Disruption of neuroendocrine stress responses to acute ferret odor by medial, but not central amygdala lesions in rats

Abstract

Investigations of the neural pathways associated with responses to predators have implicated the medial amygdala (MeA) as an important region involved in defensive behaviors. To our knowledge, however, the involvement of the MeA in neuroendocrine responses to predator odor exposure has not been investigated. Therefore, the present study examined the effects of MeA disruption in rats exposed to ferret or control odor on hypothalamo-pituitary-adrenocortical (HPA) axis activation. Bilateral lesions of the MeA were made in Sprague-Dawley rats with the neurotoxin ibotenic acid (10 microg/microl; 0.3 microl / side). As a control for regional specificity, additional groups of rats were given lesions in the central amygdala (CeA). One week after recovery, the rats were exposed to ferret or strawberry control towels in small cages to examine HPA axis responses as determined by plasma corticosterone and adrenocorticotropin hormone (ACTH) levels. Rats with complete bilateral MeA but not CeA lesions displayed significantly less corticosterone and ACTH release compared to sham-operated control rats only in the ferret odor conditions. These results suggest that the MeA is an important structure involved in the HPA axis responses to predator odors, in support of previous studies investigating behavioral responses under similar conditions.

Figure 1

Examples of NeuN immunohistochemistry verification of sham and complete MeA and CeA lesions. MeA = medial amygdala, CeA = central amygdala, LA = lateral amygdala, BLA = basolateral amygdala. Scale bar: 500 µm.

Figure 2

Depiction of the largest (light gray opaque outline) and smallest (darker filled outline) MeA lesions. Though actual lesions were bilateral, the lesions are shown unilaterally to show anatomy on the other side. Figures 47 (bregma –1.72 mm), 52 (bregma –2.28 mm), 56 (bregma –2.76), and 62 (bregma –3.48) from Paxinos and Watson’s The Rat Brain In Stereotaxic Coordinates, 5^th Edition are shown to depict the extent of the MeA lesions. MeAD = anterodorsal MeA, MeAV = anteroventral MeA, MePD = posterodorsal MeA, MePV = posteroventral MeA, CeM = medial CeA, CeC = capsular part of CeA, CeL = lateral CeA, BLA = basolateral amygdala, LA = lateral amygdala, LV = lateral ventricle.

Figure 3

Depiction of the largest (light gray opaque outline) and smallest (darker filled outline) CeA lesions. Though actual lesions were bilateral, the lesions are shown unilaterally to show anatomy on the other side. Figures 46 (bregma –1.56 mm), 52 (bregma –2.28 mm), 56 (bregma –2.76), and 61 (bregma –3.36) from Paxinos and Watson’s The Rat Brain In Stereotaxic Coordinates, 5^th Edition are shown to depict the extent of the CeA lesions. MeAD = anterodorsal MeA, MeAV = anteroventral MeA, MePD = posterodorsal MeA, MePV = posteroventral MeA, CeM = medial CeA, CeC = capsular part of CeA, CeL = lateral CeA, BLA = basolateral amygdala, LA = lateral amygdala, LV = lateral ventricle.

Figure 4

Mean hormonal responses of sham-operated control or MeA-lesioned rats exposed to strawberry (STR) or ferret (FER) odor for 30 min in small cages. Panel A depicts mean corticosterone (+/−SEM) responses in sham-operated control (Sham) or MeA lesioned (Lesion) rats. Panel B depicts mean ACTH (+/−SEM) responses in sham-operated control (Sham) or MeA lesioned (Lesion) rats. Asterisks indicate a significant difference from the Lesion/FER group (p < 0.05).

Figure 5

Mean hormonal responses of sham-operated control or CeA-lesioned rats exposed to strawberry (STR) or ferret (FER) odor for 30 min in small cages. Panel A depicts mean corticosterone (+/−SEM) responses in sham-operated control (Sham) or CeA lesioned (Lesion) rats. Panel B depicts mean ACTH (+/−SEM) responses in sham-operated control (Sham) or CeA lesioned (Lesion) rats. Asterisks indicate a significant difference from the Sham/STR group (p < 0.05). ^ indicates a significant difference from the Lesion/STR group (p < 0.05).