Basic Study
Copyright ©The Author(s) 2021. Published by Baishideng Publishing Group Inc. All rights reserved.
World J Gastroenterol. Mar 7, 2021; 27(9): 794-814
Published online Mar 7, 2021. doi: 10.3748/wjg.v27.i9.794
Effect of acetyl-L-carnitine on hypersensitivity in acute recurrent caerulein-induced pancreatitis and microglial activation along the brain’s pain circuitry
Sabrina L McIlwrath, Marlene E Starr, Abigail E High, Hiroshi Saito, Karin N Westlund
Sabrina L McIlwrath, Karin N Westlund, Research Service, New Mexico Veterans Affairs Healthcare System, Albuquerque, NM 87108, United States
Marlene E Starr, Hiroshi Saito, Department of Surgery, University of Kentucky, Lexington, KY 40536, United States
Abigail E High, College of Liberal Arts, University of Texas, Austin, TX 78712, United States
Karin N Westlund, Department of Anesthesiology and Critical Care Medicine, University of New Mexico Health Sciences Center, Albuquerque, NM 87131, United States
Author contributions: McIlwrath SL performed the animal experiments, data analysis, produced the figures, wrote the manuscript; Starr ME assisted with animal caerulein dosing and read the manuscript; High AE analyzed the microglial morphology in the brain sections; Saito H designed the study, read and edited this manuscript; Westlund KN designed the study, read and edited this manuscript; all authors discussed the results, commented on the manuscript, and approved the final version of the article.
Supported by United States Department of Veterans Affairs, VA Merit Grant, No. BX002695; and United States National Institute of Health, No. R01AG055359, No. R01GM126181 and No. R01NS39041-15.
Institutional animal care and use committee statement: This study was reviewed and approved by the Institutional Animal Care and Use Committee of the University of Kentucky (IACUC Protocol No. 2010-0736). The animal care facility of the University of Kentucky is accredited by AALAC.
Conflict-of-interest statement: The authors have nothing to disclose. This communication does not necessarily reflect the views of the Department of Veterans Affairs or the United States government.
Data sharing statement: No additional data are available.
ARRIVE guidelines statement: The authors have read the ARRIVE guidelines, and the manuscript was prepared and revised according to the ARRIVE guidelines.
Open-Access: This article is an open-access article that was selected by an in-house editor and fully peer-reviewed by external reviewers. It is distributed in accordance with the Creative Commons Attribution NonCommercial (CC BY-NC 4.0) license, which permits others to distribute, remix, adapt, build upon this work non-commercially, and license their derivative works on different terms, provided the original work is properly cited and the use is non-commercial. See: http://creativecommons.org/Licenses/by-nc/4.0/
Corresponding author: Sabrina L McIlwrath, PhD, Senior Scientist, Research Service, New Mexico Veterans Affairs Healthcare System, 1501 San Pedro SE, Albuquerque, NM 87108, United States. sabrina.mcilwrath@va.gov
Received: September 11, 2020
Peer-review started: September 11, 2020
First decision: November 25, 2020
Revised: December 8, 2020
Accepted: January 15, 2021
Article in press: January 15, 2021
Published online: March 7, 2021
Processing time: 172 Days and 11.8 Hours
Abstract
BACKGROUND

Acute pancreatitis (AP) and recurring AP are serious health care problems causing excruciating pain and potentially lethal outcomes due to sepsis. The validated caerulein- (CAE) induced mouse model of acute/recurring AP produces secondary persistent hypersensitivity and anxiety-like behavioral changes for study.

AIM

To determine efficacy of acetyl-L-carnitine (ALC) to reduce pain-related behaviors and brain microglial activation along the pain circuitry in CAE-pancreatitis.

METHODS

Pancreatitis was induced with 6 hly intraperitoneal (i.p.) injections of CAE (50 µg/kg), 3 d a week for 6 wk in male C57BL/6J mice. Starting in week 4, mice received either vehicle or ALC until experiment’s end. Mechanical hyper-sensitivity was assessed with von Frey filaments. Heat hypersensitivity was determined with the hotplate test. Anxiety-like behavior was tested in week 6 using elevated plus maze and open field tests. Microglial activation in brain was quantified histologically by immunostaining for ionized calcium-binding adaptor molecule 1 (Iba1).

RESULTS

Mice with CAE-induced pancreatitis had significantly reduced mechanical withdrawal thresholds and heat response latencies, indicating ongoing pain. Treatment with ALC attenuated inflammation-induced hypersensitivity, but hypersensitivity due to abdominal wall injury caused by repeated intraperitoneal injections persisted. Animals with pancreatitis displayed spontaneous anxiety-like behavior in the elevated plus maze compared to controls. Treatment with ALC resulted in increased numbers of rearing activity events, but time spent in “safety” was not changed. After all the abdominal injections, pancreata were translucent if excised at experiment’s end and opaque if excised on the subsequent day, indicative of spontaneous healing. Post mortem histopathological analysis performed on pancreas sections stained with Sirius Red and Fast Green identified wide-spread fibrosis and acinar cell atrophy in sections from mice with CAE-induced pancreatitis that was not rescued by treatment with ALC. Microglial Iba1 immunostaining was significantly increased in hippocampus, thalamus (intralaminar nuclei), hypothalamus, and amygdala of mice with CAE-induced pancreatitis compared to naïve controls but unchanged in the primary somatosensory cortex compared to naïves.

CONCLUSION

CAE-induced pancreatitis caused increased pain-related behaviors, pancreatic fibrosis, and brain microglial changes. ALC alleviated CAE-induced mechanical and heat hypersensitivity but not abdominal wall injury-induced hypersensitivity caused by the repeated injections.

Keywords: Acute recurrent pancreatitis; Neuropathic pain; Mechanical hypersensitivity; Heat hypersensitivity; Anxiety-like behavior; Ionized calcium-binding adaptor molecule 1

Core Tip: The caerulein- (CAE) induced pancreatitis model requiring 6 wk of repeated injections is a model of recurrent acute bouts of pancreatitis that causes pancreatic tissue damage and fibrosis. Control repeated i.p. saline injections alone caused abdominal wall injury and hindpaw secondary mechanical hypersensitivity. Treatment with acetyl-L-carnitine significantly attenuated CAE-induced hypersensitivity without alleviating pancreatic histological disruption. Mice with CAE-induced pancreatitis with secondary mechanical and heat hypersensitivity had elevated plus maze anxiety-like behavior. Post-mortem analysis revealed microglial morphology changes indicative of activation in amygdala, hippocampus, hypothalamus, and thalamus, but not in primary somatosensory cortex. These data suggest that activated microglia in these brain regions contribute to chronic hypersensitivity and anxiety-like behaviors in mice with CAE-induced pancreatitis.