Intrathecal delivery of fluorescent labeled butyrylcholinesterase to the brains of butyrylcholinesterase knock-out mice: Visualization and quantification of enzyme distribution in the brain

Exogenously delivered butyrylcholinesterase (BChE) has proven to be an efficient bioscavenger against highly toxic organophosphorus poisons and nerve agents. The scavenger properties of BChE when delivered via intramuscular, intravenous, subcutaneous, or intraperitoneal routes are limited to the body's peripheral sites because the 340 kDa enzyme does not cross the blood–brain barrier (BBB). Overcoming the BBB is an important step toward evaluating the neuroprotective properties of BChE within the central nervous system (CNS). This study examines the feasibility of delivering BChE to the brain and spinal cord by intrathecal (IT) injection. Mice completely devoid of BChE were injected intrathecally with either BChE (80 units) that was labeled with near-infrared fluorescent dye (BChE/IRDye) or a molar equivalent amount of carboxylate dye. The BChE/IRDye and carboxylate dye were tracked using an in vivo imaging system demonstrating the real-time distribution of BChE in the brain and the residence time in the brain and spinal cord through 25 h post-dosing. BChE/IRdye levels in the brain peaked at 6 h post-dosing. BChE enzyme activity was quantified in plasma and brain sections by BChE activity assays of plasma and of perfused tissues. Average BChE activity levels were 0.6 units/g in the brains of mice treated with BChE/IRDye at 4 h post-dosing. Intense fluorescent signal in the cortex, dentate gyrus and ventricles of the brain at 25 h post-dosing was visualized by confocal microscopy and the presence of BChE was confirmed with activity assays of frozen sections. This procedure proved to be an efficient, safe and rapid method to deliver BChE to the CNS of mice, providing a research tool for determining neural protection by BChE following OP exposure.