Extrahypothalamic Distribution and Action of Hypothalamic Hormones

Abstract

The highest levels of hypothalamic hormones are found, as their name suggests, in the hypothalamus. When these peptides were first discovered, it was assumed that they were present solely in the hypothalamus. Improved, more sophisticated biochemical and immunocytochemical methods, however, have revealed that they are also present in extrahypothalamic brain areas, although at relatively low concentrations. Indeed it has been shown that part of the hypothalamic hormones are not even produced in the hypothalamus, the neurons responsible for their biosynthesis being situated outside the hypothalamus and the hormones transported in axons projecting to the hypothalamus. The definition and use of the term “hypothalamic hormones” has been further complicated by the finding that a variety of neurons which contain other neuropeptides β-lipotropin, β-endorphin, ACTH, α-MSH, substance P, neurotensin, enkephalin, VIP, and cholecystokinin, for example), which are not listed in the group of hypothalamic neurohormones, have also been detected in the hypothalamus (Elde and Hokfelt, 1978; Hokfelt et al., 1978a,b; Palkovits, 1980, 1981, 1982). These substances are all present in both the median eminence and the portal blood emanating from this region, so they might justifiably be termed neurohormones. However, these neuropeptides are not known as hypothalamic neurohormones, only releasing or release-inhibiting hormones and neurohypophyseal hormones being classified in this way.