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but act as a juvenilizing hormone to inhibit metamorphosis (Youson, 1997).
Subsequently, following some unknown signal, metamorphosis begins and
much lower TH levels are necessary to drive the morphogenetic processes
associated with metamorphosis as observed in other vertebrates (R.G.
Manzon, In Press).
2.7.4.2 Hypothalamic-pituitary hormones
The past two decades have seen an explosion of new information on the
hormones of the lamprey hypothalamus and adenohypophysis (pituitary).
An extensive discussion of the lamprey hypothalamic-pituitary (HP)
hormones and the associated evolutionary interpretations is beyond the
scope of this chapter, but can be found in other recent reviews (Kawauchi
and Sower, 2006; Sower
et al
.
, 2009). Much of this work has focused on
the identifi cation and characterization of the various hormones of the HP
axis and has laid the foundation for future studies on its regulation and
function in lamprey metamorphosis. To date there are little data in support
of a role for the lamprey HP axis in the regulation of the thyroid and most
data suggest that the thyroid system is regulated by peripheral rather then
central mechanisms (Eales and Brown, 1993; Youson, 2007). Likewise, most
studies have failed to show a relationship between the HP axis and lamprey
metamorphosis, although the adenohypophysis undergoes extensive
modifi cation during metamorphosis (Wright, 1989). One notable exception
is the requirement of the adenohypophysis for normal metamorphosis of
G. australis
(Joss, 1985). Removal of the larval rostral pars distalis (RPD)
resulted in complete metamorphic stasis while metamorphosis was arrested
in stage 3 following the removal of the caudal pars distalis (CPD) (Joss, 1985).
Some recent evidence suggests that hypothalamic GnRH may be involved
in metamorphosis of
P. marinus
(Youson and Sower, 1991),
L. richardsoni
(Youson et al., 1995a), and
L. appendix
(Youson et al., 2006), and perhaps
may be connected to thyroidal regulation (Youson and Sower, 2001). This
latter point is consistent with the idea that there is overlap between the
thyroid and reproductive axes and that reproductive maturation is one of
the ancestral functions of the thyroid (see Youson and Sower, 2001).
The hypothalamus of most gnathostomes contains one or two GnRHs
and two GnRH-receptors (GnRH-Rs) (Sower et al
.
, 2009). GnRHs regulate
the synthesis and secretion of two pituitary gonadotropins (GTH): follicle
stimulating hormone (FSH) and lutenizing hormone (LH), each of which
act via one of two gonadal glycoprotein hormone (GpH) receptors. The
heterodimeric GpH family members consist of an α and β subunit, and to
date two α and fi ve β subunits have been identifi ed in vertebrates. Other
members of the GpH family include thyrotropin (TSH), thyrostimulin
(TSM), and chorionic gonadotropin (CG) (see Sower et al
.
, 2009). Lampreys
