Olfactory Cues Accelerate Reentrainment following Phase Shifts and Entrain Free-Running Rhythms in Female Octodon degus (Rodentia)

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Abstract
Social interactions between conspecifics is a type of nonphotic zeitgeber common to several species. In the diurnal rodent Octodon degus, social interactions enhance reentrainment after phase shifts and can act as a weak zeitgeber. Olfactory stimuli appear necessary for these effects since bulbectomy eliminates socially enhanced reentrainment. In Experiment 1, the authors examined whether stimulation of the main olfactory system was sufficient to enhance reentrainment after 6-h phase advances and delays in the adult female O. degus. When test animals received conspecific odor cues during reentrainment, they entrained 39% faster after phase advances (p < 0.05) and 33% faster after phase delays (p < 0.001) than when they did not receive odor cues. Thus, olfactory cues from distant female donors were sufficient to enhance rates of entrainment in female O. degus and provided results equivalent to earlier studies with donors and shifters housed in the cages together. In Experiment 2, the authors examined whether discrete 3-h and 1-h daily pulses of airborne odors from a group of 5 entrained female degus would be sufficient to produce entrainment of wheel-running activity in adult female conspecifics. During the period of exposure to 3-h pulses, 50% (4/8) of the subjects temporarily entrained to a 24-h cycle, while 12.5% (1/8) of the subjects fully entrained. Exposure to 1-h pulses allowed 37.5% (3/8) of the subjects to temporarily entrain and 12.5% (1/8) of the subjects to fully entrain. Duration of entrained episodes was positively correlated with [.Psi], daily onset of activity with respect to the timing of odor exposure (Pearson r = 0.731; p < 0.05), such that animals with the entraining odor pulse beginning during subjective day ([.Psi] = 7.8 h, CT 7.8 ± 1.4) had longer periods of entrainment (22.2 ± 5.6 days) than animals with the entraining pulse occurring during subjective night ([.Psi] =–4.6h; CT 19.4 ± 0.9; 5.6 ± 0.9 days; p < 0.001). In addition, for each animal, the combined duration of all episodes of 24-h entrainment correlated with increased period length ([.tau]) of free-running rhythms (Pearson r = 0.733; p < 0.05). Thus, daily discrete pulses of odors with durations of either 1 or3hfrom female conspecifics were sufficient to produce both temporary and full entrainment to a 24-h cycle in the majority of female O. degus, and the likelihood of long periods of entrainment correlated with long taus and coordination of the odor pulse with mid subjective day.