Repeated adrenocorticotropic hormone administration alters adrenal and thyroid hormones in free-ranging elephant seals.

Molly C McCormley,Jared S Deyarmin,Cory D Champagne,Alicia P Stephan,Daniel E Crocker,Dorian S Houser,Jane I Khudyakov

doi:10.1093/conphys/coy040

Abstract

Understanding the physiological response of marine mammals to anthropogenic stressors can inform marine ecosystem conservation strategies. Stress stimulates the activation of the hypothalamic-pituitary-adrenal (HPA) axis and synthesis of glucocorticoid (GC) hormones, which increase energy substrate availability while suppressing energy-intensive processes. Exposure to repeated stressors can potentially affect an animal's ability to respond to and recover from subsequent challenges. To mimic repeated activation of the HPA axis by environmental stressors (or challenges), we administered adrenocorticotropic hormone (ACTH) to free-ranging juvenile northern elephant seals (Mirounga angustirostris; n = 7) once daily for 4 days. ACTH administration induced significant elevation in circulating cortisol and aldosterone levels. The cortisol responses did not vary in magnitude between the first ACTH administration on Day 1 and the last administration on Day 4. In contrast, aldosterone levels remained elevated above baseline for at least 24 h after each ACTH injection, and responses were greater on Day 4 than Day 1. Total triiodothyronine (tT3) levels were decreased on Day 4 relative to Day 1, while reverse triiodothyronine (rT3) concentrations increased relative to baseline on Days 1 and 4 in response to ACTH, indicating a suppression of thyroid hormone production. There was no effect of ACTH on the sex steroid dehydroepiandrosterone. These data suggest that elephant seals are able to mount adrenal responses to multiple ACTH administrations. However, repeated ACTH administration resulted in facilitation of aldosterone secretion and suppression of tT3, which may impact osmoregulation and metabolism, respectively. We propose that aldosterone and tT3 are informative additional indicators of repeated stress in marine mammals.

Highlights

Anthropogenic disturbance can impact vulnerable wildlife populations by compounding allostatic loads experienced by individuals as a result of natural challenges (Romero et al, 2009)
We used linear mixed models (LMMs), with subject ID as a random effect, to explore hormone variation among samples repeatedly collected from individuals after adrenocorticotropic hormone (ACTH) administration
Responses of each hormone to ACTH administration were assessed with LMMs within Days 1 and 4; if significant differences were detected, we followed with a Dunnett’s post hoc test against the 0-h sample from that day

Summary

Introduction

Anthropogenic disturbance can impact vulnerable wildlife populations by compounding allostatic loads experienced by individuals as a result of natural challenges (Romero et al, 2009). Increased anthropogenic activity in coastal and marine habitats, such as noise pollution, commercial fishing and humandriven habitat loss, in addition to natural environmental challenges (e.g. prey availability), are correlated with modern declines in marine mammal populations (Springer et al, 2003; Atkinson et al, 2008; Maxwell et al, 2013). Little information exists on impacts and indicators of chronic stress (e.g. endocrine response to repeated or sustained stressors) in free-ranging marine mammals and most other wildlife species (Fair and Becker, 2000; Dickens and Romero, 2013). Evaluation of the physiological impacts of both acute and chronic stress in marine mammals is necessary to understand how populations may respond to anthropogenic and environmental disturbance over time and can help inform conservation management strategies (Cooke et al, 2010; Dantzer et al, 2014; Jessop et al, 2013)

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