To ascertain how photic stimuli disturbing overt circadian rhythms affect the endogenous rhythmicity of the suprachiasmatic nucleus (SCN), rats were subjected to constant light (LL) or to a 9-h light pulse encompassing midnight, and rhythms of abundance of the c-Fos-immunoreactive (c-Fos-ir) and the PER1-immunoreactive (PER1-ir) cells were studied during the first 1-2 cycles following release into LL or darkness (DD) within the whole SCN as well as in its ventrolateral (vl) and the dorsomedial (dm) part. LL seemingly abolished the c-Fos rhythm in the whole SCN, while the rhythm persisted in the vl- and dm-SCN. In the dm-SCN, the rhythm of c-Fos-ir was phase-delayed by about 4 h in LL, whereas the rhythm of PER1-ir was affected just slightly. In the vl-SCN, the rhythm of c-Fos photo-induction might be delayed by 5-6 h as compared with the reported rhythm [A. Sumova and H. Illnerova, Am. J. Physiol. 274 (1998) R857-R863], whereas the PER1 profile appeared to be out of phase with that in DD. After a 9-h light pulse encompassing midnight, the rhythm of PER1-ir in the dm-SCN changed just slightly, whereas the PER1 rhythm in the vl-SCN was abolished and there was just an indication of extension of elevated PER1-ir. Altogether, the data indicate that photic stimuli disturbing circadian rhythms affect more dramatically the vl- than the dm-SCN rhythmicity within the first cycles and that in the dm-SCN shifting of the c-Fos rhythm proceeds more rapidly than that of the Per1 rhythm.

Department of Neurohumoral Regulations Institute of Physiology Academy of Sciences of Czech Republic Videnska 1083 142 20 Prague 4 Czech Republic

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