Neuropeptides

Neuropeptides are short amino acid chains produced and released by neurons to act as chemical messengers throughout the nervous system. Unlike classical neurotransmitters (dopamine, serotonin, GABA) which are small molecule chemicals, neuropeptides are gene-encoded polypeptides that modulate neuronal activity, regulate behaviour, and coordinate physiological responses to internal and external stimuli. Many are also produced outside the brain and act as peripheral hormones.

Oxytocin is perhaps the most socially significant neuropeptide. Produced in the hypothalamus and released by the posterior pituitary, oxytocin mediates social bonding, trust, maternal behaviour, pair-bond formation, and the physiological aspects of sexual and reproductive function. It is the molecule underlying the 'tend-and-befriend' stress response, the bond between mother and infant, and the sensation of connection during physical intimacy. In research contexts, intranasal oxytocin has been studied for social anxiety disorder, autism spectrum disorder, borderline personality disorder, and PTSD.

Vasopressin (antidiuretic hormone, ADH) is oxytocin's closest structural relative — the two peptides differ by just two amino acids — yet their functions are complementary rather than overlapping. Vasopressin primarily regulates water retention by the kidneys, blood pressure, and in the brain, certain aspects of social memory, territorial behaviour, and pair-bonding in males. The interplay between oxytocin and vasopressin systems in social behaviour has been the subject of extensive comparative animal research.

Substance P is an 11 amino acid neuropeptide belonging to the tachykinin family. It is the primary neuropeptide of pain transmission — released by primary afferent neurons at the spinal cord level to transmit nociceptive signals. It also modulates anxiety, stress responses, and vomiting. Substance P antagonists (NK1 receptor antagonists like aprepitant) are approved as antiemetics and have been investigated for depression and anxiety treatment.

In the UK, pharmaceutical-grade oxytocin is licensed for obstetric indications. Vasopressin (desmopressin) is licensed for diabetes insipidus and nocturnal enuresis. Research-grade neuropeptides for other applications are unregulated. This category bridges endocrinology, psychiatry, and neuroscience — reflecting the deeply integrative role that neuropeptide signalling plays in human physiology.