Long-term potentiation induced by repeated paired stimulation of the stratum radiatum and alveus in CA1 pyramidal neurons of hippocampal slices

A gradual increase of the amplitude of the population spike lasting for more than 3 h was produced after repeated paired stimulation (RPS), consisting of a single 'test' stimulus applied to the stratum radiatum followed by a train of antidromic stimuli (5 pulses at 200 Hz) to the alveus with an interstimulus interval (ISI) of 20 ms. RPS was repeated every 5 s for 50 repetitions. The increases in population spike amplitude were not accompanied by detectable increase in EPSP amplitude. The enhancement of the population spike after RPS was dependent on the ISI, the number of repeated paired stimuli, and the number of antidromic stimuli delivered, as well as being specific to the paired input. Intracellular recordings showed that the increase in probability of firing to the test stimulus after RPS occurred without any clear changes in membrane resistance or membrane potential, but that there was a slight increase in EPSP amplitude in a few cases. Pairing of the test stimulus with a depolarizing current injection also increased the probability of firing. These results indicate that the repetitive activation both of pre- and post-synaptic neurons at short temporal intervals induces long-term plastic changes in post-synaptic dendritic membranes.