Dependence of LTP induction on postsynaptic depolarization: a perforated patch-clamp study in visual cortical slices of young rats.

1. To see whether there is a threshold of postsynaptic depolarization for induction of long-term potentiation (LTP) or depression (LTD) of synaptic transmission, perforated patch-clamp recordings were carried out under microscopic observation from 61 layer II/III neurons in visual cortical slices of young rats. Electrical stimulation given to nearby neurons was paired with stepwise shifts (30- or 300-ms duration) of clamped membrane potential of the recorded neurons to various levels. 2. Excitatory postsynaptic currents (EPSCs) were elicited by focal stimulation of a nearby pyramidal cell-like neuron. As the intensity of stimulation was increased, EPSCs emerged abruptly with 100% probability, and their peak latencies and amplitudes remained almost constant up to more than twice the threshold, indicating that the EPSCs were elicited monosynaptically. 3. LTP of EPSCs was induced in 11 of the 15 cells after pairing with a step to -20 mV and in 5 of the 14 cells after pairing with a step to -40 mV. No LTP was observed when the postsynaptic cells were clamped at -60, -70, or -90 mV. Significant LTD was not seen at any membrane potential level tested. There was no significant difference between the duration of potential shift of 30 and 300 ms during the pairing procedure in induction probability of LTP and magnitude of LTP, if it was induced. 4. These results suggest that LTP is induced by synaptic inputs associated with postsynaptic depolarization above the threshold around -40 mV at synapses linking layer II/III neurons in the developing visual cortex.