Regulation of synthesis and release of hypothalamic hypophysiotropic neurohormones

Abstract

There are now numerous indications that stimuli from the external and internal environment operate mainly through the hypothalamus to alter secretion of pituitary hormones. These stimuli appear to modify the secretion of hypophysiotropic neurohormones in the hypothalamus. Thus cold temperature results in increased hypothalamic TRF content and elevated TSH levels in the plasma; suckling depresses hypothalamic PIF content and increases prolactin secretion; constant light results in enhanced hypothalamic content of FSH-RF and enhanced FSH release; etc. Hormones secreted by the gonads, thyroid, or adrenals may act through the hypothalamus, directly on the pituitary or on both to alter secretion of pituitary hormones. Thus estradiol can directly stimulate the pituitary to increase release of prolactin and LH, but can also influence the hypothalamus to depress secretion of PIF and LRF. Thyroxine appears to depress pituitary secretion of TSH only by a direct action on the pituitary and has no effect on hypothalamic TRF levels. Testosterone appears to decrease FSH secretion only through the hypothalamus, by depressing FSH-RF, and has no direct action on the pituitary. In addition, pituitary hormones appear to be able to inhibit their own secretion by acting on the hypothalamic neurohormones. Thus hypothalamic implants of ACTH, FSH, or LH probably depress secretion of their respective neurohormones by the hypothalamus, whereas hypophysectomy results in increased secretion of these neurohormones. Prolactin implants increase hypothalamic content of PIF but depress secretion of prolactin by the pituitary. The hypothalamic hypophysiotropic neurohormones represent a sensitive mechanism by which environmental agents can influence secretion of pituitary hormones to meet the body's needs. The specific sites in the hypothalamus where these neuro-hormones are secreted, or the mechanisms of their synthesis and release by neuro-secretory cells, are not yet completely understood. The pathways by which environmental agents reach the hypothalamus also remain to be delineated. We know very little as yet about the mechanisms by which hypothalamic neurohormones act on the pituitary to stimulate or depress secretion of hormones. Although the hypophysiotropic neurohormones appear to be separate from each other, we do not yet know their definitive chemical structures. Also much more remains to be learned about their control by enteroceptive and exteroceptive stimuli. Despite these deficiencies in our present knowledge, further rapid progress can be expected toward elucidating the role of these newly discovered brain hormones.