Proprioceptive function is more sensitive than motor function to desflurane anesthesia

BACKGROUND: Evaluating the effects of sub-immobilizing anesthetic doses on movement will identify target neural circuits for investigation as sites of action for anesthetic-induced immobility. METHODS: Eleven pithed Northern Leopard frogs received 0, 0.4, 0.8, and 1.2 times the 50% effective dose for production of immobility (ED ₅₀) of desflurane and a further 7 received 0 and 0.4 ED ₅₀ desflurane in random order. An electric stimulus applied to the forelimb elicited a hindlimb wiping reflex that was captured on video for later analysis. Isometric tension developed in the hindlimb during the 30 s stimulus application was measured. RESULTS: Compared to 0 ED50, 0.4 ED ₅₀ desflurane significantly increased latency to wipe 0.8 (0.1, 4.0) to 17.3 (0.4, 30.0) s (median [min max]), distance traveled by the hindfoot 0.42 (0.09, 1.82) to 0.89 (0.16, 4.82) m, and proximity of the hindfoot to stimulus 1 (0, 5) to 7 (1, 40) mm. It did not alter hindlimb maximum velocity or isometric tension but significantly reduced total hindlimb force 7.3 (1.7, 23.6) to 3.2 (1.4, 13.8) N. s proportionate to a reduced number of movements from 12 (3, 28) to 8 (2, 14). From 0.4 to 0.8 ED ₅₀, motor depressant effects of desflurane became apparent with significant reductions in maximum tension from 2.0 (0.6, 5.5) to 0.8 (0.1, 1.6) N and total force from 3.2 (1.4, 13.8) to 0.9 (0.0, 2.5) N.s. CONCLUSIONS: Proprioceptive function is more sensitive to anesthetic-induced depression than motor function in frogs. This suggests that the most anestheticsensitive component of the spinal neural circuitry underlying movement generation in response to noxious stimulus is prior to the level of the motoneuron.