- A neurotransmitter, such as nitric oxide, that diffuses from the post synaptic neuron back to the presynaptic neuron 

Rutgers University

Nitric Oxide (NO) and other gas neurotransmitters are different from classic neurotransmitters: 
- Not produced in axon terminals
- Not held in or released from vesicles - it diffuses out of the neuron as soon as it's produced instead
- Does not interact with receptors - it diffuses into target cells instead
- Can serve as a retrograde transmitter 

Nitric Oxide 

Breedlove, S. & Watson, N. (2017). Behavioral neuroscience, eighth edition. Oxford University Press.

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