Greetings. Today we're going to talk about the POSTERIOR PITUITARY. The posterior pituitary is is going to govern our blood pressure as well as the osmotic balance within the body. The posterior pituitary remains connected to the brain through the region called the infundibulum. That's shown here. So we have neurons, which extend their axons from the hypothalamus and down through this neck region and into the posterior lobe. These neurons their axons terminate on the capillary beds which are perfusing all of the posterior lobe. These capillaries are fed by the inferior hypophyseal artery. It enters, breaks up into a capillary bed, and then it is drained by a vein to the systemic circulation. The cells which are of interest to us in the hypothalamus. These neurons come from two seperate sides. One's called the parventricular nucleus and the second, the suproptic nucleus. The suproptic nucleus and the parventricular nucleus secrete two types of neuro peptides. The neuropeptides are oxytocin and vasopresin. This area, posterior pituitary, then secretes two peptides into the systemic circulation. The first of the neuropeptides that I want to discuss is called oxytocin. Oxytocin is a peptide most physiologists thought was pretty boring. Oxytocin is secreted, by the suckling of breast by the infant. Mechanical stimulation of the tissue causes release of oxytocin. It in turn causes the myoepithelial cells to contract. When they contract, then milk is expelled. Oxytocin is also secreted at the time of delivery of the baby. It occurs with stretching of the cervix and of the birth canal. As this occurs, this mechanical stretching causes release of Oxytocin. Oxytocin will cause constriction or contraction of the smooth muscle of the uterus. It helps to expel the baby. So it's been known for many years that this is what oxytocin was doing. The only problem was that it was a conundrum. That is that males also secrete oxytocin. Why would a male be secreting oxytocin? Clearly human males do not give birth to the young. Secondly, males do not breastfeed the young. It was about five years ago that scientists started to look into what is actually happening with oxytocin in the male. What they found was is that oxytocin is released in high amounts at the time of orgasm, that is at sexual climax. This is occurring in both males and in females. So oxytocin then is the bonding hormone. It is the hormone that causes you to bond with your partner during the sexual act. Oxytocin is also a bonding hormone for the mother and the father to the baby. As they stroke the baby, or cuddle the baby, or hold the baby then the oxytocin levels increase. So oxytocin then is our love potion or our bonding hormone. When I first told this to my class a couple years ago, one of the students came up to me and asked me if this was available commercially. What happens if we don't have oxytocin? If we don't make enough oxytocin, then the mother actually rejects her baby. These are the individuals where they give birth to the child, but then have no have no interest in feeding the child, taking care of the child, tending to the child in any way. So with very low oxytocin levels then, there's no bonding to the offspring by the mother. This is a pathological situation. So that's oxytocin. What about vasopressin? Vasopressin is secreted to two different stimuli. Vasopresin is regulated by an osmotic stimulus and by a volume stimulus. The osmotic stimulus occurs when there is a very high rise or even a very small rise in osmolarity. This would be the concentration of sodium which is within the bloodstream. High osmolarity within plasma, releases vasopressin. In addition, vasopressin is released when there is a decrease of about 10% or more of blood volume. So a loss in blood pressure or in particularly, a loss in blood volume will cause release of antidiuretic hormone or vasopressin. These are the same entity. Vasopressin, or antidiuretic hormone, works in the kidney, on the collecting tubules of the kidney, where it moves water from the filtrate. Water in the presumptive urine moves back across the ephithelia cells which are lining these tubules into the capillaries, into the bloodstream. So vasopressin increases blood volume by moving water from presumptive urine back into the blood. By doing so it also dilutes the osmolarity of the blood. Vasopressin just moves water across this region of the body. So what is our pathology? The pathology for vasopressin is called diabetes insipidus. Many, many years ago the Greeks would recognize individuals who were drinking a lot and peeing a lot as having some type of pathological condition. What they did was to taste the urine that was coming from these individuals. If the urine tasted sweet it was called diabetes mellitus. This was urine that had glucose in it. These are individuals were secreting glucose from the body as well as water. In the individuals where the urine had no taste, they call this diabetes insipidus. Insipidus because it has no taste. That's the pathology that we see if there's a problem with the vasopressin or the anti-diuretic hormone. We can have two separate problems. One would be a central problem. THat occurs if there's a lesion or damage from trauma which prevents secretion from the posterior pituitary. Or there may be a tumor constricting the release from the posterior pituitary. Under these conditions you would have very low levels of vasopressin or anti-diaretic hormone. The individual is not able to concentrate their urine. Consequently, they are secreting or excreting from the body 18 liters of urine per day. The second type of pathology actually is found within the kidney.This is called a nephrogenic problem or a nephrogenic lesion. These can be two separate problems. One is where there is receptor resistance. We have a defective vasopressin receptor present on the principal cells, which are lining the collecting ducts. If the receptor is absent or not active, and vasopressin is present, the target cells cannot recognize the vasopressin. They don't respond. This would be receptor resistance. The second type of defect that can occur, is that the aquaporin 2 channels are defective. What vasopressin does in these cells, is tto move aquaporin channels, or water channels up to the luminal surfaces of the cells. and inserts these aquaporin channels into the plasma membrane. In the presence of aquaporin, water can move across these cells and into the blood. In the absence of aquaporin, no water can move from the lumen of these tubules, to the blood. So one of our general concepts, if you recall is that the pituitary has two distinct lobes, anterior and posterior. They have different embryonic origins. They're regulated separately. They produce different hormone products. The posterior pituitary has two products, oxytocin and vasopressin. The anterior pituitary is regulated by negative feedbacks from hormones from the target cells and we said there are four major feedback loops. The posterior pituitary secretes neural endocrines in response to changes in osmolarity of the blood, or the volume of the blood. Also to mechanical stimulation. So, if osmolarity and/or volume change then the posterior pituitary secretes vasopressin. With mechanical stimulation, it secretes oxytocin. Notice the major feedback loops are not mediated by hormones coming back to regulate the release of these two neuropeptides. Instead, the feedback for vasopressin is the change in osmolarity, or the change in blood the volume. For oxytocin, the feed back is to stop the mechanical stimulation either at the breast, or within the birth canal. Okay. So the next time we come in, then, we're going to start talking about the hypothalamus-pituitary axis and how it regulates energy or basal metabolic rates within the body. So see you then.
