3 - The Endocrinology of the Stress Response in Fish: An Adaptation-Physiological View

Abstract

1. Introduction 1.1. The Fish Forebrain 1.2. Stress 2. Stress and the Brain: The (Neuro-)Endocrine Hypothalamus 2.1. Fundamental Axes Interact 2.2. The CRF System 2.3. Ontogeny of the CRF System 2.4. Control Over the Pituitary Gland 2.5. CRF and Behavior 3. Stress and the Pituitary Gland 3.1. Adrenocorticotropic Hormone (ACTH) 3.2. Alpha-MSH 4. Stress and the Head Kidney 4.1. Catecholamine-Producing Cells 4.2. Steroid-Producing Cells 4.3. Communication Within the Head Kidney 4.4. Stress and Energy 5. Synthesis and Perspective For any organism dealing with environmental challenges, proper handling of stressful conditions is key to survival. Extant fishes represent the earliest vertebrates on earth and must have been masters in doing so, given their vast and sometimes fast radiation. Ancestral genome expansions (two or three whole genome duplication rounds) and stable water conditions contributed to their great ability to evolve and the eventual rise of tetrapods. An elaborate endocrine machinery provides the chemical mediation of a hypothalamically integrated signal to properly spend energy and allow for fight or flight when confronted with stressful conditions. We discuss developments in fish forebrain and (nonexhaustively) hypothalamic lay-out from the newest insights, obtained mostly from zebrafish studies. Corticotropin releasing factor, adrenocorticotropic hormone (ACTH), α-melanocyte-stimulating hormone, adrenaline, and cortisol, the key chemical mediators in the hypothalamic–pituitary–interrenal (HPI) axis, are passed in review and in the context of allostatic regulation of stress responses. We dedicate this chapter to Sjoerd E. Wendelaar Bonga, friend and teacher, who introduced us to the concept of stress and taught us to deal with it.