Single-cell morphological characterization of CRH neurons throughout the whole mouse brain

Yu Wang,Jiang-Ning Zhou,Hui Gong,Qinghong Shan,Pu Hu,Peng Chen,Chuan Huang,Anan Li,Zhaohuan Huang

doi:10.1186/s12915-021-00973-x

Abstract

BackgroundCorticotropin-releasing hormone (CRH) is an important neuromodulator that is widely distributed in the brain and plays a key role in mediating stress responses and autonomic functions. While the distribution pattern of fluorescently labeled CRH-expressing neurons has been studied in different transgenic mouse lines, a full appreciation of the broad diversity of this population and local neural connectivity can only come from integration of single-cell morphological information as a defining feature. However, the morphologies of single CRH neurons and the local circuits formed by these neurons have not been acquired at brain-wide and dendritic-scale levels.ResultsWe screened the EYFP-expressing CRH-IRES-Cre;Ai32 mouse line to reveal the morphologies of individual CRH neurons throughout the whole mouse brain by using a fluorescence micro-optical sectioning tomography (fMOST) system. Diverse dendritic morphologies and projection fibers of CRH neurons were found in various brain regions. Follow-up reconstructions showed that hypothalamic CRH neurons had the smallest somatic volumes and simplest dendritic branches and that CRH neurons in several brain regions shared a common bipolar morphology. Further investigations of local CRH neurons in the medial prefrontal cortex unveiled somatic depth-dependent morphologies of CRH neurons that exhibited three types of mutual connections: basal dendrites (upper layer) with apical dendrites (layer 3); dendritic-somatic connections (in layer 2/3); and dendritic-dendritic connections (in layer 4). Moreover, hypothalamic CRH neurons were classified into two types according to their somatic locations and characteristics of dendritic varicosities. Rostral-projecting CRH neurons in the anterior parvicellular area had fewer and smaller dendritic varicosities, whereas CRH neurons in the periventricular area had more and larger varicosities that were present within dendrites projecting to the third ventricle. Arborization-dependent dendritic spines of CRH neurons were detected, among which the most sophisticated types were found in the amygdala and the simplest types were found in the hypothalamus.ConclusionsBy using the CRH-IRES-Cre;Ai32 mouse line and fMOST imaging, we obtained region-specific morphological distributions of CRH neurons at the dendrite level in the whole mouse brain. Taken together, our findings provide comprehensive brain-wide morphological information of stress-related CRH neurons and may facilitate further studies of the CRH neuronal system.

Highlights

Corticotropin-releasing hormone (CRH) is an important neuromodulator that is widely distributed in the brain and plays a key role in mediating stress responses and autonomic functions
In the olfactory bulb (OB), the transgenic fluorescent proteins were mainly distributed in the glomerular layer (Gl), external plexiform layer (EPl), and the mitral cell layer (Mi) in all three mouse lines
CRHIRES-Cre;Ai14 mice were labeled with bright cell bodies and dense fibers were labeled in the EPl (Fig. 1B, b’), but only a few dendritic structures were labeled in the Gl (Fig. 1B, b, indicated by the dotted box)

Summary

Introduction

Corticotropin-releasing hormone (CRH) is an important neuromodulator that is widely distributed in the brain and plays a key role in mediating stress responses and autonomic functions. CRH-expressing neurons are broadly distributed in other brain regions, including the inferior olivary nucleus, Barrington’s nucleus, pontine tegmentum, cerebral cortex, hippocampus, and central amygdala. Depending on their region-specific somatic locations, CRH neurons participate in various functional activities, such as learning memory, synaptic plasticity, food intake, and drug addiction, as well as anxiety-like and depression-like behaviors [4,5,6,7]. The precision of imaging via fMOST can reveal complex fiber orientations and can even distinguish individual dendrites Such quantitative three-dimensional (3D) neuronal morphologies obtained at a brain-wide scale can provide highly accurate arborization details and comprehensive mapping of CRH neuronal connections throughout the brain

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Discussion

Conclusion