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Relationship between Phase Resetting and the Free-Running Period in Djungarian Hamsters

Department of Biology, Wesleyan University, Middletown, Connecticut 06459-0170

G. Robert Lynch

Department of Biology, Wesleyan University, Middletown, Connecticut 06459-0170

A data set of 293 phase shifts was analyzed in order to determine the relationship between phase resetting and the free-running period () in Djungarian hamsters. Phase shifts in response to a 15-min light pulse were assigned to one of two groups (_shot, <24 hr; _long, >24 hr), and two phase response curves (PRCs) were constructed. The two PRCs differed predominantly in the advance region, which extended so far into the subjective day of PRC_long that a dead zone was lacking. The functional significance of PRC differences was assessed by computer simulations of entrainment to varying skeleton photoperiods and entrainment to a 12-hr skeleton photoperiod with varying 's. Results from these simulations confirmed the theoretical predictions by Pittendrigh and Daan: Stability of entrainment under varying photoperiods depended on the ratio of the PRC slopes at the phases illuminated by light (S_E/S _M). This ratio was always larger than 1 for PRC_long. It approached 0 for PRC_short as soon as the evening light illuminated the dead zone; this occurred for entrainment to very short photoperiods. Stability of entrainment to lights-off was in general better for PRC_long than for PRC_short, especially if PRC_long was used in combination with _long This suggests that it can be advantageous for stability of entrainment to lights-off to express a > 24 hr in combination with a PRC lacking a dead zone. Stability of entrainment under varying 's was not much different for PRC_long or PRC _short. However, stability of entrainment deteriorated for PRC _long in combination with short 's, whereas it deteriorated for PRC_short in combination with long 's.

Key Words: entrainment • phase response curve • free-running period • Phodopus sungorus

Journal of Biological Rhythms, Vol. 7, No. 1, 75-83 (1992)
DOI: 10.1177/074873049200700107


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