Prolactin is a luteotrophic agent, acting from mid-luteal phase in both pregnant and non-pregnant animals. Its role in stimulating
and maintaing the corpus luteum, allowing the gestation period to be maintained, as well as the role in preparing
and maintaining lactation (in concert with estrogens, progesterone, corticosteroids, somatropin and insulin) in the bitch has
been recognized. Prolactin seems to be also involved in ensuring maternal behaviour, including the preparations for delivery
and the care of the litter thereafter.Among domestic animals, the dog (Canis familiaris) is a monoestric
species, with a long oestrous cycle (12 months in most
non-domesticated species; 7 ± 1.5 months in the dog) and
with a period of about 4 months of anestrus, in which there
is no ovarian activity. Reproduction and pregnancy in dogs
are regulated by many hormones, among which prolactin.
Prolactin (PRL) is a single-chain peptide hormone, composed
of almost 200 amino acids, produced by the lactotrotropic
cells of the anterior pituitary. Prolactin secretion is pulsatile
and this pulse rate is probably a result of the combined
effect of different hypothalamic factors.
Dopamine activatesprolactin synthesis. Also thyrotropin-releasing factor (TRH),
serotonin, vasoactive intestinal peptide (VIP), and estrogen
substances activate prolactin synthesis, but about 100 times
less than dopamine does. Gamma-aminobutyric acid (GABA)
inhibits the synthesis of prolactin with an inhibitory effect
100-fold lower than the activating effect of prolactin.
In dogs, prolactin is secreted in a pulsatile fashion and
most prolactin is released during the second half of the luteal
phase. Prolactin is an essential luteotropin in the dog from
mid-luteal phase in both pregnant and non-pregnant animals.
It appears to act by sustaining corpus luteum lifespan and
function rather than by direct stimulatory effects on progesterone
secretion. Indeed, prolactin stimulates and maintains
the corpus luteum of the ovary, which is the source of
progesterone, allowing the gestation period to be maintained
from the second half of the pregnancy. However, prolactin
does not act alone; during pregnancy, prolactin, estrogen and
progesterone stimulate the development of the mammary
gland.
Originally, the observation in the bitch that PRL blood values
rise during the second half of pregnancy, while progesterone
values were simultaneously declining, led to the conclusion
that PRL might have an anti-luteotropic effect. The use of
PRL inhibiting drugs, i.e. dopamine agonists like bromocriptine
and cabergoline during the second half of pregnancy have
shown that PRL must have a luteotrophic effect as their use
results in an immediate sharp drop in the blood PRL concentrations
followed by a drop in levels of blood progesterone. In addition, using the same PRL-inhibitors in the treatment
of dogs with clinical anoestrus (abnormally prolonged
anoestrous period) reliably induces a fertile oestrus.
In pregnant bitches, the plasma prolactin concentration starts
to rise about 1 month after ovulation, which is when the
plasma progesterone concentration begins to decline. Also in
healthy cyclic bitches, most prolactin is released during the
second half of the luteal phase. The changes in growth hormone
(GH) and prolactin release during the luteal phase may
promote the physiological proliferation and differentiation of
mammary gland tissue in the bitch. In the early part of the
luteal phase progesterone-induced mammary GH initiates proliferation of the mammary epithelium, whereas in the late
luteal phase, when progesterone concentrations decrease,
prolactin release increases and promotes lobulo-alveolar differentiation. Hence, the declining plasma progesterone
concentrations during the second half of the luteal phase appear
to influence prolactin secretion.
In monogastric animals, such as dogs, cats, primates and
women, prolactin is also fundamental during the preparation
and maintenance of lactation after birth. As a matter of fact,
after delivery and during lactation, prolactin levels continue to
be elevated.
Prolactin allows the growth and differentiation of the mammary
gland, maintains segregation of breast milk, reduces
fertility during the lactation period, facilitates immunological
regulation in the female, as well as an exchange of water
and electrolytes during pregnancy.
The tactile stimulationof the nipple or breast in the mother after birth inhibits the
release of dopamine into the hypothalamus, increasing the
concentration of prolactin in blood. On the other hand, high
concentrations of prolactin inhibit the secretion of gonadotropin-releasing
hormone (LH, luteinizing hormone and FSH,
follicle-stimulating hormone), which prevents gonadotropins
to act on the gonads. Thanks to this mechanism, fertility levels
are reduced during lactation, preventing females to become
pregnant while feeding their offspring.
In pregnant bitches, plasma prolactin levels exceed 100 ng/
ml. Weaning, which can be defined as the phase of maternal
care during which lactation decreases most rapidly,
causes a decrease in prolactin blood values.
The use of potent prolactin inhibitors, mostly dopamine agonists
like bromocriptine, metergoline and cabergoline, has
confirmed that prolactin is the luteotropic hormone from day
30 of pregnancy onward and that prolactin is essential for the
preparation, commencement and maintenance lactation, as
well as for the activation maternal of maternal and sexual behavior. Prolactin seems to be involved in ensuring maternal
behaviour, including the preparations for delivery and the
care of the litter thereafter, although it is not yet clear how it
shares these effects with oxytocin.
Prolactin secretion is under the tonic inhibitory control of hypothalamic
dopaminergic neurons and the stimulatory action
of estrogens, with a number of other hypothalamic hormones
playing a modulatory role in the control of prolactin secretion. In most vertebrates, neuroendocrine functions are
organized in regular cycles of different periodicity. Hormone
secretion patterns are mainly regulated endogenously, although
they are also under environmental influence.
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