title: The Severity of Acute Stress Is Represented by Increased Synchronous Activity and Recruitment of Hypothalamic CRH Neurons.
abstract: The hypothalamo-pituitary-adrenocortical (HPA) axis regulates stress physiology and behavior. To achieve an optimally tuned adaptive response, it is critical that the magnitude of the stress response matches the severity of the threat. Corticotropin-releasing hormone (CRH) released from the paraventricular nucleus of the hypothalamus is a major regulator of the HPA axis. However, how CRH-producing neurons in an intact animal respond to different stressor intensities is currently not known. Using two-photon calcium imaging on intact larval zebrafish, we recorded the activity of CRH cells, while the larvae were exposed to stressors of varying intensity. By combining behavioral and physiological measures, we first determined how sudden alterations in environmental conditions lead to different levels of stress axis activation. Then, we measured changes in the frequency and amplitude of Ca(2+) transients in individual CRH neurons in response to such stressors. The response magnitude of individual CRH cells covaried with stressor intensity. Furthermore, stressors caused the recruitment of previously inactive CRH neurons in an intensity-dependent manner, thus increasing the pool of responsive CRH cells. Strikingly, stressor-induced activity appeared highly synchronized among CRH neurons, and also across hemispheres. Thus, the stressor strength-dependent output of CRH neurons emerges by a dual mechanism that involves both the increased activity of individual cells and the recruitment of a larger pool of responsive cells. The synchronicity of CRH neurons within and across hemispheres ensures that the overall output of the HPA axis matches the severity of the threat.
  
  
  CONCLUSIONS
  Stressors trigger adaptive responses in the body that are essential for survival. How the brain responds to acute stressors of varying intensity in an intact animal, however, is not well understood. We address this question using two-photon Ca(2+) imaging in larval zebrafish with transgenically labeled corticotropin-releasing hormone (CRH) cells, which represent a major regulator of the stress axis. We show that stressor strength-dependent responses of CRH neurons emerge via an intensity-dependent increase in the activity of individual CRH cells, and by an increase in the pool of responsive CRH cells at the population level. Furthermore, we report striking synchronicity among CRH neurons even across hemispheres, which suggests tight intrahypothalamic and interhypothalamic coordination. Thus, our work reveals how CRH neurons respond to different levels of acute stress in vivo.
authors.affiliation: Developmental Genetics of the Nervous System, and.
authors.forename: Colette M
authors.initials: CM
authors.lastname: Vom Berg-Maurer
authors.affiliation: Department of Biomedical Optics, Neural Circuits and Behavior Research Group, Max Planck Institute for Medical Research, 69120 Heidelberg, Germany, and.
authors.forename: Chintan A
authors.identifier: http://orcid.org/0000-0003-3890-0744
authors.initials: CA
authors.lastname: Trivedi
authors.affiliation: Department of Biomedical Optics, Neural Circuits and Behavior Research Group, Max Planck Institute for Medical Research, 69120 Heidelberg, Germany, and.
authors.forename: Johann H
authors.initials: JH
authors.lastname: Bollmann
authors.affiliation: Developmental Genetics of the Nervous System, and.
authors.forename: Rodrigo J
authors.initials: RJ
authors.lastname: De Marco
authors.affiliation: Developmental Genetics of the Nervous System, and Focus Program Translational Neuroscience, University Medical Center, Johannes Gutenberg University Mainz, 55131 Mainz, Germany soojin.ryu@mpimf-heidelberg.mpg.de.
authors.forename: Soojin
authors.identifier: http://orcid.org/0000-0002-7059-0160
authors.initials: S
authors.lastname: Ryu
mesh_terms: D000818:Animals; D030801:Animals, Genetically Modified; D001362:Avoidance Learning / Q000502:physiology; D002118:Calcium / Q000378:metabolism; D003346:Corticotropin-Releasing Hormone / Q000235:genetics / Q000378:metabolism*; D005786:Gene Expression Regulation / Q000502:physiology*; D006360:Heat-Shock Proteins / Q000235:genetics / Q000378:metabolism; D006854:Hydrocortisone / Q000378:metabolism; D007031:Hypothalamus / Q000473:pathology*; D007814:Larva; D008164:Luminescent Proteins / Q000235:genetics / Q000378:metabolism; D008564:Membrane Potentials / Q000235:genetics / Q000502:physiology*; D009043:Motor Activity / Q000235:genetics; D009474:Neurons / Q000502:physiology*; D013312:Stress, Physiological / Q000502:physiology*; D015027:Zebrafish
chemical_list: D006360:Heat-Shock Proteins; D008164:Luminescent Proteins; D003346:Corticotropin-Releasing Hormone; D002118:Calcium; D006854:Hydrocortisone
keywords: HPA axis; calcium imaging; corticotropin releasing hormone; stress; synchronicity; zebrafish