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fi ed inputs represent a small percentage of the total kisspeptin input to GnRH neurons,
but it is not clear whether this refl ects limitations of the use of peptide co-markers (see
above) or the presence of inputs from other kisspeptin cell populations. Again, the use
of transgenic approaches where all axonal projections from a given kisspeptin popula-
tion can be completely labeled should help to resolve this question.
Finally, in addition to kisspeptin inputs to GnRH neurons and terminals, there is
also evidence that the reciprocal connection exists, that is, GnRH afferent input to
kisspeptin cells. Specifi cally, confocal studies have shown close contacts between
GnRH fi bers in the MBH, and kisspeptin cells of the ARC in the rhesus monkey [
30
]
and sheep (Fig.
3.2d
). Thus, in these species, kisspeptin (KNDy) neurons, which are
in themselves reciprocally interconnected, may comprise part of a larger reciprocal
circuitry that includes GnRH neurons and the POA kisspeptin population (see above).
One may speculate that if both GnRH and kisspeptin neurons are capable of display-
ing intrinsic pulsatile activity [
37
,
124
], a reciprocal network involving interconnec-
tions at multiple levels may be important in conferring synchronization of phase upon
this rhythm in order to generate a coherent GnRH pulse.
Development and Sex Differences in the Kisspeptin
Signaling System
In comparing kisspeptin and
Kiss1
expression across mammalian species, it is
important to keep in mind developmental changes and sexual differences in expres-
sion. Sexually dimorphic expression of at least some kisspeptin populations has
been found in all mammals in which it has been examined. Moreover, it is not sur-
prising that the developmental trajectory of both kisspeptin and
Kiss1
varies with
the time course of maturation in each species. In this section, we will review the
data available on these two issues for the limited number of species in which it is
available. In-depth discussion of the development and sexual differentiation of
kisspeptin neurons is also available in Chap.
11
.
Development of Kisspeptin and Kiss1 Expression in Females
Because kisspeptin was implicated in the onset of puberty at the time that its repro-
ductive function was discovered, it is not surprising that most developmental studies
have focused on the pubertal transition. Thus, although the role of kisspeptin in
puberty is discussed in detail elsewhere in this topic (Chaps.
11
and
12
), it will also
be considered here, as it represents an integral part of most reports on its develop-
mental expression. Early studies in rats reported an increase in
Kiss1
mRNA expres-
sion associated with pubertal development [
125
]. However, those studies used
RT-PCR to measure mRNA levels from whole hypothalami, so individual areas of
the hypothalamus were not examined. As detailed previously, kisspeptin cells in the
RP3V area and the ARC are differentially regulated. Thus, expression changes in
each area must be considered separately.
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