Endorphins are named for their endogenous morphinelike activity. These substances are produced in the hypothalamus from the precursor proopiomelanocortin (POMC) and have diverse activities, including regulation of temperature, appetite, mood, and behavior (30).
2.
Enkephalins are the most widely distributed opioid peptides in the brain, and they function primarily in regulation of the autonomic nervous system. Proenkephalin A is the precursor for the two enkephalins of primary importance: methionine–enkephalin and leucine–enkephalin.
3.
Dynorphins are endogenous opioids produced from the precursor proenkephalin B that serve a function similar to that of the endorphins, producing behavioral effects and exhibiting a high analgesic potency.
The endogenous opioids play a significant role in the regulation of hypothalamic–pituitary function. Endorphins appear to inhibit GnRH release within the hypothalamus, resulting in inhibition of gonadotropin secretion (31). Ovarian sex steroids can increase the secretion of central endorphins, further depressing gonadotropin levels (32).
Endorphin levels vary significantly throughout the menstrual cycle, with peak levels in the luteal phase and a nadir during menses (33). This inherent variability, although helping to regulate gonadotropin levels, may contribute to cycle-specific symptoms experienced by ovulatory women. For example, the dysphoria experienced by some women in the premenstrual phase of the cycle may be related to a withdrawal of endogenous opiates (34).
Anterior Pituitary The anterior pituitary is responsible for the secretion of the major hormone-releasing factors—FSH, LH, TSH, and ACTH—as well as GH and prolactin. Each hormone is released by a specific pituitary cell type.
The gonadotropins FSH and LH are produced by the anterior pituitary gonadotroph cells and are responsible for ovarian follicular stimulation. Structurally, there is great similarity between FSH and LH (Fig. 7.6). They are both glycoproteins that share identical α subunits
Figure 7.6 The structural similarity between follicle-stimulating hormone (FSH), luteinizing hormone (LH), and thyroid-stimulating hormone (TSH). The α subunits are identical, and the β subunits differ.
and differ only in the structure of their β subunits, which confer receptor specificity (35,36). The synthesis of the β subunits is the rate-regulating step in gonadotropin biosynthesis (37). Thyroid-stimulating hormone and placental hCG also share identical α subunits with the gonadotropins. There are several forms of each gonadotropin, which differ in carbohydrate content as a result of posttranslation modification. The degree of modification varies with steroid levels and is an important regulator of gonadotropin bioactivity.
Prolactin, a 198–amino acid polypeptide secreted by the anterior pituitary lactotroph, is the primary trophic factor responsible for the synthesis of milk by the breast (38). Several forms of this hormone, which are named according to their size and bioactivity, are normally secreted (39). Prolactin gene transcription is principally stimulated by estrogen; other hormones promoting transcription are TRH and a variety of growth factors.