1. The effects of low-frequency stimulus trains on synaptically evoked responses in piriform cortex pyramidal cells were studied by the use of intracellular recording techniques in an in vitro slice preparation. Afferent and association fiber systems were differentially stimulated with electrodes placed in layer 1a or layer 1b, respectively. To quantify synapse modifiability, the heights of postsynaptic potentials (PSPs) elicited by paired-pulse stimulation (100-ms interval) were averaged over a 50-s period before and after a set of 10 stimulus trains (10 pulses each, 20 Hz, 5-s interpulse interval). 2. Afferent and association fibers showed consistent differences in their response to stimulation during the period lasting from ~10 to 200 s after presentation of trains. During this time period, the responses to stimulation of association fibers in layer 1b displayed a short-term potentiation, which over the 10 posttrain trials, produced an average increase in PSP height of 23.2 ± 3.7% (mean ± SE). On the other hand, responses to layer 1a stimulation showed an average depression of 10.9 ± 3.6%. Layer 1b potentiation decayed with a time constant roughly estimated at 79 s. Layer 1b potentiation appeared even at very low stimulus voltages and after local association fiber input had been cut, suggesting that it was largely a monosynaptic effect. 3. In the period immediately after train presentations, responses evoked by both layers showed a short-term augmentation with a time constant around 3 s. In layer 1a, this augmentation was superimposed on a depression with slow recovery. At longer times after train presentation (>5 min), 2 cells out of 46 showed changes (increases) in synaptic efficacy in response to layer 1b stimulation. 4. In the current experiments both layers 1a and 1b showed statistically significant facilitation before the presentation of stimulus trains. However, layer 1b facilitation decreased from 22.7 ± 3.5% to a statistically insignificant 3.9 ± 3.3% after the presentation of trains, whereas layer 1a facilitation remained at a statistically significant level of 23.1 ± 5.7%. 5. These experiments show that pyramidal cell responses to stimulation of the afferent and association fiber systems are affected directly by the previous presentation of trains of stimuli. This suggests that mechanisms of synaptic modification may differ between the afferent and intrinsic association synaptic projections onto single pyramidal cells in olfactory cortex. These results are important for our ongoing attempts to model the functional organization of this cortex. In addition, the presynaptic or postsynaptic mechanisms possibly underlying these differences are considered in light of our data showing an interaction of short-term potentiation with paried-pulse facilitation.
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