PIRIFORM CORTEX EFFERENTS TO THE ENTORHINAL CORTEX IN-VIVO - KINDLING-INDUCED POTENTIATION AND THE ENHANCEMENT OF LONG-TERM POTENTIATION BYLOW-FREQUENCY PIRIFORM CORTEX OR MEDIAL SEPTAL STIMULATION

The entorhinal cortex receives input from many cortical areas and medi ates the flow of information between these sites and the hippocampal f ormation. Long-term synaptic plasticity in cortical efferents to the e ntorhinal cortex may contribute to the transmission of neural activity to the hippocampus, as well as the storage of information, but little is known about plasticity in these pathways. We describe here the use of evoked field potential recordings from chronically implanted elect rodes in the rat entorhinal cortex to investigate synaptic plasticity in the large piriform (olfactory) cortex projection to the superficial layers of the entorhinal cortex. Both kindling-induced potentiation a nd long-term potentiation (LTP) were tested. In addition, we attempted to modulate LTP induction by the co-induction of frequency potentiati on and by the co-activation of the medial septum. Epileptogenic kindli ng stimulations of the piriform cortex (1-s, 60-Hz trains 3 times/day for 5 days) were found to result in a reliable potentiation of field r esponses evoked by piriform cortex test pulses. Non-epileptogenic teta nization of the piriform cortex with 400-Hz 16-pulse trains reliably r esulted in LTP effects. These effects could be augmented by embedding brief LTP induction stimuli within Ii-pulse, 15-Hz trains that alone p roduce only frequency potentiation. Go-activating the medial septum wi th 10-Hz trains, just prior to tetanization of the piriform cortex, au gmented LTP of piriform cortex inputs to the entorhinal cortex in an i nput-specific manner. All potentiation effects were found to last for periods of weeks. These findings demonstrate that both epileptogenic a nd non-epileptogenic piriform cortex stimulation induces lasting poten tiation of population field responses in the entorhinal cortex of the awake rat. The LTP effects were inducible in a graded manner and were sensitive to the temporal context of stimulation. The finding that low -frequency activation of the septum can enhance plasticity in the ento rhinal cortex adds to a body of data indicating a role for the medial septum in contributing to theta activity and plasticity in both the en torhinal cortex and hippocampal formation. (C) 1997 Wiley-Liss, Inc.