c-Fos rhythm in subdivisions of the rat suprachiasmatic nucleus under artificial and natural photoperiods

Recent studies have shown that the waveform of the rhythm of c-Fos photoind uction in the ventrolateral (vl) part of the suprachiasmatic nucleus (SCN) and that of the rhythm in the spontaneous c-Fos production in the dorsomedi al (dm) part of the SCN in rats released into constant darkness depend on t he photoperiod under which the animals were previously maintained. The aim of the present study was to find out how the rhythms of c-Fos immunoreactiv ity in both SCN subdivisions are affected by actual light-dark (LD) cycles with various photoperiods, either artificial or natural ones, that animals may usually experience. Rats were maintained under artificial LD cycles, wi th either a long (16-h photoperiod) or a short (8-h photoperiod) or under n atural daylight. In the latter case, c-Fos rhythms were followed in the sum mer when the photoperiod lasted about 16 h or in winter when it lasted only 8 h. The rhythms of c-Fos immunoreactivity under natural daylight did not differ significantly from those under corresponding artificial photoperiods . Under a long photoperiod, the morning c-Fos rise in the dm- as well as in the vl-SCN occurred about 4 h earlier than under a short one. In both SCN subdivisions, the interval when the nighttime c-Fos immunoreactivity was lo w, was shorter under a long than under a short photoperiod by roughly 6 h. The morning c-Fos rise in the dm-SCN always preceded that in the vl-SCN. Wh ereas in the former one the rise was due to the endogenous dm-SCN rhythmici ty, in the latter one the rise was induced by the morning light onset. The results show that whereas c-Fos rhythmicity under actual LD cycles is affec ted by the photoperiod in both SCN subdivisions, mechanism of c-Fos inducti on in the dm-SCN differs from that in the vl-SCN.