CHAPTER 18
Hypothalamic Regulation of Hormonal Functions 279VASOPRESSIN & OXYTOCIN
IN OTHER LOCATIONS
Vasopressin-secreting neurons are found in the suprachias-
matic nuclei, and vasopressin and oxytocin are also found in
the endings of neurons that project from the paraventricular
nuclei to the brain stem and spinal cord. These neurons ap-
pear to be involved in cardiovascular control. In addition, va-
sopressin and oxytocin are synthesized in the gonads and the
adrenal cortex, and oxytocin is present in the thymus. The
functions of the peptides in these organs are unsettled.
Vasopressin Receptors
There are at least three kinds of vasopressin receptors: V
1A
,
V
1B
, and V
2
. All are G protein-coupled. The V
1A
and V
1B
re-
ceptors act through phosphatidylinositol hydrolysis to in-
crease the intracellular Ca
2+
concentration. The V
2
receptors
act through G
s
to increase cAMP levels.
Effects of Vasopressin
Because one of its principal physiologic effects is the retention
of water by the kidney, vasopressin is often called the
antidi-
uretic hormone (ADH).
It increases the permeability of the
collecting ducts of the kidney so that water enters the hyper-
tonic interstitium of the renal pyramids (see Chapter 38). The
urine becomes concentrated and its volume decreases. The
overall effect is therefore retention of water in excess of solute;
consequently, the effective osmotic pressure of the body fluids
is decreased. In the absence of vasopressin, the urine is hypo-
tonic to plasma, urine volume is increased, and there is a net
water loss. Consequently, the osmolality of the body fluid rises.
Effects of Oxytocin
In humans, oxytocin acts primarily on the breasts and uterus,
though it appears to be involved in luteolysis as well (see
Chapter 25). A G protein-coupled serpentine oxytocin recep-
tor has been identified in human myometrium, and a similar
or identical receptor is found in mammary tissue and the ova-
ry. It triggers increases in intracellular Ca
2+
levels.
The Milk Ejection Reflex
Oxytocin causes contraction of the
myoepithelial cells,
smooth-
muscle-like cells that line the ducts of the breast. This squeezes
the milk out of the alveoli of the lactating breast into the large
ducts (sinuses) and thence out of the nipple
(milk ejection).
Many hormones acting in concert are responsible for breast
growth and the secretion of milk into the ducts (see Chapter 25),
but milk ejection in most species requires oxytocin.
Milk ejection is normally initiated by a neuroendocrine
reflex. The receptors involved are the touch receptors, which are
plentiful in the breast—especially around the nipple. Impulses
generated in these receptors are relayed from the somatic touch
pathways to the supraoptic and paraventricular nuclei. Dis-
charge of the oxytocin-containing neurons causes secretion of
oxytocin from the posterior pituitary (Figure 18–8). The infant
suckling at the breast stimulates the touch receptors, the nuclei
are stimulated, oxytocin is released, and the milk is expressed
into the sinuses, ready to flow into the mouth of the waiting
infant. In lactating women, genital stimulation and emotional
stimuli also produce oxytocin secretion, sometimes causing
milk to spurt from the breasts.Other Actions of Oxytocin
Oxytocin causes contraction of the smooth muscle of the uter-
us. The sensitivity of the uterine musculature to oxytocin is
enhanced by estrogen and inhibited by progesterone. The in-
hibitory effect of progesterone is due to a direct action of the
steroid on uterine oxytocin receptors. In late pregnancy, the
uterus becomes very sensitive to oxytocin coincident with a
marked increase in the number of oxytocin receptors and ox-
ytocin receptor mRNA (see Chapter 25). Oxytocin secretion is
increased during labor. After dilation of the cervix, descent of
the fetus down the birth canal initiates impulses in the afferent
nerves that are relayed to the supraoptic and paraventricular
nuclei, causing secretion of sufficient oxytocin to enhance la-
bor (Figure 25-32). The amount of oxytocin in plasma is nor-
mal at the onset of labor. It is possible that the marked increase
in oxytocin receptors at this time causes normal oxytocin le-
vels to initiate contractions, setting up a positive feedback.
However, the amount of oxytocin in the uterus is also in-
creased, and locally produced oxytocin may also play a role.
Oxytocin may also act on the nonpregnant uterus to facilitate
sperm transport. The passage of sperm up the female genital
tract to the uterine tubes, where fertilization normally takes
place, depends not only on the motile powers of the sperm but
also, at least in some species, on uterine contractions. The geni-
tal stimulation involved in coitus releases oxytocin, but it has
not been proved that it is oxytocin which initiates the rather
specialized uterine contractions that transport the sperm. The
secretion of oxytocin is increased by stressful stimuli and, like
that of vasopressin, is inhibited by alcohol.
Circulating oxytocin increases at the time of ejaculation in
males, and it is possible that this increase causes increased
contraction of the smooth muscle of the vas deferens, propel-
ling sperm toward the urethra.CONTROL OF ANTERIOR
PITUITARY SECRETION
ANTERIOR PITUITARY HORMONES
The anterior pituitary secretes six hormones:
adrenocortico-
tropic hormone (corticotropin, ACTH), thyroid-stimulat-
ing hormone (thyrotropin, TSH), growth hormone, follicle-
stimulating hormone (FSH), luteinizing hormone (LH),