Mechanisms of presynaptic inhibition studied using paired-pulse facilitation.

An investigation was made of the effect of presynaptic inhibition on paired-pulse facilitation (PPF) of group Ia afferent excitatory postsynaptic potentials (EPSPs). The main finding from this study was that PPF was enhanced during presynaptic inhibition of compound Ia EPSPs. This increase in PPF is identical to that seen at other synapses when the probability of transmitter release is decreased by lowering the extracellular calcium or raising the extracellular magnesium concentration, providing unequivocal evidence that presynaptic inhibition is associated with a decrease in the probability of transmitter release. Further, by analogy with the effects of reduced calcium influx on PPF at other synapses, the results support the idea that presynaptic inhibition is associated with reduced calcium influx into nerve terminals.