Developmental changes of thyroliberin (TRH) in the rat brain.

Levels of thyroliberin (TRH) were measured by radioimmunoassay in the hypothalamus and in the extrahypothalamic brain tissue of rats from fetal day 12 up to day 47 of postnatal life. TRH was detectable as early as on day 12 of intrauterine life in both the hypothalamic primordia and the extrahypothalamic brain tissue. The increase of hypothalamic TRH content was marked between fetal days 12 and 15, then up to the end of gestation it exhibited a slower tendency of increase. From fetal day 15 up to postnatal day 2 the TRH content of the hypothalamus was always higher than that of the extrahypothalamic brain. Both hypothalamic and extrahypothalamic TRH content increased up to day 35 of postnatal life.