Ganong's Review of Medical Physiology, 23rd Edition

280
SECTION III
Central & Peripheral Neurophysiology

and
prolactin (PRL).
An additional polypeptide,
β
-lipotropin
(
β
-LPH), is secreted with ACTH, but its physiologic role is un-
known. The actions of the anterior pituitary hormones are
summarized in Figure 18–9. The hormones are discussed in
detail in the chapters on the endocrine system. The hypothal-
amus plays an important stimulatory role in regulating the se-
cretion of ACTH,
β
-LPH, TSH, growth hormone, FSH, and
LH. It also regulates prolactin secretion, but its effect is pre-
dominantly inhibitory rather than stimulatory.

NATURE OF HYPOTHALAMIC CONTROL

Anterior pituitary secretion is controlled by chemical agents
carried in the portal hypophysial vessels from the hypothala-
mus to the pituitary. These substances used to be called releas-
ing and inhibiting factors, but now they are commonly called
hypophysiotropic hormones.
The latter term seems appro-
priate since they are secreted into the bloodstream and act at a
distance from their site of origin. Small amounts escape into

the general circulation, but they are in high concentration in

portal hypophysial blood.

HYPOPHYSIOTROPIC HORMONES

There are six established hypothalamic releasing and inhibiting

hormones (Figure 18–10):

corticotropin-releasing hormone

(CRH); thyrotropin-releasing hormone (TRH); growth hor-

mone-releasing hormone (GRH); growth hormone-inhibit-

ing hormone (GIH),

now generally called

somatostatin;

luteinizing hormone-releasing hormone (LHRH),

now gener-

ally known as

gonadotropin-releasing hormone (GnRH);

and

prolactin-inhibiting hormone (PIH).

In addition, hypotha-

lamic extracts contain prolactin-releasing activity, and a

prolac-

tin-releasing hormone (PRH)

has been postulated to exist.

TRH, VIP, and several other polypeptides found in the hypothal-

amus stimulate prolactin secretion, but it is uncertain whether

one or more of these peptides is the physiologic PRH. Recently,

an orphan receptor was isolated from the anterior pituitary, and

FIGURE 18–9
Anterior pituitary hormones.
In women, FSH and LH act in sequence on the ovary to produce growth of the ovarian follicle,
ovulation, and formation and maintenance of the corpus luteum. Prolactin stimulates lactation. In men, FSH and LH control the functions of the
testes.

Anterior pituitary

β-LPH ACTH hormoneGrowth TSH FSH LH Prolactin

? Breast

17-Hydroxy-

corticoids

Aldosterone,

sex hormones

Somato-

medins

Thyroxine Estrogen Progesterone

FIGURE 18–10
Effects of hypophysiotropic hormones on the secretion of anterior pituitary hormones.

Hypothalamus

CRH TRH

Anterior

pituitary

GnRH

GRH

GIH

PRH

PIH

β-LPH ACTH TSH LH FSH Growth Prolactin

hormone