Oxytocin

Identity a 9-amino-acid cyclic peptide (with a disulfide bridge) that is both a hormone and a central neurotransmitter. It's made in the hypothalamus, stored in and released from the posterior pituitary, and acts through the oxytocin receptor (OTR), a Gq-coupled GPCR. It has two quite separate lives: a peripheral hormonal role (uterine contraction, milk let-down) and a central neuromodulatory role (social/maternal behavior) — and the register distinction between them is essential, because the peripheral role is approved medicine and the central one is mostly hype-outpacing-evidence. (Historically notable: oxytocin was the first peptide hormone ever chemically synthesized — du Vigneaud, Nobel Prize 1955.) ## Mechanism (as proposed) oxytocin acts through the oxytocin receptor (OTR), a Gq/11-coupled GPCR signalling via phospholipase C → IP3 → intracellular Ca²⁺. Peripherally, in the myometrium, this Ca²⁺ rise (plus prostaglandin production) drives uterine contraction — the basis of its labor-inducing action; OTR density rises dramatically toward term (why it works then). It also contracts mammary myoepithelial cells for milk let-down. Centrally, oxytocin released within the brain modulates amygdala, reward, and social-cognition circuits, influencing maternal behavior, pair-bonding, social salience, and fear/trust processing — robustly in animal models. The translational problem is that central human effects are small, context-/genotype-dependent (OXTR variants), and inconsistently replicated, and intranasal delivery to the brain is itself uncertain — so the elegant "give oxytocin → more bonding/less social deficit" logic hasn't reliably materialized in human trials.