Biology Reference
In-Depth Information
the conversion of T
4
to T
3
is not required for negative feedback. TH receptor
b
is required for transcriptional repression of the
tsh
b
and
trh
genes in mammals
(
Flamant & Samarut, 2003; Guissouma, Dupre, & Demeneix, 2005
).
tr
b
mRNA increased throughout metamorphosis
in the tadpole pituitary
(
Manzon & Denver, 2004
).
Despite the presence of functional negative feedback in the
prometamorphic tadpole, TSH production shows a progressive increase
throughout metamorphosis reaching a peak at metamorphic climax. Hypo-
thalamic neurosecretory neurons and the median eminence, the structure
necessary for the delivery of neurohormones to the pituitary, develop during
prometamorphosis under the influence of TH (
Denver, 1998b
). The expres-
sion of neuropeptide receptors by anterior pituitary cells, and the responsive-
ness of these cells to secretagogues increases during metamorphosis (
Kaneko
et al., 2005; Manzon & Denver, 2004
).
Etkin (1968)
first proposed that the
maturation of the hypothalamus, median eminence, and pituitary under the
influence of TH is responsible for the sustained rise in plasma TH concen-
tration that drives metamorphosis. Thus, combined with a slight decrease in
the sensitivity of the pituitary to negative feedback at metamorphic climax,
the hypothalamic drive for TSH production may be sufficient to overcome
negative feedback exerted by the elevated plasma TH concentration at this
time. Negative feedback is likely to be physiologically important for limiting
TSH secretion once the system has matured, and perhaps during maturation
of the neuroendocrine system; that is, the coordination of morphogenesis
may require the temperance of TSH expression by TH throughout meta-
morphosis. However, the sustained rise in thyroid activity during metamor-
phosis is likely to be due primarily to the maturational effects of TH on the
CNS (and perhaps the pituitary) rather than the absence of negative feed-
back. The relatively lower levels of pituitary
Dio2
and
TR
b
expression dur-
ing early prometamorphosis might be permissive for the sustained rise in
TSH during prometamorphosis.
2.2. Adrenocorticotropic hormone
Expression of proopiomelanocortin mRNA in the anterior pituitary is low
in premetamorphic tadpoles and increases during prometamorphosis
peaking at metamorphic climax (
Aida, Iwamuro, Miura, & Kikuyama,
1999
). To my knowledge, there have been no direct measures of ACTH
during tadpole development. Functional ACTH receptors are expressed
