Neurotransmitters are chemicals that are used to relay, amplify and modulate signals between a neuron and another cell. According to the prevailing beliefs of the 1960s, a chemical can be classified as a neurotransmitter if it meets the following conditions:
There are precursors and/or synthesis enzymes located in the presynaptic neuron;
The chemical must be present in the presynaptic element
It is available in sufficient quantity in the presynaptic neuron to affect the postsynaptic neuron;
There must be postsynaptic receptors and the ability for the chemical to bind to said receptors
A biochemical mechanism for inactivation must be present.
Over 50 neuroactive peptides (vasopressin, somatostatin, neurotensin, etc.) have been found, among them hormones such as LH or insulin that have specific local actions in addition to their long-range signalling properties.
Single ions, such as synaptically-released zinc, are also considered neurotransmitters by some.
Nitrogen monoxide (NO)
The major "workhorse" neurotransmitters of the brain are glutamic acid (=glutamate) and GABA.
Diseases may affect specific neurotransmitter systems. For example, Parkinson's disease is at least in part related to failure of dopaminergic cells in deep-brain nuclei, for example the substantia nigra. Treatments potentiating the effect of dopamine precursors have been proposed and effected, with moderate success.
A brief comparison of the major neurotransmitter systems follows:
^ ab Rang, H. P. (2003). Pharmacology. Edinburgh: Churchill Livingstone, page 474 for noradrenaline system, page 476 for dopamine system, page 480 for serotonin system and page 483 for cholinergic system.. ISBN 0-443-07145-4.