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Long-term potentiation (LTP) of synaptic transmission is a widely studied model of neuronal plasticity. The induction of LTP is known to require processes in the postsynaptic neuron, while experimental evidence suggests that the expression of LTP may occur in the presynaptic terminal. This has led to speculation that a retrograde messenger travels from the post- to the presynaptic cell during induction of LTP. Extracellular application or postsynaptic injection of two inhibitors of nitric oxide synthase, N-nitro-L-arginine or NG-methyl-L-arginine, blocks LTP. Extracellular application of hemoglobin, which binds nitric oxide, also attenuates LTP. These findings suggest that nitric oxide liberated from postsynaptic neurons may travel back to presynaptic terminals to cause LTP expression.
Schuman et al. (Fri,) studied this question.