Nitric oxide and memory.

Nitric oxide (NO) is widely used in neural circuits giving rise to learning and memory. NO is an unusual neurotransmitter in its modes of release and action. Is its association with learning and memory related to its unusual properties? Reviewing the literature might allow the formulation of a general principle on how NO and memory are related. However, other than confirming that there is indeed a strong association between NO and memory, no simple rules emerge on the role of NO in learning and memory. The effects of NO are not associated with a particular stage or form of memory and are highly dependent on species, strain, and behavior or training paradigm. Nonetheless, a review does provide hints on why NO is associated with learning and memory. Unlike transmitters acting via receptors expressed only in neurons designed to respond to the transmitter, NO is a promiscuous signal that can affect a wide variety of neurons, via many molecular mechanisms. In circuits giving rise to learning and memory, it may be useful to signal some events via a promiscuous messenger having widespread effects. However, each circuit will use the promiscuous signal in a different way, to achieve different ends.