The Hypothalamus and Its Hormones

The hypothalamus is a small region located within the brain that controls many bodily functions, including eating and drinking, sexual functions and behaviors, blood pressure and heart rate, body temperature maintenance, the sleep-wake cycle, and emotional states (e.g., fear, pain, anger, and pleasure). Hypothalamic hormones play pivotal roles in the regulation of many of those functions.

Because the hypothalamus is part of the central nervous system, the hypothalamic hormones actually are produced by nerve cells (i.e., neurons). In addition, because signals from other neurons can modulate the release of hypothalamic hormones, the hypothalamus serves as the major link between the nervous and endocrine systems. For example, the hypothalamus receives information from higher brain centers that respond to various environmental signals. Consequently, hypothalamic function is influenced by both the external and internal environments as well as by hormone feedback. Stimuli from the external environment that indirectly influence hypothalamic function include the light-dark cycle; temperature; signals from other members of the same species; and a wide variety of visual, auditory, olfactory, and sensory stimuli. The communication between other brain areas and the hypothalamus, which conveys information about the internal environment, involves electrochemical signal transmission through molecules called neurotransmitters (e.g., aspartate, dopamine, gammaaminobutyric acid, glutamate, norepinephrine, and serotonin). The complex interplay of the actions of various neurotransmitters regulates the production and release of hormones from the hypothalamus.

The hypothalamic hormones are released into blood vessels that connect the hypothalamus and the pituitary gland (i.e., the hypothalamic-hypophyseal portal system). Because they generally promote or inhibit the release of hormones from the pituitary gland, hypothalamic hormones are commonly called releasing or inhibiting hormones. The major releasing and inhibiting hormones include the following.

• Corticotropin-releasing hormone (CRH), which is part of the hormone system regulating carbohydrate, protein, and fat metabolism as well as sodium and water balance in the body.
• Gonadotropin-releasing hormone (GnRH), which helps control sexual and reproductive functions, including pregnancy and lactation (i.e., milk production)
• Thyrotropin-releasing hormone (TRH), which is part of the hormone system controlling the metabolic processes of all cells and which contributes to the hormonal regulation of lactation
• Growth hormone-releasing hormone (GHRH), which is an essential component of the system promoting the organism’s growth 
•  Somatostatin, which also affects bone and muscle growth but has the opposite effect as that of GHRH
• Dopamine, a substance that functions primarily as a neurotransmitter but also has some hormonal effects, such as repressing lactation until it is needed after childbirth.

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