Long-term potentiation phenomena in the rat limbic forebrain.

Long-term potentiation (LTP) phenomena were investigated in several limbic forebrain pathways. With the possible exception of the lateral olfactory tract, LTP could be produced in all pathways tested. LTP effects tended to increase as the stimulation site moved caudally along the pyriform lobe. The largest effects were produced by stimulation of pathways into and out of the hippocampus. Target sites also differed, with the hippocampal sites showing the strongest and longest lasting LTP effects. The time course of LTP appeared to be best fitted by the sum of two exponential curves with time constants of about 1.5 h and 5 days respectively. We also looked at the potentiation effects produced by repeated epileptogenic (kindling) stimulations, and the effect of this potentiation on subsequent tests of short-term and long-term potentiation. In most cases, the short-term effects, and the first component of the LTP effect, were still intact after kindling. The longest lasting component, however, could no longer be produced with either amygdala or perforant path stimulation. This result indicates that the potentiation produced by kindling may be based upon the same mechanism as the LTP effect.