Emergence of VIP rhythmicity following somatostatin depletion in the rat suprachiasmatic nucleus.

Administration of a somatostatin (SS) depletor, cysteamine, markedly reduced SS levels in rat suprachiasmatic nucleus (SCN). At the same time, cysteamine administration induced a circadian rhythm of vasoactive intestinal polypeptide (VIP) content in the SCN, which otherwise remains constant under constant environmental conditions. These results suggest that the stable level of VIP in the SCN under constant conditions is not an intrinsic property of VIP neurons but a consequence of interactions with other components in the SCN.