Research Intra-Amygdala Muscimol Injections Impair Freezing and Place Avoidance in Aversive Contextual Conditioning

Rats were trained by shocking them in a closed compartment. When subsequently tested in the same closed compartment with no shock, normal rats showed an increased tendency to freeze. They also showed an increased tendency to actively avoid the compartment when given access to an adjacent neutral compartment for the first time. Amygdala inactivation with bilateral muscimol injections before training attenuated freezing and eliminated avoidance during the test. Rats trained in a normal state and given intra-amygdala muscimol injections before the test did not freeze or avoid the shock-paired compartment. This pattern of effects suggests that amygdala inactivation during training impaired acquisition of a conditioned response (CR) due either to inactivation of a neural substrate essential for its storage or to elimination of a memory modulation effect that facilitates its storage in some other brain region(s). The elimination of both freezing and active avoidance by amygdala inactivation during testing suggests that neither of these behaviors is the CR. The possibility that the CR is a set of internal responses that produces both freezing and avoidance as well as other behavioral effects is discussed. In aversive conditioning procedures, neutral cues such as those that comprise an experimental context (conditioned stimuli; CS) are paired with an aversive event such as footshock (uncondi-tioned stimulus; US). Usually only a few CS–US pairings are re