Reduced synaptic facilitation between pyramidal neurons in the piriform cortex after odor learning

Learning-related cellular modifications were studied in the rat piriform co rtex after operand conditioning. Rats were trained to discriminate positive cues in pairs of odors. In one experimental paradigm, rats were trained to memorize 35-50 pairs of odors ("extensive training"). In another paradigm, training was continued only until rats acquired the rule of the task, usua lly after learning the first two pairs of odors ("short training"). "Pseudo -trained" and "naive" rats served as controls. We have previously shown tha t "rule learning" of this task was accompanied by reduced spike afterhyperp olarization in pyramidal neurons in brain slices of the piriform cortex. In the present study, synaptic inputs to the same cells were examined. Pairs of electrical stimuli applied to the intrinsic fibers that interconnect lay er II pyramidal neurons revealed significant reduction in paired-pulse faci litation (PPF) in this pathway even after short training. PPF in shortly tr ained rats was reduced to the same extent as in extensively trained rats. P PF reduction did not result from modification of membrane properties in the postsynaptic cells, change in postsynaptic inhibition, or impairment of th e facilitation mechanism. Extracellular field potential recordings showed e nhanced synaptic transmission in these synapses. The reduction in PPF becam e apparent only 3 d after task acquisition and returned to control value 5 d later. PPF evoked by stimulating the afferent fibers to the same neurons was increased 1 d after training for 2 d. We suggest that the transient enh ancement in connectivity in the intrinsic pathway is related to the enhance d learning capability and not to memory for specific odors, which lasts for weeks.