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The Posterior Lobe of the Pituitary Gland
The human endocrine system is responsible for a wide array of functions resulting in homeostasis in the body. This collection of glands utilizes chemical messengers, called hormones. The “master gland", the pituitary, controls the function of all other glands, though, it itself is under the control of the hypothalamus.
The pituitary gland is composed of three lobes; anterior, intermediate and posterior. The anterior lobe, or adenohypophysis, is formed from glandular tissue. The intermediate lobe is just a thin layer of cells separating the anterior and posterior lobes, but it does produce and secrete a single hormone. The posterior lobe, or neurohypophysis, is constructed of neural tissue. The entire structure is pea sized and hangs from the hypothalamus by the pituitary stalk—a short length of neural axons. It is located on the underside of the brain where the optic nerves intersect, called the optic chiasm. All of the hormones which it secretes enter the bloodstream directly, without ducts. There is a vast network of capillaries surrounding each lobe.
The neurohypophysis secretes only two hormones in contrast to the several secreted by the adenohypophysis. The posterior pituitary hormones are, however, produced in the hypothalamus, though they are stored and secreted by the pituitary gland. The hormones are transported through the pituitary stalk and, following secretion, travel freely in the blood. From the hypothalamus, the hormones are paired with a protein molecule called neurophysins. Each of these two posterior pituitary hormones have primary physiological effects, as well as secondary effects. This article will only briefly touch on the secondary effects, as they are not widely documented and therefore, not used in therapy for medical conditions.
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Antidiuretic hormone (ADH) is a nonapeptide amino acid chain, meaning that its chemical structure is composed of nine individual peptides. It is known by several names including arginine vasopressin (AVP), vasopressin and argipressin.
Receptors in the walls of blood vessels measure the amount of water in the blood. When the concentration of water falls, the posterior pituitary releases ADH. Antidiuretic hormone acts on the distal and collecting tubules in the kidneys; in the absence of this hormone, these tubules are impermeable to water. In the presence of ADH, the distal and collecting tubules become permeable and result in the reabsorption of water, which dilutes the blood. Permeability of the tubules is accomplished by the insertion of molecules, called aquaporins, into the wall of the tubules. This also causes excretion of a concentrated, hypertonic urine. The release of ADH is inhibited by atrial natriuretic factor (ANF) released by the heart, when the blood is too diluted. It is also inhibited by alcohol, which is a diuretic, and results in diluted, hypotonic urine.
The name vasopressin is derived from the hormone's effect on the cardiovascular system. The other major effect of ADH is mild, systemic vasoconstriction. The hormone acts upon the smooth muscle in the walls of the arteries. The mild effects of vasoconstriction have little effect in the large arteries, such as the aorta. The effects are more widely observable in smaller arterioles. This constriction causes an increase in arterial blood pressure, particularly during instances of hemorrhage or dehydration.
Research and experimentation have shown several possible secondary effects of ADH. It has been suggested that this hormone plays a part in memory formation and learning. It has also been shown to have anti-fever and pain relief properties. In social behavior, it has been hypothesized that ADH plays a part in fatherly behavior, that is aggression towards strangers and intruders of one's territory, as well as pair bonding.
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Oxytocin is also a nonapeptide hormone produced by the hypothalamus. It is produced by both sexes, however the effects in women have been well documented for a number of years. It is secreted by the neurohypophysis in response to stimulation of the nipple, in particular the suckling reflex, visual or auditory stimulation from the baby, during intercourse and as a result of expansion of the uterus and cervix during birth. The presence of estrogen increases the uterus's response to oxytocin. Fear, anxiety, pain and alcohol inhibit the release of oxytocin. The presence of progesterone decreases the uterus's sensitivity to oxytocin.
The major effects of oxytocin are the milk “letdown" response, contraction of the uterine wall during birth and, possibly, the formation of maternal behaviors. The last effect has come under debate as experiments have shown conflicting conclusions. In the “letdown" response, milk has been formed and filled the alveoli of the mammary glands within the breast tissue. Oxytocin stimulates the ejection of the milk from the alveoli through the nipple in response to suckling. During birth, oxytocin causes the smooth muscle in the uterine wall to contract in order to aid in the birth process.
Several secondary effects of oxytocin have been posited. As the function of the hormone in men is not clearly elucidated, there is a hypothesis that it aids in the transport of sperm post-coitus, to increase the chances of reproduction. Likewise, it is suggested that it aids in egg transport in women. It has also been postulated that oxytocin plays a role in sexual response, particularly in men and non-pregnant women. The high levels of oxytocin in breast milk have led to the association of it with the mutual bonding of the offspring to the mother. Also, it may be somewhat responsible for the release of prolactin from the adenohypophysis; prolactin causes growth in the alveoli of the mammary glands and milk production.
Research has suggested a strong link between oxytocin and social behaviors, such as cooperation, generosity and, in men, empathy. This link has fostered several compelling research studies into the use of oxytocin as a possible treatment for patients with autism and Asperger's syndrome. Experimentally, oxytocin has shown social improvement in the condition, but is not currently a medically-recognized treatment.
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Clinton Community College: Endocrine System
The University of Manchester: Posterior Pituitary Hormones
University of Virginia School of Medicine: Posterior Pituitary Hormones
The University of Texas Arlington: Posterior Pituitary Hormones
Colorado State University: Antidiuretic Hormone (Vasopressin)
Stanford University's Pharmacogenomics Knowledge Base (PharmGKB): Vasopressin
Colorado State University: Oxytocin
Current Opinion in Neurobiology (2004): Vasopressin, Oxytocin and Social Behaviour
Keverne, Eric B. and James P. Curley
Neuropsychopharmacology (2003): Oxytocin Infusion Reduces Repetitive Behaviors in Adults with Autistic and Asperger's Disorders
Hollander, Eric; et al.
Proceeding of the National Academy of Sciences of the United States (2010): Promoting Social Behavior With Oxytocin in High-Functioning Autism Spectrum Disorders (abstract only)
Andari, Elissar; et al.