The action of norepinephrine in the rat hippocampus. IV. The effects of locus coeruleus stimulation on evoked hippocampal unit activity.

Activity of neurons in the hippocampus (HPC) was recorded in awake, freely moving rats. Most cells were inhibited by either a loud auditory stimulus (tone) or by electrical stimulation of the nucleus locus coeruleus (LC). The inhibitory responses to the tone were antagonized by drugs that interfere with central noradrenergic transmission. When LC stimulation was used as the unconditioned stimulus in a classical conditioning paradigm, previously inhibitory responses to the tone were reinstituted. When behaviorally subthreshold LC stimulation preceded a tone which was correlated with food, the existing conditioned response to the tone was potentiated. These data suggest that the generalized inhibitory response of HPC neurons to a tone is modulated by the noradrenergic pathway and that experimental activation of LC can potentiate HPC responses to behaviorally significant conditioned stimuli.