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The anterior pituitary is the glandular part of the pituitary gland, containing biological cells that synthesize and release hormones. Signals from the hypothalamus reach it through the hypophyseal portal system: blood passes through one network of capillaries, carries releasing or inhibiting hormones down connecting blood vessels, and enters a second network beside the hormone-secreting biological cells. This provides a direct chemical route for control by the brain.^[ref]

Its products include growth hormone, prolactin and thyroid-stimulating hormone. For example, prolactin promotes milk production, whereas oxytocin released by the posterior pituitary promotes milk ejection. Making milk and moving it outward are distinct controlled processes. Similarly, thyroid-stimulating hormone stimulates release of thyroid hormones, which participate in negative feedback on the control pathway. The anterior pituitary therefore both produces hormones and relays upstream signals; it is not simply a storage site for hormones made in the hypothalamus.^[ref]

Oxytocin is a nine-amino acid peptide hormone synthesized by neurons in the hypothalamus and released into the circulation from the posterior pituitary. It promotes contraction of uterine smooth muscle during childbirth and contraction of cells around mammary secretory units, ejecting milk. Milk ejection is distinct from producing the milk.^[ref]

Childbirth illustrates positive feedback: stretching of the cervix stimulates a neural pathway that increases oxytocin release; stronger uterine contractions can increase the stretch until delivery removes the initiating stimulus. During suckling, sensory input similarly couples an external event to hormone release and milk ejection. These circuits connect sensory neurons, endocrine signaling and mechanical action.^[ref]

The pituitary gland is an endocrine gland at the base of the brain, connected to the hypothalamus. Its anterior pituitary contains hormone-secreting glandular cells; its posterior pituitary contains the terminals of hypothalamic neurons. The two regions consequently communicate with the hypothalamus in different ways.^[ref]

For example, the anterior pituitary secretes prolactin, which promotes milk production, while the posterior pituitary releases oxytocin, which promotes milk ejection. One organ thus participates in distinct, coordinated stages of a physiological response.^[ref]

The posterior pituitary is the neural part of the pituitary gland. Peptide hormones made in the soma of hypothalamic neurons travel down their axons and are released from terminals into the blood. The location of release is therefore different from the location of synthesis.^[ref]

Its released hormones include oxytocin and vasopressin. The latter helps the kidneys conserve water. This is an example of a neuron producing an endocrine output: an electrical signal at its terminal can cause a chemical signal to enter the circulation.^[ref]

Vasopressin, also called antidiuretic hormone, is a peptide hormone made by neurons in the hypothalamus and released into blood through the posterior pituitary. An important action is to conserve water by increasing its reabsorption in the kidney.^[ref]

In the collecting duct, vasopressin regulates the abundance of aquaporin-2 water channels in the cell membrane facing the fluid that will become urine. More channels increase membrane permeability to water, allowing more water to return toward the blood by osmosis. This changes a route through the membrane rather than making the channel an active water pump.^[ref]

A revealing 1995 experiment perfused isolated rat collecting ducts and combined measured water flow with electron microscopy locating labeled channels. During vasopressin exposure, water permeability and channel abundance at the surface increased together. Withdrawal reversed both changes. Measuring function and channel location in the same tubules connected the physiological response to movement of channels from internal vesicles to the cell membrane.^[ref]