The Maternal Brain

Oxytocin builds up in the posterior pituitary during pregnancy because less is released and more is produced. Oestrogen stimulates the oxytocin gene when progesterone secretion falls, although we do not yet know how the oestrogen receptor expressed in oxytocin neurones regulates this gene. Oxytocin neurones are strongly inhibited by three mechanisms which prevent them from releasing the stored oxytocin prematurely. First, progesterone, acting through an intermediary, intensifies actions of the inhibitory neurotransmitter GABA on oxytocin neurones ... brakes on. Second, stimulated oxytocin neurones produce nitric oxide, which diCuses from the cells and their terminals, restraining oxytocin cell activation and secretion ... more braking. During pregnancy, oestrogen and progesterone increase neuronal nitric oxide synthase (nNOS) activity and also activate the third mechanism, which uses brain peptides with opiate-like activity. These opioid peptides restrain oxytocin secretion first via receptors in the posterior pituitary and later at the nerve cell bodies ... safety stop? The brakes come oC ... when progesterone secretion collapses near term: GABA is less eCective and the nNOS gene is turned down. However, the opioid restraint remains, stopping the oxytocin neurones from running out of control, as excitation now predominates. A boost is given by oxytocin itself, breaking out from the nerve cell dendrites and driving the oxytocin neurones to operate near full power. Finally, oxytocin released in the brain acts on the circuitry for maternal behaviour, sensitized through induction of oxytocin receptors by oestrogen.