A. Location of Releasing Hormones in the Central Nervous System

The location of the releasing hormones in various cells of the CNS has been accomplished frequently by the use of fluorescently labeled monospecific antibodies and microscopic detection of fluorescence. The localization of TRH by this method is to the nerve terminals of the hypothalamic nuclei of the dorsomedial nucleus, the paraventricular nucleus, and the periforni-cal region. There are some other locations for TRH outside of the hypothalamus, suggesting a more diversified role for this peptide. Thus, the TRH neuroendocrine system has its cell bodies in the periventricular region, and these neurons extend to the median eminence. The cell bodies are located in an area described as the thyrotropic area. Thus, TRH is widely distributed in the hypothalamus and also occurs in extrahy-pothalamic parts of the CNS and in many peripheral tissues and organs. It may coexist with other neurotransmitters and neuromodulators in brain neurons. In addition to releasing TSH, it also releases PRL and norepinephrine.

GnRH is present in neurons located in the external layer of the median eminence closest to the stalk. It is located caudally from this area of the median eminence as well.

Somatostatin (GIH)-containing neurons reside in the median eminence, the periventricular nuclei of the hypothalamus, the peripheral nervous system, and the gut. GIH has also been located in cells of the gastroent-eropancreatic endocrine system (see Chapter 7) and in the thyroid, as well as in several areas of the CNS.

3. Hypothalamic Releasing Hormones TRH

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