Effects of Different Anesthetics on the Paired-Pulse Depression of the H Reflex in Adult Rat

Hyperreflexia is a common feature of spinal cord injury (SCI), and changes in reflex excitability have been reported to be useful in assessing treatments in animal models of cord damage. However, spinal reflexes are known to be dependent on anesthetic level. As a preliminary to its use in SCI, the excitability of the Hoffman reflex (H reflex) has been assessed under several commonly used anesthetics. The H reflex was recorded in the distal foot muscles (dorsal interossei) of adult rats, while the lateral plantar nerve was stimulated. Five different anesthetics were used: ketamine, halothane, Nembutal, Etomidate, and Saffan. Recording and stimulating electrodes were inserted directly through the skin to minimize the surgical procedure for each experiment, allowing repeated recording to be made in the same animal on a weekly basis. Suppression of the H reflex was tested using twin-pulse stimulation. Halothane and ketamine produced suppression of the H response when interpulse intervals were shortened to less than 1 s. The H-reflex suppression profiles recorded under Etomidate, Saffan, and Nembutal anesthesia were less sensitive to the stimulation rate, with little reduction until intervals were 200 ms or less. The suppression profiles of halothane and ketamine resemble that seen in unanesthetized humans, whereas that under the other anesthetics tried here resembles that observed in spinal-cord-injured animals. The results suggest a preferential action of some anesthetics on descending pathways involved in reflex modulation and the importance of assessing reflex excitability under anesthetics such as ketamine or halothane.