Signals to the hypothalamus from either outside or within the body may be filtered and transmitted through central nervous system neurons. Primary inputs to the hypothalamus are from the limbic system, brain stem reticular formation, thalamus, subthalamus, basal ganglia, retina, and possibly the neocortex. These neural signals, along with chemical signals (hormones) from the peripheral glands, affect the hypothalamic secretion of either releasing or release-inhibiting hormones of which the primary ones are listed in Table 1. The hypothalamic hormonal molecules are transported to the anterior pituitary where they bind to specific receptors on the membranes of their target cells in the anterior pituitary, activating the appropriate intracellular signaling system(s) to bring about the secretion (or inhibition) of the hormone for which that cell is responsible. The secretion pattern of several hypothalamic (and therefore pituitary) hormones is pulsatile, as illustrated by the individual examples we will discuss in the following paragraphs.
Table1. Hypothalamic Releasing and Release-Inhibiting Hormones
As shown in Figure 1, the anterior pituitary is the source of several polypeptide hormones, each of which is controlled by at least one hypothalamic input (positive, negative or both). Taken together, these hormones gov ern a wide variety of important bodily functions. They are: growth hormone (GH, somatotropin), which stimulates growth, particularly that of the skeleton, and often acts through somatomedins (insulin-like growth factors, IGFs) produced in the liver and elsewhere; thyroid stimulating hormone (thyrotropin, TSH), which stimulates the thyroid gland to secrete thyroxine (T4) and triiodothyronine (T3) for control of metabolism, development and differentiation; luteinizing hormone (LH) and follicle stimulating hormone (FSH), which play major roles in reproductive functions; adrenocorticotropic hormone (corticotropin, ACTH), a principal mediator of the stress adaptation through its stimulation of adrenocortical steroid hormones; prolactin (PRL), important for the synthesis of milk constituents during lactation and may have other actions. Not shown in Figure 1 are some important anterior pituitary hormones, which will be discussed later in the chapter: melanocyte-stimulating hormone (melanotropin, MSH), which plays a role in skin-darkening reactions; β-lipotropin (β-LPH), which through its proteolytic products β-endorphin and Met enkephalin, may promote analgesia in stress and act as a neurotransmitter in signaling the release of other hormones. The sources and plasma half-lives of the trophic anterior pituitary hormones are summarized in Table 2.
Fig1. Summary of hypothalamic-pituitary-hormonal systems. Specific systems that operate according to the principles outlined in Figure 2 are shown. For each, the predominant hypothalamic releasing hormone (in green) or release-inhibiting factor (red) is shown. The main target tissues of the anterior pituitary hormones are indicated, along with the hormones they produce and, in the green boxes, major biological actions.
Fig2. Overview of hypothalamic-pituitary-peripheral systems. An archetypal cascade of hormonal events is shown, beginning with an external or internal environmental signal. These signals (green boxes) are transmitted to the central nervous system and then propagated to the hypothalamus, the anterior pituitary (adenohypophysis), and the peripheral target gland (pink boxes) for generating the appropriate biological response. The order of magnitude of the amount of hormone released in each of the three final steps is given in parentheses. The increasing amounts of hormones secreted at each step provide for amplification and the cascade is kept in check by both short (e.g., anterior pituitary to hypothalamus) and long (peripheral gland to hypothalamus) negative feedback loops.
Table2. Secretion of Anterior Pituitary Hormones
