Free-running rhythms and light- and dark-pulse phase response curves for diurnal Octodon degus (Rodentia)

Only rarely have precise, short-duration light pulses been used to generate phase response curves (PRCs) in diurnal mammals as done for nocturnal mammals, and a dark-pulse PRC has never been generated for a diurnal mammal. In addition, the relationship between free-running rhythms in different light intensities and PRCs has not been explored in diurnal mammals. We examined these relationships in Octodon degus, a diurnal hystricomorph rodent. Male degus lengthened the circadian period (tau) and duration of daily activity (alpha) after an increase in light intensity from 0 (DD) to 250 lx, and tau was furthered lengthened when light intensity increased from 580 to 5,800 lx. To generate a light-pulse PRC, degus were housed in DD and exposed to 20-min light pulses (250 lx) and phase shifts recorded across the circadian day. Two different PRCs were generated in response to 20-min light pulses. The majority of animals produced significant phase delays between circadian time (CT) 0 and CT 6, phase advances between CT 13 and CT 22, and a nonsignificant response period between CT 8 and CT 13. Two animals produced a PRC devoid of significant phase delays, producing only significant phase advances between CT 17 and CT 24. To generate a dark-pulse PRC, animals were moved to LL (580 lx) and exposed to 1-h dark pulses. After dark pulses degus produced significant phase delays between CT 20 and CT 8, advances from CT 10 to CT 17, and nonsignificant responses between CT 18 and CT 20. This is the first report of a PRC to dark-pulse stimuli for a diurnal mammal. Thus light- and dark-pulse PRCs can be generated in a comparable way to those of nocturnal rodents, and we conclude that nocturnal and diurnal rodents use similar photic signals to produce somewhat different PRCs.