Releasing and Inhibiting Hormones

Since axons do not enter the anterior pituitary, hypothalamic control of the anterior pituitary is achieved through hormonal rather than neural regulation. Releasing and inhibiting hormones, produced by neurons in the hypothalamus, are transported to axon endings in the basal portion of the hypothalamus. This region, known as the median eminence (fig 11.15), contains blood capillaries that are drained by venules in the stalk of the pituitary.

The venules that drain the median eminence deliver blood to a second capillary bed in the anterior pituitary. Since this second capillary bed is downstream from the capillary bed in the median eminence and receives venous blood from it, the vascular link between the median eminence and the anterior pituitary forms a portal system. (This is analogous to the hepatic portal system that delivers venous blood from the intestine to the liver, as described in chapter 18.) 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)

■ Figure 11.14 Hormones secreted by the anterior pituitary and their target organs. Notice that the anterior pituitary controls some (but by no means all) of the other endocrine glands.

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Table ll.7 Hypothalamic Hormones Involved

in the Control of 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

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

-Cell body

Axons to primary capillaries

Primary capillaries

Primary capillaries

Pituitary stalk

Posterior pituitary

Anterior pituitary

Median eminence

Pituitary stalk

Posterior pituitary

Anterior

pituitary

hormones

■ Figure 11.15 Hypothalamic control of the anterior pituitary.

Neurons in the hypothalamus secrete releasing hormones (shown as dots) into the blood vessels of the hypothalamo-hypophyseal portal system. These releasing hormones stimulate the anterior pituitary to secrete its hormones into the general circulation.

stimulates the secretion of TSH, and corticotropin-releasing hormone (CRH) stimulates the secretion of ACTH from the anterior pituitary. A single releasing hormone, gonadotropin-releasing hormone, or GnRH, stimulates the secretion of both gonadotropic hormones (FSH and LH) from the anterior pituitary. The secretion of prolactin and of growth hormone from the anterior pituitary is regulated by hypothalamic inhibitory hormones, known as prolactin-inhibiting hormone (PIH) and somatostatin, respectively.

A specific growth hormone-releasing hormone (GHRH) that stimulates growth hormone secretion has been identified as a polypeptide consisting of forty-four amino acids. Experiments suggest that a releasing hormone for prolactin may also exist, but no such specific releasing hormone has yet been discovered.

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