The control of metabolism, growth, and reproduction is mediated by a combination of neural and endocrine systems located in the hypothalamus and pituitary gland. The pituitary weighs about 0.6 g and rests at the base of the brain in the bony sella turcica near the optic chiasm and the cavernous sinuses. The pituitary consists of an anterior lobe (adenohypophysis) and a posterior lobe (neurohypophysis) (Figure 37–1). It is connected to the overlying hypothalamus by a stalk of neurosecretory fibers and blood vessels, including a portal venous system that drains the hypothalamus and perfuses the anterior pituitary. The portal venous system carries small regulatory hormones (Figure 37–1, Table 37–1) from the hypothalamus to the anterior pituitary.
The posterior lobe hormones are synthesized in the hypothalamus and transported via the neurosecretory fibers in the stalk of the pituitary to the posterior lobe; from there they are released into the circulation.
DA Posterior pituitary Endocrine glands, liver, bone, & other tissues Portal venous system Anterior pituitary +–HypothalamusSST Oxytocin ADH PRL GHRH TRH CRH GnRH GH TSH ACTH LH FSH
FIGURE 37–1 The hypothalamic-pituitary endocrine system. Hormones released from the anterior pituitary stimulate the production of hormones by a peripheral endocrine gland, the liver, or other tissues, or act directly on target tissues. Prolactin and the hormones released from the posterior pituitary (vasopressin and oxytocin) act directly on target tissues. Hypothalamic factors regulate the release of anterior pituitary hormones. ACTH, adrenocorticotropin; ADH, antidiuretic hormone [vasopressin]; CRH, corticotropin-releasing hormone; DA, dopamine; FSH, follicle-stimulating hormone; GH, growth hormone; GHRH, growth hormone-releasing hormone; GnRH, gonadotropin-releasing hormone; LH, luteinizing hormone; PRL, prolactin; SST, somatostatin; TRH, thyrotropin-releasing hormone; TSH, thyroid-stimulating hormone.
Drugs that mimic or block the effects of hypothalamic and pituitary hormones have pharmacologic applications in three primary areas: (1) as replacement therapy for hormone deficiency states; (2) as antagonists for diseases caused by excess production of pituitary hormones; and (3) as diagnostic tools for identifying several endocrine abnormalities.
All the hormones produced by the anterior pituitary except prolactin are key participants in hormonal systems in which they regulate the production of hormones and autocrine-paracrine factors by endocrine glands and other peripheral tissues. In these systems, the secretion of the pituitary hormone is under the control of one or more hypothalamic hormones. Each hypothalamic-pituitary-endocrine gland system or axis provides multiple opportunities for complex neuroendocrine regulation of growth and development, metabolism, and reproductive function.