CIRCADIAN CHANGES IN THE EXPRESSION OF VASOACTIVE-INTESTINAL-PEPTIDE-2 RECEPTOR MESSENGER-RNA IN THE RAT SUPRACHIASMATIC NUCLEI

The suprachiasmatic nuclei (SCN) in the hypothalamus function as the p rimary circadian pacemaker. A receptor for vasoactive intestinal pepti de (VIP), denoted as VIP2, is abundantly expressed in the SCN. Since t he rodent circadian clock demonstrates phase-dependent sensitivity to exogenous VIP, we investigated the possibility that VIP2 receptor mRNA is differentially expressed in the SCN across the 24 h cycle. To esta blish whether VIP2 receptor mRNA levels change across the 12:12 h ligh t-dark (LD) cycle (lights on designated as Zeitgeber time (ZT) 0), rat s were killed at ZT 0, 2, 6, 10, 12, 14, 18 and 22. To determine if va riation in this mRNA occurs in the absence of LD entrainment cues, Lig hts were not turned on at the time of transition from dark to light (d esignated as CT 0); the animals in this group were killed in constant darkness (DD) at CT 0, 2, 6, 10, 12, 14, 18 and 22. In situ hybridizat ion histochemistry indicated no variations in VIP2 receptor mRNA in th e cingulate cortex under either LD or DD conditions. There was, howeve r, significant variation in the expression of VIP2 receptor mRNA withi n the SCN during the LD cycle, with one peak at ZT 6 and at ZT 22. A c omparable biphasic pattern of mRNA expression was observed in DD anima ls with peaks at CT 10 and another at CT 22. The results suggest that the phase-dependent actions of VIP on the clock may involve phase-spec ific changes in the availability of VIP2 receptor within the SCN. (C) 1998 Elsevier Science B.V.