Central and peripheral inhibition of milk ejection

Abstract

Milk ejection can be inhibited as a result of a lack of release of oxytocin (OT) from the pituitary gland, or, when OT is released normally, as a lack of effect on the mammary gland. The former can be overcome by administration of exogenous OT, whereas the latter cannot. Central inhibition of OT release can be caused by several factors, such as relocating cows to unfamiliar surroundings for milking, oestrus, change from suckling to machine milking. In such cases peripheral blood concentrations of β-endorphin and cortisol are elevated. Administration of exogenous cortisol does not cause inhibition of milk ejection. On the other hand, the administration of morphine inhibits OT release from the pituitary, and, therefore, milk ejection. The effect of morphine can be abolished by naloxone (opioid antagonist) administration. However, premilking administration of naloxone does not abolish spontaneous inhibition of milk ejection in cows during milking in unfamiliar surroundings. The role of β-endorphin in central inhibition of milk ejection remains unclear. Peripheral inhibition of milk ejection is caused by exogenous α-adrenergic receptor agonists or OT receptor blocking agents, while OT release from the pituitary is normal. Administered β-adrenergic agonists cause elevated milk flow rates as compared with normal milking. Inhibition of milk ejection by catecholamines cannot be abolished by administration of OT but α-adrenergic receptor blocking agents. Effects of OT receptor blocking agents can be abolished by administration of high dosages of exogenous OT. However, milk flow rates are not reduced as long as milk is present in the cistern. The presence of high concentrations of adrenergic receptors in the teat and lower amounts in more proximal regions of the bovine udder has been demonstrated. Their influence on milkability has been reported and relations between adrenergic receptor concentrations in the udder or on blood platelets and milking characteristics are most likely. Under stress free conditions the sympathetic tone in the mammary gland is reduced during milking and increases between milkings. All these investigations reflect the importance of the sympathic nervous system in the process of milk removal. Milk removal seems to be regulated by the adrenergic system, although the mechanism is not clear.