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treatment had no effect in short days, leading to the conclusion that GnIH is not
regulated by this gonadal steroid. Pinealectomy prevented the reduction in GnIH
gene expression under short days, and melatonin treatment of long-day animals led
to a reduction in GnIH gene expression within 3 weeks [
87
]. Most recently, another
study in Siberian hamsters confi rmed the results of the earlier work, confi rming the
changes in GnIH expression with long- and short-day photoperiod and regulation by
melatonin, with corroborative immunohistochemical data [
86
]. In addition, these
authors showed that GnIH fi ber input to GnRH cells was reduced under short-day
photoperiod. Importantly, it was shown that intracerebroventricular infusion of
GnIH inhibited plasma LH levels in animals under long-day photoperiod but stimu-
lated LH levels when animals were under short-day photoperiod. This led the authors
to speculate that GnIH is inhibitory when LH levels are high and stimulatory when
LH levels are low. These data implicate GnIH in “seasonality” in hamsters, but the
higher levels of activity of GnIH neurons under long-day photoperiod (the breeding
condition) indicate that it is unlikely that changes in GnIH are of major importance
in the reproductive state that is controlled by photoperiod. However, GnIH certainly
has signifi cant roles in the regulation of appetite [
84
,
88
] and stress [
89
], and the
former is controlled by photoperiod in seasonally breeding mammals.
In summary, in sheep at least, there is good indication that seasonality is due to
reciprocal changes in the infl uence of kisspeptin and GnIH on GnRH secretion, with
the added effect of an increase in GnIH action on pituitary gonadotropes in anestrus;
the same is not true for hamsters.
Response to Kisspeptin in Breeding and Nonbreeding “Seasons”
In sheep, the response to kisspeptin is greater in the nonbreeding season than in the
breeding season [
90
], which appears to be due to a higher level of kisspeptin recep-
tor expression in GnRH neurons and a higher GnRH response to kisspeptin in the
nonbreeding season [
31
]. In a consistent manner, a higher response to kisspeptin, in
terms of LH secretion, is seen in Siberian hamsters under short-day (nonbreeding)
photoperiod compared to long-day (breeding) photoperiod [
91
]. The higher response
to kisspeptin in nonbreeding animals may be a refl ection of the lower GnRH/LH
pulse frequency at this time, allowing for greater build up of releasable pools of
GnRH and LH respectively.
Induction of Reproductive Function with Kisspeptin During
Photoperiodic Quiescence
The recognition that kisspeptin cells are major regulatory elements of the gonado-
tropic axis (for review, see ref. [
92
]), together with the above-mentioned observa-
tions, strongly indicates that acyclicity in ewes during the nonbreeding season is
associated with reduced kisspeptin function in the ARC. In order to determine
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