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have three hypothalamic GnRHs (GnRH-I, GnRH-II and GnRH-III), one
GnRH-R (partial/full length clones of two additional putative receptors
have been identifi ed but not characterized), one GTH, a second GpH with
homology to TSM, one gonadal GpH receptor and one thyroidal GpH
receptor (Sower et al
.
, 2009).
There is a clear trend in
P. marinus
,
L. appendix
, and
L. richardsoni
that
GnRH-I and GnRH-III increase in the brain throughout metamorphosis
with the most notable increase occurring towards the end of metamorphosis
(Youson and Sower, 1991; Youson et al., 1995a, 2006). Although developmental
data are currently lacking on the more recently identifi ed lamprey GnRH-II,
like GnRH-I and -III its transcript has been localized to the hypothalamus
and it has been shown to stimulate the pituitary-gonadal axis both
in
vitro
and
in vivo
(Kavanaugh
et al
.
, 2008). The concentration of GnRH-I
and GnRH-III in
P. marinus
brains, as determined by radioimmunoassay
following extraction, is low in larvae of year classes 2, 3, and 4, and remains
low until stage 3 of metamorphosis (Youson and Sower, 1991; Youson
et
al
.
, 1995a). A moderate increase in GnRH-I occurs at stage 6 followed by a
dramatic increase at stage 7 and levels remain elevated in feeding parasites
(Fig. 4). GnRH-III also increases at stage 6 and remains elevated through
parasitic feeding although peak levels of GnRH-I are approximately 3 times
greater than those of GnRH-III (Youson et al
.
, 1995a). These data are further
corroborated by both immunohistochemical (Wright
et al
.
, 1994; Tobet
et
al
.
, 1995,1996,1997) and
in situ
hybridization (Root
et al
.
, 2005) studies in
P. marinus
. This rise in GnRH during metamorphosis is also observed in the
non-parasitic species
L. richardsonii
(Youson et al
.
, 1995a) and
L.
appendix
(Youson
et al
.
, 2006). As with
P. marinus
, peak immunostaining for GnRH in
L. appendix
is observed at stage 7 of metamorphosis. One curious difference
between the two species (which have two different life history strategies)
is the observation that GnRH levels begin to increase earlier in
L. appendix
(during stage 1 and 2) and gradually increase throughout metamorphosis
(Youson et al
.
, 2006). In fact, there was an earlier activity of GnRH-I in
L. appendix
compared to
P. marinus
and this feature is most likely related to the
fact that the sexual maturation of non-parasitic species commences shortly
after metamorphosis and, therefore, their gonadal maturation is advanced
as compared to parasitic species at the same stage of development.
There is no doubt that a rise in brain GnRH is a characteristic of
metamorphosis in both parasitic and non-parasitic life history strategies.
What requires further clarifi cation is the precise function of GnRH at this time
in development. Is the role of GnRH related strictly to gonadal development,
does it function in the regulation of the thyroid or other endocrine axes
or does it also play a more direct role in the metamorphic process? The
fi nding that there is a rise in GnRH during spontaneous metamorphosis
prompted an investigation into GnRH levels following the manipulation
