Analysis of vigilant scanning behavior in mice using two-point digital video tracking

Abstract

Vigilant scanning of the environment is a major risk assessment activity in many species. However, due to difficulties in its manual scoring, scanning has rarely been quantified in laboratory rodent studies. We developed a novel method for automated measurement of vigilant scanning in mice, based on simultaneous tracking of an animal's nose- and center-points. The studied scanning parameters included the frequency and duration of scans and scanning (nose-point) speed. The sensitivity of these parameters to anxiolytic diazepam (1-2mg/kg) and anxiogenic FG-7142 (5mg/kg) was evaluated upon exposure to the context (conditioning chamber) before and 24h after footshock. Scanning behavior was observed in all C57BL/6, 129xC57BL/6, and DBA/2 mice, as recurrent stationary episodes accompanied by observatory head movements. These episodes respectively comprised 28 ± 1%, 29 ± 1%, and 24 ± 2% of preexposure time. Diazepam dose-dependently decreased the scanning frequency and duration, without affecting the scanning speed. Fear conditioning increased freezing and inhibited other behaviors upon reexposure, with scanning being only marginally affected and still comprising 17 ± 2%, 16 ± 2%, and 19 ± 1% of reexposure time, respectively. Consequently, scanning accounted for most (DBA/2) or virtually all (C57BL/6 and 129xC57BL/6) gross motor activities upon reexposure. FG-7142 mirrored the effects of conditioning, inducing behavioral inhibition with scanning being least affected. Two-point tracking is effective for studying vigilant scanning in mice. Using this approach, we show that scanning is a key risk assessment activity in both unconditioned and conditioned mice; scanning is resistant to threat-induced behavioral inhibition and is highly sensitive to anxiolytic treatment.