Neurotransmitters can be grouped in different classes, such as Small molecules (eg, glutamate, gamma-aminobutyric acid, glycine, adenosine, acetylcholine, serotonin, histamine, noradrenaline) Neuropeptides (eg, endorphins) Gaseous molecules (eg, nitric oxide, carbon monoxide) Endocannabinoids Glutamate and aspartate These amino acids (glutamate and aspartate) are the major excitatory neurotransmitters in the CNS.