Nitric oxide facilitates long-term potentiation, but not long-term depression.

Reports that nitric oxide synthase (NOS) inhibition prevents the induction of long-term potentiation (LTP) have been controversial. Recent evidence suggests that NO may help to regulate the threshold for LTP induction. We have tested this hypothesis by examining the effects of stimulus frequency and train duration on synaptic plasticity in the presence of either NO donors or NOS inhibitors. Two different NO donors facilitated LTP induction by stimuli that normally produced only short-term potentiation, whereas NOS inhibitors blocked LTP to stimuli that normally produce small LTP. NO donors facilitated LTP induction even when NMDA receptors were blocked, indicating that NO need not act via NMDA receptors. NO donors and NOS inhibitors were without effect on long-term depression (LTD), suggesting that they act on a distinct potentiating mechanism. Thus, NO could contribute to the establishment of plasticity under physiologically relevant conditions by selectively increasing the probability of LTP induction.