Hippocampal evoked potentials in novel environments: a behavioral clamping method.

The hippocampus is involved in the detection of novelty and is essential for certain forms of learning about environmental events and relationships. The cellular and molecular mechanisms of one form of hippocampal synaptic plasticity, long-term potentiation (LTP), are thought to overlap significantly with the neural mechanisms of learning. In this study changes in hippocampal synaptic efficacy were measured in awake, freely behaving rats during exploration of novel environments. Because hippocampal physiology is modulated by on-going behavior, evoked potentials collected during Type 1 versus Type 2 behavior were evaluated separately. The effect of prior LTP induction at perforant path-dentate synapses on exploration-induced changes was evaluated. The results show that exploration causes an increase in population spike amplitude with no change in excitatory postsynaptic potential during Type 1 behavior that lasts longer than 5 min. Prior induction of hippocampal LTP occludes the change induced by exploration. This change is not likely to be due to a reduction of GABAergic inhibition induced by novelty.