<P> The most prevalent transmitter is glutamate, which is excitatory at well over 90% of the synapses in the human brain . The next most prevalent is Gamma - Aminobutyric Acid, or GABA, which is inhibitory at more than 90% of the synapses that do not use glutamate . Although other transmitters are used in fewer synapses, they may be very important functionally: the great majority of psychoactive drugs exert their effects by altering the actions of some neurotransmitter systems, often acting through transmitters other than glutamate or GABA . Addictive drugs such as cocaine and amphetamines exert their effects primarily on the dopamine system . The addictive opiate drugs exert their effects primarily as functional analogs of opioid peptides, which, in turn, regulate dopamine levels . </P> <Table> <Tr> <Th> Category </Th> <Th> Name </Th> <Th> Abbreviation </Th> <Th> Metabotropic </Th> <Th> Ionotropic </Th> </Tr> <Tr> <Td> Small: Amino acids (Arg) </Td> <Td> Arginine </Td> <Td> </Td> <Td> α - Adrenergic receptors, imidazoline receptors </Td> <Td> NMDA receptors </Td> </Tr> <Tr> <Td> Small: Amino acids </Td> <Td> Asparagine </Td> <Td> Asp </Td> <Td>--</Td> <Td> NMDA receptors </Td> </Tr> <Tr> <Td> Small: Amino acids </Td> <Td> Glutamate </Td> <Td> Glu </Td> <Td> Metabotropic glutamate receptors </Td> <Td> NMDA receptors, kainate receptors, AMPARs </Td> </Tr> <Tr> <Td> Small: Amino acids </Td> <Td> Gamma - aminobutyric acid </Td> <Td> GABA </Td> <Td> GABA receptors </Td> <Td> GABA receptors, GABA - ρ receptors </Td> </Tr> <Tr> <Td> Small: Amino acids </Td> <Td> Glycine </Td> <Td> Gly </Td> <Td>--</Td> <Td> NMDA receptors, glycine receptors </Td> </Tr> <Tr> <Td> Small: Amino acids </Td> <Td> D - serine </Td> <Td> Ser </Td> <Td>--</Td> <Td> NMDA receptors </Td> </Tr> <Tr> <Td> Small: Acetylcholine </Td> <Td> Acetylcholine </Td> <Td> Ach </Td> <Td> Muscarinic acetylcholine receptors </Td> <Td> Nicotinic acetylcholine receptors </Td> </Tr> <Tr> <Td> Small: Monoamine (Phe / Tyr) </Td> <Td> Dopamine </Td> <Td> DA </Td> <Td> Dopamine receptors, trace amine - associated receptor 1 </Td> <Td>--</Td> </Tr> <Tr> <Td> Small: Monoamine (Phe / Tyr) </Td> <Td> Norepinephrine (noradrenaline) </Td> <Td> NE, NAd </Td> <Td> Adrenergic receptors </Td> <Td>--</Td> </Tr> <Tr> <Td> Small: Monoamine (Phe / Tyr) </Td> <Td> Epinephrine (adrenaline) </Td> <Td> Epi, Ad </Td> <Td> Adrenergic receptors </Td> <Td>--</Td> </Tr> <Tr> <Td> Small: Monoamine (Trp) </Td> <Td> Serotonin (5 - hydroxytryptamine) </Td> <Td> 5 - HT </Td> <Td> Serotonin receptors (all except 5 - HT) </Td> <Td> 5 - HT </Td> </Tr> <Tr> <Td> Small: Monoamine (His) </Td> <Td> Histamine </Td> <Td> </Td> <Td> Histamine receptors </Td> <Td>--</Td> </Tr> <Tr> <Td> Small: Trace amine (Phe) </Td> <Td> Phenethylamine </Td> <Td> PEA </Td> <Td> Human trace amine - associated receptors: hTAAR1, hTAAR2 </Td> <Td>--</Td> </Tr> <Tr> <Td> Small: Trace amine (Phe) </Td> <Td> N - methylphenethylamine </Td> <Td> NMPEA </Td> <Td> hTAAR1 </Td> <Td>--</Td> </Tr> <Tr> <Td> Small: Trace amine (Phe / Tyr) </Td> <Td> Tyramine </Td> <Td> TYR </Td> <Td> hTAAR1, hTAAR2 </Td> <Td>--</Td> </Tr> <Tr> <Td> Small: Trace amine (Phe / Tyr) </Td> <Td> Octopamine </Td> <Td> Oct </Td> <Td> hTAAR1 </Td> <Td>--</Td> </Tr> <Tr> <Td> Small: Trace amine (Phe / Tyr) </Td> <Td> Synephrine </Td> <Td> Syn </Td> <Td> hTAAR1 </Td> <Td>--</Td> </Tr> <Tr> <Td> Small: Trace amine (Trp) </Td> <Td> Tryptamine </Td> <Td> </Td> <Td> hTAAR1, various serotonin receptors </Td> <Td>--</Td> </Tr> <Tr> <Td> Small: Trace amine (Trp) </Td> <Td> N - methyltryptamine </Td> <Td> NMT </Td> <Td> hTAAR1, various serotonin receptors </Td> <Td>--</Td> </Tr> <Tr> <Td> Lipid </Td> <Td> Anandamide </Td> <Td> AEA </Td> <Td> Cannabinoid receptors </Td> <Td>--</Td> </Tr> <Tr> <Td> Lipid </Td> <Td> 2 - Arachidonoylglycerol </Td> <Td> 2 - AG </Td> <Td> Cannabinoid receptors </Td> <Td>--</Td> </Tr> <Tr> <Td> Lipid </Td> <Td> 2 - Arachidonyl glyceryl ether </Td> <Td> 2 - AGE </Td> <Td> Cannabinoid receptors </Td> <Td>--</Td> </Tr> <Tr> <Td> Lipid </Td> <Td> N - Arachidonoyl dopamine </Td> <Td> NADA </Td> <Td> Cannabinoid receptors </Td> <Td> TRPV1 </Td> </Tr> <Tr> <Td> Lipid </Td> <Td> Virodhamine </Td> <Td> </Td> <Td> Cannabinoid receptors </Td> <Td>--</Td> </Tr> <Tr> <Td> Small: Purine </Td> <Td> Adenosine </Td> <Td> Ado </Td> <Td> Adenosine receptors </Td> <Td>--</Td> </Tr> <Tr> <Td> Small: Purine </Td> <Td> Adenosine triphosphate </Td> <Td> ATP </Td> <Td> P2Y receptors </Td> <Td> P2X receptors </Td> </Tr> <Tr> <Td> PP: Galanins </Td> <Td> Galanin </Td> <Td> </Td> <Td> GALR1, GALR2, GALR3 </Td> <Td>--</Td> </Tr> <Tr> <Td> PP: Galanins </Td> <Td> Galanin - like peptide </Td> <Td> </Td> <Td> GALR1, GALR2, GALR3 </Td> <Td>--</Td> </Tr> <Tr> <Td> PP: Gastrins </Td> <Td> Gastrin </Td> <Td> </Td> <Td> Cholecystokinin B receptor </Td> <Td>--</Td> </Tr> <Tr> <Td> PP: Gastrins </Td> <Td> Cholecystokinin </Td> <Td> CCK </Td> <Td> Cholecystokinin receptors </Td> <Td>--</Td> </Tr> <Tr> <Td> PP: Neurohypophyseals </Td> <Td> Vasopressin </Td> <Td> AVP </Td> <Td> Vasopressin receptors </Td> <Td>--</Td> </Tr> <Tr> <Td> PP: Neurohypophyseals </Td> <Td> Oxytocin </Td> <Td> OT </Td> <Td> Oxytocin receptor </Td> <Td>--</Td> </Tr> <Tr> <Td> PP: Neurohypophyseals </Td> <Td> Neurophysin I </Td> <Td> </Td> <Td>--</Td> <Td>--</Td> </Tr> <Tr> <Td> PP: Neurohypophyseals </Td> <Td> Neurophysin II </Td> <Td> </Td> <Td>--</Td> <Td>--</Td> </Tr> <Tr> <Td> PP: Neuropeptide B / W </Td> <Td> Neuropeptide B </Td> <Td> NPB </Td> <Td> NPBW1, NPBW2 </Td> <Td>--</Td> </Tr> <Tr> <Td> PP: Neuropeptide B / W </Td> <Td> Neuropeptide S </Td> <Td> NPS </Td> <Td> Neuropeptide S receptors </Td> <Td>--</Td> </Tr> <Tr> <Td> PP: Neuropeptide Y </Td> <Td> Neuropeptide Y </Td> <Td> NY </Td> <Td> Neuropeptide Y receptors </Td> <Td>--</Td> </Tr> <Tr> <Td> PP: Neuropeptide Y </Td> <Td> Pancreatic polypeptide </Td> <Td> PP </Td> <Td>--</Td> <Td>--</Td> </Tr> <Tr> <Td> PP: Neuropeptide Y </Td> <Td> Peptide YY </Td> <Td> PYY </Td> <Td>--</Td> <Td>--</Td> </Tr> <Tr> <Td> PP: Opioids </Td> <Td> Enkephalin </Td> <Td> </Td> <Td> δ - Opioid receptor </Td> <Td>--</Td> </Tr> <Tr> <Td> PP: Opioids </Td> <Td> Dynorphin </Td> <Td> </Td> <Td> κ - Opioid receptor </Td> <Td>--</Td> </Tr> <Tr> <Td> PP: Opioids </Td> <Td> Endorphin </Td> <Td> </Td> <Td> μ - Opioid receptors </Td> <Td>--</Td> </Tr> <Tr> <Td> PP: Opioids </Td> <Td> Endomorphin </Td> <Td> </Td> <Td> μ - Opioid receptors </Td> <Td>--</Td> </Tr> <Tr> <Td> PP: Orexins </Td> <Td> Orexin A </Td> <Td> OX - A </Td> <Td> Orexin receptors </Td> <Td>--</Td> </Tr> <Tr> <Td> PP: Orexins </Td> <Td> Orexin B </Td> <Td> OX - B </Td> <Td> Orexin receptors </Td> <Td>--</Td> </Tr> <Tr> <Td> PP: Secretins </Td> <Td> Secretin </Td> <Td> </Td> <Td> Secretin receptor </Td> <Td>--</Td> </Tr> <Tr> <Td> PP: Secretins </Td> <Td> Motilin </Td> <Td> </Td> <Td> Motilin receptor </Td> <Td>--</Td> </Tr> <Tr> <Td> PP: Secretins </Td> <Td> Glucagon </Td> <Td> </Td> <Td> Glucagon receptor </Td> <Td>--</Td> </Tr> <Tr> <Td> PP: Secretins </Td> <Td> Vasoactive intestinal peptide </Td> <Td> VIP </Td> <Td> Vasoactive intestinal peptide receptors </Td> <Td>--</Td> </Tr> <Tr> <Td> PP: Secretins </Td> <Td> Growth hormone--releasing hormone </Td> <Td> GHRH </Td> <Td> Growth hormone--releasing hormone receptor </Td> <Td>--</Td> </Tr> <Tr> <Td> PP: Somatostatins </Td> <Td> Somatostatin </Td> <Td> </Td> <Td> Somatostatin receptors </Td> <Td>--</Td> </Tr> <Tr> <Td> PP: Tachykinins </Td> <Td> Neurokinin A </Td> <Td> </Td> <Td>--</Td> <Td>--</Td> </Tr> <Tr> <Td> PP: Tachykinins </Td> <Td> Neurokinin B </Td> <Td> </Td> <Td>--</Td> <Td>--</Td> </Tr> <Tr> <Td> PP: Tachykinins </Td> <Td> Substance P </Td> <Td> </Td> <Td>--</Td> <Td>--</Td> </Tr> <Tr> <Td> PP: Tachykinins </Td> <Td> Neuropeptide K </Td> <Td> </Td> <Td>--</Td> <Td>--</Td> </Tr> <Tr> <Td> PP: Other </Td> <Td> Adrenocorticotropic hormone </Td> <Td> ACTH </Td> <Td> ACTH receptor </Td> <Td>--</Td> </Tr> <Tr> <Td> PP: Other </Td> <Td> N - Acetylaspartylglutamate </Td> <Td> NAAG </Td> <Td> Metabotropic glutamate receptor 3 (mGluR3) </Td> <Td>--</Td> </Tr> <Tr> <Td> PP: Other </Td> <Td> Cocaine - and amphetamine - regulated transcript </Td> <Td> CART </Td> <Td> Unknown G / G - coupled receptor </Td> <Td>--</Td> </Tr> <Tr> <Td> PP: Other </Td> <Td> Bombesin </Td> <Td> </Td> <Td>--</Td> <Td>--</Td> </Tr> <Tr> <Td> PP: Other </Td> <Td> Gastrin releasing peptide </Td> <Td> GRP </Td> <Td>--</Td> <Td>--</Td> </Tr> <Tr> <Td> PP: Other </Td> <Td> Kisspeptin </Td> <Td> </Td> <Td> GPR54 </Td> <Td>--</Td> </Tr> <Tr> <Td> Gasotransmitter </Td> <Td> Nitric oxide </Td> <Td> NO </Td> <Td> Soluble guanylyl cyclase </Td> <Td>--</Td> </Tr> <Tr> <Td> Gasotransmitter </Td> <Td> Carbon monoxide </Td> <Td> CO </Td> <Td>--</Td> <Td> Heme bound to potassium channels </Td> </Tr> <Tr> <Td> Gasotransmitter </Td> <Td> Hydrogen sulfide </Td> <Td> H2S </Td> <Td>--</Td> <Td>--</Td> </Tr> </Table> <Tr> <Th> Category </Th> <Th> Name </Th> <Th> Abbreviation </Th> <Th> Metabotropic </Th> <Th> Ionotropic </Th> </Tr> <Tr> <Td> Small: Amino acids (Arg) </Td> <Td> Arginine </Td> <Td> </Td> <Td> α - Adrenergic receptors, imidazoline receptors </Td> <Td> NMDA receptors </Td> </Tr>

Four categories of neurotransmitters based on chemical composition