Nitric oxide, synaptic plasticity and memory formation in the brain

Nitric oxide (NO) is an original biological effector mediating three-dimensional intercellular signaling owing to its diffusion properties. NO production in neurons and long-term potentiation (LTP), a synaptic plasticity phenomenon believed to underlie memory formation, both depend on an intracellular calcium rise via activation of excitatory amino-acid receptors. Therefore, NO has been proposed to be involved in information storage in the brain. Electrophysiology experiments demonstrate that inhibition of endogeneous NO production blocks LTP in the hippocampus and that exogeneous NO mimics it. Behavioral experiments indicate that NOS-inhibitors impair performances in models of spatial and olfactory learning, but leave shock-avoidance conditioning unaffected. NO therefore appears to be a physiological messenger of synaptic plasticity that helps to distinguish hippocampus-dependent forms of memory from others depending less on this brain structure.