Long-term potentiation in rat piriform cortex following discrimination learning

The behavioral conditions for induction of long-term potentiation (LTP) elicited by unilateral patterned electrical stimulation of the lateral olfactory tract (LOT) was studied in piriform cortex. A group of animals was trained to discriminate two natural odors while another group was natural odor. Both groups were successful in the discrimination and no statistical significant difference was observed in behavioral data between these two groups on series of 5 successive daily training sessions. With animals trained to perform the task with the artificial cue, monosynaptic responses evoked by single pulse stimulation of the LOT were collected, prior to and just after each of the successive training sessions. Comparisons with behavioral data collected at the beginning and the end of a training session revealed that the population synaptic responses increased with the percentage of correct responses performed by the animals. This increase (LTP) was progressive and present only when significant discrimination between the two cues was observed. A positive correlation was found between the increase in monosynaptic responses and the level of performance. Responses elicited by control electrodes were slightly depressed at the end of the discrimination learning series. In addition, when a group of naive animals was pseudoconditioned, giving the patterned electrical stimulation for the same number of sessions but without any olfactory training, no LTP was recorded. These results support the hypothesis that progressive learning of olfactory discrimination gradually potentiates cortical synapses in a defined cortical terminal field and may explain why LTP observed in the piriform cortex is not elicited by the patterned stimulation by itself, but only in an associative context.