Alpha 2 modulation of type 1 and type 2 hippocampal theta in the freely moving rat

In order to determine the brain sites involved in the noradrenergic modulation of hippocampal theta, the alpha 2 agonist detomidine was infused into one of three brain sites in the freely moving animal: hippocampus, median raphe nucleus (MRN), or locus coeruleus (LC). Bilateral hippocampal recording electrodes were implanted in 45 animals. Infusions of detomidine into the hippocampus produced an attenuation of type 1 (movement) theta. Detomidine infused into the MRN produced a release of type 2 (immobility) theta, although having no effect on type 1 theta. No significant changes were noted in the hippocampal EEG activity following infusions of detomidine into the LC. Infusions of Xylocaine into each of the above brain sites were ineffective in eliciting changes in hippocampal EEG activity. No behavioral effects were noted following infusions of detomidine or Xylocaine. The present results suggest that the attenuation of type 1 theta occurs at the hippocampus, while the release of type 2 theta occurs at the MRN.