Spontaneous c-Fos rhythm in the rat suprachiasmatic nucleus: location and effect of photoperiod

A recently reported circadian rhythm in the spontaneous c-Fos immunoreactiv ity in the rat suprachiasmatic nucleus (SCN) is expressed mostly in the dor somedial (dm) SCN, where vasopressinergic cells are located. The aim of the present study is to find out whether day length, i.e., photoperiod, affect s the dm-SCN rhythm and, if so, how the rhythm adjusts to a change from a l ong to a short photoperiod. In addition, a question of whether the spontane ous c-Fos production is localized in vasopressin-producing cells or in othe r cells is also studied to characterize further the dm-SCN rhythmicity. Com bined immunostaining for c-Fos and arginine vasopressin (AVP) revealed that most of c-Fos immunopositive cells were devoid of AVP; the results suggest that c-Fos-producing cells in the dm-SCN are mostly not identical with tho se producing AVP. In rats maintained under a long photoperiod with 16:8-h l ight-dark cycle (LD 16:8) daily and then released into darkness, the time o f the afternoon and evening decline of the spontaneous c-Fos immunoreactivi ty in the dm-SCN differed just slightly from the time in rats maintained or iginally under a short LD 8: 16 photoperiod; however, the morning c-Fos ris e occurred about 4 h earlier under the long than under the short photoperio d. After a change from a long to a short photoperiod, a rough but not yet a fine adjustment of the morning c-Fos rise to the change was accomplished w ithin 3-6 days. The results show that similar to the recently reported vent rolateral SCN rhythmicity, the intrinsic dm-SCN rhythmicity is also affecte d by the photoperiod and suggest that the whole SCN state is photoperiod de pendent.