SHORT DAY-INDUCED DEPLETION OF LIPID STORES IS FAT PAD-SPECIFIC AND GENDER-SPECIFIC IN SIBERIAN HAMSTERS

The naturally occurring, short day (SD)-induced decrease in body mass in male Siberian hamsters is due almost exclusively to decreases in body fat during the first few weeks of SD exposure. This decrease in body fat is not uniformly distributed among the white adipose tissue (WAT) pads. The purpose of the present experiment was to answer four questions: (i) Are the SD-induced preferential decreases in internally located fat pad mass gender specific?; (ii) Does gonadectomy and/or steroid replacement therapy alter this pattern of body fat depletion?; (iii) What is the role of lipoprotein lipase (LPL) activity in this process?; and (iv) Does SD-induced functional castration affect food intake, body mass and lipid-related fat responses similarly to surgical castration? Adult male and female Siberian hamsters were housed in long days (LD) for 6 wk following castration (CAST) or ovariectomy (OVX) combined with either SC implants of testosterone (T), estradiol or the cholesterol control (CHOL). While the remaining animals of both sexes remained gonadally intact. At this time, half the animals in each group were transferred to SDs for 6 wk. SD-housed gonad-intact males and females exhibited gonadal regression and decreased body mass and carcass lipid; however the pattern of lipid depletion from the WAT pads was gender specific. SD-exposed gonad-intact males showed disproportionate decreases in fat pad mass in the internally located epididymal WAT (EWAT) and retroperitoneal WAT (RWAT) fat pads compared with the more externally located intraperitoneal WAT (IWAT) pad, whereas females showed uniform relative decreases for all pads and regardless of their reproductive status. CAST + CHOL or T eliminated the SD-induced fat pad-specific pattern of lipid depletion in males. Therefore, the slowly decreasing serum T concentrations trigged by the initial exposure to SDs may underlie the fat pad-specific differential depletion of lipid. The photoperiod-, gonadectomy-, and hormone replacement therapy-induced changes in fat pad mass were seldom associated with parallel changes in specific LPL activity. CAST and SD exposure both reduce serum T concentrations, decrease body mass via reductions in the same carcass components and decrease food intake; however, these the two treatments differ with respect to their effects on WAT mass and LPL activity. Specifically, SD exposure decreases IWAT, EWAT and RWAT mass, whereas CAST decreases only EWAT mass. In addition, specific LPL activity is not affected by SD exposure, whereas CAST generally increases specific LPL activity in all pads compared to their gonad-intact controls.