Memory enhancing effect of low-dose sevoflurane does not occur in basolateral amygdala-lesioned rats

Certain anesthetics might enhance aversive memory at doses around 0.1 minimum alveolar concentration. This issue was investigated in a rat model of learning and memory. In addition, evidence for basolateral amygdala (BLA) involvement in mediating memory enhancement was sought.

First, the memory-enhancing potential of various anesthetics was determined. Rats underwent single-trial inhibitory avoidance training (0.3 mA shock/1 s) during exposure to air, 0.11% sevoflurane, 0.10% halothane, 0.77% desflurane, or 0.12% isoflurane. Memory was assessed at 24 h. Second, the BLA contribution to sevoflurane memory enhancement was determined. Rats received bilateral excitotoxic N-methyl-D-aspartate (12.5 mg in 0.2 microl per BLA) lesions of the BLA 1 week before training. Memory of lesioned and control rats was compared 24 h after training in air or sevoflurane.

Sevoflurane exposure during training significantly enhanced 24-h retention performance for both nonoperated and sham-operated rats (P < 0.005 for both vs. their respective controls). Halothane, but not desflurane or isoflurane, also enhanced retention performance (P < 0.05). However, halothane-induced hyperalgesia during learning clouds interpreting enhanced retention performance solely as a memory consolidation effect. BLA lesions significantly reduced and equalized retention performance for both sevoflurane- and air-exposed animals. Lesions blocked memory enhancement without also causing a generalized inability to learn, because additional training revealed essentially normal task acquisition and 24-h memory.

Sevoflurane enhances aversive memory formation in the rat. The BLA likely contributes to this effect. The risk of aversive memory formation may be enhanced during exposure to low-dose sevoflurane.