Functional correlates of selective long-term potentiation in the olfactory cortex and olfactory bulb.

High-frequency stimulation of the granule cell layer of the olfactory bulb (OB) has previously been shown to result in a selective long-term potentiation (LTP) of late components of potentials evoked in the OB and piriform cortex (PC). The functional impact of this potentiation was explored in male Long-Evans rats with chronically implanted electrodes by comparing the effects of paired-pulse stimulation of the OB in potentiated and control animals. Effects were examined on two components of the potential evoked in the PC: A1, which represents the population EPSP produced by OB mitral cells in PC pyramidal cells via the lateral olfactory tract (LOT), and B1, which represents the subsequent population EPSP produced by PC pyramidal cells in other pyramidal cells. Two separate functional correlates of selective LTP were found. First, there was enhanced paired-pulse depression of B1, indicating increased inhibition of PC pyramidal cells. Second, there was a shift from paired-pulse facilitation to depression of A1, which was accompanied by a decrease in amplitude of the LOT volley, indicating that fewer mitral cells were activated by the stimulation. This shift was most prominent in animals with stimulating electrodes closest to the mitral cell layer, suggesting that it is dependent upon direct stimulation of mitral cell somata. These observations, together with other results reported in the manuscript, support the conclusion that there is an enhanced inhibition of mitral cells following selective LTP. Thus a primary consequence of selective LTP appears to be enhanced inhibition of principal neurons in both the PC and OB. These findings are consistent with our previous proposal that selective LTP represents potentiation at excitatory synapses made by PC pyramidal cells on inhibitory interneurons in the PC and OB.