Co-localization of preprosomatostatin mRNA and preprotachykinin A mRNA in neurons of the rat suprachiasmatic nucleus.
The suprachiasmatic nucleus (SCN), a circadian oscillator, contains various peptides arranged in the compartment-specific manner. Somatostatin (SS) and substance P (SP), a peptide derived from preprotachykinin A (PPT-A), are expressed in neurons in the intermediate zone, a narrow area between the major dorsomedial and ventrolateral subdivisions. In the present study, we examined the possibility of co-localization of SS and SP in the SCN by a double-labeling in situ hybridization method using 35S-labeled and digoxigenin-labeled cRNA probes. In the SCN, most of preprosomatostatin (PPSS) mRNA-containing neurons expressed PPT-A mRNA (86%) and, in turn, almost all preprotachykinin A (PPT-A) mRNA-expressing neurons expressed PPSS mRNA signals (97%). Both PPSS and PPT-A mRNAs were also detected in the cerebral cortex and the caudate-putamen, however, their co-existence was extremely rare (< 4%) in these regions. Since the pharmacological effects of SS and SP are similar to that of the light pulses exposed on animals under constant darkness, the co-release of peptides might be an important process for entraining the circadian clock in the SCN.