Kisspeptin and GnRH Pulse Generation - Kisspeptin Signaling in Reproductive Biology - page 292

regions from the rest of the brain. They found that isolating the mediobasal hypo-
thalamus (MBH), including the pituitary, abolishes ovulation but not follicular
development [ 20 ]. This was confi rmed later by Blake and Sawyer [ 21 ], who demon-
strated that complete hypothalamic deafferentation spares LH pulses in ovariecto-
mized (OVX) rats. These experiments clearly showed that the brain center generating
GnRH/LH pulses was located within the hypothalamic area isolated by the Halasz's
knife, namely, the MBH. According to this data, the GnRH pulse generator may not
involve GnRH neurons themselves, because very few GnRH cell bodies are located
in the MBH of most animal species (with the exception of primates, in which most
of GnRH neurons are located in the area [ 22 ]). The MBH location of the GnRH
pulse generator was also confi rmed by fetal MBH transplantation in rats that had
brain lesions which abolished GnRH pulses [ 23 ]. A type of deafferentation called
posterior-anterior deafferentation (PAD), which cuts the anterior part of the arcuate
nucleus (ARC) off, abolished pulsatile LH secretion in rats, but the pulse was
restored with transplantation of fetal MBH tissues (but not fetal cortical tissues).
These fi ndings indicate the presence of a GnRH pulse-generating mechanism in the
MBH region.
On the other hand, evidence also suggests that GnRH neurons themselves are
equipped with an intrinsic GnRH pulse-generating mechanism. This was fi rst dem-
onstrated in GT-1 cells, which are immortalized by introducing T antigen to the
mouse genome to induce GnRH-producing tumor cells. GT-1 cells show periodic
excitation, resulting in pulsatile GnRH release into the culture medium [ 24 ].
Further evidence came from primary cultures of rhesus monkey GnRH neurons
taken from the fetal olfactory placode, the anatomical region where GnRH neurons
originate and migrate from to the hypothalamus during development. The idea to
obtain a pure population of GnRH neurons from the monkey fetus came from the
laboratory of Terasawa and enabled the demonstration of pulsatile activation of
GnRH neurons in vitro. These primary GnRH neurons displayed periodic increases
in intracellular calcium concentrations [ 25 ]. Terasawa's group also found that the
periodic increases in intracellular calcium levels in cultured GnRH neurons are
synchronized with each other [ 26 ]. The authors considered that these calcium
increases cause GnRH pulses.
It is evident that GnRH is released in fi xed intervals from GnRH neuronal termi-
nals. The synchronized release of GnRH from each nerve terminal appears to require
coordinated activation of GnRH neurons from neuronal afferents. There are three
mechanistic possibilities for synchronizing GnRH neuronal output. First, GnRH cell
bodies make contacts with each other, as evidenced by reports of morphological
contacts between GnRH neuronal processes [ 27 ]. However, somatosomatic or den-
drodendritic contacts between GnRH neurons are quite rare in the POA of rats [ 27 ].
Second, the synchronization of GnRH releases from each nerve terminal might be
achieved by contact between multiple GnRH terminals in the median eminence,
because the median eminence is one of the sites where there is a convergence of
various bioactive substances acting to regulate the release GnRH [ 28 ]. There might
be the third possibility that GnRH cells may all be synchronized by an upstream
“clock” that affects all GnRH cells at the same time, resulting in simultaneous