Lactation, pituitary hormone release

Secretion of pituitary hormones is responsive to behavior and effects of experience. Consider, for example, the phenomenon of lactation, in which the sucking stimulus to the nipple triggers the release of oxytocin,... 

Prolactin Prolactin is an anterior pituitary hormone which induces milk secretion from the breasts of lactating women. Nipple stimulation promotes prolactin release and dopamine inhibits prolactin release. 

A polypeptide hormone secreted by the anterior pituitary gland in both males and females. The highest serum levels occur in pregnancy, when, along with other hormones, it promotes lactation and mammary gland growth. Prolactin secretion, unlike other pituitary hormones, is limited by a hypothalamic prolactin-release inhibiting factor (PIF). 

The posterior pituitary is innervated by direct nervous stimulation from the hypothalamus, resulting in the release of specific hormones. The hypothalamus synthesizes two hormones, oxytocin and vasopressin. These hormones are stored in and released from the posterior pituitary lobe. Oxytocin exerts two actions (1) it promotes uterine contractions during labor, and (2) it contracts the smooth muscles in the breast to stimulate the release of milk from the mammary gland during lactation. Vasopressin is an antidiuretic hormone (ADH) essential for proper fluid and electrolyte balance in the body. Specifically, vasopressin increases the permeability of the distal convoluted tubules and collecting ducts of the nephrons to water. This causes the kidney to excrete less water in the urine. Consequently, the urine becomes more concentrated as water is conserved. 

Oxytocin is a peptide hormone secreted by the posterior pituitary that elicits milk ejection in lactating women. It may contribute to the initiation of labor. Oxytocin is released during sexual orgasm. 

Since prolactin has no target organ-inhibitory feedback system similar to those of TSH, ACTH, LH or FSH, it was proposed that high circulating levels of prolactin may inhibit the release of the hormone by the pituitary gland [251]. Ample experimental confirmation of this hypothesis has appeared in recent years. Either systemic injection or implantation of minute amounts of prolactin into the median eminence significantly reduces prolactin in the adenohypophysis and blood and inhibits mammary development and lactation in the rat. The inhibitory action of prolactin is believed to be exerted at the hypothalamic level since increases in hypothalamic PIF activity [252,253] and in the activity of the... 

Thus it would appear that the immediate effect of a single suckling episode, i.e. acute suckling, is to reduce the prolactin content of the anterior pituitary gland with a release of the hormone into the circulation. The overall importance of suckling to lactation has been adequately reviewed (20-24). 

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