Melatonin

Melatonin (N-acetyl-5-methoxytryptamine) is the primary biochemical output of the pineal gland and the central timing signal of the mammalian circadian system. Synthesised from tryptophan through a four-step enzymatic pathway in pinealocytes — rate-limited by AANAT (arylalkylamine N-acetyltransferase) which is activated only during darkness — melatonin is secreted exclusively during the dark phase of the light-dark cycle, producing a sharp nocturnal peak (100-300 pg/mL in young adults) that signals nighttime duration to the entire body.

Melatonin acts primarily through two high-affinity Gi-coupled GPCRs: MT1 (MTNR1A, Kd approximately 20-40 pM) and MT2 (MTNR1B, Kd approximately 100-160 pM). MT1 is expressed in the SCN master pacemaker neurons, anterior pituitary, hippocampus, retina, immune cells, and peripheral vascular tissue. MT1 Gi coupling reduces cAMP in SCN neurons, acutely suppressing their firing rate — the direct neural mechanism by which the pineal melatonin signal resets and entrains the central pacemaker. MT2 is expressed in SCN (mediating phase-shifting responses to melatonin), retina, and hippocampus, coupling to Gq/11 in addition to Gi.

The SCN circadian pacemaker operates through a transcription-translation feedback loop: CLOCK/BMAL1 heterodimers drive transcription of PER1/2/3 and CRY1/2 genes; PER and CRY proteins accumulate, dimerize, and return to the nucleus to inhibit CLOCK/BMAL1 — completing a 24-hour cycle. Melatonin/MT1 signalling in SCN neurons modulates the amplitude and phase of this molecular clock, coordinating it with the external light-dark cycle and broadcasting the synchronised circadian timing signal to peripheral oscillators throughout the body.

Melatonin's lipophilic indole structure (partition coefficient logP approximately 1.4) allows passive diffusion across all biological membranes without requiring transporters. This membrane permeability enables: receptor-independent antioxidant chemistry through direct radical scavenging (melatonin and its oxidation metabolites AFMK and AMK neutralise hydroxyl radicals, superoxide, and peroxynitrite); mitochondrial accumulation without the electrochemical gradient dependence required by most targeted antioxidants; and CNS penetration without active transport.

Published research has connected melatonin to mitochondrial biology: melatonin accumulates in mitochondria and has been proposed to scavenge ROS generated at the electron transport chain, protecting cardiolipin from peroxidation. This mitochondrial antioxidant dimension places Melatonin research in a complementary context to SS-31 (Elamipretide) mitochondrial biology — both compounds interact with the mitochondrial inner membrane and influence ROS biology, but through distinct mechanisms (SS-31 via cardiolipin binding; Melatonin via direct free radical scavenging).

Immunomodulatory research: Melatonin receptors on T lymphocytes, NK cells, and macrophages connect the pineal gland to circadian immune regulation. The nocturnal melatonin peak has been proposed to coordinate timing of immune surveillance activities, with published data showing MT1/MT2-mediated effects on IL-2 production, NK cell cytotoxicity, and T cell proliferation.

Research applications: MT1/MT2 receptor binding (competitive displacement with [125I]-melatonin) and cAMP functional assays; SCN electrophysiology in hypothalamic slices; circadian gene expression profiling (CLOCK, BMAL1, PER1/2, CRY1/2); antioxidant capacity assays (DPPH, ABTS, ORAC); and mitochondrial ROS measurement with melatonin pre-treatment.

MW: 232.28 g/mol. CAS: 73-31-4. Photosensitive indole — store in amber container at -20°C. Limited aqueous solubility (approximately 1mg/mL); prepare 100mM stocks in DMSO, dilute to less than 0.1% DMSO for cell work. For laboratory and analytical research purposes only.

Solubility and handling note: Melatonin has limited aqueous solubility (approximately 1mg/mL at pH 7.4). For cell culture, prepare a 100mM stock in DMSO, then dilute to working concentrations (1nM-100uM) in culture media — maintain final DMSO concentration below 0.1% to avoid solvent toxicity. For in vitro antioxidant assays (DPPH, ABTS), melatonin dissolves well in ethanol (up to 10mM); ensure matched ethanol vehicle controls. Light sensitivity: melatonin undergoes UV-driven oxidation to N-formylkynurenine and subsequent metabolites — all handling and incubation should use amber containers and dim lighting. Positive controls for MT1/MT2 research: Ramelteon (selective MT1/MT2 agonist, higher receptor affinity than melatonin); 2-iodomelatonin (high-affinity agonist used in radioligand binding). MW: 232.28 g/mol. CAS: 73-31-4. Store at -20°C, amber container. For laboratory and analytical research purposes only.