Selective long-term potentiation in the pyriform cortex

Electrical stimulation of the olfactory bulb (OB) produces an evoked potential in the pyriform cortex (PC) characterized by an initial surface-negative wave (period 1) representing activation of PC pyramidal cells via the lateral olfactory tract, followed by a surface-positive wave (period 2) which is temporally associated with recurrent and feed-forward inhibition. The experiment reported here examined the changes that occur in the PC evoked potential following a pattern of stimulation that has been found to produce short- and long-term potentiation (LTP) in other areas of the forebrain. Male Long-Evans rats with electrodes in the OB and PC were divided into two groups. LTP animals received high-frequency stimulation of the OB (30 trains of 10 pulses each at a frequency of 100 Hz). Control animals received the same number of pulses at a lower frequency (1 Hz). This procedure was repeated 6 times at 2-day intervals. Neither high- nor low-frequency stimulation altered period 1 of the PC evoked potential, indicating that synaptic input arriving via the lateral olfactory tract was unaffected. However, LTP animals exhibited a marked increase in the amplitude and duration of period 2 which appeared to reflect two separate processes: a short-term change that peaked within 30 min of the trains; and a long-term change that accumulated across the 6 treatments. LTP of period 2 persisted in latent form for at least 32 days after the last treatment. Control animals exhibited only small changes that were attributed to the paired-pulse stimulation used for testing rather than the 1 Hz Control trains. These results suggest that repeated high-frequency stimulation of the OB causes a persistent alteration in the way information is processed within the PC. The form of LTP demonstrated here is markedly different from that found in the hippocampal formation, where potentiation of the monosynaptic excitatory postsynaptic potential is a prominent effect. The functional significance of this change cannot be determined with certainly from the present experiment, but available evidence suggests that it represents an enhancement of inhibitory processes within the PC.