Long-Term Potentiation Involves Increases in the Probability of Neurotransmitter Release

There is great interest in understanding the mechanisms of expression underlying long-term potentiation (LTP). They are agreed to involve an increase in synaptic efficacy, which is described by three multiplicative parameters: p, the probability of neurotransmitter release; n, the number of active release sites; and q, the postsynaptic unit response to transmitter release. We report three new lines of evidence suggesting that increases in p contribute to LTP expression. (i) When the contributions to LTP by p, n, and q are maximized, and p alone is decreased, another high-frequency stimulation elicits additional LTP. The additional potentiation is only associated with decreases in paired-pulse facilitation (PPF) suggesting an increase in p. (ii) There is an inverse relationship between baseline Ip and the magnitude of LTP elicited, consistent with Ip having more or less room to increase when p is smaller or greater. (iii) It has been shown that there is an inverse relationship between the magnitude of LTP induced and the associated changes in PPF. Now I find that decreasing p before inducing LTP moves the set-point for measuring those changes in PPF from before to after p is decreased, which would only occur if p contributes to LTP. Three lines of evidence, then, suggest that increases in p contribute to LTP expression, which is consistent with a presynaptic contribution to LTP. These experiments do not address potential postsynaptic contributions.