| Glutamate ejection in the vicinity of locus coeruleus (LC) in urethane anesthetized rats was used to selectively activate norepinephrine (NE) release from LC fibers. Extracellular units, 0.1 Hz perforant path-evoked potentials (PP-EP), and hippocampal EEG were concurrently monitored in the dentate gyrus. In all experiments, LC activation induced potentiation of the PP-EP. Significant potentiation of the PP-EP was not seen until an average of 20 s. after glutamate ejection. Significant changes in spontaneous neuronal activity occurred both immediately and with a delay following LC activation. Subgranular layer, nonbursting neurons with narrow action potentials were immediately excited by NE for an average duration of 26.6 s. Distal subgranular layer, nonbursting neurons with wider action potentials were inhibited by LC activation for an average duration of 88 s. Bursting hilar neurons showed a delayed suppression, beginning an average of 10 s. after the flag and lasting for an average of 276 s. Finally, a group of three cells were unaffected. Theta was present before and after the ejection in the majority of experiments (11) and increases in theta frequency were seen in 3 animals. A loss of theta occurred within an average of 36 s. after the ejection in 8 experiments and lasted an average of 6 min.;Based on the electrophysiological and anatomical results, a model is suggested in which there are three different interneuron types which are affected differently by LC-NE (excitiation, inhibition, and unaffected) and contribute to modulating excitation and inhibition in the system. Bursting cells are either unaffected or show prolonged reduced activity similar to the period of LC suppression.;It seems that conditions are set up by LC-NE release which contribute to the promotion of an increased coupling between PP input and granule cell output which would increase DG circuit throughput. |
