Qualitative and quantitative criteria indicate that TRH is present in methanolic extracts prepared from all regions of the rat brain. The conclusion that TRH in the extracts of rat brain is the same as synthetic TRH is based on the following criteria: (a) immunoreactivity with a specific antiserum to synthetic TRH, (b) chromatographic properties on Sephadex G-10, (c) electrophoretic mobility on cellulose acetate, (d) susceptibility to degradation by plasma, and (e) capacity of the extracts to stimulate TSH release in vitro from anterior pituitary tissue. The greatest concentration of TRH, determined by radioirnmunoassay, was found in the hypothalamus, followed by the posterior pituitary gland, thalamus, brain stem, cerebrum, anterior pituitary gland, and cerebellum. The total quantity of TRH in the rat brain was 18.2 ng. Of this amount, 33.6% was present in the thalamus; 26.7% in the cerebrum; 18.8% in the hypothalamus; 18.6% in the brain stem; 1.6% in the cerebellum; 0.42% in the posterior pituitary gland; and 0.09% in the anterior pituitary gland. On the basis of these findings, it is concluded that significant quantities of TRH are present in the extrahypothalamic regions of the brain as well as in the hypothalamic regions.
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