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Depolarization and Neurotransmitter Regulation of Vasopressin Gene Expression in the Rat Suprachiasmatic Nucleus In Vitro

Vasopressin (VP) transcription in the rat suprachiasmatic nucleus (SCN) in organotypic culture was studied by in situ hybridization histochemistry using an intron-specific VP heteronuclear RNA probe. The circadian peak of VP gene transcription in the SCN in vitro is completely blocked by a 2 h exposure to tetrodotoxin (TTX) in the culture medium, and this TTX inhibition of VP gene transcription is reversed by exposure of the SCN to either forskolin or potassium depolarization. This suggests that an intrinsic, spontaneously active neuronal mechanism in the SCN is responsible for the cAMP- and depolarization-dependent pathways involved in maintaining peak VP gene transcription. In this paper, we evaluate a variety of neurotransmitter candidates, membrane receptors, and signal-transduction cascades that might constitute the mechanisms responsible for the peak of VP gene transcription. We find that vasoactive intestinal peptide (VIP) and a VPAC2 (VIP receptor subtype 2) receptor-specific agonist, Ro-25-1553, are the most effective ligands tested in evoking a cAMP–mitogen-activated protein kinase signal transduction cascade leading to an increase in VP gene transcription in the SCN. In addition, a second independent pathway involving depolarization activating L-type voltage-gated calcium channels and a Ca-dependent kinase pathway [inhibited by KN62 (1-[N,O-bis(5-isoquinolinesulphonyl)-N-methyl-l-tyrosyl]-4-phenylpiperazine)] rescues VP gene transcription in the presence of TTX. In the absence of TTX, these independent pathways appear to act in a cooperative manner to generate the circadian peak of VP gene transcription in the SCN.