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either one of the genes,
kiss1
or
kiss2
, and
gpr54
-
1
or
gpr54
-
2
, acquired predominant
functions in the HPG axis regulation, which were different among branches. That is
because Kiss1 and Kiss2 show similar binding activity to Gpr54-1 and Gpr54-2
immediately after their divergence. Interestingly, there is an observed tendency for
the species that lost Gpr54-1 to also have lost Kiss1. Obviously, much more exten-
sive analyses using different species in different phylogenetic branches are needed
to verify the general signifi cance of this phenomenon.
Functional Evolution of Kiss1 and Kiss2
As described above, the kisspeptin system is well conserved among vertebrate spe-
cies, except for the avian species. However, the general physiological functions of
kisspeptin in vertebrates still remain to be elucidated. To date, numerous studies
have reported on the involvement of kisspeptin(s) in the regulation of the HPG axis.
In mice, kisspeptins have been reported to act on GnRH neurons directly, by acting
on some intrinsic ion channels to produce strong persistent depolarization [
10
-
15
]
(see Chap.
6
)
. From the initial reports that loss of
Kiss1
or
GPR54
genes in human
and rodents disrupts puberty and leads to hypogonadotropic hypogonadism
[
16
-
18
], and Kiss1 peptide administration induces LH release by activating Gpr54
localized in GnRH1 neurons [
11
,
19
,
20
], it is clear that the kisspeptin-Gpr54 sys-
tem plays a critical role in the regulation of HPG axis at least in placental mammals
(reviewed in ref. [
21
]). On the other hand, there are fewer studies supporting similar
regulatory mechanisms of the kisspeptin system in nonmammalian vertebrates.
Moreover, it should be noted that the avian lineage can reproduce in spite of the fact
that they lack both the
kiss1
and
kiss2
systems (see Fig.
2.3
). Therefore, there are
presumably other mechanisms in addition to the kisspeptin system for the central
regulation of reproduction, at least in birds.
Studies in teleost kisspeptin systems seem to have yielded more complex situa-
tions. There are confl icting results that either support the presence of co-expression
of
gpr54
in GnRH1 neurons in European seabass [
22
] and a tilapia
Oreochromis
niloticus
[
23
] or their absence in another species of tilapia
Astatotilapia burtoni
[
24
].
Our unpublished results in medaka also showed the absence of
gpr54
mRNA in
GnRH1 neurons. Thus, the results of previous reports suggest that the situation is
different among different species. Studies of exogenous administration of kisspeptins
have been performed both in mammals and teleosts. In mammals, to our knowledge,
all the studies to date showed an increase in plasma LH [
20
,
25
-
34
]. In contrast to the
wealth of knowledge in mammals, a much smaller number of studies have been per-
formed in nonmammalian vertebrates. Kiss1 and/or Kiss2 increased LH mRNA or
serum LH concentration in zebrafi sh [
35
], sea bass [
6
], and goldfi sh [
5
]. The experi-
mental conditions for the occurrence of a rise in LH induced by kisspeptin, and the
time of LH/FSH rise after administration of kisspeptin, vary among the different
studies. Thus, in teleosts, the situation is somewhat different from those obtained
from the placental mammals, where kisspeptin is essential for reproduction.
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