 Catecholamines such as norepinephrine (NE), epinephrine (EPI, adrenaline), and dopamine (DA). These neurotransmitters are synthesized in a series of enzymatic reactions from the amino acid tyrosine. Neurons that use catecholamines as their neurotransmitter are called catecholaminergic neurons. Catecholamines are secreted by cells in the CNS that are involved in the regulation of movement, mood, and attention. Neurons that utilize epinephrine (adrenaline) as their neurotransmitter are called adrenergic neurons. They all contain an enzyme that converts NE to adrenaline (EPI), which serves as a transmitter between postsynaptic sympathetic axons and effectors in the ANS. EPI is also released into the bloodstream by the endocrine cells (chromaffin cells) of the adrenal medulla during the fight-or-ﬂight response.
 Serotonin or 5-hydroxytryptamine (5-HT). Serotonin is formed by the hydroxylation and decarboxylation of tryptophan. It functions as a neurotransmitter in neurons of the CNS and enteric nervous system. Neurons that use serotonin as their neurotransmitter are called serotonergic. After the release of serotonin, a portion is recycled by reuptake into presynaptic serotonergic neurons.
 Amino acids such as -aminobutyrate (GABA), glutamate (GLU), aspartate (ASP), and glycine (GLY) also act as neurotransmitters, mainly in the CNS.
 Nitric oxide (NO), a simple gas with free radical properties, also has been identified as a neurotransmitter. At low concentrations, NO carries nerve impulses from one neuron to another. Unlike other neurotransmitters, which are synthesized in the nerve cell body and stored in synaptic vesicles, NO is synthesized within the synapse and used immediately. It is postulated that excitatory neurotransmitter GLU induces a chain reaction in which NO synthase is activated to produce NO, which in turn diffuses from the presynaptic knob via the synaptic cleft and post-synaptic membrane to the adjacent cell, resulting ultimately in generation of an action potential.
 Small peptides have also been shown to act as synaptic transmitters. Among these are substance P (so named because it was originally found in a powder of acetone extracts of brain and intestinal tissue), hypothalamic releasing hormones, enkephalins, vasoactive intestinal peptide (VIP), cholecystokinin (CCK), and neurotensin. Many of these same substances are synthesized and released by enteroendocrine cells of the intestinal tract. They may act immediately on neighboring cells (paracrine secretion) or be carried in the bloodstream as hormones to act on distant target cells (endocrine secretion).
They are also synthesized and released by endocrine organs and by the neurosecretory neurons of the hypothalamus.
Neurotransmitters released into the synaptic cleft may be degraded or recaptured.