Photic phase response curve in Octodon degus: assessment as a function of activity phase preference

Light exposure during the early and late subjective night generally phase d elays and advances circadian rhythms, respectively. However, this generalit y was recently questioned in a photic entrainment; study in Octodon degus. Because degus can invert their activity phase preference from diurnal to no cturnal as a function of activity level, assessment of phase preference is critical for computations of phase reference [circadian time (CT) 0] toward the development of a photic phase response curve. After determining activi ty phase preference in a 24-h light-dark cycle (LD 12:12), degus were relea sed in constant darkness. In this study, diurnal (n = 5) and nocturnal (n = 7) degus were randomly subjected to 1-h light pulses (30-35 lx) at many ci rcadian phases (CT 1-6: n = 7; CT 7-12: n = 8; CT 13-18: n = 8; and CT 19-2 4: n = 7). The circadian phase of body temperature (Tb) onset was defined a s CT 12 in nocturnal animals. In diurnal animals, CT 0 was determined as Tb onset + 1 h. Light phase delayed and advanced circadian rhythms when deliv ered during the early (CT 13-16) and late (CT 20-23) subjective night, resp ectively. No significant phase shifts were observed during the middle of th e subjective day (CT 3-10). Thus, regardless of activity phase preference, photic entrainment of the circadian pacemaker in Octodon degus is similar t o most other diurnal and nocturnal species, suggesting that entrainment mec hanisms do not determine overt diurnal and nocturnal behavior.