Melatonin vs Oxytocin

Melatonin

Melatonin (N-acetyl-5-methoxytryptamine) is a small indoleamine made from tryptophan, chiefly in the pineal gland; it is not a peptide. It is made from serotonin in two steps, acetylation and then methylation, in the pineal gland and in smaller amounts in the retina, gut and other tissues. Pineal production follows a daily rhythm set by the suprachiasmatic nucleus of the hypothalamus: it rises in darkness and is suppressed by light. Human research is substantial and concentrates on circadian physiology and sleep, alongside a large animal literature that includes seasonal breeding. Work on its chemistry as a free-radical scavenger is mostly in vitro or in animals, often at concentrations well above those found in blood.

Oxytocin

Oxytocin is a nine-residue peptide hormone, cyclised by a disulphide bond between its two cysteines and amidated at the C-terminus. It is made by neurons of the hypothalamus and released both from the posterior pituitary into the circulation and within the brain. It differs from vasopressin at two of its nine positions, and their receptors belong to one family. Its classical physiological roles, in uterine contraction and milk ejection, were established first, and in the 1950s it became the first peptide hormone to be synthesised in the laboratory. More recent research concerns its signalling in the brain and social behaviour; the animal literature is substantial, while human findings have been inconsistent and remain debated.

How Melatonin acts

Two high-affinity melatonin receptors, MT1 and MT2, have been characterised. Both are G-protein-coupled receptors that couple mainly to Gi, lowering cyclic AMP, and both are expressed in the suprachiasmatic nucleus, where rodent studies have linked MT1 to suppression of neuronal firing and MT2 to shifts in the timing of the circadian clock. They are also found in the retina, blood vessels and several peripheral tissues. A third binding site, once labelled MT3, was later identified as the enzyme quinone reductase 2. Apart from receptor signalling, melatonin and several of its metabolites react directly with reactive oxygen and nitrogen species in chemical systems.

How Oxytocin acts

The oxytocin receptor (OXTR) is a class A G-protein-coupled receptor that signals mainly through Gq/11, activating phospholipase C and raising intracellular calcium; coupling to other G proteins has also been reported. It is expressed in the myometrium, mammary myoepithelial cells and several brain regions, among other tissues, and uterine expression rises sharply late in pregnancy. Because oxytocin and vasopressin are so alike, each can act at the other's receptors at higher concentrations, and oxytocin has measurable activity at the vasopressin V1a receptor. How much oxytocin in the circulation crosses into the brain is still debated.

The records side by side

• Type: Melatonin, Amino acid derivative (non-peptide); Oxytocin, Cyclic nonapeptide hormone (neurohypophysial peptide)
• Length: Melatonin, not recorded; Oxytocin, 9 amino acids
• Molecular weight: Melatonin, 232.28 g/mol; Oxytocin, 1007.2 g/mol
• Molecular formula: Melatonin, C13H16N2O2; Oxytocin, C43H66N12O12S2
• Half-life: Melatonin, not recorded; Oxytocin, Plasma half-life of oxytocin ranges from 3 to 20 min.

Reading a comparison

A comparison places two records side by side; it does not merge them. Each description above is drawn from that compound's own record, and the two were not necessarily characterised by the same methods, at the same time, or to the same level of detail. Before comparing any attribute, it is worth establishing that both records describe the substance you think they describe — names are not identifiers, and two entries sharing a name can differ in salt form, stereochemistry or the convention used to state a mass. Where a figure carries no method beside it, the comparison is between two reported numbers rather than between two measurements.