CIRCADIAN MODULATION IN PHOTIC INDUCTION OF FOS-LIKE IMMUNOREACTIVITYIN THE SUPRACHIASMATIC NUCLEUS CELLS OF DIURNAL CHIPMUNK, EUTAMIAS-ASIATICUS

Photic induction of immediate early genes including c-fos in the supra chiasmatic nucleus (SCN) has been well demonstrated in the nocturnal r odents. On the other hand, in diurnal rodents, no data is available wh ether the light can induce c-fos or Fos in the SCN. We therefore exami ned whether 60 min light exposure induces Fos-like immunoreactivity (F os-lir) in the SCN cells of diurnal chipmunks and whether the inductio n is phase dependent, comparing with the results in nocturnal hamsters . We also examined an effect of light on the locomotor activity rhythm under continuous darkness. Fos-lir was induced in the chipmunk SCN. T he induction was clearly phase dependent. The light during the subject ive night induced strong expression of Fos-lir. This phase dependency is similar to that in hamsters. However, unlike in hamsters, the Fos-l ir was induced in some SCN cells of chipmunks exposed to light during the subjective day. In the locomotor rhythm, on the other hand, the li ght pulse failed to induce the phase shift at phases at which the Fos- lir was induced. These results suggest that the photic induction of Fo s-lir in the diurnal chipmunks is gated by a circadian oscillator as w ell as in the nocturnal hamsters. However, the functional role of Fos protein may be different in the diurnal rodents from in the nocturnal rodents.