Measuring Pavlovian fear with conditioned freezing and conditioned suppression reveals different roles for the basolateral amygdala

Abstract

In Pavlovian fear conditioning, pairing a neutral cue with aversive foot shock endows a cue with fear-eliciting properties. Studies of Pavlovian fear conditioning measuring freezing have demonstrated the basolateral amygdala (BLA) to be critical to both fear learning and memory. The nucleus accumbens core (NAc), while not important to freezing, is important to the enhancement of instrumental responding by cues paired with food reward. In the present study we investigated the role of the BLA and the NAc in another property of fear cues, the ability to suppress instrumental responding for food rewards (conditioned suppression). Sham, BLA and NAc-lesioned rats received a fear discrimination procedure in which one visual cue (CS+) predicted foot shock while a second cue (CS-) did not. Conditioning took place over a baseline of instrumental responding, allowing for concurrent measure of freezing and instrumental suppression. NAc lesions left fear conditioning fully intact. BLA lesions impaired acquisition and discrimination of fear when assessed with conditioned freezing. However, BLA lesions only altered fear acquisition and left discrimination completely intact when assessed with conditioned suppression. These findings suggest a critical role for the BLA in fear when assessed with conditioned freezing but a diminished role when assessed with conditioned suppression.

Figure 1

Histology. Photomicrographs showing representative brain sections of the BLA and NAc of rats that received Sham, BLA or NAc lesions. A, B, Sections stained with Nissl in the intact and lesioned BLA, respectively. C, D, Sections stained with Nissl in the intact and lesioned NAc, respectively. E, F, Sections subjected to immunohistochemistry with Neu-N in the intact and lesioned NAc, respectively. LA, Lateral amygdala; B, Basal amygdala NAc, Nucleus accumbens core; NAs, Nucleus accumbens shell.

Figure 2

Freezing and instrumental suppression in Pavlovian fear conditioning. A-C, Mean ± SEM % Time freezing over the 3 fear conditioning days (f1-3) and 6 fear discrimination blocks (d1-d6) during CS+ (closed symbols), CS− (open symbols) and pre-CS (grey symbols) for A, Sham, B, NAc and C, BLA rats. D-F, Mean ± SEM Kamin ratios over the 3 fear conditioning days (f1-3) and 6 fear discrimination blocks (d1-d6) during CS+ (closed symbols) and CS− (open symbols) for D, Sham, E, NAc and F, BLA rats. A Kamin ratio of 0.5 (dotted line) indicates the absence of instrumental suppression. (*p < 0.05, Tukey’s Honestly Significant Difference)

Figure 3

Unconditioned responses to foot shock. A, Mean ± SEM activity/min during pre-CS (white bars) and foot shock (black bars) over all days of conditioning for Sham, NAc and BLA rats. B, Mean ± SEM lever-press/min during pre-CS (white bars) and foot shock (black bars) over all days of conditioning for Sham, NAc and BLA rats. (**p < 0.01, Tukey’s Honestly Significant Difference)