Thyrotropin-releasing hormone in rat brain: nyctohemeral variations.

The concentration of thyrotropin-releasing hormone (TRH) was measured by specific radioimmunoassay in three different brain regions of rats sacrificed every 4 h over a 24 h period. TRH concentration reached a zenith at 1200 h both in the hypothalamus and the amygdala but did not vary in the forebrain. These results indicate that a nyctohemaral variation of TRH levels exists in hypothalamic and extrahypothalamic brain areas.