Alterations in levels of thyrotrophin releasing hormone and rates of hormone degradation in hypothalamic tissue of the developing rat.

Developmental changes in levels of hypothalamic thyrotrophin releasing hormone (TRH) and the role that hypothalamic degrading enzymes may play in these alterations were investigated. Levels of TRH in male and female fetal rats and in male neonatal rats were measured by radioimmunoassay. The hormone content of the hypothalamus was shown to increase from less than 1 ng at 1--4 days of age to approximately 10 ng at 20 days of age. Thereafter, the content of TRH declined to the adult level of about 5 ng. The ability of fractionated hypothalamic homogenates to degrade TRH was measured over the same time. The 27 000 g supernatant fraction contained a degrading activity that yielded only radiolabelled deamido-TRH upon incubation with [L-proline 2,3-3H]TRH. The corresponding particulate fraction contained at least two distinct TRH degrading activities as determined by the number of metabolites present. Changes in rates of degradation were not large enough to account for the differences observed in levels of TRH.