Faster immobilization and recovery with remimazolam compared to midazolam in medetomidine–butorphanol balanced anesthesia in male Sprague–Dawley rats: a randomized crossover study

BACKGROUND: Balanced injectable anesthesia combining an α2-adrenoceptor agonist, a benzodiazepine, and an opioid is widely used in laboratory rodents because it provides sedation, muscle relaxation, and analgesia through complementary pharmacological mechanisms. In Japan, a mixture of medetomidine, midazolam, and butorphanol (MMB) is commonly used as an injectable anesthetic regimen in mice and rats. Remimazolam is an ultra-short-acting benzodiazepine that is rapidly metabolized by tissue esterases and produces no active metabolites. Because of its rapid metabolism and predictable pharmacokinetic profile, remimazolam has attracted attention as a sedative agent in clinical anesthesia and procedural sedation. Replacing midazolam with remimazolam generates a novel balanced anesthetic combination consisting of medetomidine, remimazolam, and butorphanol (MRB). Although this regimen has been investigated in mice, its anesthetic characteristics in rats have not yet been fully evaluated. The present study aimed to compare the anesthetic and physiological effects of MMB and MRB anesthesia in Sprague-Dawley (SD) rats. RESULTS: Both mixtures induced anesthesia in all animals. The MRB regimen produced faster immobilization than the MMB regimen, whereas the time to loss of righting reflex did not differ significantly between regimens. Reflex-based anesthetic depth during the observation period was comparable between the two regimens. Physiological monitoring revealed regimen-dependent differences in time-course dynamics, particularly for oxygen saturation and respiratory rate. For oxygen saturation, pointwise comparisons showed no significant difference after Holm correction, whereas the area under the curve (AUC) analysis did show a significant difference. Linear mixed-effects modeling (LMM) further demonstrated significant group, time, and group × time interaction effects for oxygen saturation, and significant time and group × time interaction effects for respiratory rate. Recovery after atipamezole (ATI) administration was faster in the MRB group, as shown by both return of righting reflex and behavioral recovery score analyses. CONCLUSIONS: MRB produced a reflex-score profile comparable to that of MMB while accelerating early immobilization and antagonist-assisted recovery. Substitution of midazolam with remimazolam also altered SpO₂ and respiratory-rate trajectories. MRB may therefore be useful when rapid recovery is desired. However, comparability was limited to the reflex-based scoring system; these findings do not establish equivalence for painful or invasive surgical procedures.