EFFECT OF PHOTOPERIOD AND COLD-ACCLIMATION ON SURVIVAL OF MICE IN COLD

Male albino mice (Swiss-Webster) were raised at 5.degree. C under short (8L[light]:16D[dark]) and long (16L:8D) light periods. All mice were housed in groups of 3-5 individuals in plastic mouse cages (16 .times. 12 .times. 28 cm) until 42 days of age with food and water ad lib and cold exposed to -40.degree. C between 1000 and 1600 h to determine survival time or time until loss of righting response occurred (CT min). Under short photoperiods, survival time was 49.3 .+-. 4.4 min and under long photoperiods it was 38.7 .+-. 1.9 min (P < 0.05). A 2nd group of mice was maintained from birth at thermoneutral temperature (22.degree. C) under constant darkness, short day lengths (4L:20D) or constant light as described. When exposed to -20.degree. C, survival time was 80.0 .+-. 5.0 min for the animals kept in constant darkness, 61.1 .+-. 2.3 min for animals raised in short photoperiods (4L:20D) (P < 0.01), and 52.4 .+-. 2.3 min for mice raised in constant light (P < 0.05). After 30 min, mean rectal temperature was 32.1 .+-. 0.47.degree. C for constant-darkness animals, 30.5 .+-. 0.43.degree. C for short-day animals (P < 0.02) and 28.5 .+-. 0.74.degree. C for animals raised in constant light (P < 0.05). After 60 min, mean rectal temperatures for constant-dark, 4L:20D and constant-light animals were compared, and body temperature was 23.7 .+-. 1.6, 17.3 .+-. 1.5 (P < 0.01) and 12.8 .+-. 0.87.degree. C (P < 0.05), respectively. Photoperiod influences cold resistance at cold and thermoneutral acclimation temperatures, although when considered individually, cold acclimation enhances cold survival to a greater degree than does reduced light exposure.