CHANGES IN VASOACTIVE-INTESTINAL-PEPTIDE MESSENGER-RNA LEVELS IN THE RAT SUPRACHIASMATIC NUCLEUS FOLLOWING P-CHLOROPHENYLALANINE (PCPA) TREATMENT UNDER LIGHT-DARK CONDITIONS

Photic stimulus and serotonin (5-hydroxytryptamine; 5-HT) are two fact ors known to regulate vasoactive intestinal peptide (VIP) synthesis in the suprachiasmatic nucleus (SCN). To explore the role of 5-HT in the photic stimulus-induced change in VIP synthesis, we investigated the changes in level of VIP mRNA under a 12 h light/12 h dark cycle follow ing depletion of 5-HT by intraperitoneal administration of p-chlorophe nylalanine (PCPA) methyl ester (200 mg/kg concentration) for 3 success ive days. To estimate VIP mRNA expression, we performed in situ hybrid ization using imaging plates combined with microcomputer-based imaging analysis. In light-phase, total signals of VIP mRNA from the PCPA-tre ated rats showed a significant decrease compared with those from the s aline-treated control rats. However, in dark-phase, there were no sign ificant decreases between the PCPA-treated rats and the saline-control rats. The present results strongly suggest that 5-HT neuronal inputs to the SCN interfere with the effect of photic stimulus on VIP synthes is at the mRNA level.