Hippocampal excitability related to the phase of theta rhythm in urethanized rats.

Urethanized rats were studied to determine if excitability in the hippocampus is related to the phase of the theta rhythm. The responses recorded from area CA1 were clearly modulated at the theta frequency. Those recorded from the dentate region were not. The most excitable phase for CA1 differed significantly from that previously reported for walking rats. These findings suggested that the mechanisms for theta production differ for normal and urethanized rats.