Open AccessReversal of photoschedule in spring does not prevent photorefractoriness in Siberian hamsters
Author(s) -
Jefimow Małgorzata,
Wojciechowski Michał S.,
Tęgowska Eugenia
Publication year - 2005
Publication title -
journal of experimental zoology part a: comparative experimental biology
Language(s) - English
Resource type - Journals
eISSN - 1552-499X
pISSN - 1548-8969
DOI - 10.1002/jez.a.225
Subject(s) - entrainment (biomusicology) , rhythm , phodopus , circadian rhythm , acclimatization , medicine , endocrinology , biology , brown adipose tissue , thermogenesis , thermoregulation , period (music) , photoperiodism , zoology , hamster , adipose tissue , ecology , physics , acoustics
We studied the influence of light–dark (L:D) cycle reversal on daily variations in the brown adipose tissue (BAT) capacity for nonshivering thermogenesis (NST) in Siberian hamsters ( Phodopus sungorus ). Continuous and simultaneous measurements of BAT temperature ( T BAT ) and preferred ambient temperature ( PT a ) were made after noradrenaline (NA) injections administered every 4 hr. First, hamsters were acclimated for 4 weeks to an ambient temperature ( T a ) of 23°C and 12L:12D, and then to a reversed photoschedule 12D:12L for 8 weeks. The same was done after a 4‐ and 8‐week acclimation period at the same T a . We found that after photoschedule reversal, the re‐entrainment of T BAT and PT a rhythms preceded re‐entrainment of the NST rhythm. The daily rhythms of T BAT and PT a were fully re‐entrained after 4 weeks of acclimation to the reversed photoschedule, but rhythmicity of the response to NA disappeared. This rhythm was restored in hamsters acclimated to a reversed photoschedule for 8 weeks. We suggest that the daily rhythm of NST capacity is not responsible for generating the rhythm of body temperature ( T b ). Rather, it is a result of the daily rhythm of T b , but adjusts to the new environment more slowly than the T b rhythm. When a daily rhythm of NST was present, the increase in T BAT after NA injection was inversely correlated with the pre‐injection T BAT . In addition, NA‐induced changes in PT a reflected the intensity of NST in BAT; namely, increased T BAT was correlated with the post‐injection decrease in PT a . When the increase in T BAT was large, animals chose a lower T a to dissipate excessive heat and prevent overheating. In the course of the experiments, we recorded a decreased mean NST capacity and increased body mass of hamsters. These changes are representative of the time of photorefractoriness and a transition to a summer status. Despite prolonged exposure to an intermediate day length (12 hr of light) and photoschedule reversal, hamsters continued to change towards their summer condition and were able to acclimate to the new D:L cycle. J. Exp. Zool. 303A:976–986, 2005 . © 2005 Wiley‐Liss, Inc.