AJP - Regulatory, Integrative and Comparative Physiology, Vol 267, Issue 1 191-R201, Copyright © 1994 by American Physiological Society
Photoperiodic time measurement in Djungarian hamsters evaluated from T-cycle studies
W. Puchalski and G. R. Lynch
Department of Environmental, Population and Organismic Biology, University of Colorado, Boulder 80309-0334.
We investigated the photoperiodic response to T-cycles (0.5 h of light at
intervals ranging from 23.0 h to 25.3 h) of two phenotypes of Djungarian
hamsters that either exhibit or lack physiological short-day adjustments
under a photoperiod of 9 h light:15 h darkness. Illumination of the same
circadian time caused a similar photoperiodic response in both phenotypes.
Thus hamsters found to be insensitive under a full short-day photoperiod
can exhibit short-day adjustments after exposure to certain T-cycles. Given
these results we conclude that the absence of photoperiodic adjustments
normally found in short-day-insensitive hamsters results from their
atypical entrainment under a full short-day photoperiod. We further suggest
that the photoperiodic phenomena seen in Djungarian hamsters cannot be
adequately explained by an external coincidence model of photoperiodic time
measurement. As a more suitable model, we propose one form of internal
coincidence where duration of daily motor activity reflects the phasing of
multiple, endogenous oscillators. This conclusion is supported by the close
relationship between duration of activity and the photoperiodic response as
well as by the observation that a light pulse modulates duration of
activity in a phase-dependent manner.
