Model Systems for Studying Kisspeptin Signalling: Mice and Cells - Kisspeptin Signaling in Reproductive Biology - page 479

was reduced to around 9% and the GnRH neurons that remained showed no evidence
of expression of the kisspeptin receptor. Thus, these mice lack the downstream path-
ways for kisspeptin signalling, but surprisingly the female mice showed normal sex-
ual maturation and fertility even though they had signifi cantly smaller ovaries. These
data suggest that a small number of GnRH neurons (around 10% of normal) that do
not express the kisspeptin receptor are capable of sustaining the reproductive axis.
Genetic Ablation of Kiss1 Neurons
Unexpectedly, genetic ablation of Kiss1 neurons by CRE-induced expression of a
Diphtheria toxin A protein did not cause sterility in female mice [ 39 ]. In contrast,
when the Kiss1 neurons were ablated in adult females at post-natal day 20 (P20) by
cell-specifi c expression of a Diphtheria toxin receptor and toxin delivery, the
females were sterile. The authors suggest that this may be due to developmental
compensation for loss of the Kiss1 neurons before P20. One hypothesis is that
absence of a signalling molecule from Kiss1 neurons during development may alter
the physiology of GnRH neurons and allow them to respond to other factors that can
stimulate GnRH release. The identity of this signalling molecule is not known, nor
the compensatory changes that might be induced in the GnRH neurons. These could
include induction of new receptor molecules by the GnRH neurons or increased
dendritic branching to alter afferent inputs.
One important caveat to these neuronal ablation studies is that a small number of
neurons survive and these may still exert physiological effects. Ablation of the
GnRH neurons leaves a residual population of around 10%, but it is known that as
little as 12% of the GnRH neuron population can initiate puberty in mice, although
higher numbers of these neurons are required for full fertility [ 51 ]. While these
residual neurons do not seem to express the kisspeptin receptor, this was established
by lack of activation of a Rosa-YFP transgene [ 52 ], which is known to give low
expression levels in neurons and may have been below the level of detection.
Similarly, there is a residual Kiss1 neuronal population in the AVPV nucleus of
around 3%. The minimum number of Kiss1 neurons that are required for fertility is
not known, but as kisspeptins are extremely potent neuropeptides, relatively few
may be suffi cient.
Transgenic Mice with Fluorescently—Marked GnRH Neurons
A number of transgenic mouse lines have been developed with GnRH neurons
marked with a green fl uorescent protein ([ 53 ], for review, see [ 54 ]) and these have
been used to examine the electrophysiological effects of kisspeptins on GnRH neu-
rons. Kisspeptins act on the majority of GnRH neurons to initiate a sustained depo-
larization event and increase the action potential fi ring rate [ 55 - 57 ]. Pharmacological
studies and an assessment of the current-voltage (I-V) relationships indicate that