An automated flinch detecting system for use in the formalin nociceptive bioassay

1 Department of Anesthesiology, University of California, San Diego, La Jolla, California 92093; and 2 Yaksh Magnetic Solutions, Lilburn, Georgia 30047 The biphasic display of paw-flinch behavior in the rat after injection of formalin into the dorsum of the hind paw is used for the screening of anti-hyperalgesic agents. Described and characterized here is a less labor-intensive system for counting flinch activity by detecting movement of a small metal band placed on the formalin-injected paw. A signal is generated as the band breaks the electromagnetic field of a loop antenna located under the rat and processed through an algorithm that determines flinch activity using 1 ) amplitude, 2 ) zero-voltage crossing, and 3 ) signal duration. Flinches are summed and stored over a selected collection interval throughout the assay for later analysis. Studies have validated the measures with respect to 1 ) system stability over time; 2 ) system-to-"practiced observer" correlation on flinch detection, r 2 = 0.94; 3 ) system variables including time of day, sex, age, and body weight; and 4 ) 50% effective dose values similar to those previously reported for intrathecal morphine and the NMDA antagonist MK-801. formalin test; spinal sensitization; pain models; flinching behavior