Does the Amygdala Mediate Anesthetic-induced Amnesia?

Background Amnesia for aversive events caused by benzodiazepines or propofol depends on the basolateral amygdala (BLA). Whether the amnesia of volatile anesthesia is also mediated through the BLA is unknown. If so, a general principle of anesthetic-induced amnesia may be emerging. Here, using an inhibitory avoidance paradigm, the authors determine whether BLA lesions prevent sevoflurane-induced amnesia. Methods Male Sprague-Dawley rats were separated into two groups: sham-operated controls (n = 22) and rats given bilateral N-methyl-D-aspartate lesions of the BLA (n = 32). After a 1-week recovery, the rats were randomly assigned to be trained during either air or sevoflurane (0.3% inspired, 0.14 minimum alveolar concentration) exposure. Animals learned to remain in the starting safe compartment of a step-through inhibitory avoidance apparatus for 100 consecutive seconds by administering foot shock (0.3 mA) whenever they entered an adjacent shock compartment. Memory was assessed at 24 h. Longer latencies to enter the shock compartment at 24 h imply better memory. Results Sham-air (n = 10) animals had a robust memory, with a median retention latency of 507 s (interquartile range, 270-600 s). Sham-sevoflurane (n = 6) animals were amnesic, with a latency of 52 s (27-120 s) (P < 0.01, vs. sham-air). Both the air-exposed (n = 5) and the sevoflurane-exposed (n = 8) animals with BLA lesions showed robust memory, with latencies of 350 s (300-590 s) and 378 s (363-488 s), respectively. The latencies for both did not differ from the performance of the sham-air group and were significantly greater than the latency of the sham-sevoflurane group (both P < 0.01). Conclusions BLA lesions block sevoflurane-induced amnesia. A role for the BLA in mediating anesthetic-induced amnesia may be a general principle of anesthetic action.

The Overshadowing of Background Stimuli: Some Effects of Varying Amounts of Training and UCS Intensity

Two experiments investigated the capacity of a nominal CS to overshadow background stimuli. Rats received CS-shock pairings in one compartment of a double compartment apparatus. After training the shock compartment was represented, but in the absence of both the CS and shock. Overshadowing was then assessed by measuring rats’ latency to enter this compartment. If rats readily entered the shock compartment this indicated that the background cues in that compartment had acquired little or no associative strength during training, and overshadowing was therefore inferred. If however rats avoided the shock compartment, then this indicated that the background stimuli must have become aversive during training, and overshadowing could not then be inferred. In both experiments it was found that the capacity of a CS to overshadow background stimuli was directly related to the amount of training given. The experiments also showed that this capacity is inversely related to shock intensity. The results are discussed in terms of Rescorla and Wagner's (1972) model of conditioning.

Differential Effects of Fear on Activity in Novel and Familiar Environments

The locomotor activity of mice was observed in an open field which was either novel or had been explored for 20 min. Immediately before entry into the open field, groups of Ss were given either 0, 4, or 12 shocks in an adjoining compartment. Ss given either 4 or 12 shocks showed a suppression of activity relative to the performance of nonshocked control Ss, suppression being more marked in the familiar than in the novel environment. Within the novel environment shocked Ss were less active than nonshocked Ss near the shock compartment but slightly more active in far areas. In the familiar environment, by contrast, suppression of activity among shocked Ss was manifested in all areas of the open field. It was concluded that activity by fearful animals is influenced by their familiarity with the environment in which the activity occurs and that this relationship can be understood in terms of the adaptive consequences of such behavior in the history of the animal.