Early polysynaptic potentiation recorded in the dentate gyrus during an associative learning task.

In this report, we investigated the electrophysiological dynamics of the neuronal circuit including the dentate gyrus during an associative task. A group of rats was trained to discriminate between a patterned electrical stimulation of the lateral olfactory tract, used as an artificial cue associated with a water reward, and a natural odor associated with a light flash. Polysynaptic field potential responses, evoked by a single electrical stimulation of the same lateral olfactory tract electrode, were recorded in the molecular layer of the ipsilateral dentate gyrus prior to and just after each training session. An increase in this response was observed when a significant discrimination of the two cues began. A positive correlation was found between the change in the polysynaptic potentiation and behavioral performances. The onset latency of the potentiated polysynaptic response was 35-45 ms. When a group of naive animals was pseudoconditioned, no change in field potential was observed. These results are consistent with the hypothesized dynamic activation of the dentate gyrus early in the making of association, allowing gradual storage of associative information in a defined set of synapses. Moreover, the onset latency of the potentiated response suggests the existence of reactivating hippocampal loops during the processing of associative information.