Differential effects of isoflurane, halothane, and ketamine on the regional methionine-enkephalinlike immunoreactivity in the mouse brain.

The widely used measurement index for anesthetic potency, minimum alveolar concentration (MAC), is hypothesized to be the sum of the effects on multiple neural systems whose contribution to anesthesia differs depending on the agents used. The present study, which compared the effects of halothane, isoflurane, and ketamine, at equipotent level of anesthesia, on the methionine-enkephalinergic neurons in 9 brain regions, showed a significant difference in the methionine-enkephalin-like immunoreactivity (Met-ENK-like IR) among the anesthetics in each region. The order of the Met-ENK-like IR was: halothane > ketamine > isoflurane in the caudatus putamen; halothane > isoflurane ≊ketamine in the nucleus accumbens and the ventral pallidum; halothane ≊isoflurane > ketamine in the globus pallidus, the nucleus dorsomedialis hypothalami, and the nucleus ventromedialis hypothalami; and halothane > isoflurane > ketamine in the arcuate nucleus, the periaqueductal gray, and the nucleus reticularis parvocellularis. These findings indicate that these three anesthetics affect the methionine-enkephalinergic neurons in the motor and pain controlling pathways in different fashions.