Differential potentiation of early and late components evoked in olfactory cortex by stimulation of cortical association fibers.

The present study examined in detail the development and decay of potentiation induced in vivo by repeated high-frequency stimulation of cortical association fibers (AF) in piriform cortex (PC). Male Long-Evans rats with chronically-implanted stimulating and recording electrodes were administered potentiating AF stimulation (thirty 10-pulse 100-Hz trains) on 8 consecutive days, followed by a ninth administration after an 8-day layoff. The time course of potentiation was monitored by local field potentials evoked in the PC and olfactory bulb (OB) by 0.1 Hz single-pulse AF test stimulation before, during, and following each potentiating treatment. AF test stimulation evoked two distinct components in the PC, an early component (EC) and a late component (LC). High-frequency AF stimulation produced potentiation of each component, but with very different characteristics. EC potentiation consisted of a brief augmentation during each bout of potentiating stimulation that persisted <2 min after the last high-frequency train and showed no cumulative effects following repeated induction across days. In contrast, LC potentiation developed gradually, requiring several daily potentiation treatments to reach maximum amplitude, and decayed more slowly each time it was induced. Furthermore, LC potentiation persisted in latent form for at least 8 days following its apparent decay and could be reinstated by repeated test stimulation that was without effect at the beginning of the experiment. Potentiation in the OB resembled LC potentiation in its characteristics, but with less latent potentiation. These results indicate that the potentiation reported here is distinctly different from the long-term potentiation previously demonstrated in vitro in the PC, and suggest that this potentiation represents an increase in excitability within the cortical association fiber system that can be stored in latent form and retrieved at a later time. These characteristics make this potentiation a suitable candidate for participation in long-term functional changes within olfactory cortex.