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kisspeptin activates TRPC channels, and the sources of calcium mobilization
following kisspeptin activation of GnRH neurons. Although the majority of fi ndings
seem to indicate that the initial calcium signal comes via plasma membrane chan-
nels [
14
,
57
,
58
], there is also evidence that kisspeptin induces the release of cal-
cium from intracellular stores in GnRH neurons via inositol-1,4,5-trisphosphate
(IP3) receptors [
50
,
72
]. However, intracellular dialysis with 2-APB, which abro-
gates the store release of calcium, does not inhibit the effects of kisspeptin [
14
].
Certainly, sustained calcium release is not required for kisspeptin's actions since
calcium mobilization is transient [
50
].
The mammalian TRPC channels can be activated by G protein-coupled receptors
and receptor tyrosine kinases (see refs. [
59
,
73
]). In a heterologous cell expression
system (i.e., Chinese hamster ovary K1 cells expressing Kiss1R), kisspeptin is capa-
ble of activating multiple signaling pathway resulting in increased IP
3
formation,
calcium mobilization, arachidonic acid release, and MAP kinase phosphorylation
[
7
]. Although the kisspeptin induction of GnRH release in hypothalamic explants
from immature animals incubated in vitro is reported to involve recruitment of
ERK1/2 and p38 kinases, these actions of kisspeptin have not been confi rmed in
adult GnRH neurons [
50
,
72
]. In native GnRH neurons, the PLC inhibitor U73122
inhibits the effects of kisspeptin [
14
,
50
], and indeed, it is known that all mamma-
lian TRPC channels require PLC for activation [
60
]. Therefore, it appears that
Gq-coupled GPR54 activates PLC
to signal downstream to open TRPC channels
in GnRH neurons, thereby allowing the infl ux of sodium and calcium. Interestingly,
in POMC neurons, PLC
β
γ
1 appears to be the isozyme coupled to TRPC channel
activation by leptin [
69
].
Classically, the TRPC3, 6, and 7 subfamily is DAG sensitive [
59
,
73
].
Although TRPC3 and 7, and to a lesser extent TRPC6, transcripts are expressed
in GnRH neurons, the surrogate DAG signaling molecule 2-acetyl sn-glycerol
(OAG) has only a small effect to activate an inward current (~25% of the kiss-
peptin-induced current) in GnRH neurons [
14
]. A potential explanation is that
both hydrolysis of PIP
2
by PLC
and the calcium trigger that facilitates the
TRPC channel opening (i.e., the infl ux of Ca
2+
through calcium channels [
74
])
might be missing when applying OAG alone to GnRH neurons. In addition, La
3+
at a 100
β
M concentration, which potentiates TRPC4 and 5 and blocks TRPC3,
6, and 7 channels [
75
], did not attenuate or augment the kisspeptin-induced cur-
rent, which indicates that an ensemble of these channel subunits must exist in
GnRH neurons as revealed by single-cell RT-PCR [
14
]. Indeed, extracellular
2-APB (100
μ
M), which is a potent blocker of TRPC3, 4, 5, and 6 channels, and
FFA, which is a potent blocker of TRPC4 and 5 channels, inhibit the effects of
kisspeptin in GnRH neurons (Fig.
6.3
). These blockers have a similar effects on
the leptin activation of TRPC channels in arcuate POMC and kisspeptin neurons,
although lanthanum clearly potentiates the leptin-induced activation of TRPC
currents in POMC and kisspeptin neurons [
69
,
70
], suggesting subtle differences
between the GnRH neurons and the other two cell types. Collectively, these data
suggest that, although all of the “brain” TRPC channels are expressed in GnRH
μ
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