ENVIRONMENTAL-STRESS AS A DEVELOPMENTAL CUE - CORTICOTROPIN-RELEASINGHORMONE IS A PROXIMATE MEDIATOR OF ADAPTIVE PHENOTYPIC PLASTICITY IN AMPHIBIAN METAMORPHOSIS
Rj. Denver, ENVIRONMENTAL-STRESS AS A DEVELOPMENTAL CUE - CORTICOTROPIN-RELEASINGHORMONE IS A PROXIMATE MEDIATOR OF ADAPTIVE PHENOTYPIC PLASTICITY IN AMPHIBIAN METAMORPHOSIS, Hormones and behavior, 31(2), 1997, pp. 169-179
Environmentally induced phenotypic plasticity allows developing organi
sms to respond adaptively to changes in their habitat. Desert amphibia
ns have evolved traits which allow successful development in unpredict
able environments. Tadpoles of these species can accelerate metamorpho
sis as their pond dries, thus escaping mortality in the larval habitat
. This developmental response can be replicated in the laboratory, whi
ch allows elucidation of the underlying physiological mechanisms. Here
I demonstrate a link between a classical neurohormonal stress pathway
(involving corticotropin-releasing hormone, CRH) and the developmenta
l response to habitat desiccation. Injections of CRH-like peptides acc
elerated metamorphosis in western spadefoot toad tadpoles. Conversely,
treatment with two CRH antagonists, the CRH receptor antagonist alpha
-helical CRH(9-41) and anti-CRH serum, attenuated the developmental ac
celeration induced by habitat desiccation. Tadpoles subjected to habit
at desiccation exhibited elevated hypothalamic CRH content at the time
when they responded developmentally to the declining water level. CRH
injections elevated whole-body thyroxine, triiodothyronine, and corti
costerone content, the primary hormonal regulators of metamorphosis. I
n contrast, alpha-helical CRH(9-41) reduced thyroid activity. These re
sults support a central role for CRH as a neurohormonal transducer of
environmental stimuli into the endocrine response which modulates the
rate of metamorphosis. Because in mammals, increased fetal/placental C
RH production may initiate parturition, and CRH has been implicated in
precipitating preterm birth arising from fetal stress, this neurohorm
onal pathway may represent a phylogenetically ancient developmental re
gulatory system that allows the organism to escape an unfavorable larv
al/fetal habitat. (C) 1997 Academic Press.