structure, the medial prefrontal cortex (mPFC). The midline thalamus appears to be an intermediate between these two structures. We recorded neurons of a midline nucleus (nucleus reuniens, RE) during theta and non-theta states. Additionally, we recorded hippocampal CA1 population responses to RE stimulation. RE cell firing patterns are classified as (i) spike rate response to stimulation (ii) determination of bursting events (iii) coherence estimation between hippocampal EEG and RE response to stimulation (within the theta frequency band of 5 - 12 Hz). The present data suggests an increase in RE spike rate due to tail pinch elicited theta activity, with no evidence of bursting activity and a weak coherence within the theta band. Furthermore, we evaluated evoked excitatory post-synaptic potentials (EPSPs) in the hippocampal CA1 to RE stimulation, as well as entorhinal cortex (EC) stimulation. We demonstrated a consistent reduction in evoked potential (EP) latency at CA1 to RE and EC stimulation during theta compared to non-theta states.