Changes in the paired-pulse ratio after long-term potentiation induced by 2- and 100-Hz tetanus in rat visual cortical slices

The effects of 2- and 100-Hz tetanus on long-term potentiation (LTP) of field potentials recorded from layers II/III and induced in layer IV in rat visual cortical slices were examined. Paired-pulse stimulation was used to probe the different mechanisms of LTP induced by 2- and 100-Hz tetanus. The paired-pulse ratio (PPR) decreased after the LTP induced by 2-Hz tetanus, with the changes in PPR being correlated with LTP amplitude. However, in the LTP induced by 100-Hz tetanus, the changes in PPR were not correlated with LTP expression. These experiments suggest that an enhanced probability of presynaptic transmitter release underlies LTP induced by 2-Hz tetanus, but not LTP induced by 100-Hz tetanus.