Cycle of period gene expression in a diurnal mammal (Spermophilus tridecemlineatus): Implications for nonphotic phase shifting

Ground squirrels, Spermophilus tridecemlineatus, were kept in a 12:12 h lig ht-dark cycle. As expected for a diurnal species, their locomotor activity occurred almost entirely in the daytime. Expression of mPer1 and mPer2 in t he suprachiasmatic nucleus was studied at six time points by in situ hybrid ization. For both these genes, mRNA was highest in the first part of the su bjective day (about zeitgeber time 5). This is close to the time when mPer1 and mPer2 expression is maximal in nocturnal rodents. These results have i mplications for understanding nonphotic phase response curves in diurnal sp ecies and thereby for guiding research on nonphotic phase shifting in peopl e.