Neurotransmitters are endogenous chemicals that transmit, strengthen or modulate signals from a neuron to a target cell across at a chemical synapse. They are released by vesicles into the synaptic cleft, usually following the arrival of an action potential at the synapse. The action potential is translated from electrical into chemical information. Neurotransmitters diffuse through the synaptic cleft to receptors at the postsynaptic site. After their release, they are quickly inactivated and removed by reuptake.
There are different types of neurotransmitters, mostly classified as
Amino acids: glutamate, γ-aminobutyric acid (GABA), glycine, etc.
Monoamines & biogenic amines: dopamine, noradrenaline, adrenaline, histamine, serotonin, etc.
Peptides: substance P, opioid peptides, etc.
Others: acetylcholine, adenosine, nitric oxide, etc.