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et al., 2006; Denver, 2009b, 2009c; Okada et al., 2009
), and injections of
CRF-like peptides elevated whole-body TH content in tadpoles of several
species (
Boorse &Denver, 2004; Denver, 1993, 1997; Gancedo et al., 1992
).
Commensurate with their positive actions on tadpole thyroid activity,
CRF-like peptides have been shown to accelerate tadpole metamorphosis
(
Boorse & Denver, 2002; Denver, 1993, 1997; Gancedo et al., 1992;
Miranda, Affanni, & Paz, 2000
). Conversely, blocking endogenous CRF
by passive immunization with CRF antiserum, or by injection of the
CRF receptor antagonist
a
-helical CRF
(9-41)
slowed spontaneous meta-
morphosis, or blocked simulated pond drying-induced metamorphosis
(
Denver, 1997
). Further, hypothalamic
crf
mRNA and peptide content in-
creased during spontaneous metamorphosis (
Denver, 2009b
), and hypotha-
lamic CRF peptide content was increased in spadefoot toad tadpoles that
accelerated metamorphosis in response to simulated pond drying
(
Denver, 1997
).
Kulkarni, Singamsetty, and Buchholz (2010)
recently
showed that CRF accelerates development of the direct developing frog
Eleutherodactylus coqui
. Because CRF is a stress neurohormone, endogenous
CRF may participate in environmentally induced (stress-induced) meta-
morphosis (
Boorse & Denver, 2004; Denver, 1997
).
Work from Sakae Kikuyama's laboratory found that a majority of the
TSH-releasing activity of tadpole and adult frog hypothalamic extracts on
dispersed adult pituitary cells can be blocked by coincubation with the
CRF receptor antagonist
a
-helical CRF
(9-41)
(
Ito et al., 2004; Okada
et al., 2009
). These findings suggest that a significant proportion of TSH-
releasing activity in the amphibian hypothalamus is contributed by CRF-
like peptides. They also suggest that other factors could be involved in
the regulation of TSH, or that
a
-helical CRF
(9-41)
may have only partial
antagonist activity in amphibia as has been found in mammals (
Rivier,
Rivier, & Vale, 1984
).
CRF actions are mediated by two G protein-coupled receptors (CRF
1
and CRF
2
;
Dautzenberg & Hauger, 2002
) and are modulated by a secreted-
binding protein (CRF-BP;
Seasholtz, Valverde, & Denver, 2002
). The ac-
tion of CRF-like peptides on TSH release in the tadpole is mediated by the
CRF
2
receptor expressed in thyrotropes (
Okada et al., 2007, 2009
);
whereas, ACTH release may be controlled by the CRF
1
receptor in am-
phibians as it is in mammals (
De Groef, Geris, et al., 2003; De Groef,
Goris, Arckens, Kuhn, & Darras, 2003; Okada et al., 2009; Tonon et al.,
1986; Van Pett et al., 2000
). In
X. laevis
,
crf
1
mRNA was expressed during
premetamorphosis and its level increased during prometamorphosis, reaching
