The oligopeptides vasopressin and oxytocin are elaborated by cells of the supraoptic and paraventricular nuclei and are transported, via their axons, through the stalk of the pituitary to its posterior lobe, where these substances are stored. Together, these elements constitute the neurohypophysis (posterior pituitary), which develops as an evagination of the floor of the third ventricle. Some of the vasopressin-containing nerve endings also terminate on cells of origin of the autonomic nervous system and on the capillary plexus of the hypophyseal portal circulation, through which they influence the secretion of CRH and GH. The peptide components of vasopressin and oxytocin, whose chemical nature was determined by DuVigneaud, are almost identical, differing from one another by only two amino acids.
Vasopressin, acting on the V2 receptors in kidney tubules, serves as the antidiuretic hormone (ADH) and, complemented by thirst mechanisms, maintains the osmolality of the blood. Plasma osmolality modifies vasopressin secretion by acting directly on the supraoptic and paraventricular neurons as well as separate osmoreceptors in the hypothalamus. If serum osmolality falls below 280 mOsm/L, the release of ADH is completely inhibited. Antidiuresis is maximal when plasma levels of vasopressin reach 5 pg/mL. This system is most effective in maintaining homeostasis when serum osmolality is in the range between 280 and 295 mOsm/L.
Alterations in blood volume and pressure also affect vasopressin release through neural mechanisms originating in the baroand mechanoreceptors of the aortic arch, carotid sinus, and right atrium. Afferent signals from these regions are conveyed in the vagus and glossopharyngeal nerves, which synapse in the nucleus of the tractus solitarius; the precise pathways leading to the hypothalamus have not been delineated, however. With severe hypotension, ADH release will continue even if there is a low serum osmolality; the effects of blood pressure predominate over osmolarity as a stimulus. Vasopressin secretion is also influenced by nonosmotic factors. Nausea, for example, is associated with a rise in the levels of the hormone, as much as 100-fold. Hypoglycemia also has an effect, but less pronounced. Drugs such as morphine, nicotine, alcohol, and certain chemotherapeutic agents (cyclophosphamide) also cause release of the stored peptide. Pain, emotional stress, and exercise have long been thought to cause release of vasopressin, but it is unclear whether this is a direct effect or is mediated through hypotension or nausea.
Oxytocin initiates uterine contraction and promotes lactation. Its release is stimulated by distention of the cervix, labor, breastfeeding, and estrogen. The effects of oxytocin are inhibited by alcohol.
Role of the Hypothalamus in Sexual Development (See Also Chap. 27)
The hypothalamus also plays a critical role in the development of human sexuality and its expression, a theme elaborated further in the next chapter. The suprachiasmatic nucleus and the number of neurons it contains are considerably larger in men than in women, a dimorphism that becomes evident during postnatal development. Studies by LeVay have suggested that the interstitial nucleus of the hypothalamus is smaller in homosexual males compared to heterosexual males, although the biologic evidence has been sharply challenged (Byne) and more research is needed to understand the implications of the reported association. These issues are addressed further in the section on sexual development in Chap. 27.