Evaluation of acetylcholine, seizure activity and neuropathology following high-dose nerve agent exposure and delayed neuroprotective treatment drugs in freely moving rats

Abstract

Organophosphorus nerve agents such as soman (GD) inhibit acetylcholinesterase, producing an excess of acetylcholine (ACh), which results in respiratory distress, convulsions and status epilepticus that leads to neuropathology. Several drugs (topiramate, clobazam, pregnanolone, allopregnanolone, UBP 302, cyclopentyladenosine [CPA], ketamine, midazolam and scopolamine) have been identified as potential neuroprotectants that may terminate seizures and reduce brain damage. To systematically evaluate their efficacy, this study employed in vivo striatal microdialysis and liquid chromatography to respectively collect and analyze extracellular ACh in freely moving rats treated with these drugs 20 min after seizure onset induced by a high dose of GD. Along with microdialysis, EEG activity was recorded and neuropathology assessed at 24 h. GD induced a marked increase of ACh, which peaked at 30 min post-exposure to 800% of control levels and then steadily decreased toward baseline levels. Approximately 40 min after treatment, only midazolam (10 mg/kg) and CPA (60 mg/kg) caused a significant reduction of ACh levels, with CPA reducing ACh levels more rapidly than midazolam. Both drugs facilitated a return to baseline levels at least 55 min after treatment. At 24 h, only animals treated with CPA (67%), midazolam (18%) and scopolamine (27%) exhibited seizure termination. While all treatments except for topiramate reduced neuropathology, CPA, midazolam and scopolamine showed the greatest reduction in pathology. Our results suggest that delayed treatment with CPA, midazolam, or scopolamine is effective at reducing GD-induced seizure activity and neuropathology, with CPA and midazolam capable of facilitating a reduction in GD-induced ACh elevation.

Keywords:

• Acetylcholine
• adenosine receptor agonist
• anticholinergics
• anticonvulsant
• benzodiazepines
• glutamate receptor antagonist
• microdialysis
• midazolam
• N6-Cyclopentyladenosine
• nerve agent
• neuropathology
• neuroprotectant
• neurosteroids
• scopolamine
• soman

Disclosure statement

The views expressed in this article are those of the authors and do not reflect the official policy of the Department of Army, Department of Defense, or the United States Government. The experimental protocol was approved by the Animal Care and Use Committee at the United States Army Medical Research Institute of Chemical Defense and all procedures were conducted in accordance with the principles stated in the Guide for the Care and Use of Laboratory Animals and the Animal Welfare Act of 1966 (P.L. 89-544), as amended. The facility where this research was conducted is fully accredited by the Association for Assessment and Accreditation of Laboratory Animal Care International.

Funding information

This research was supported by an Interagency Agreement (IAA) between NIH/NIAID (IAA #AOD14018-001-00000) and the USAMRICD (#A120-B.P2014-02) for Chemicals Affecting the Nervous System – Anticonvulsants/Neuroprotectants. This research was supported in part by an appointment (Cindy Acon-Chen, Jeffrey A. Koenig, Garrett R. Smith, and Amber R. Truitt) to the Postgraduate Research Participation Program at the U.S. Army Medical Research Institute of Chemical Defense administered by the Oak Ridge Institute for Science and Education through an interagency agreement between the U.S. Department of Energy and USAMRMC.