INHALATION ANESTHETICS SUPPRESS THE EXPRESSION OF C-FOS PROTEIN EVOKED BY NOXIOUS SOMATIC STIMULATION IN THE DEEPER LAYER OF THE SPINAL-CORD IN THE RAT

The effects of inhalation anesthetics, nitrous oxide (N2O) and halotha ne, on the expression of c-Fos protein evoked by formalin injection we re studied in the spinal cord in the rat. The expression of c-Fos prot ein was detected by immunocytochemistry following the injection of for malin (5%, 100 mu l) into the plantar surface of the left hindpaw. Aft er 15 min of halothane (F) anesthesia, the anesthetics was switched to 40% or 70% of N2O, 0.5% or 1.5% of F or room air (for control) immedi ately following the formalin injection. Two hours later the rats were sacrificed and perfused. Sections of the IA level of spinal cord were immunostained with anti c-Fos antibody. We counted the number of Fos-l ike immunoreactive (FLI) cells in every specific lamina as follows; su perficial layer (laminae I and II), nucleus proprius (laminae III and TV), neck of the dorsal horn (laminae V and VI) and ventral gray (lami nae VII-X). Then we compared the results of each category of sample. B oth N2O and halothane suppressed the expression of c-Fos in the neck o f the dorsal horn and ventral gray in a dose-dependent manner, but no effects were seen at the superficial layer or nucleus proprius. Suppre ssion of c-Fos expression was greater under N2O than halothane anesthe sia. This finding suggests that N2O had a stronger analgesic effect th an halothane. The current study indicates that inhalation anesthetics do not act equally on every kind of spinal neurons. Both N2O and halot hane have effects on spinal neurons in the deeper layers but not on th e neurons existed in laminae I-II, some of which directly receive noxi ous inputs. Pretreatment with 2 mg/kg of naloxone, which completely re versed the effects of morphine, did not alter the effect of 70%N2O, su ggesting that the analgesic effect of N2O is not mediated by an intrin sic opioid mechanism at the spinal cord level. (C) 1997 Elsevier Scien ce B.V.