Abstract:
The entrainment of the circadian rhythm of locomotor activity was
studied in the field mouse Mus booduga in order to examine the relationship
between the free-running period (r) and minimum tolerable light pulse interval
of the skeleton photoperiods. The animals were entrained under three
different lightldark (LD) schedules, each out of phase with the other. They
were then subjected to various skeleton photoperiods created by two repeated
light pulses (LPs) interrupting darkness. Animals that selected the shorter
interval between the LPs as their “subjective night” had significantly shorter
z (23.13 f 0.38h) as compared to those that selected the longer dark interval
as subjective night (T = 23.87 k 0.18h). When the longer dark interval was
12h, animals selecting that interval as their subjective night included both
long-z and short-z individuals. When both intervals of darkness were of equal
duration, no difference in the selection of subjective night was seen between
short and long-r animals. When the “dusk” LP for the animals that selected
the longer dark interval as subjective night was advanced by 2h to create a
new skeleton photoperiod, the number of transient cycles appearing before
steady-state entrainment was found to depend on the duration of the photoperiods.
When the night defined by the two LPs was reduced below 6h, a
dramatic “phase jump” in the activity rhythm was observed, and the initial
phase relationship was restored after a relaxation in the night duration. We observed considerable interindividual variation in the “minimum tolerable
light pulse interval of skeleton photoperiods,” which we suggest may be due
to the observed variation in T among individuals.
