Neurochemical Regulation of Oxytocin Secretion in Lactation*

THE nonapeptide oxytocin (OT) is an excellent example of a messenger molecule with diverse physiological actions as well as modes of delivery to its target cells (1, 2). After release from neurosecretory terminals in the neurohypophysis, OT exerts effects as a hormone carried by the systemic circulation to distant target organs, perhaps the most important of which are the mammary gland and uterus (1). In addition, OT may serve as a hypophyseotropic factor, released from nerve terminals in the median eminence into the pituitary portal vasculature, to affect anterior pituitary secretion (2–7), as a peptidergic neurotransmitter or neuromodulator acting within the central nervous system to influence a variety of neuroendocrine, behavioral, and autonomic functions (2, 8–13), and as a paracrine regulatory peptide in the ovary and testis (14–18). Although its most widely investigated physiological actions across all of these modalities are related to reproduction, including the processes of lactation and maternal care, OT also appears to participate in a variety of physiological responses to physical and metabolic challenges.