To avoid breeding during unsuitable environmental or physiological circumstances,
the reproductive axis adjusts its output in response to fluctuating internal and external
conditions. The ability of the reproductive system to alter its activity appropriately
in response to these cues has been well established. However, the means by which reproductively
relevant cues are interpreted, integrated and relayed to the reproductive axis remain
less well specified. The neuropeptide kisspeptin has been shown to be a potent positive
stimulator of the hypothalamo-pituitary-gonadal (HPG) axis, suggesting a possible
neural locus for the interpretation/integration of these cues. Because a failure to
inhibit reproduction during winter would be maladaptive for short-lived female rodents,
female Siberian hamsters (Phodopus sungorus) housed in long and short days were examined.
In long "summer" photoperiods, kisspeptin is highly expressed in the anteroventral
periventricular nucleus (AVPV), with low expression in the arcuate nucleus (Arc).
A striking reversal in this pattern is observed in animals held in short, "winter"
photoperiods, with negligible kisspeptin expression in the AVPV and marked staining
in the Arc. Although all studies to date suggest that both populations act to stimulate
the reproductive axis, these contrasting expression patterns of AVPV and Arc kisspeptin
point to disparate roles for these two cell populations. Additionally, we found that
the stimulatory actions of exogenous kisspeptin are blocked by acyline, a gonadotropin-releasing
hormone (GnRH) receptor antagonist, suggesting an action of kisspeptin on the GnRH
system rather than pituitary gonadotropes. Finally, females held in short day lengths
exhibit a reduced response to exogenous kisspeptin treatment relative to long-day
animals. Together, these findings indicate a role for kisspeptin in the AVPV and Arc
as an upstream integration center for reproductively relevant stimuli and point to
a dual mechanism of reproductive inhibition in which kisspeptin expression is reduced
concomitant with reduced sensitivity of the HPG axis to this peptide.