VISUAL SENSITIVITIES OF NUR77 (NGFI-B) AND ZIF268 (NGFI-A) INDUCTION IN THE SUPRACHIASMATIC NUCLEUS ARE DISSOCIATED FROM C-FOS INDUCTION AND BEHAVIORAL PHASE-SHIFTING RESPONSES

Mammalian circadian rhythms are regulated by a pacemaker in the suprac hiasmatic nucleus of the hypothalamus. Recent work from several labora tories has shown that light induces the IEGs, c-fos and jun-B, in the rodent suprachiasmatic nucleus. In hamsters, there is a strong correla tion between circadian entrainment and the induction of c-Sos and jun- B in the suprachiasmatic nucleus by light. Previous work has shown tha t the IEGs, nur77 and zif268, both of which encode transcription facto rs, are also light-inducible in the rat suprachiasmatic nucleus [Rusak , B., McNaughton, L., Robertson, H.A. and Hunt, S.P., Circadian variat ion in photic regulation of IEG mRNAs in rat suprachiasmatic nucleus c ells, Mel. Brain Res., 14 (1992) 124-130.; Sutin, E.L. and Kilduff, T. S., Circadian and light-induced expression of IEG mRNAs in the rat sup rachiasmatic nucleus, Mel. Brain Res., 15 (1992) 281-290.]. To charact erize the photic-regulation of these genes in the suprachiasmatic nucl eus of golden hamsters, we used in situ hybridization to measure nur77 and zif268 mRNA levels with P-33-labeIed complementary RNA probes. 5- min monochromatic light pulses at CT19 induced a dramatic increase in both nur77 and zif268 mRNA levels. Peak mRNA levels occurred 45-60 min after light onset for both nur77 and zif268. In addition, the inducti on of both nur77 and zif268 mRNA levels was gated by the circadian pac emaker. Light pulses during subjective day (CT3 and CT9), which do not cause behavioral phase-shifts, did not significantly alter mRNA level s of either nur77 or zif268; whereas light pulses during the subjectiv e night (CT14 and CT19), which induce phase-shifts, dramatically incre ased both nur77 and zif268 mRNA levels. In contrast to c-fos induction , which has a photic threshold indistinguishable from that of the beha vioral phase-shifting response, nur77 and zif268 mRNA induction were f ound to have Visual sensitivities greater than the phase-shifting resp onse by 1-2 log units (10-100-fold). Although light and circadian phas e regulate nur77 and zif268 expression in the SCN, these results demon strate that their induction is not rate-limiting for photic entrainmen t of the hamster circadian system. (C) 1997 Elsevier Science B.V.