The hypothalamus is the source of two
hormones of the posterior lobe of the
pituitary (Table 36-1). They are formed
in neurosecretory cells in the hypothalamus,
whose axons extend down
the infundibular stalk and into the
posterior lobe. The hormones are
secreted from axon terminals ending
in close proximity to blood capillaries,
which the hormones enter when
released (see Figure 36-5). In a sense
the posterior lobe is not a true
endocrine gland, but a storage and
release center for hormones manufactured
entirely in the hypothalamus.
The two posterior lobe hormones of
mammals, oxytocin and vasopressin,
are chemically very much alike. Both
are polypeptides consisting of eight
amino acids and are called octapeptides
(Figure 36-7). These hormones
are among the fastest-acting hormones,
since they are capable of producing
a response within seconds of
their release from the posterior lobe.
|Figure 36-7 Posterior lobe hormones of mammals.
oxytocin and vasopress in consist of eight amino
(the two sulfur-linked cysteine molecules
a single amino acid, cystine).
Oxytocin and vasopressin are
for amino acid substitutions in the blue positions.
Abbreviations represent amino acids.
has two important specialized
reproductive functions in adult
female mammals. It stimulates contraction
of uterine smooth muscles during
parturition (birth of the young). In clinical
practice, oxytocin is used to
induce delivery during a difficult labor
and to prevent uterine hemorrhage
after birth. A second action of oxytocin
is that of milk ejection by the mammary
glands in response to suckling.
Recent work also has established a role
for oxytocin in pair-bonding behavior
in both sexes in monogamous voles.
, the second posterior
lobe hormone, acts on collecting ducts
of the kidney to increase water reabsorption
and thus restrict urine flow,
as already described on. It is
therefore often called antidiuretic
. Vasopressin also increases
blood pressure through its generalized
constrictor effect on smooth muscles
of arterioles. Finally, vasopressin acts
centrally to increase thirst, and therefore,
All jawed vertebrates secrete two
posterior lobe hormones that are quite
similar to those of mammals. All are
octapeptides, but their structures vary
because of amino acid substitutions in
three of eight amino acid positions in
One GnRH hormone regulates both FSH and LH, but some recent research experiments suggest a separate FSH-releasing hormone (FSH-RH).
Birds and some mammals lack an intermediate lobe. In these forms, MSH is produced by the anterior lobe.
In all vertebrate classes except mammals.
Of all posterior lobe hormones, vasotocin
(Table 36-1) has the widest
phylogenetic distribution and is believed
to be the parent hormone from
which other octapeptides evolved. It is
found in all vertebrate classes except
mammals. It is a water-balance hormone
in amphibians, especially toads, in which
it acts to conserve water by (1) increasing
permeability of skin (to promote
water absorption from the environment),
(2) stimulating water reabsorption from
the urinary bladder, and (3) decreasing
urine flow. Action of vasotocin is best understood in amphibians, but it appears
to play some water-conserving role in
birds and reptiles as well.