Increases in glutamate release and phosphoinositide metabolism associated with long-term potentiation and classical conditioning.
Long-term potentiation (LTP) is a widely studied model of the kind of activity-dependent modulation of synaptic efficacy which is assumed to provide the physical basis for learning. Whether LTP, in the hippocampus or elsewhere in the brain, does in fact serve such a role is still a matter for debate. One approach to answering this question is to identify physiological or biochemical changes which are common to both learning and LTP; in the hippocampus, for example, one can ask whether the biochemical changes associated with LTP are also associated with learning. In this chapter we summarize the results which we have obtained in a study of glutamate release and phosphoinositide turnover in the dentate gyrus of rats trained in a classical conditioning task. The similarity between the changes occurring after classical conditioning and those associated with LTP is consistent with the hypothesis that LTP is one of the mechanisms by which a neural trace of the learned association is formed. We discuss this interpretation in the light of the observation that classical conditioning does not appear to affect synaptic responses in the hippocampus.