Rats exposed to the odor of a predator or to the elevated plus maze express fear behaviors without a prior exposure to either stimulus. The expression of innate fear provides for an ideal model of anxiety which can aid in the elucidation of brain circuits involved in anxiety-related behaviors. The current experiments compared activation of neuropeptide-containing neuronal populations in the amygdala of rats exposed to either the elevated plus maze (EPM; 5 minutes) versus home cage controls, or predator ferret odor versus butyric acid, or no odor (30 minutes). Sections of the brains were prepared for dual-labeled immunohistochemistry and counts of c-Fos co-localized with somatostatin (SOM) or neuropeptide Y (NPY) were made in the basolateral (BLA), central (CEA), medial (MEA) nucleus of the amygdala. Ferret odor and butyric acid exposure significantly decreased the percentage of SOM–positive neurons also immunoreactive for c-Fos in the anterior BLA compared to controls, whereas EPM exposure yielded a significant increase in the activation of SOM-positive neurons versus home cage controls. In the CEA, ferret odor and butyric exposure significantly decreased the percentage of SOM-positive neurons also immunoreactive for c-Fos compared to no-odor controls whereas EPM exposure yielded no change versus controls. In the MEA, both ferret odor exposure and EPM exposure resulted in increased SOM co-localized with c-Fos compared to control groups whereas NPY co-localized with c-Fos occurred following ferret odor exposure, but not EPM exposure. These results indicate that phenotypically distinct neuronal populations of the amygdala are differentially activated following exposure to different anxiogenic stimuli. These studies further elucidate the fundamental neurocircuitry of anxiety and could possibly explain the differential behavioral effects of predator versus novelty-induced stress.

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