CHAPTER 10: Neural and Neuroendocrine Control of the Internal Milieu

KEY CONCEPTS

• Hormones are signaling molecules that reach their target cells via the bloodstream.

• The hypothalamus is the key regulator of the endocrine system by releasing hormones directly into the blood and by regulating the anterior pituitary.

• The hypothalamic–neurohypophyseal (posterior pituitary) system secretes two peptide hormones directly into the blood, vasopressin and oxytocin.

• To regulate the anterior pituitary, hypothalamic neurons synthesize peptide releasing and inhibitory factors.

• The hypothalamic–pituitary–adrenal (HPA) axis comprises corticotropin–releasing factor (CRF; also called corticotropin–release hormone or CRH), released by the hypothalamus; adrenocorticotropic hormone (ACTH; also called corticotropin), released by the anterior pituitary; and glucocorticoids, released by the adrenal cortex.

• The hypothalamic–pituitary–thyroid axis consists of hypothalamic thyrotropin–releasing hormone (TRH), the anterior pituitary hormone thyroid–stimulating hormone (TSH; also called thyrotropin), and the thyroid hormones triiodothyronine (T₃) and tetraiodothyronine or thyroxine (T₄).

• The hypothalamic–pituitary–gonadal axis comprises hypothalamic gonadotropin–releasing hormone (GnRH), the anterior pituitary luteinizing hormone (LH) and follicle–stimulating hormone (FSH), and the gonadal steroids, which include testosterone, estrogen, and progesterone.

• Prolactin is synthesized and released by the anterior pituitary and triggers lactation in the breast; its release is inhibited by dopamine acting at D₂ dopamine receptors.

• Growth hormone, also released by the anterior pituitary, is under the stimulatory control of growth hormone–releasing hormone (GHRH) and inhibitory control of somatostatin, a peptide transmitter.

• The hypothalamus contains a complex network of peptide–releasing neurons that project broadly throughout the brain to regulate diverse functions such as feeding and energy expenditure, mood and motivation, and sleep. Hypothalamic neurons also utilize glutamate and GABA as neurotransmitters.

• Leptin, released by adipocytes, regulates the long–term state of the body’s energy reserves largely via its actions on the hypothalamus.

• The arcuate nucleus of the hypothalamus plays a key role in feeding and energy expenditure. It contains one set of neurons that express proopiomelanocortin (POMC) that suppresses feeding and another set that express neuropeptide Y (NPY) that stimulates feeding.

• The POMC product, melanocortin (also called α-melanocyte–stimulating hormone or MSH) and NPY, in turn, regulate several other feeding peptides (eg, melanin–concentrating hormone or MCH) to produce their ultimate effects on food intake and energy metabolism.

• The hypothalamus responds to inflammatory cytokines by causing fever and sickness behavior.