Period-dependent oscillatory free-run in the locomotor activity rhythm of the field mouse mus booduga under skeleton photoperiodic regimes

Abstract

The entrainment behaviour of the circadian rhythm of locomotor activity in the field mouse Mus booduga was studied in order to evaluate the role of the animals' free-running period (t) and the duration of skeleton photoperiods in determining entrainment of animals with t values beyond and close to the "limits of entrainment". We predicted that animals with t lesser than the lower "limit of entrainment" would entrain only to short skeleton photoperiods (= 6 h) and not to longer skeleton photoperiods. Experimental animals (n = 25) were entrained to light/dark (LD) 12:12 h schedule, and then subjected to various skeleton photoperiods in which the duration of one of the two intervals of darkness was successively reduced while holding the zeitgeber period (T) constant. Some animals (n = 9) entrained to long as well as short photoperiods, whereas others (n = 5) entrained only to extremely short skeleton photoperiods of 6 h or less. The mean t of the animals entraining to all photoperiods (23.78 ¥ 0.22 h) was significantly greater than that of the animals that entrained only to very short skeleton photoperiods (22.43 ¥ 0.41 h) (t _{df 12} = 5.3, p < 0.001). We also selected a few animals (n = 11) with average t value of 23.13 ± 0.38 h and studied them under several skeleton photoperiods. To our surprise the animals which were subjected to restricted dark intervals invariably underwent "phase-jump" assuming the longer dark interval as "subjective night". We suggest that the observed variation in entrainment behaviour might be due to the variation seen among individual animals in t and the shape of their PRC. These results support the view that the duration of the skeleton photoperiod and the t of an individual animal interact to determine its entrainment, and underscore the relevance of inter-individual variation in circadian organisation to studies of circadian rhythms.