Page 477 - The Toxicology of Fishes

10

The Endocrine System

Peter Thomas

CONTENTS
Introduction............................................................................................................................................457
Hypothalamic–Pituitary–Gonadal Axis .................................................................................................458
Endocrine Control of the Reproductive Cycle ............................................................................460
Neuroendocrine Control of Gonadotropin Secretion...................................................................462
Actions of Hormones at Target Tissues ................................................................................................463
Activation of Intracellular Second Messengers...........................................................................463
Genomic Steroid Actions via Nuclear Steroid Receptors...........................................................463
Nuclear Steroid Receptors in Fish .....................................................................................464
Nongenomic Steroid Actions via Membrane Steroid Receptors ................................................465
Membrane Steroid Receptors in Fish.................................................................................465
Sites of Chemical Interference on the Hypothalamic–Pituitary–Gonadal Axis...................................466
Mechanisms of Endocrine Toxicity.......................................................................................................467
Alterations of Hormone Secretion...............................................................................................467
Neuroendocrine Secretion ..................................................................................................467
Gonadal Steroidogenesis ....................................................................................................468
Interference with Hormone Actions.............................................................................................469
Intracellular Second Messengers........................................................................................469
Interference with Genomic Steroid Actions via Nuclear Steroid Receptors..................... 469
Factors That Influence Steroid-Receptor-Mediated Endocrine Disruption ....................... 471
Interference with Nongenomic Steroid Actions via Membrane Steroid Receptors.................... 472
Hypothalamic–Pituitary–Interrenal Axis ...............................................................................................474
Interference of the HPI Axis........................................................................................................475
Hypothalamic–Pituitary–Thyroid Axis..................................................................................................476
Interference of the HPT Axis.......................................................................................................477
Summary ................................................................................................................................................477
Acknowledgments..................................................................................................................................478
References..............................................................................................................................................478

Introduction

The endocrine, nervous, and immune systems are the primary means of communication between cells
and organs in multicellular animals. The regulation of many physiological functions involves integration
of nervous and endocrine systems, although typically endocrine responses are slower and longer lasting
than neural ones. Specific chemical messengers, hormones, are secreted by endocrine glands directly
into the blood. Although the hormones are distributed in the bloodstream throughout the body, they only
act at specific target tissues. This specificity of hormone action is due to the presence of specific receptor
molecules that preferentially bind the hormone in the target cells. Hormone binding activates the receptor,
causing stimulation of intracellular second messenger pathways or direct binding of the hormone–receptor

457