Neuroendocrinology of Amphibian Metamorphosis - Current Topics in Developmental Biology - page 191

CHAPTER SEVEN

Neuroendocrinology of
Amphibian Metamorphosis

Robert J. Denver *

,1
* Department of Molecular, Cellular and Developmental Biology, The University of Michigan, Ann Arbor,
Michigan, USA
† Department of Ecology and Evolutionary Biology, The University of Michigan, Ann Arbor, Michigan, USA
1

, †

Corresponding author: e-mail address: [e-mail]

Contents
1. Hormonal Control of Metamorphosis 196

1.1 Thyroid hormone 196

1.2 Corticosteroids 203

2. Neuroendocrine Control of Metamorphosis 205

2.1 Thyroid-stimulating hormone 206

2.2 Adrenocorticotropic hormone 210

2.3 Growth hormone and prolactin 211

3. Role of the Neuroendocrine System in Mediating Environmental Influences on the
Timing of Metamorphosis 214

Acknowledgment 217

References 217

Abstract
The timing of metamorphosis is a central amphibian life history trait and is controlled by
the interplay of developmental progression, body size and condition, and environmen-
tal signals. These different processes and signals are integrated by the neuroendocrine
system to regulate production of hormones by the thyroid gland. Thyroid hormone (TH)
is the primary morphogen controlling metamorphosis, while corticosteroids (CSs) pro-
duced by the interrenal glands synergize with TH to promote metamorphic changes.
The actions of TH are modulated by monodeiodinase enzymes expressed in TH target
tissues. CSs act by sensitizing tissues to the actions of TH via the upregulation of TH recep-
tors andmonodeiodinases. The increase in thyroid gland activity duringmetamorphosis is
controlled by the hypothalamus and pituitary gland. The hypothalamo - pituitary - thyroid
and hypothalamo - pituitary - interrenal axes are regulated at multiple levels. Hypothalamic
corticotropin-releasing factor (CRF) functions as a common, central regulator of pituitary
thyroid-stimulating hormone (TSH) and adrenocorticotropic hormone (ACTH) secretion in
tadpoles. CRF neurons transduce the signals of environmental change (e.g., pond drying,
resource availability, etc.) on metamorphic timing by regulating TSH and ACTH secretion,
and consequently the production of TH and CS.