Human Physiology, 14th edition (2016)

Endocrine Glands 335

The anterior pituitary secretion of growth hormone is under

the control of two polypeptide hormones from the hypothala-

mus. The secretion of growth hormone–releasing hormone

(GHRH) by the hypothalamus stimulates the anterior pituitary

to secrete growth hormone, whereas somatostatin from the

hypothalamus inhibits growth hormone secretion. A prolactin-

inhibiting hormone, identified as the neuro-transmitter dopa-

mine, inhibits the secretion of prolactin from the anterior pituitary.

This is the most physiologically important regulator of prolactin

secretion, although several factors (including oxytocin and TRH)

have been shown to promote prolactin secretion when dopamine

release declines.

Feedback Control of the Anterior Pituitary

In view of its secretion of releasing and inhibiting hormones,

the hypothalamus might be considered the “master gland.” The

chain of command, however, is not linear; the hypothalamus

and anterior pituitary are controlled by the effects of their own

actions. In the endocrine system, to use an analogy, the general

takes orders from the private. The hypothalamus and anterior

pituitary are not master glands because their secretions are

controlled by the target glands they regulate.

Anterior pituitary secretion of ACTH, TSH, and the gonad-

otropins (FSH and LH) is controlled by negative feedback

inhibition from the target gland hormones. Secretion of ACTH

is inhibited by a rise in corticosteroid secretion, for example,

and TSH is inhibited by a rise in the secretion of thyroxine from

the thyroid. These negative feedback relationships are easily

demonstrated by removal of the target glands. Castration (surgi-

cal removal of the gonads), for example, produces a rise in the

secretion of FSH and LH. In a similar manner, removal of the

adrenals or the thyroid results in an abnormal increase in ACTH

or TSH secretion from the anterior pituitary.

The effects of removal of the target glands demonstrate

that, under normal conditions, these glands exert an inhibi-

tory effect on the anterior pituitary. This inhibitory effect

can occur at two levels: (1) the target gland hormones can

act on the hypothalamus to inhibit the secretion of releasing

Figure 11.15 Hypothalamic control of the anterior
pituitary. Neurons in the hypothalamus secrete releasing
hormones (shown as green spheres) into the blood vessels of
the hypothalamo-hypophyseal portal system. These releasing
hormones stimulate the anterior pituitary to secrete its hormones
(pink spheres) into the general circulation.

Axons to primary

capillaries

Primary

capillaries

Portal venules

Cell body

Median

eminence

Pituitary stalk

Anterior

pituitary

Releasing

hormones

Secondary

capillaries

Posterior pituitary

Anterior

pituitary

hormones

the liver; chapter 18, section 18.5.) The vascular link between
the hypothalamus and the anterior pituitary is thus called the
hypothalamo-hypophyseal portal system.
Regulatory hormones are secreted into the hypothalamo-
hypophyseal portal system by neurons of the hypothalamus. These
hormones regulate the secretions of the anterior pituitary ( fig. 11.15
and table  11.7 ). Thyrotropin-releasing hormone (TRH) stimu-
lates the secretion of TSH, and corticotropin-releasing hormone
(CRH) stimulates the secretion of ACTH from the anterior pitu-
itary. A single releasing hormone, gonadotropin-releasing
hormone, or GnRH, stimulates the secretion of both gonadotropic
hormones (FSH and LH) from the anterior pituitary.

Hypothalamic Hormone Structure Effect on Anterior Pituitary

Corticotropin-releasing hormone (CRH) 41 amino acids Stimulates secretion of adrenocorticotropic hormone (ACTH)

Gonadotropin-releasing hormone (GnRH) 10 amino acids Stimulates secretion of follicle-stimulating hormone (FHS)

and luteinizing hormone (LH)

Prolactin-inhibiting hormone (PIH): dopamine Catecholamine Inhibits prolactin secretion

Somatostatin 14 amino acids Inhibits secretion of growth hormone

Thyrotropin-releasing hormone ( TRH) 3 amino acids Stimulates secretion of thyroid-stimulating hormone (TSH)

Growth hormone-releasing hormone (GHRH) 44 amino acids Stimulates growth hormone secretion

Table 11.7 | Hypothalamic Hormones Involved in the Control of the Anterior Pituitary