Photoperiod, sex, gonadal steroids, and housing density affect body fat in hamsters.

Siberian hamsters (Phodopus sungorus sungorus) show seasonal cycles in body mass with the peak mass occurring in the summer and the nadir in the winter. These naturally occurring changes in body mass are triggered by changes in the photoperiod and are mimicked in the laboratory by transferring male or female hamsters from long "summer-like" days (LDs) to short "winter-like" days (SDs). During the initial exposure to SDs (1-6 weeks), the decreases in body fat are not uniform and are associated with the relative preferential depletion of internally located intraperitoneal white adipose tissue (WAT) pads compared with the more externally located subcutaneous WAT pads. This pattern of lipid depletion only occurs in males. The purpose of the present experiments was to explore further the SD-induced gender- and fat pad-specific relative decreases in fat pad mass by answering the following questions: 1) What are the underlying alterations in adipose tissue cellularity associated with the SD-induced changes in fat pad mass? 2) Does housing density affect these decreases in fat pad mass? and 3) Can these changes in WAT mass be altered by gonadal steroid manipulation? In Experiment 1, adult male and female gonadally intact Siberian hamsters were housed singly or 10 hamsters per cage. In Experiment 2, adult male and female hamsters were left gonadally intact or gonadectomized. Castrated males received a cholesterol control (CHOL), testosterone (T), or dihydrotestosterone (DHT) implant, whereas ovariectomized females received CHOL or estradiol implant. Six weeks after surgery and implantation, half the animals in each group in both experiments were transferred to SDs for an additional 6 weeks, whereas the others remained in LDs. It was found that: 1) the SD-induced decrease in fat pad mass was reflected primarily as decreased fat cell size, 2) the relative pattern of SD-induced lipid depletion only was seen in single-housed males, 3) T or estradiol treatment reversed the effects of gonadectomy in both photoperiods, 4) despite the restoration of LD serum concentrations for both hormones, T only reversed the SD-induced decrease in body mass and food intake, whereas estradiol only reversed the SD-induced decrease in RWAT pad mass, and 5) DHT treatment exaggerated the SD-induced decreases in body mass and food intake. Collectively, these results showed that: a) short photoperiod-mediated changes in fat pad mass are due to fluctuations only in fat cell size, b) housing density affected the relative pattern of SD-induced lipid depletion in males, whereas most other photosensitive responses did not differ between single- and group-housed hamsters, and c) many of the SD-induced responses were gonadal steroid independent.