Although the rat is usually not considered to be sensitive to photoperiod, under some experimental conditions photoperiod responses are unmasked. In addition, we have observed photoperiod-induced changes in body weight gain in lean and obese Zucker rats. In this experiment, body mass, food intake, body composition, brown adipose tissue (BAT) thermogenic state, and blood concentrations of corticosterone, insulin, and glucose were evaluated under one of two lighting conditions: a short (10 h light: 14 h dark) or a long (14 h light: 10 h dark) photoperiod. Plasma corticosterone and glucose concentrations measured under fasting conditions were unaffected by photoperiod in either genotype. The amount of BAT mitochondrial protein isolated was less in long photoperiod rats. BAT mitochondrial GDP binding was unaffected by photoperiod in the lean rats, but tended to be lower in long photoperiod obese rats than in short photoperiod obese rats. Although, photoperiod had no effect on daily food intake of rats exposed to the short versus long photoperiod, body mass was heaviest in obese rats raised in long photoperiod. Plasma insulin was increased in both lean and obese rats in long photoperiod. In addition, fat storage appeared to shift to internal depots in the lean rats exposed to long photoperiod. Our data demonstrate that photoperiod does have an effect on male Zucker rats with respect to body weight and fat distribution, with the obese rats being more sensitive to changes in photoperiod than the lean rats. Photoperiod modifies fat accumulation, food intake, body composition, and body weight gain in a variety of mammals. These effects are pronounced in rodents such as Syrian hamsters and voles, whose reproductive cycle is sensitive to photoperiod (1). Rats are generally not thought to be as sensitive to changes in photoperiod as are the more "wild-type" rodents even though rats breed more succesfully when placed in long photoperiod. Moreover, under some experimental conditions such as bulbectomy and pinealectomy, photoperiod responses are unmasked in rats (2,3). This study evaluated the effects of long versus short photoperiod on weight gain, body composition, and food intake as well as on several hormonal variables in Zucker lean and obese male rats. A pilot study from our laboratory indicated that pinealectomy increased weight gain without a change in food intake of lean but not obese Zucker rats. In addition, obese Zucker rats (fa/fa) lack the normal diurnal feeding pattern and periodicity in plasma corticosterone levels seen in most lean rats,(4), adrenalectomy restores the feeding patterns to those observed in lean rats, and corticosterone replacement restores the patterns to those of sham-operated obese rats (5). These observations suggest that photoperiod may have a greater effect on the lean than on the obese rat. The present study negates this hypothesis.
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