A circadian rhythm of somatostatin messenger RNA levels, but not of vasoactive intestinal polypeptide/peptide histidine isoleucine messenger RNA levels in rat suprachiasmatic nucleus.

Circadian profiles of somatostatin and vasoactive intestinal polypeptide/peptide histidine isoleucine (VIP/PHI) messenger RNA levels in the suprachiasmatic nucleus (SCN) of blinded rats were measured by Northern analysis using synthesized oligonucleotide probes; the SCN is the site of a circadian oscillator. Somatostatin mRNA level in the SCN had a clear circadian rhythm with the peak around the onset of the subjective day and the trough around the onset of the subjective night. This rhythmicity was not observed in the adjacent anterior hypothalamus or the cortex and was thus specific to the SCN. No significant circadian fluctuation was observed in VIP/PHI mRNA levels in the SCN in this experiment. These results indicate the presence of an intrinsic circadian rhythm in the mRNA content of somatostatin, one of the peptides in dorsomedial SCN, and suggest a potential role of the peptide in circadian oscillation and the possible involvement of the transcriptional process in circadian rhythm generation in the SCN.