MODULATION OF SYNAPTIC PLASTICITY IN THE HIPPOCAMPUS AND PIRIFORM CORTEX BY PHYSIOLOGICALLY MEANINGFUL OLFACTORY CUES IN AN OLFACTORY ASSOCIATION TASK

Animals were trained to discriminate two natural odors while another g roup was trained to discriminate between a patterned electrical stimul ation distributed on the lateral olfactory tract (LOT), labelled olfac o-mimetic stimulation (OMS), used as an olfactory cue versus a natural odor. No statistically significant difference was observed in behavio ral data between these two groups. The animals trained to learn the me aning of the OMS exhibited a gradual long-term potentiation (LTP) phen omenon in the piriform cortex. When a group of naive animals was pseud o-conditioned, giving the OMS for the same number of sessions but with out any olfactory training, no LTP was recorded. These results indicat e that the process of learning olfactory association gradually potenti ates cortical synapses in a defined cortical terminal field, and may e xplain why LTP in the piriform cortex is not elicited by the patterned stimulation itself, but only in an associative context. As olfactory and hippocampus regions are connected via the lateral entorhinal corte x, the olfactomimetic model was used to study the dynamic of involveme nt of the dentate gyrus (DG) in learning and memory of this associativ e olfactory task. Polysynaptic field potentials? evoked by the LOT sti mulation, were recorded in the molecular layer of the ipsilateral DG. An early and rapid (2nd session) potentiation was observed when a sign ificant discrimination of the two cues began to be observed. The onset latency of the potentiated response was 30-40 ms. When a group of nai ve animals was pseudoconditioned, no change was observed. Taken togeth er, these results support the hypothesis that early activation of the DG during the learning of olfactory cue allows the progressive storage of olfactory information in a defined set of potentiated cortical syn apses. The onset latency of the polysynaptic potentiated responses sug gests the existence of a reactivating hippocampal loops during the pro cessing of olfactory information.