There are many different ways to classify<unk> s. Dividing them into amino acids, peptides, and monoamines is sufficient for some classification purposes. Major<unk> s: *Amino acids: glutamate, aspartate, D-serine, gamma-Aminobutyric acid (GABA), glycine *Gasotransmitters: nitric oxide (NO), carbon monoxide (CO), hydrogen sulfide (H2S) *Monoamines: dopamine (DA), norepinephrine (noradrenaline; NE, NA), epinephrine (adrenaline), histamine, serotonin (SER, 5-HT) *Catecholamines: dopamine, norepinephrine (noradrenaline), epinephrine (adrenaline) *Trace amines: phenethylamine, N-methylphenethylamine, tyramine, 3-iodothyronamine, octopamine, tryptamine, etc. *Peptides: oxytocin, somatostatin, substance P, cocaine and amphetamine regulated transcript, opioid peptides *Purines: adenosine triphosphate (ATP), adenosine *Others: acetylcholine (ACh), anandamide, etc. In addition, over 100 neuroactive peptides have been found, and new ones are discovered regularly. Many of these are co-released along with a small-molecule transmitter. Nevertheless, in some cases, a peptide is the primary transmitter at a synapse. Beta-Endorphin is a relatively well-known example of a peptide<unk>  because it engages in highly specific interactions with opioid receptors in the central nervous system. Single ions (such as synaptically released zinc) are also considered<unk> s by some, as well as some gaseous molecules such as nitric oxide (NO), carbon monoxide (CO), and hydrogen sulfide (H2S). The gases are produced in the neural cytoplasm and are immediately diffused through the cell membrane into the extracellular fluid and into nearby cells to stimulate production of second messengers. Soluble gas<unk> s are difficult to study, as they act rapidly and are immediately broken down, existing for only a few seconds. 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<unk>  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. List of<unk> s, peptides, and gaseous signaling molecules +<unk> s Category Name Abbreviation Metabotropic Ionotropic : Amino acids (Arg) Arginine Arg, R α2-Adrenergic receptors, imidazoline receptors NMDA receptors Small: Amino acids Aspartate Asp, D – NMDA receptors Small: Amino acids Glutamate Glu, E Metabotropic glutamate receptors NMDA receptors, kainate receptors, AMPARs Small: Amino acids Gamma-aminobutyric acid GABA GABAB receptors GABAA receptors, GABAA-ρ receptors Small: Amino acids Glycine Gly, G – NMDA receptors, glycine receptors Small: Amino acids D-serine Ser, S – NMDA receptors Small: Acetylcholine Acetylcholine ACh Muscarinic acetylcholine receptors Nicotinic acetylcholine receptors Small: Monoamine (Phe/Tyr) Dopamine DA Dopamine receptors, trace amine-associated receptor 1 – Small: Monoamine (Phe/Tyr) Norepinephrine (noradrenaline) NE, NAd Adrenergic receptors – Small: Monoamine (Phe/Tyr) Epinephrine (adrenaline) Epi, Ad Adrenergic receptors – Small: Monoamine (Trp) Serotonin (5-hydroxytryptamine) 5-HT Serotonin receptors (all except 5-HT3) 5-HT3 Small: Monoamine (His) Histamine H Histamine receptors – Small: Trace amine (Phe) Phenethylamine PEA Human trace amine-associated receptors: , – Small: Trace amine (Phe) N-methylphenethylamine NMPEA hTAAR1 – Small: Trace amine (Phe/Tyr) Tyramine TYR hTAAR1, hTAAR2 – Small: Trace amine (Phe/Tyr) octopamine Oct hTAAR1 – Small: Trace amine (Phe/Tyr) Synephrine Syn hTAAR1 – Small: Trace amine (Trp) Tryptamine hTAAR1, various serotonin receptors – Small: Trace amine (Trp) N-methyltryptamine NMT hTAAR1, various serotonin receptors – Lipid Anandamide AEA Cannabinoid receptors – Lipid 2-Arachidonoylglycerol 2-AG Cannabinoid receptors – Lipid 2-Arachidonyl glyceryl ether 2-AGE Cannabinoid receptors – Lipid N-Arachidonoyl dopamine NADA Cannabinoid receptors TRPV1 Lipid Virodhamine Cannabinoid receptors – Small: Purine Adenosine Ado Adenosine receptors – Small: Purine Adenosine triphosphate ATP P2Y receptors P2X receptors Small: Purine Nicotinamide adenine dinucleotide β-NAD P2Y receptors P2X receptors + Neuropeptides Category Name Abbreviation Metabotropic Ionotropic Bombesin-like peptides Bombesin BBR1-2-3 – Bombesin-like peptide Gastrin releasing peptide GRP – – Bombesin-like peptide Neuromedin B NMB Neuromedin B receptor – Bradykinins Bradykinin B1, B2 – Calcitonin/CGRP family Calcitonin Calcitonin receptor – Calcitonin/CGRP family Calcitonin gene-related peptide CGRP CALCRL – Corticotropin-releasing factors Corticotropin-releasing hormone CRH CRHR1 – Corticotropin-releasing factors Urocortin CRHR1 – Galanins Galanin GALR1, GALR2, GALR3 – Galanins Galanin-like peptide GALR1, GALR2, GALR3 – Gastrins Gastrin Cholecystokinin B receptor – Gastrins Cholecystokinin CCK Cholecystokinin receptors – Granins Chromogranin A ChgA – – Melanocortins Adrenocorticotropic hormone ACTH ACTH receptor – Melanocortins Proopiomelanocortin POMC Melanocortin 4 receptor – Melanocortins Melanocyte-stimulating hormones MSH Melanocortin receptors – Neurohypophyseals Vasopressin AVP Vasopressin receptors – Neurohypophyseals Oxytocin OT Oxytocin receptor – Neurohypophyseals Neurophysin I – – Neurohypophyseals Neurophysin II – – Neurohypophyseals Copeptin – – Neuromedins Neuromedin U NmU NmUR1, NmUR2 – Neuropeptide B/W Neuropeptide B NPB NPBW1, NPBW2 – Neuropeptide B/W Neuropeptide S NPS Neuropeptide S receptors – Neuropeptide Y Neuropeptide Y NY Neuropeptide Y receptors – Neuropeptide Y Pancreatic polypeptide PP – – Neuropeptide Y Peptide YY PYY – – Opioids Enkephalins δ-Opioid receptor – Opioids Dynorphins κ-Opioid receptor – Opioids Neoendorphins κ-Opioid receptor – Opioids Endorphins μ-Opioid receptors – Opioids Endomorphins μ-Opioid receptors – Opioids Morphine μ-Opioid receptors – Opioids Nociceptin/orphanin FQ N/OFQ Nociceptin receptors – Orexins Orexin A OX-A Orexin receptors – Orexins Orexin B OX-B Orexin receptors – Parathyroid hormone family Parathyroid hormone-related protein PTHrP – – RFamides Kisspeptin KiSS GPR54 – RFamides Neuropeptide FF NPFF NPFF1, NPFF2 – RFamides Prolactin-releasing peptide PrRP PrRPR – RFamides Pyroglutamylated RFamide peptide QRFP GPR103 – Secretins Secretin Secretin receptor – Secretins Motilin Motilin receptor – Secretins Glucagon Glucagon receptor – Secretins Glucagon-like peptide-1 GLP-1 Glucagon-like peptide 1 receptor – Secretins Glucagon-like peptide-2 GLP-2 Glucagon-like peptide 2 receptor – Secretins Vasoactive intestinal peptide VIP Vasoactive intestinal peptide receptors – Secretins Growth hormone–releasing hormone GHRH Growth hormone–releasing hormone receptor – Secretins Pituitary adenylate cyclase-activating peptide PACAP ADCYAP1R1 – Somatostatins Somatostatin Somatostatin receptors – Tachykinins Neurokinin A – – Tachykinins Neurokinin B – – Tachykinins Substance P – – Tachykinins Neuropeptide K – – Other Agouti-related peptide AgRP Melanocortin receptor – Other N-Acetylaspartylglutamate NAAG Metabotropic glutamate receptor 3 (mGluR3) – Other Cocaine- and amphetamine-regulated transcript CART Unknown Gi/Go-coupled receptor – Other Gonadotropin-releasing hormone GnRH GnRHR – Other Thyrotropin-releasing hormone TRH TRHR – Other Melanin-concentrating hormone MCH MCHR 1,2 – + Gasotransmitters Category Name Abbreviation Metabotropic Ionotropic Gaseous signaling molecule Nitric oxide NO Soluble guanylyl cyclase – Gaseous signaling molecule Carbon monoxide CO – Heme bound to potassium channels Gaseous signaling molecule Hydrogen sulfide H2S – –
Neurotransmitter
Neurohormone
Xenohormone
Depressogen
Stimulant
Monoamine neurotransmitter
Gastrointestinal hormone
Local hormone