Visualising oxytocin neurone activity in vivo: The key to unlocking central regulation of parturition and lactation

During parturition and lactation, oxytocin neurones in the supraoptic and paraventricular nuclei fire high-frequency bursts of action potentials that are coordinated across the entire population. Each burst generates a large pulse of oxytocin release into the circulation to induce uterine contraction for parturition and mammary duct contraction for milk ejection. Bursts are stimulated by cervical stretch during parturition and by suckling during lactation. However, the mechanisms by which these stimuli are translated into episodic bursts are poorly understood, as are the mechanisms that coordinate bursts across the oxytocin neurone population. An elegant series of experiments conducted in the 1980s and 1990s used serial paired recordings to show that oxytocin neurones do not act as a syncytium during bursts; rather, they start each burst within a few hundred milliseconds of each other but with no distinct "leaders" or "followers". In addition to afferent noradrenergic inputs that relay the systemic stimuli to oxytocin neurones, bursts depend on somato-dendritic oxytocin release within the hypothalamus. Hence, bursts are considered to be an emergent property of oxytocin neurones that is bootstrapped by appropriate afferent stimulation. Although much progress was made using traditional electrophysiological recordings in head-fixed anaesthetised animals, research has effectively stalled in the last few decades. However, the emergence of new technologies to monitor neuronal activity in freely-behaving animals has reinvigorated efforts to understand the biology underpinning burst firing in oxytocin neurones. Here, we report the use of fibre photometry to monitor the dynamics of milk ejection bursts in the oxytocin neurone population of freely-behaving mice. This approach will shed light on the neural mechanisms that control the oxytocin bursts underpinning parturition and lactation.