OXYTOCIN AND MILK EJECTION

by

Margaret C. Neville


Table of Contents

INTRODUCTION
THE LET-DOWN REFLEX
THE NEUROENDOCRINOLOGY OF OXYTOCIN SYNTHESIS AND RELEASE
Effects of emotional input on oxytocin release.
Effects of opioids on oxytocin release.
INTERACTION OF OXYTOCIN WITH MYOEPITHELIAL CELLS

BIBLIOGRAPHY

Additional information on milk secretion

INTRODUCTION

Myoepithelial cells

Figure 1. Myoepithelial cells surrounding a mammary alveolus


Milk removal from the breast is accomplished by the contraction of myoepithelial cells, whose processes form a basket-like network around the alveoli where milk is stored (Figure 1), in concert with sucking by the infant. When the infant is suckled, afferent impulses from sensory stimulation of nerve terminals in the areolus travel to the central nervous system where they promote the release of oxytocin from the posterior pituitary. In the woman oxytocin release is often associated with such stimuli as the sight or sound or even the thought of the infant indicating a large cerebral component in this "neuroendocrine reflex". The oxytocin is carried through the blood stream to the mammary gland where it interacts with specific receptors on myoepithelial cells, initiating their contraction and expelling milk from the alveoli into the ducts and sub-areolar sinuses. The passage of milk through the ducts is facilitated by longitudinally arranged myoepithelial cell processes whose contraction shortens and widens the ducts, allowing free flow of milk to the nipple. The process by which milk is forceably moved out of the alveoli is called milk ejection or let-down and is essential to milk removal from the lactating breast.

THE LET-DOWN REFLEX

The temporal pattern of milk ejections varise remarkably from species to species (Cross, 1977). At one extreme, the rabbit nurses once a day, apparently ejecting about 250 grams o milk in two to five minutes in response to a single spurt of oxytocin. At the other extreme, the rat nurses her litter on the average about half of each hour. Letdown is delayed for at least 15 minutes after the attachment of the pups (Crowley & Armstrong, 1992). Thereafter increases in mammary pressure corresponding to oxytocin-induced milk ejection can be measured every 5 to 12 minutes. Humans fall between these two extremes. In the woman ejection can be measured as a rise in prssure sensed with a small catheter placed in a mammary duct or noted subjectively by the mother as a "tingling sensation" in the breast prior to or shortly after the start of suckling. The contractions last about 1 minute and occur with a frequency of 4 to 10 contractions per 10 minutes (Cobo et al, 1967). When properly measured, e.g in blood samples drawn at 2 minute intervals, the pulses of oxytocin in the blood stream are spaced at similar intervals (McNeilly et al., 1983).

In addition to its action on the mammary gland, oxytocin causes uterine contractions particularly in the term and postpartum uterus. Women who put their infants to the breast soon after delivery often experience uterine cramping that probably aids in uterine involution.

THE NEUROENDOCRINOLOGY OF OXYTOCIN SYNTHESIS AND RELEASE

Oxytocin holds the distinction of being the first naturally-occuring peptide hormone to b synthesized, a feat for which du Vigneaud received the Nobel prize in 1955. Immunostaining techniques have been used to show that oxytocin is synthesized mainly in specialized magnocellular neurons in the supraoptic and paraventricular nucleus of the hypothalamus, separate from the vasopressin-synthesizing neurons (Zimmerman & Defendini, 1977). Fibers from the paraventricular nucleus course to the level of the supraoptic nucleus where they are joined by fibers from this nucleus as well. Both sets of fibers then travel down the median eminence to the posterior pituitary.

Because of their large size the magnocellular neurons that release oxytocin have received extensive study. The neuronal bodies have diameters of 25-30 micrometers and short dendrites that may be joined with gap junctions with other oxytocin neurons. Some of the dendrites have varicosities that suggest that they release oxytocin locally within the hypothalamus, a suggestion that is supported by several observations suggesting that local oxytocin is an essential regulator of the milk ejection reflex.

Brownstein and coworkers (Brownstein et al.1980) used pulse chase experiments to show that oxytocin is synthesized as part of a 30 kD prohormone in the hypothalamus. After cleavage to smaller subunits the prohormone is packaging into secretory granules and transported down the axonal processes of the magnocellular neurons to the posterior pituitary. There it is further cleaved to oxytocin and its binding protein, a 10 kD neurophysin. The process is very rapid; fully formed radioactive oxytocin begins to accumulate in the neurohypophysis 1.5 hours after injection of radioactive amino acids into the cerebrospinal fluid. Like the release of neurotransmitters and peptide hormones, oxytocin release involves exocytosis of secretory granules containing both oxytocin and its related neurophysin following an burst of impulses carried to the neurohypophysis from the oxytocin neurones in the hypothalamus (Crowley & Armstrong, 1992).

The neural controls underlying oxytocin release during lactation in the rat have received extensive investigation using an experimental paradigm in which lactating rats are separated from their pups for several hours, anesthetized with urethane and then reunited with the pups. The reader is referred to an excellent reveiw by Crowley and Armstrong (1992) for more detail. The afferent somatosensory pathway from the nipple is thought to project through the spinal cord via the ipsilateral dorsolateral funiculus to the lateral cervical nucleus. From here ascending fibers cross in the medulla and ascend to the mesencephalon and thalamus. The final links from these these areas to the hypothalamus are not yet clear. What is clear is that many types of neurochemicals and hormones modulate the response of the oxytocin neurons, including glutamate, alpha-adrenergic agents, neuropeptide Y, beta-adrenergic agents, endogenous opioids and several others. Oxytocin release is also modulated at the level of the posterior pituitary primarily by opioids, acting preferentially at the K-subtype of opioid receptor. Finally there is mounting evidence for stimulatory effects of prolactin acting at the level of the neurohypophysis to play a role in oxytocin release possibly as part of a bidirectional feedback positive feedback loop in which oxytocin also facilitates prolactin release.

Effects of emotional input on oxytocin release. Newton and Newton showed in 1948 that psychological stress or pain decreased milk output (Newton & Newton, 1948). In relaxed, undisturbed women suckling their infants oxytocin release begins with the onset of suckling or even prior to suckling (McNeilly et al.,1983) when the infant cries or becomes restless. When the suckling women were asked to carry out difficult mental calculations while nursing the infant or fed traffic noise through earphones the number of oxytocin pulses was significantly reduced.

Effects of alcohol on oxytocin release. Ethyl alcohol is a potent inhibitor of oxytocin release. Chronic ethanol ingestion by lactating rats led to both a decrease in milk production and a change in milk composition, with decreased lactose and increased lipid content (Vilaro et al.1987). An elegant, early study in which intramammary pressure was measured in response to suckling by the infant demonstrated that ethanol inhibited milk ejection in a dose-dependent manner (Cobo, 1973). In this study Cobo found that doses of alcohol up to 0.45 g/kg, doses that produce a blood level less than 0.1%, had no effect on intramammary pressure although they did abolish uterine contractures, suggesting that the myoepithelial cells in the breast are more sensitive to the hormone than is the myometrium.

Effects of opioids on oxytocin release. A potent effect of opioids on oxytocin release is suggested by the observation in rats that morphine inhibits the let-down reflex (Rayner et al.1995) and the mechanism of this response has been extensively studied in this species. In one carefully done study evidence for involvement of kappa receptors on magnocellular neurons was obtained, whereas morphine, a mu-receptor agonist depressed the mammary response to oxytocin (Russell et al., 1993) with no effect on the oxytocin-secreting neurons. The effects of opioids have not been extensively studied in lactating women. In a single report (Coiro et al., 1992), naloxone, an opioid antagonist, had no effect on oxytocin release but partially abrogated the inhibition produced by alcohol, suggesting that both that ethanol acts through an opioid pathway and that opioids do not play a role in the normal release of oxytocin during lactation.

INTERACTION OF OXYTOCIN WITH MYOEPITHELIAL CELLS

The recent cloning of the oxytocin receptor shows that it belongs to a family of nonapeptide hormone receptors that includes the vasopressin receptors and that appear to be coupled to the inositol phospholipid-protein kinase C system. The human oxytocin receptor is a 389 amino acid protein whose secondary structure resembles G-protein coupled, rhodopsin type receptors. The gene spans about 17 kilobases on chromosome 3 (3p26.2) and consists of 3 introns and 4 exons (Inoue et al.,1994). The rat gene has also been cloned (Rozen et al.1995) and should provide the basis for future functional studies of the receptor in the mammary myoepithelial cells.

Oxytocin appears to stimulate the growth of myoepthelial cells both in vitro and in vivo. When administered with estrogen and progesterone via implantable pellets into a virgin moue mammary gland it also enhanced myoepthelial differentiation of the cap cells surrounding the terminal end bud (Sapino et al., 1993). However, in the mammary gland its more characteristic and physiological action is to promote contraction of the myoepithelial cells. In an elegant study Olins and Bremel (Olins & Bremel, 1984) found that oxytocin stimulated the phosphorylation of the myosin light chain in rat mammary myoepithelial cells within 30 seconds of binding. Influx of extracellular calcium ions regulated the duration of the response. There is also evidence for involvement of intracellular calcium stores from studies of the effects of oxytocin on tissue pieces (Da Costa et al., 1995). In this study the response of the tissues i was also shown to increase from pregnancy to lactation.

That oxytocin release is necessary for the maintenance of lactation is shown by studies on oxytocin null mice (LINK TO ARTICLE). Since it has long been known that milk removal from the breast was necessary for continued milk secretion, it is not surprising to find that homozygous oxytocin null mice are unable to maintain their litters. These mice will be useful to determine the role of oxytocin in other reproductive processes at both the end-organ and hypothalamic levels.