Diurnal changes in arginine vasopressin gene transcription in the rat suprachiasmatic nucleus.

The diurnal changes in arginine vasopressin (AVP) mRNA and heteronuclear (hn) RNA, an indicator for gene transcription, were examined in the hypothalamus of Sprague-Dawley rats using in situ hybridization. AVP hnRNA levels in the suprachiasmatic nucleus (SCN) varied during a 24-h cycle and showed a peak at day-time [Zeitgeber time (ZT) 5], which preceded the peak in AVP mRNA levels by 4 h. AVP hnRNA was undetectable at ZT 13 and 17, indicating that the gene transcription was almost shut down at these time points. AVP mRNA levels in the SCN continued to decrease at night (ZT 13, 17 and 21) when there were minimal changes in transcription, suggesting rapid turnover of mRNA. Similar diurnal changes in AVP hnRNA levels were observed without photic cues. On the other hand, AVP hnRNA or mRNA levels in the supraoptic nucleus, where AVP is synthesized in response to plasma osmolarity and/or volume, did not show any circadian rhythm. These data suggest that both dynamic changes in AVP gene transcription and rapid turnover of mRNA contribute to the diurnal variation in AVP mRNA levels in the SCN.