Evidence for synaptic potentiation in a cortical network during learning.

The connections between the lateral olfactory tract (LOT) and layer I of the piriform cortex were used to test the idea that certain forms of learning involve potentiation of cortical synapses. Rats were trained on a series of two-odor discriminations over a period of several days after which patterned electrical stimulation (short, high frequency bursts with 5-6 bursts per second) of the LOT was used as a discriminative cue. The animals reacted to the stimulation as though it were an odor and quickly learned to respond appropriately and to distinguish between 'positive' and 'negative' electrodes. Comparisons of the monosynaptic responses in the piriform cortex evoked by single pulse stimulation of the LOT before and after learning revealed that the population synaptic responses were substantially potentiated by the training. This effect was present in an unchanged form 24 h later. Responses elicited by control stimulating electrodes were slightly or not at all affected by training to stimulation with another electrode. Synaptic potentiation was not found in a small group of rats that did not learn to respond to patterned stimulation and was also absent when the stimulation was applied to naive rats. These results provide evidence that rapid learning of a specific cue potentiates cortical synapses in a defined terminal field.