Biology Reference
In-Depth Information
during embryogenesis grew more rapidly than controls, suggesting that
pituitary GH is under inhibitory hypothalamic control; however, the
tadpoles failed to metamorphose, supporting that a hypothalamic neurohor-
mone was required to stimulate TSH secretion (
Etkin, 1968
). Destruction of
the preoptic area or surgical removal of the primordium of the posterior hy-
pothalamus (and thus isolation of the pituitary from the brain) prevented
metamorphosis (reviewed by
Denver, 1996
). Investigations of the normal
development of the neurosecretory centers of the hypothalamus and the me-
dian eminence further support Etkin's hypothesis (
Etkin, 1968
).
A striking example of the role of the hypothalamus in controlling meta-
morphosis, in particular, the role of hypothalamic CRF, comes from studies
of desert toad species. The most important environmental variable for a tad-
pole is water availability, and duration of the aquatic habitat can profoundly
influence the rate of metamorphosis in many species. This is especially true
for desert amphibians that tend to breed in ephemeral habitats. As discussed
earlier, CRF-like peptides control TSH secretion in tadpoles, acting via the
CRF
2
receptor. Because the secretion of CRF is activated by stressors, CRF
plays a central role in mediating a tadpole's developmental response to a de-
teriorating larval habitat (e.g., pond drying in the case of the Western
spadefoot toad) (
Boorse & Denver, 2004; Denver, 1997; Denver, 1998a;
Denver, Mirhadi, & Phillips, 1998
). The timing of the expression of recep-
tors for neurohormones in the pituitary gland, particularly the CRF
2
recep-
tor, could be important in establishing competence of pituitary thyrotropes
to respond to stimulation by CRF-like peptides (
Manzon & Denver, 2004;
Okada et al., 2009
).
Other environmental factors that are known to alter the timing of
metamorphosis (e.g., food availability, crowding, predation) may also act
through the neuroendocrine stress axis. For example, whole-body CORT
content was elevated in tadpoles that were food restricted or subjected to
high conspecific density, compared to their high resource, low density
counterparts (
Glennemeier & Denver, 2002b
). Both low food and in-
creased density resulted in slowed growth and development in
premetamorphic tadpoles, which agrees with other studies showing
growth- and development-inhibiting effects of these factors in
premetamorphs (but contrast this with prometamorphic animals which ac-
celerate development in response to food restriction or crowding). This
slowed growth caused by crowding stress was reversed by treatment of tad-
poles with the CORT synthesis inhibitor metyrapone, again suggesting a
functional
role for
the hypothalamo- hypophyseal-interrenal axis
in
