Disrupting basolateral amygdala function impairs unconditioned freezing and avoidance in rats

Lesions of the lateral/basolateral amygdala nuclei (BLC) disrupt freezing b ehaviour in response to explicit or contextual cues (conditioned stimuli - CS) paired previously with footshock (unconditioned stimulus). This deficit in expression of defensive behaviour in response to conditioned stimuli is often interpreted as inability of lesioned rats to learn CS-US association s. However, findings of several studies indicate that BLC-lesioned rats can rapidly learn CS-US associations. Such findings suggest that lesioned rats can learn CS-US associations but are impaired in the expression of freezin g behaviour. In the present study we report that both temporary inactivatio n (lidocaine) and permanent excitotoxic (NMDA) lesions of the BLC impair th e unconditioned freezing and avoidance behaviours of rats in response to a novel fear-eliciting stimulus, a ball of cat hair. These findings suggest t hat the BLC influences the expression of freezing and avoidance behaviours, and/or that it potentiates rats' experience of fear. Along with prior evid ence of spared memory for aversive learning after BLC lesions, these findin gs suggest that disrupted freezing to conditioned cues in BLC-lesioned rats does not necessarily reflect inability to form CS-US associations.