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Oxytocin secreted by nerve terminals in the posterior pituitary has important actions for ensuring a successful outcome of pregnancy: it stimulates uterine contractions that lead to birth and it is essential in the milk-ejection reflex, enabling milk to be expelled from the mammary glands into the mouths of suckling young. Oxytocin also has important actions in the brain: released from dendrites of neurones that innervate the posterior pituitary, oxytocin auto-excites the neurones to fire action potentials in co-ordinated bursts, causing secretion of pulses of oxytocin. Central oxytocin actions are blocked by an oxytocin antagonist given into the brain and, consequently, milk transfer stops. Systemic peptide oxytocin antagonist (atosiban) treatment is used clinically in management of pre-term labour, a major obstetric problem. Hence, it is important to know whether an oxytocin antagonist given peripherally can enter the brain and interfere with central oxytocin actions. In the present study, we tested F792, a peptide oxytocin antagonist. In urethane-anaesthetised suckled rats, we show that the mammary gland responsiveness to oxytocin is blocked by i.v. injections of 7 μg/kg of F792, and the milk-ejection reflex is blocked when F792 is given directly into the brain at a dose of 0.2 μg. To critically test whether F792 given systemically can enter the brain, we recorded the suckling- and oxytocin-induced burst-firing of individual antidromically identified oxytocin neurones in the paraventricular nucleus. Given systemically at 100 μg/kg i.v., F792 acted only peripherally, blocking the milk-ejecting actions of oxytocin, but not the burst-firing of oxytocin neurones during suckling (n = 5 neurones in five rats). Hence, this peptide oxytocin antagonist does not enter the brain from the circulation to interfere with an essential oxytocin function in the brain. Furthermore, the functions of oxytocin in the brain evidently cannot be explored with a systemic peptide antagonist.
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