Effects of continuous light and dark exposure on hypothalamic thyrotropin-releasing hormone in rats

The thyrotropin-releasing hormone (TRH) content in whole hypothalamus was measured by specific radioimmunoassay in rats exposed to a daily light-dark cycle (06:00–18:00 light, 18:00–06:00 dark) and in animals exposed to constant light or constant darkness for 12 days. A nyctohemeral variation of hypothalamic TRH was observed in light-dark exposed animals, while this variation was abolished by exposure either to constant light or constant darkness. These data indicate that any study involving hypothalamic TRH determination should be carried out taking into account the diurnal variation and the effect of environmental light exposure.