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however, a signifi cant increase in immunoreactive kisspeptin cells was seen in both
the ARC and the POA of ewes transferred to from long-day (16 h light:8 h dark) to
short-day (8 h light:16 h dark) photoperiod (replicating the shift to the breeding
season). In this latter study, the sheep were ovariectomized and given estrogen
implants, and the increase in kisspeptin-ir neuron number was seen in the same time
frame as the release from estrogen negative feedback. This adds further weight to
the assertion that the seasonality of reproduction in the ewe involves a change in the
responsiveness of kisspeptin cells to estrogen.
In addition to seasonal changes in kisspeptin gene expression and synthesis in
the ovine brain, there is also greater kisspeptin input to the GnRH neurons in the
breeding than nonbreeding season [
30
]. This most likely originates from the POA
kisspeptin cells (see above). Thus, at least in sheep, both the level of kisspeptin
expression and the level of kisspeptin input to GnRH neurons are higher during the
breeding season, while the negative feedback effects of estrogen on kisspeptin are
lower at this time of the year. This further asserts the case for a major role of kiss-
peptin in the seasonality of reproduction.
As the seasonal change in kisspeptin expression in ewes is replicated by manipu-
lation of photoperiod, it appears this change in the diurnal pattern of the light/dark
cycle may be the primary stimulus governing kisspeptin change. Work in hamsters
supports this notion, although there are differences between Syrian and Siberian
hamsters that complicate interpretation in these species. Transfer from long days
(16 h light:8 h dark) to short days (8 h light:16 h dark) reduced the number of cells
expressing
Kiss1
in male and female Syrian hamsters, in both the AVPV and the
ARC, as measured via in situ hybridization [
55
,
62
-
64
]. These data are consistent
with those obtained in the sheep [
49
], but different to those seen in the Siberian
hamster, in which immunohistochemistry was used to measure an increase in kiss-
peptin-ir cell number in the ARC under short-day photoperiod [
21
]. Interestingly,
Siberian hamsters that did not respond to short days (with a reduction in reproduc-
tive function) showed no increase in kisspeptin cell numbers, which remained simi-
lar to those seen under long days [
21
]. Although these papers indicate a critical
species difference between Syrian and Siberian hamsters, it should be noted that
one study [
21
] counted kisspeptin-ir cells, whereas the other [
57
] counted
Kiss1 -
expressing
cells, and thus, different techniques were used. However, it has been
recognized for some time that Syrian and Siberian hamsters may differ in the mech-
anisms that underlie seasonality of reproductive function [
65
]. These differences
may, at least in part, be due to the differences in the neural sites of action of melato-
nin. Thus, in Siberian hamsters, lesion of the SCN prevents the inhibitory effect of
exogenous melatonin infusions [
66
], but this is not the case in Syrian hamsters [
67
].
Role of the A14/A15 Dopaminergic Nucleus
In sheep, dopaminergic neurons located in the A15 region of the forebrain appear to
be involved in the seasonality of breeding (reviewed in refs. [
68
,
69
]). These dopami-
nergic cells act in some way to reduce GnRH pulse frequency during the nonbreeding
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