The four chemical classes of neurotransmitters are small-molecule transmitters, neuropeptides, neurotransmitter-containing vesicles, and co-transmitters.
Small-molecule transmitters are the simplest type of neurotransmitter and are composed of non-protein molecules, such as amino acids or monoamines. They are released directly into the synaptic cleft when an action potential reaches the axon terminal and bind to postsynaptic receptors. Examples of small-molecule transmitters include acetylcholine, dopamine, serotonin, and glutamate.
Neuropeptides are small protein molecules that are synthesized in the endoplasmic reticulum of neurons and packaged into vesicles before release. These peptides are usually released in conjunction with a small-molecule neurotransmitter and bind to their own unique set of receptors. Examples of neuropeptides include endorphins and oxytocin.
Neurotransmitter-containing vesicles are small membrane-enclosed packages which contain neurotransmitter molecules and are released when stimulated. They are found primarily in the axon terminals of neurons where they are stored until needed. Examples of neurotransmitter-containing vesicles include GABA vesicles and acetylcholine vesicles.
Co-transmitters are neurotransmitter molecules that are released along with a classical neurotransmitter and are also known as co-activators. They act synergistically with the neurotransmitters to amplify the response to a given stimulus. Examples of co-transmitters include noradrenaline, adenosine, and gamma-aminobutyric acid (GABA).
The chemical class of neurotransmitters that is not on this list is lipid-soluble molecules. Lipid-soluble molecules are molecules that are soluble in lipids and make up the majority of the chemical species in living organisms, including neurotransmitters. Examples of lipid-soluble molecules include steroids, prostaglandins, and eicosanoids. These molecules are not considered to be part of the four chemical classes of neurotransmitters.