Essay: Prolactin

1. Which glands and which associated organ(s) create and secrete this hormone. As appropriate describe the structure and important features of these tissues as they pertain to hormone production. (i.e. is it in a particular part of the gland? A special cell type? Etc.)
Prolactin is a hormone created primarily in the anterior portion of the pituitary gland by lactotropic cells. These cells produce prolactin in response too signals from dopamine and thyrotropin-releasing hormone. Some scientists have proposed that the hormone is also synthesized in the hypothalamus based on evidence of prolactin immunoreactivity in rats. They found that Rats possess a large number of prolactin – like molecules that are very structurally similar to the pituitary prolactin in the placenta and in the decidua. The non pregnant uterus is another source of prolactin. Through studies done on non pregnant rats, scientists have found that the prolactin in their uteruses is indistinguishable from the prolactin secreted by the pituitary gland. Additionally, it is extremely likely that the hormone is also secreted by the mammary gland itself. The epithelial cells that uptake prolactin from the blood most likely synthesize the hormone. Finally, there is evidence to suggest that lymphocytes can also secrete prolactin in the immune system.
2. What triggers the creation and secretion of this hormone? (i.e.what event or change precipitates the creation of your hormone)
The primary things that triggers prolactin release are the stimulation of the nipples, stress, and increased levels of estrogen and other ovarian steroids. The suckling that a baby does to receive milk from its mother temporarily blocks the secretion of dopamine which normally inhibits prolactin. This action allows for a rise in serum prolactin concentrations allowing the baby to drink milk. Additionally, prolactin secretion is impacted by numerous factors released by the lactotrophs and other cells within the pituitary gland. Finally, Environmental stimuli such as light, odor, sound, and stress can also be included in causes for the secretion of prolactin.
3. Determine the location(s) that this hormone effects. What type of receptor interacts with your hormone, and where are the receptors located? How does this hormone enter or affect the cell?
Prolactin primarily impacts the mammary glands and the ovaries. Prolactin binding sites have been discovered in chemosensitive areas of the brain as well as the heart, lung, thymus, spleen, liver, pancreas, kidney, adrenal gland, skeletal muscle, and skin. The Prolactin receptor (PRLR) is a class 1 cytokine receptor. It contains a extracellular region, a transmembrane region, and a cytoplasmic region. (Chon-Hwa Tsai-Morris, 2012). These receptors “are connected through tyrosine phosphorylation to a predominant downstream mediator” (Nelson D. Horseman, 2016) in order to enter the cell.
4. What is the physiological effect of this hormones action?
The most well known effect of prolactin is its impact on the mammary gland. It plays a big role in the growth and development of the gland, and the synthesis and secretion of milk after pregnancy. Prolactin not only increases lipoprotein lipase activity in the mammary gland in order for a mother to produce milk, it also acts as a neuropeptide to promote physiological responses related to reproduction, stress adaptation, neurogenesis, and neuroprotection (Luz Torner, 2016). Data collected shows the influence of prolactin around reproductive behavior: “In humans, high prolactin levels are associated with psychosomatic reactions including pseudopregnancy” (Mark E. Freeman, 2000). Additionally, it may stimulate synthesis of alpha-lactalbumin which is the regulatory protein of the lactose enzyme system. (Dept. of Nutritional Services, 1990). Prolactin acts as a luteotrophic hormone because it helps maintain the corpus luteum’s functionality for 6 days after mating. Finally, prolactin plays a role in maintaining homeostasis by regulating the immune system. Studies in the 1970’s conducted by several scientists including Nagy and Berczi found that “a large number of immune perturbations were found to be associated with prolactin deficiency” (Mark E. Freeman, 2000). Thus, the main role of Prolactin in the human body is to promote physiological and psychological adaptations in order to help with overall reproductive success.
5. How is this hormone regulated? (not just type of regulation, but overall mechanism)
Prolactin secretion is primarily regulated by the hypothalamus: the neurotransmitter dopaine is secreted from neurons in the hypothalamus and transported to the pituitary lactotrophs where DA (dopamine) binds to D2 receptors in order to inhibit the release of PRL (Ben-Jonathan, 1985). The most likely mechanism for the release of prolactin is disinhibition of dopamine. When dopamine releases its inhibitory affect on the hypothalamus it frees the pituitary gland to release prolactin. It has been found that mice with a disrupted D2 receptor gene have an overproduction of prolactin and leading to lactotroph adenoma. Although the inhibitory influence of dopamine on prolactin is almost certain, the inverse relationship is not always present. “This inconsistency between dopaminergic activity and prolactin secretion could be easily resolved by assuming that additional PIF may also contribute to the negative control of prolactin secretion (Mark E. Freeman, 2000). This suggests that there might also be other inhibitors that regulate its release. Some researchers believe that among them could be GABA and somastatin. Finally, prolactin secretion could also be influenced by the lactrotrophs and by other cells within the pituitary gland.
6. What is the effect on the organism if this hormone is over/underproduced?
Prolactin deficiency is caused when one has a general shortage of the pituitary hormones. Deficiency can cause Sheehan syndrome, in which the pituitary gland is destroyed during or shortly after giving birth. Additionally, a decrease in prolactin can cause a mother to produce insufficient milk. People who are found to have low prolactin levels are also expected to be more susceptible to some infections. Increased prolactin secretion can be caused by damage to the pituitary stalk. This causes prolactin to be less regulated because it blocks the flow of dopamine from the hypothalamus to the lactotrophs. An increase of prolactin can also be caused by the development of Pituitary hormone-secreting tumors. The prolactin secreting tumors are called prolactionomas, and cause abnormal lactation (galactorrhea), menstrual abnormalities, and infertility. Additionally, for women of reproductive age, a surplus of prolacin can cause decreased secretion of gonadotropins which means a decreased cyclis ovarian function. This can cause women to experience loss of sexual desire, dryness of the vagina, infertility and sometimes galactorrhea (abnormal lactation). For men, a high concentration of prolactin can decrease testicular function causing loss of sexual desire, erectile dysfunction, muscle weakness, and infertility.