Pharmacological activation of the amygdala, but not single prolonged footshock-induced acute stress, interferes with cue-induced motivation toward food rewards in rats

Abstract

In the face of threats, animals adapt their behaviors to cope with the situation. Under such circumstances, irrelevant behaviors are usually suppressed. In this study, we examined whether food-seeking motivation would decrease under activation of the amygdala, an important nucleus in the regulation of stress response in the central nervous system, or after a physical acute stress session. In Experiment 1, we pharmacologically activated the basolateral nucleus (BLA) or the central nucleus of the amygdala (CeA) before a cue-induced reinstatement test in rats. Our results showed that activation of the BLA or the CeA abolished cue-induced motivation toward food rewards, while locomotor activity and free food intake were not affected. In Experiments 2 and 3, we further assessed anxiety and despair levels, as well as cue-induced reinstatement, after a single prolonged footshock-induced acute stress in rats. Behaviorally, acute stress did not affect anxiety level, despair level, or cue-induced motivation toward food rewards. Physiologically, there was no difference in cellular activities of the amygdala immediately after acute stress. To conclude, our results suggested that pharmacological activation of the amygdala decreased cue-induced motivation toward food reward. However, physiological acute stress did not immediately interfere with the negative emotions, motivation, or amygdala activities of the animals.

Keywords: acute stress; basolateral nucleus of the amygdala; central nucleus of the amygdala; food-seeking; motivated behavior.

Figure 1

Example of cannula placements in the BLA and the CeA in Experiment 1. The location of injector tips on the nissl-stained brain sections was defined as the drug infusion site. LA, lateral nucleus of the amygdala; BLA, basolateral nucleus of the amygdala; CeA, central nucleus of the amygdala.

Figure 2

Example of c-fos + labelings in the amygdala. (A) The subregions of the amygdala were sampled at four anterior–posterior (AP) levels relative to bregma (−2.04, −2.52, −3.00, and −3.48 mm). The lines indicated the sampled area of the LA, the BLA, and the CeA. (B) Example of c-fos + neurons in the amygdala. The magnified images illustrate the rectangle areas shown in panel A. The c-fos + neurons were labeled in blue-black (four neurons were pointed with black arrowheads as examples). LA, lateral nucleus of the amygdala; BLA, basolateral nucleus of the amygdala; CeA, central nucleus of the amygdala.

Figure 3

Example of c-fos + labelings in the NAcc. The NAcc was sampled at three anterior–posterior (AP) levels relative to bregma (+2.28, +1.80, and + 1.32 mm). The squares (200 × 200 μm²) represented the sampled area in each AP level. The c-fos + neurons were labeled in blue-black, as the same in Figure 2B. NAcc, nucleus accumbens core.

Figure 4

Example of c-fos + labelings of dopaminergic neurons in the VTA. (A) The VTA was sampled at two anterior–posterior (AP) levels relative to bregma (−5.28 and − 5.76 mm). (B) Example of labeled neurons in the VTA. The magnified images illustrate the rectangle areas shown in panel A. The c-fos+/TH+ neurons were identified as brown cell bodies with blue-black nuclei (three neurons were pointed with black arrowheads as examples). VTA, ventral tegmental area.

Figure 5

Activation of the BLA or the CeA decreased cue-induced motivation toward food rewards in the reinstatement test. (A) The experimental timeline in Experiment 1. The arrows indicate the time of drug administration. (B) The histology summary in Experiment 1. The numbers indicated anterior–posterior (AP) levels relative to bregma. (C) All groups of animals behaved equivalently on the last VR5 training day. Compared to the control group, activation of the BLA or the CeA decreased the reinstatement effect. (D) The locomotor activity and free-feeding test did not show differences among the three groups. All statistics were presented as mean ± SEM. BLA, basolateral nucleus of the amygdala; CeA, central nucleus of the amygdala; VR5, variable-ratio 5; EXT, extinction; Rein, Reinstatement test.

Figure 6

Example of neuronal morphology after three times of saline or NMDA infusion. For every subject, the brain sections were present with cannula implant location and 0.35 mm anterior or posterior relative to implant location. The magnified images illustrate the square areas shown in low-magnification images. Scale bar, 500 μm in low magnification images, 50 μm in high magnification images. BLA, basolateral nucleus of the amygdala; CeA, central nucleus of the amygdala; S, Saline, N, N-methyl-D-aspartate.

Figure 7

Footshock-induced acute stress did not affect anxiety-and despair-like behaviors. (A) The experimental timeline in Experiment 2. The arrows indicate the time of a single prolonged footshock-induced acute stress. There was no statistical difference between groups in (B) percentage of time spent in open arms and entries into open arms in EPM and (C) time spent in immobility in FST. All statistics were presented as mean ± SEM. EPM, elevated plus maze; FST, forced swim test.

Figure 8

Footshock-induced acute stress did not interfere with cue-induced motivation toward food rewards in the reinstatement test. (A) The experimental timeline in Experiment 3. The arrow indicates the time of a single prolonged footshock-induced acute stress. (B) The freezing level during the first 10 min and the last 10 min of the total 2 h session. (C) The two groups of animals that underwent the reinstatement procedure behaved equivalently on the last VR5 training day. The footshock-induced acute stress did not decrease the motivation of food-seeking. Both the Stress and Ctrl group of the animals showed reinstatement to cues. (D) The footshock-induced acute stress did not decrease nosepoke counts from the last extinction session to the reinstatement test in both groups. All statistics were presented as mean ± SEM. VR5, variable-ratio 5; EXT, extinction; Rein, Reinstatement test.

Figure 9

The cell density in (count/nm²) in (A) the amygdala (the LA, the BLA, and the CeA) and (B) the NAcc. All statistics were presented as mean ± SEM. LA, lateral nucleus of the amygdala; BLA, basolateral nucleus of the amygdala; CeA, central nucleus of the amygdala; NAcc, nucleus accumbens core.