MORPhA Scale: Behavioral and electroencephalographic validation of a rodent anesthesia scale

Background: Laboratory practice commonly implies rodent anesthesia. Such is instrumental not only for animal welfare, but also for standardized assessment of new anesthetics.

New method: We developed and validated a grading system for a temporal follow-up of anesthesia. The Minho Objective Rodent Phenotypical Anesthesia (MORPhA) scale was tested in mice (C57BL/6 J) and rats (Wistar-Han) anesthetized with ketamine/dexmedetomidine (ket/dex). The scale comprises 12 behavioral readouts organized in 5 stages - (i) normal/(ii) hindered voluntary movement, elicited response to (iii) non-noxious/(iv) noxious stimuli and (v) absence of response - evaluated at regular time-points. Progression across stages was monitored by electroencephalography (EEG) in rats during anesthesia induction and reversal (atipamezole) and during induction with a second anesthetic drug (pentobarbital).

Results: Higher anesthetic doses decreased the time to reach higher levels of anesthesia during progression, while increasing the time to regain waking behavior during reversal in both mice and rats. A regular decrease in high frequencies (low and high gamma) power was observed as the MORPhA score increased during anesthesia induction, while the opposite pattern was observed during emergence from anesthesia through reversion of dex effect.

Comparison with existing methods: Degree of anesthesia in laboratory rodents is normally evaluated by testing loss of reflexes. While these are useful endpoint assessments, they are of limited application to study induction/reversal kinetics or factors affecting individual susceptibility. MORPhA allows graded standardized assessment of this progression in a biologically-relevant fashion.

Conclusions: The devised anesthetic scale is of simple application and provides a semi-quantifiable readout of anesthesia induction/reversal.