D. Sidote et al., DIFFERENTIAL-EFFECTS OF LIGHT AND HEAT ON THE DROSOPHILA CIRCADIAN CLOCK PROTEINS PER AND TIM, Molecular and cellular biology, 18(4), 1998, pp. 2004-2013
Circadian (congruent to 24-h) rhythms are governed by endogenous bioch
emical oscillators (clocks) that in a wide variety of organisms can be
phase shifted (i.e., delayed or advanced) by brief exposure to light
and changes in temperature. However, how changes in temperature reset
circadian timekeeping mechanisms is not known, To begin to address thi
s issue, we measured the effects of short-duration heat pulses on the
protein and mRNA products from the Drosophila circadian clock genes pe
riod (per) and timeless (tint). Heat pulses at all times in a daily cy
cle elicited dramatic and rapid decreases in the levels of PER and TIM
proteins, PER is sensitive to heat but not light, indicating that ind
ividual clock components can markedly differ in sensitivity to environ
mental stimuli. A similar resetting mechanism involving delays in the
per-tint transcriptional-translational feedback loop likely underlies
the observation that when heat and light signals are administered in t
he early night, they both evoke phase delays in behavioral rhythms, Ho
wever, whereas previous studies showed that the light-induced degradat
ion of TIM in the late night is accompanied by stable phase advances i
n the temporal regulation of the PER and TIM biochemical rhythms, the
heat-induced degradation of PER and TIM at these times in a daily cycl
e results in little, if any, long-term perturbation in the cycles of t
hese clock proteins, Rather, the initial heat-induced degradation orf
FER and TIM in the late night is followed by a transient and rapid inc
rease in the speed of the PER-TIM temporal program. The net effect of
these heat-induced changes results in an oscillatory mechanism with a
steady-state phase similar to that of the unperturbed control situatio
n. These findings can account for the lack of apparent steady-state sh
ifts in Drosophila behavioral rhythms by heat pulses applied in the la
te night and strongly suggest that stimulus-induced changes in the spe
ed of circadian clocks can contribute to phase shifting responses.