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 trigge
red by changes in the photoperiod and are mimicked in the laboratory b
y 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 loc
ated 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 re
lative 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 housin
g density affect these decreases in fat pad mass? and 3) Can these cha
nges in WAT mass be altered by gonadal steroid manipulation? In Experi
ment 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. Castrat
ed males received a cholesterol control (CHOL), testosterone (T), or d
ihydrotestosterone (DHT) implant, whereas ovariectomized females recei
ved CHOL or estradiol implant. Six weeks after surgery and implantatio
n, 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 ref
lected primarily as decreased fat cell size, 2) the relative pattern o
f SD-induced lipid depletion only was seen in single-housed males, 3)
T or estradiol treatment reversed the effects of gonadectomy in both p
hotoperiods, 4) despite the restoration of LD serum concentrations for
both hormones, T only reversed the SD-induced decrease in body mass a
nd food intake, whereas estradiol only reversed the SD-induced decreas
e in RWAT pad mass, and 5) DHT treatment exaggerated the SD-induced de
creases in body mass and food intake. Collectively, these results show
ed that: a) short photoperiod-mediated changes in fat pad mass are due
to fluctuations only in fat cell size, b) housing density affected th
e relative pattern of SD-induced lipid depletion in males, whereas mos
t other photosensitive responses did not differ between single- and gr
oup-housed hamsters, and c) many of the SD-induced responses were gona
dal steroid independent.