In estrous cycling females, hunger for food is
lowest when sexual motivation and circulating estradiol concentrations are
highest, and these fluctuations are amplified when food availability is limited.
We hypothesize that the amplifying effects of mild food restriction are
mediated by increases in the secretion of gonadotropin-inhibiting hormone
(GnIH) and its interaction with ovarian steroids. GnIH treatment increases food
hoarding and inhibits sexual motivation in Syrian hamsters, and thus, we
predicted that levels of cellular activation in GnIH-immunoreactive (ir) cells
would be increased by mild food restriction on most days of the estrous cycle,
except on the day of estrous in this species. Female Syrian hamsters were
either fed ad libitum or food restricted to 75% of their average ad
libitum intake for a period of 8 days. After treatment, blood and brains
were collected from groups representing every day of the 4-day estrous cycle.
Brain sections were double-labeled for GnIH-ir and for the protein Fos, a
marker for cellular activation. Food restriction 1) significantly elevated Fos-ir/GnIH-ir
on all days of the estrous cycle except the periovulatory day, 2) had no effect
on circulating concentrations of estradiol or progesterone, and 3) decreased
circulating concentrations of leptin similarly on every day of the estrous
cycle. In summary, food restriction, and possibly decreased plasma leptin, activates
GnIH cells differentially according to the day of the estrous cycle, ensuring
that food hoarding is increased in preparation for the energetic costs of
pregnancy and lactation, and mating is restricted to the most fertile period.
This work was funded by NSF grants IOS-1257638 and IOS-1257876.