Effects of prolonged acclimation to intermediate photoperiod and photoschedule reversal in photosensitive golden hamsters

Abstract

We investigated the effect of prolonged acclimation to 12 hr of light and photoschedule reversal during the time of photosensitivity in golden hamsters (Mesocricetus auratus). Before the experiments, animals were housed under natural photoperiod and then transferred to 12L:12D (light 12 hr:dark 12 hr) in autumn for 12 weeks. After 4 weeks of acclimation, photoschedule was reversed (12D:12L). First experiments were done after 4 weeks of acclimation to an ambient temperature (Ta) of 23°C and a 12L:12D photoschedule. We examined the daily variations in brown adipose tissue (BAT) capacity for nonshivering thermogenesis (NST). Noradrenaline (NA) injections were given every 4 hr while BAT temperature (TBAT) and preferred ambient temperature (PTa) were monitored continuously and simultaneously in a thermal gradient system. Then, we investigated the effect of light-dark cycle reversal on a daily rhythm of NST. The hamsters were acclimated to the photoschedule reversed by 12 hr and the same Ta. After 4 and 8 weeks of acclimation to a reversed photoschedule, the experiments were repeated. We found that the daily rhythm of the response to NA was entrained to the new light-dark cycle after 4 weeks of acclimation to a reversed photoschedule. Maximum effect of NA was always recorded during the light phase and in the latter part of the dark phase of the day. NA-induced increase in TBAT was correlated with the decrease in PTa, and was also inversely correlated with pre-injection TBAT. These data imply that the daily rhythm of the capacity for NST opposes the daily rhythm of body temperature (Tb). After 8 weeks of acclimation to the reversed photoschedule, the rhythmicity of the response to NA disappeared, and the daily fluctuations in TBAT were the smallest. This lack of rhythm may be a physiological adaptation to winter conditions when the daily amplitude of Tb rhythm is markedly reduced and, as a consequence, NST capacity does not vary within the day. Moreover, after 8 weeks of acclimation to reversed photoschedule, NST capacity decreased while response to saline increased. During the experiments, hamsters were photosensitive and were changing to their winter status. However, because of the lack of cold during acclimation, the capacity for NST did not increase. Increased responsiveness to saline, indicating an increase in stress-induced thermogenesis, might be advantageous for "fight or flight" reaction. © 2005 Wiley-Liss, Inc.