Thyrotropin-Releasing Hormone in the Hypothalamus of the Rat

The hypothalamic content of thyrotropin-releasing hormone (TRH) has been determined in normal and thyroxine-treated, thyroidectomized and hypophysectomized rats. Preliminary studies showed that hypothalamic and stalk median eminence homogenates destroyed TRH immunoreactivity, though neither was as effective as cerebral cortex in this regard. Consequently, TRH was extracted from hypothalamic homogenates with methanol. Recovery of TRH added to hypothalamic tissue ranged from 83.0 to 103.8%. The immunoreactivity of hypothalamic extracts was destroyed by incubation in human serum and eluted from Sephadex G-25 in the same fractions as synthetic TRH. In normal adult male rats of increasing size, the mean hypothalamic TRH contents ranged from 7.1 to 8.4 ng, not differing significantly in four groups of animals whose mean weights varied from 152 to 359 g. Hypothalamic TRH was 4.7 ± 0.2 ng/100 g body wt in the smallest and fell progressively to 2.2 ± 0.1 ng/100 g body wt in the largest animals. Serum T4 and TSH levels were similar in all groups. Single 5 or 25 μg/100 g body wt doses of T4 resulted in no changes in hypothalamic TRH in rats killed 4 and 24 hr later. Chronic (1, 2, and 4 weeks) daily T4 treatment (5 μg/100 g body wt) resulted in slight but not sustained reductions in hypothalamic TRH content. Serum T4 levels ranged from 11.3 to 13.1 μg/100 ml and TSH levels were undetectable in the treated animals. Hypothalamic TRH content was not altered in rats thyroidectomized 2 and 4 weeks previously. In hypophysectomized rats, hypothalamic TRH content was 54% of control after 2 weeks and 74% of control after 4 weeks. Maintenance of normal serum T4 levels in hypophysectomized rats did not restore body weight or hypothalamic TRH content. Significant increases in serum TSH in normal rats after exposure to 5 C for 20 to 60 min were not associated with changes in hypothalamic TRH content. These results show that hypothalamic TRH can be readily extracted and precisely quantitated and demonstrate that wide variations in serum T4 and TSH levels have little effect on hypothalamic TRH content. (Endocrinology94: 188, 1974)