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In-Depth Information
gpr54
-
2
seems to have signifi cant expression in POA in zebrafi sh and medaka,
while
gpr54
-
1
has been lost in some species.
In zebrafi sh, Kiss1 neurons, located in the habenula, were demonstrated to proj-
ect to the interpeduncular and raphe nuclei. On the other hand, Kiss2 neurons,
whose cell bodies were shown to be localized in the dorsal and ventral hypothala-
mus, widely projected to the ventral telencephalon, POA, thalamus, and ventral/
caudal hypothalamus, suggesting that Kiss2 neurons mainly function as a
homeostatic regulator in this species [
58
].
Gpr54
-
2
was shown to be predominantly
expressed in POA, ventral telencephalon, hypothalamus, and several nuclei of the
brain. However,
gpr54
-
1
was only expressed in habenula. Close apposition of
Kiss2-ir fi bers to GnRH1 neurons was also observed in zebrafi sh, although the
authors did not examine the co-expression of
gpr54
in GnRH neurons.
On the other hand, in medaka, in addition to the habenulo-interpeduncular path-
way, the Kiss1 neurons were shown to project many fi bers to the POA, ventral telen-
cephalon, and hypothalamus, but not to the rest of the brain [
59
], which is consistent
with our recent results of immunohistochemical demonstration of the projections of
Kiss1 neurons in the Kiss1-EGFP medaka established by us (Shimada et al., unpub-
lished observations). This predominant distribution of Kiss1 fi bers in the hypothala-
mus and POA correspond to the dense distribution of
kiss1
mRNA expressing
neurons in the hypothalamus of medaka, whereas zebrafi sh lacks such Kiss1 neurons
in the hypothalamus. Because the Kiss1 neurons in the medaka hypothalamic nucleus
NVT show high sex steroid sensitivity [
36
] it is suggested that the release of Kiss1 in
the POA, hypothalamus, and ventral telencephalon will also vary according to the
breeding state. Furthermore, the receptor distribution was also examined for
gpr54
-
1
and -
2
by in situ hybridization, and
gpr54
-
2
was shown to be widely expressed in
the medaka brain, especially in regions that are involved in homeostatic regulation,
consistent with the Kiss1 neuron projections. On the other hand, as in the case of
zebrafi sh, the expression of
gpr54
-
1
was practically confi ned to the habenula and
POA (Kanda et al., unpublished data). Recently, indirect effects of kisspeptin on non-
hypophysiotropic GnRH3 neurons was reported [
60
]. Further studies of hypophysio-
tropic and non-hypophysiotropic function of kisspeptin neurons are important for the
understandings of physiological functions of kisspeptin systems.
In a Cichlid fi sh,
A. burtoni
, the distribution of kisspeptin receptor was examined
by in situ hybridization [
24
]. The authors reported that the fi sh lacked
gpr54
-
1
. The
cells expressing
gpr54
-
2
were localized in hypothalamus, POA, and ventral telen-
cephalon, which is similar to the results of zebrafi sh and medaka. In addition,
gpr54
-
2
was shown to be expressed in the dorsal telencephalon and some other
brain regions, with the most prominent expression in the olfactory bulb. Since
gpr54
-
2
was not expressed in the GnRH1 neurons, the authors suggested the exis-
tence of non-GnRH1 neurons that express
gpr54
-
2
and may exhibit sensitivity to
kisspeptin, according to the social state or sexual maturation of the fi sh [
24
].
From these studies, it may be suggested that
gpr54
-
2
is likely to play a wide
variety of roles in teleosts, because
gpr54
-
1
positive cells are localized mainly in
habenula and other restricted regions in the brain in medaka and zebrafi sh, or even
absent in some species [
24
,
61
-
63
], whereas there is no report of teleost species that
lack
gpr54
-
2
. Therefore, it may be suggested that
gpr54
-
1
became predominant in
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