Oxytocin

Oxytocin is a cyclic nine-residue peptide closed by a disulfide bridge, synthesised in the paraventricular and supraoptic nuclei of the hypothalamus and released from the posterior pituitary. It differs from vasopressin at only two positions.

Its peripheral endocrinology — uterine contraction, milk ejection — is settled science with a century of work behind it. Its central role as a social signalling molecule is the newer and far more contested subject.

• Social recognition and memory, including the strong animal-model evidence in voles and rodents

• Pair bonding and affiliative behaviour, where the comparative work across species is the most persuasive part

• Trust and cooperation in human behavioural economics paradigms — the area with the most publicised findings and the most failed replications

• Anxiety and stress reactivity, including interactions with the HPA axis

• Autism research, where intranasal administration has been trialled with mixed results

Two methodological problems recur and are worth knowing before reading the field: whether intranasal administration meaningfully reaches the brain is disputed, and oxytocin's cross-reactivity at vasopressin receptors means effects are not always attributable to the oxytocin receptor.

PT-141 and Kisspeptin-10 are the other neuroendocrine peptides here acting on behaviour and the reproductive axis, and Selank the anxiolytic-focused research peptide.