| DEFINITION OF A NEUROTRANSMITTER
|
Traditionally, for a molecule to be labeled as a neurotransmitter a number of criteria have to be met. These include:
- Synthesis of the molecule must occur within the neurone, i.e. all biosynthetic enzymes, substrates, cofactors, etc., are present for de novo synthesis.
- Storage of the molecule occurs within the nerve ending prior to release, e.g. in synaptic vesicles.
- Release of the molecule from the presynaptic ending occurs in response to an appropriate stimulus such as action potential.
- There is binding and recognition of the putative neurotransmitter molecule on the postsynaptic target cell.
- Mechanisms exist for the inactivation and termination of the biological activity of the neurotransmitter.
|
Rigorous adherence to the above criteria means that some molecules that are involved in the crosstalk between neurons are not in the strict sense classified as neurotransmitters. Thus, nitric oxide (NO), adenosine, neurosteroids, polyamines, etc., are often termed neuromodulators rather than neurotransmitters.
|
|
