Kisspeptin Antagonists - Kisspeptin Signaling in Reproductive Biology - page 177

proestrus, with a progressive rise of serum concentrations between 16:00 and 20:00,
followed by a decrease in LH levels on the morning of oestrus. However, when
peptide 234 was infused, 7 out of 9 females failed to display the prototypical surge
of LH at proestrus (Fig. 8.13d ) [ 31 ]. This experiment provides direct evidence for
the indispensable role of KP signalling in mediating the pre-ovulatory surge of
gonadotropins (Fig. 8.14 ). The fi nding supports a previous demonstration that
administration of KP antiserum abolishes the ovulatory LH surge [ 34 ].
Design of KP Antagonists to Penetrate the Blood-Brain Barrier
KP-10 and its peptide antagonists can be modifi ed at the NH2 terminus without loss
of activity. We have therefore exploited this property by adding the penetrating peptide
sequence (which allows passage across the cell membrane) to the NH2 terminus of
KP antagonist, peptide 234. This potentially facilitates transfer across the blood-
brain barrier. When administered systemically, this antagonist (Peptide 271) inhib-
ited KP-10 stimulation of LH in rats following both i.c.v. and systemic administrations
of KP-10 [ 31 ]. Peptide 271 has also been shown to decrease both amplitude and
frequency of LH pulses in OVX ewes. In these animals, peptide 271 was also shown
to inhibit EB-induced LH surge [ 35 ], providing further evidence for the role of KP
in positive steroid feedback (Fig. 8.15 ).
Kobayashi et al. also modifi ed the small molecule antagonist, compound 9l, to
increase penetration of the blood-brain barrier, as compound 9l had relatively low
brain exposure when given by i.v. administration to male castrated rats. The modifi ed
compound 15a was shown to have high brain exposure levels in castrated male rats
and also suppressed plasma LH levels in these animals (Fig. 8.10d ) [ 25 ].
KP-Independent GnRH Secretion
The majority of the studies described above indicated that KP antagonists did not
affect basal LH secretion in intact and castrated male mice and rats, in ovariect-
omised oestrogen treated ewes, in studies on the ovulatory LH surge in rats and
ewes, and when administered into the ARC in ovariectomised, oestrogen-replaced
rats. It cannot be ruled out that this is due to the dose of antagonist used in these
studies, as higher doses have not yet been tested. However, if this is shown not to be
due to the dosage, then these fi ndings suggest that, although KP is clearly involved
in regulating LH pulses and the increase in LH secretion after orchidectomy and
during the LH surge, the maintenance of basal LH appears to be KP-independent.
This proposal is supported by the demonstration that peptide 234 decreased
KP-stimulated GnRH neuron fi ring rate in mouse brain but not the non-stimulated
fi ring rate, and also decreased GnRH pulses, but not basal GnRH secretion, in the
rhesus monkey. Moreover, peptide 234 does not lower LH to the same extent as