Biology Reference
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that increased ARC
Kiss1
expression may be involved in puberty onset. More recently,
initial elevations in ARC
Kiss1
expression in neonatal/juvenile female rats were
reportedly followed by a decrease in
Kiss1
levels by 3 weeks of age [
47
,
64
], perhaps
concurrent with increased sex steroid production (and negative feedback) at this time.
One study also detected another increase in ARC
Kiss1
of females between weeks 5
and 8 [
64
]. Several studies reported that ARC
Kiss1
levels in male rats remained
essentially unchanged during the fi rst several weeks of life (up to around 4 weeks of
age) [
47
,
64
], followed in one case by a small temporary increase in ARC
Kiss1
levels
around week 5 [
64
]. Overall, the current data on rodent ARC
Kiss1
expression during
development, especially at prepubertal and pubertal stages, is incomplete and fairly
inconsistent. Thus, full characterization of
Kiss1
expression in the ARC during devel-
opment and puberty is still needed in rodents, as well as other species (see below).
Very few studies on developmental changes in ARC kisspeptin neurons have
been conducted in non-rodent species. One study using agonadal male and intact
female monkeys described an increase in
Kiss1
expression in the ARC during
puberty [
25
]. In peripubertal female lambs with controlled sex steroid levels, a non-
signifi cant increase in ARC
Kiss1
cell number was seen over time when comparing
weeks 25, 30, and 35. When analyzed specifi cally in lambs that showed increased
LH pulse frequency, the number of
Kiss1
cells in the middle ARC was found to have
a signifi cant linear increase between weeks 25, 30, and 35, possibly implicating the
middle ARC in puberty onset [
65
]. However, another recent report in sheep found
that kisspeptin-ir cell number increased in the caudal ARC of females between the
prepubertal and postpubertal periods [
109
]. Thus, the specifi c location(s) within the
ovine ARC where
Kiss1
might increase during puberty occurs still needs to be
resolved. Collectively, these initial studies in monkeys and sheep suggest that
increased levels of
Kiss1
in the ARC of pubertal animals correlate with increased
activation of the reproductive axis, but these studies surveyed a low resolution of
ages over development, necessitating additional data on this subject.
Sex Differences in ARC Kisspeptin Neurons During Development
Most data agree that, unlike the sexually dimorphic AVPV/PeN [
31
,
47
,
70
], the
number of ARC
Kiss1
or kisspeptin cells and the quantity of
Kiss1
mRNA per cell
are not majorly different between sexes in adult rodents, especially when circulating
sex steroid levels are equalized between the sexes [
28
,
36
,
40
,
42
,
44
,
66
,
94
]. In the
few cases where an ARC
Kiss1
sex difference was observed, it was typically in
gonadally intact adult animals that have unequal sex steroid levels between the
sexes. For example, Adachi et al. [
42
] reported that intact adult male and female rats
had equivalent ARC
Kiss1
expression at all stages of the female estrous cycle except
for at diestrous 2. Moreover, adult males and females exhibit similar increases in
Kiss1
levels in the ARC after gonadectomy (GDX) and similar inhibition of ARC
Kiss1
expression when treated equivalently with sex steroids [
36
,
40
,
42
,
44
].
It should be noted that sex differences exist in the ARC in several other parameters,
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