The effect of low and high concentrations of halothane, enflurane, and isoflurane on posttraining memory function was studied in male ddN mice. Mice were trained to escape an aversive electric foot shock as an unconditioned stimulus within 3 s after being exposed to light and a buzzer as a conditioned stimulus. Immediately after training (first session: 30 trials), the animals were exposed to halothane, enflurane, or isoflurane for 120 min and then were tested again on the avoidance task (second session: 30 trials) 22 h after cessation of exposure. The performance ratios, [B/A] (i.e., A is the score in the first session, and B the score in the second) were compared between the anesthetized groups and their respective control (nonanesthetized) groups. Mean performance ratios in the control groups ([B/A]c) ranged from 136.8% to 163.9% and those in the anesthetized groups ([B/A]a) ranged from 151.4% to 174.7%. [B/A] in each anesthetized group exceeded [B/A] in its corresponding control group. [B/A]a significantly exceeded [B/A]c by 13.1% in the 1.23 minimum alveolar concentration (MAC) enflurane group (P < 0.05) and by 12.4% in the 0.29 MAC isoflurane group (P < 0.05). These results suggest that posttraining exposure to volatile anesthetics facilitates memory.
We determined whether enflurane-induced opisthotonus in ddN mice is mediated by N-methyl-D-aspartate (NMDA) receptor using NMDA receptor antagonists dizocilpine (MK-801) and ketamine. Animals were given intraperitoneal injections of 0.2 ml saline (control), 2.5 or 5.0 mg.kg(-1) dizocilpine in saline, or 20 or 40 mg.kg(-1) ketamine is saline 20 min prior to exposure to 2.0% enflurane. Incidence of opisthotonus measured during exposure to enflurane for 20 min was 49% (n = 51) in saline (control) group, 6.7 (P < 0.01 vs control, n = 30) and 15.0% (P < 0.01, n = 40) in 2.5 and 5.0 mg.kg(-1) dizocilpine group, respectively, and 43.9 (NS, n = 41) and 40.0% (NS, n = 40) in 20 and 40 mg.kg(-1) ketamine group, respectively. These results strongly suggest that enflurane-induced opisthotonus is mediated by NMDA receptor. Ketamine failed to suppress significantly due to possibly small dosages. Further, dizocilpine itself produced severe seizures during preenflurane period (30.0 and 40.0% in 2.5 and 5.0 mg.kg(-1), respectively), which may be a novel finding.
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