Patterns of wheel running are related to Fos expression in neuropeptide-Y-containing neurons in the intergeniculate leaflet of Arvicanthis niloticus

A variety of nonphotic influences on circadian rhythms have been documented in mammals. In hamsters, one such influence, running in a novel wheel, is mediated in part by the pathway extending from neuropeptide-Y (NPY)-contain ing cells within the intergeniculate leaflet (IGL) of the thalamus to the h ypothalamic suprachiasmatic nucleus (SCN). Arvicanthis niloticus is a speci es in which all individuals are diurnal with respect to general activity an d body temperature when they are housed without a running wheel, but access to a running wheel induces a subset of individuals to become nocturnal. In the first study, the authors evaluated the possibility that nocturnal and diurnal patterns of wheel running in Arvicanthis are correlated with differ ences in IGL function. Adult male Arvicanthis housed in a 12:12 light-dark (LD) cycle were monitored in wheels, classified as nocturnal or diurnal, an d then perfused either 4 h after lights-on or 4 h after lights-off. Section s through the intergeniculate leaflet were processed for immunohistochemica l labeling of Fos and NPY. The percentage of NPY cells that expressed Fos w as significantly influenced by an interaction between time of day and pheno type such that it rose from night to day in diurnal animals, and from day t o night in nocturnal animals. In the second experiment, the authors establi shed that running in a wheel actually induces Fos in the IGL of Arvicanthis . Specifically, the proportion of NPY cells expressing Fos was increased by access to wheels in nocturnal animals at night and in diurnal animals duri ng the day. In the third experiment, the authors established that lesions o f the IGL eliminate NPY fibers within the SCN, suggesting that these IGL ce lls project to the SCN in this species as has been established in other rod ents. Together, these data demonstrate a clear difference in NPY cell funct ion in nocturnal and diurnal Arvicanthis that appears to be caused, at leas t in part, by the differences in their wheel-running patterns, and that NPY cells within the IGL project to the SCN in Arvicanthis.