Week 3 Study Guide11

Week 3 Study Guide11 - Page |1 Lecture#5 Lecture Outline Hypothalamus-Anterior Pituitary Gland Axes Hypothalamic Hormones Dopamine GHRH

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P a g e | 1 Lecture#5: 1-19-2010 Lecture Outline: Hypothalamus-Anterior Pituitary Gland Axes Hypothalamic Hormones: Dopamine, GHRH, Somatostatin, CRH, TRH, GnRH Anterior Pituitary: Non-Trophic: PRL, GH (Antagonistic Regulation) Trophic: ACTH, TSH, Gonadotropins (Negative Feedback Regulation) Reflex and Feedback Regulation Primary and Secondary Secretion Hyposecretion (Deficiency) Hypersecretion (Excess) Hypothalamus-Anterior Pituitary Gland Axes Prolactin (PRL) Regulation and Secretion Prolactin is tonically secreted from the lactotroph cells of the anterior pituitary with a diurnal peak secretion at night. Its secretion is regulated by antagonistic control via the dual action of the hypothalamic hormones Prolactin Releasing Factors (PRFs) and Prolactin Inhibiting Hormone (PIH), which was recently identified as the neurohormone dopamine. Note that hypothalamic secretion of Thyrotropin Releasing Hormone (TRH) stimulates prolactin secretion, yet dopamine and somatostatin inhibit TSH secretion. Dopamine exerts dominant control over prolactin regulation, thereby limiting its secretion during the course of the lifetime, both in males and females. However, pregnancy represents a break in dopamine’s regulatory control as particularly high levels of hormones, such as estrogen, progesterone, growth hormone, and cortisol, promote increased prolactin secretion. During pregnancy, prolactin serves to stimulate the production of milk and enlarge the mammary glands. Though estrogen and progesterone work permissively with prolactin to foster its effects in further developing the glandular structure of the mammary glands, they simultaneously inhibit milk production and ejection. In other words, high circulating levels of estrogen, progesterone, growth hormone, and cortisol play a dichotomous role in that they promote the secretion of prolactin and its effect in developing the glandular structure of the mammary glands, yet they simultaneously inhibit milk production in the breast. Progesterone, especially, is known to contribute to this inhibitory effect. Instead of milk, the breast lobules produce colustrum, which is a low-fat liquid that bears very little nutritional value. It is only after the delivery of the baby that progesterone levels diminish and milk production and letdown occur. Interestingly, the delivery of the baby signifies the loss of the perpetual secretion of prolactin as high circulating levels of the aforementioned hormones fall after the birth of the baby. Thus, the stimulation of prolactin by the intrinsic hormonal cues halts and elevated prolactin secretion becomes solely dependent upon the stimulation of the mechanoreceptors of the nipple by the infant suckling. The infant suckling activates the nipple’s mechanoreceptors which send afferent signals to the hypothalamus that inhibit dopamine secretion and give rise to elevated prolactin secretion. This mechanism explains the loss of milk production once breastfeeding stops and the baby is weaned. Further, the nipples must be
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This note was uploaded on 04/06/2012 for the course BICD 150 taught by Professor Fortes during the Winter '09 term at UCSD.

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Week 3 Study Guide11 - Page |1 Lecture#5 Lecture Outline Hypothalamus-Anterior Pituitary Gland Axes Hypothalamic Hormones Dopamine GHRH

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