Somatostatin neurons form a distinct peptidergic neuronal group in the rat suprachiasmatic nucleus: a double labeling in situ hybridization study

Abstract

Accumulating evidence indicates that somatostatin (SS) is a key substance for the circadian rhythm of rodents. In the present study, we investigated whether SS mRNA coexists with arginine-vasopressin (AVP) mRNA, vasoactive intestinal peptide/peptide histidine isoleucine amide (VIP/PHI) mRNA and glutamate decarboxylase (GAD) mRNA in neurons of the rat suprachiasmatic nucleus (SCN) by double labeling in situ hybridization technique. SS mRNA-positive neurons were scattered in the whole region of rostral SCN, in the intermediate region between dorsomedial and ventrolateral region at the middle level, and in the mid to lateral region at the caudal level. These neurons were located in the close vicinities of the dorsomedial AVP and ventrolateral VIP/PHI mRNA-positive cell clusters. They rarely coexpressed AVP mRNA or VIP/PHI mRNA, but mostly coexpressed GAD mRNA. Thus, SS-synthesizing neurons are GABAergic and form a distinct cell group different from AVP or VIP/PHI cell groups.