Disrupting basolateral amygdala function impairs unconditioned freezing and avoidance in rats Almira Vazdarjanova, Larry Cahill and James L. McGaugh Center for the Neurobiology of Learning and Memory, and Department of Neurobiology and Behaviour, University of California, Irvine, CA 92697-3800, USA Keywords: cat hair, excitotoxic lesions, fear conditioning, lidocaine, predator odour Abstract Lesions of the lateral/basolateral amygdala nuclei (BLC) disrupt freezing behaviour in response to explicit or contextual cues (conditioned stimuli ± CS) paired previously with footshock (unconditioned stimulus). This de®cit in expression of defensive behaviour in response to conditioned stimuli is often interpreted as inability of lesioned rats to learn CS±US associations. However, ®ndings of several studies indicate that BLC-lesioned rats can rapidly learn CS±US associations. Such ®ndings suggest that lesioned rats can learn CS±US associations but are impaired in the expression of freezing behaviour. In the present study we report that both temporary inactivation (lidocaine) and permanent excitotoxic (NMDA) lesions of the BLC impair the unconditioned freezing and avoidance behaviours of rats in response to a novel fear-eliciting stimulus, a ball of cat hair. These ®ndings suggest that the BLC in¯uences the expression of freezing and avoidance behaviours, and/or that it potentiates rats' experience of fear. Along with prior evidence of spared memory for aversive learning after BLC lesions, these ®ndings suggest that disrupted freezing to conditioned cues in BLC-lesioned rats does not necessarily re¯ect inability to form CS±US associations. Introduction According to one view of emotionally in¯uenced learning, the basolateral amygdala (BLC) forms and stores associations between conditioned and aversive unconditioned stimuli (LeDoux, 1998; Maren, 1999a; Shi & Davis, 1999). The ®ndings that rats with BLC lesions (Maren et al., 1996b; Maren, 1998; Amorapanth et al., 2000) or rats in which the NMDA receptors in the BLC were blocked (Maren et al., 1996a) show lessened fear-related behaviour, such as freezing, when returned to an environment in which they previously received a footshock are often cited as supporting this view. However, these data are equally consistent with the possibility that the BLC is involved in the expression of freezing, regardless of what provokes it (Vazdarjanova & McGaugh, 1998; Cahill et al., 1999). That is, the impaired freezing may re¯ect a performance de®cit rather than a memory de®cit. A similar hypothesis regarding the function of the entire amygdala was ®rst proposed by Blanchard & Blanchard (1972) to explain their ®ndings that amygdala lesions disrupt freezing and approach behaviour to both conditioned and unconditioned stimuli. Consistent with this hypothesis are ®ndings that blockade of the AMPA receptors in the amygdala of rats increases exploratory locomotion (Mesches et al., 1996), a behaviour negatively correlated with freezing in fear-eliciting situations (Blanchard et al., 1989). Moreover, selective amygdala lesions severely decrease freezing and increase approach to a toy snake in monkeys that have never experienced snakes (Meunier et al., 1999; Nelson et al., 1999). Although the central and medial amygdala nuclei have been implicated in the expression of unconditioned defensive behaviours (Kemble et al., 1990; Davis, 1997), other evidence suggests a role for the BLC as well. Lesions of the BLC decrease unconditioned analgesia (Bellgowan & Helmstetter, 1996), and the ¯ight distance and chase duration of rats pursued by a predator (Kemble et al., 1990). Additionally, inactivation of the AMPA receptors in the BLC abolishes the increase in startle to a loud sound given it the presence of a bright light not paired previously with an aversive event (Walker & Davis, 1997). Lastly, blocking of the NMDA receptors in the BLC decreases neophobia (Burns et al., 1996). If the BLC mediates the expression of freezing to both uncondi- tioned and conditioned stimuli, then tests that use freezing to assay emotional memory may, at the very least, overestimate the contri- butions of the BLC in memory processes. It has long been known that exposure to a predator (Blanchard & Blanchard, 1971; Canteras et al., 1997) or to its odour (Blanchard et al., 1989; Zangrossi & File, 1992, 1994; File et al., 1993; Wallace & Rosen, 2000) can elicit fear, as assessed by the display of several defensive behaviours, including freezing. In the present experiments we compared the freezing and avoidance behaviours of rats with sham or BLC lesions in response to either a ball of real cat hair, or a ball of hair-like plastic stuf®ng. We further compared the effects of temporary inactivation and permanent excitotoxic lesions of the BLC on the expression of these uncondi- tioned behaviours. General methods Subjects and surgery All experimental procedures and handling of the subjects were approved by the institutional animal care and use committee. The subjects were 90 adult male Sprague±Dawley rats (Charles River Breeding Laboratories, Raleigh, NC, USA) weighing 270±350 g at Correspondence: Dr Almira Vazdarjanova, ARL, Division of Neural Systems, Memory & Ageing, PO Box 245115, University of Arizona, Tucson, AZ 85724-5115, USA. E-mail: almira@nsma.arizona.edu Received 8 November 2000, revised 25 June 2001, accepted 25 June 2001 European Journal of Neuroscience, Vol. 14, pp. 709±718, 2001 ã Federation of European Neuroscience Societies