Changes of interhemispheric hippocampal responses during conditioning in the awake rat.

The responses of the hippocampus of the awake rat to stimulation of one of its afferent pathways were measured during classical conditioning. It was found that when the contralateral hippocampus is stimulated concurrently with the presentation of a conditioned stimulus preceding either food or an aversive shock, a late (30-40 msec) negative component in the averaged evoked response can be recorded. This late component was absent when the interhemispheric stimulation was applied prior to presentation of a conditioned stimulus or when the rat was satiated or pretreated with drugs which interfere with noradrenergic or serotonergic neurotransmission. It is suggested that classical conditioning changes neurotransmission in certain pathways in the brain and that the monoamines are involved in mediation of this change.