Page 72 - Clinical Small Animal Internal Medicine

40 Section 2 Endocrine Disease

Oxytocin Cerebral Cortex
VetBooks.ir Similar to VP, OT is produced in hypothalamic nuclei,
Oxytocin (OT) regulates lactation and parturition.
and transported to the posterior pituitary where it is
stored for release. OT release is stimulated by suckling,
during lactation, and in response to vaginal or cervical Hypothalamus
distension. Dopamine and opioids are also known to
modulate OT secretion from the posterior pituitary, as
well as extrareproductive factors such as stress. TRH
In the uterus, OT binds to its receptors and stimu-
lates the myometrium to contract. The sensitivity of
uterine receptors to circulating OT is variable during
different stages of reproduction. Receptor sensitivity is Pituitary
reduced during pregnancy, and greatly enhanced at
the time of delivery when OT stimulates contraction of
uterine smooth muscle and results in expulsion of
the newborn. This OT‐mediated uterine contraction TSH
is further augmented by the presence of estrogen,
which increases at parturition. Conversely, OT‐
mediated uterine contraction is inhibited by progester-
one, which is high throughout pregnancy but plummets Thyroid Gland
at parturition.
During lactation, OT release is triggered by suckling.
Circulating OT stimulates contraction of myoepithelial Thyroid
cells in mammary gland ducts, which results in milk Hormone
ejection. As for PRL, its function in the male is not fully
understood.
Target Tissue
Regulation of the Figure 5.3 Typical feedback control of hormone secretion in the
Neuroendocrine System neuroendocrine system. The hypothalamus releases thyrotropin‐
releasing hormone (TRH) in response to a stimulus. TRH stimulates
The neuroendocrine system tightly regulates hormone the anterior pituitary to releases thyroid‐stimulating hormone
(TSH). TSH targets cells in the thyroid gland to produce thyroid
production by means of feedback mechanisms, which hormone, which circulates to target cells and elicits a response
help ensure optimal hormone levels and activity. The (increase energy expenditure). Thyroid hormone exerts negative
hypothalamus and pituitary gland synthesize various feedback on the hypothalamus and pituitary to downregulate TRH
regulatory hormones – both releasing and inhibitory in and TSH production, respectively. Plus signs indicate stimulation
nature – that circulate to target endocrine glands (such of hormone production and release; minus signs indicate
inhibition hormone production and release.
as adrenal, thyroid, gonads) and modulate hormone
release. These hormones bind to receptors in target tis-
sues to generate a physiologic response, for example GH stimulates release of TSH, which then is transported in
stimulating growth in long bones. This pathway is the blood to the thyroid gland where it stimulates pro-
referred to as the hormone cascade, and it allows ampli- duction of thyroid hormone. As thyroid hormone levels
fication of a signal in a closed system. When sufficient rise above a certain threshold, there is negative feedback
amounts of target hormone are produced, the target hor- on production of TRH and TSH, which thereby decreases
mone exerts feedback on the hypothalamus and pituitary thyroid hormone production. Subsequently, as thyroid
gland to inhibit previous steps in the pathway. Often hormone levels decay, negative feedback is relieved
there is also negative feedback on the hypothalamus by on the hypothalamus and pituitary, and production of
pituitary hormones, a principle referred to as short‐loop thyroid hormone is once again restored through the
feedback. hormone cascade. Neuroendocrine regulation predomi-
An excellent example of typical feedback regulation is nantly relies on negative feedback, but there are instances
illustrated by the thyroid hormone pathway (Figure 5.3). when positive feedback is used in which target hormones
The hypothalamus produces TRH, which reaches influence the hypothalamus and pituitary to further aug-
the anterior pituitary via the portal circulation. Here, it ment their own production.