Involvement of subcortical and cortical afferents to the lateral nucleus of the amygdala in fear conditioning measured with fear- potentiated startle in rats trained concurrently with auditory and visual conditioned stimuli
The goal of this work was to test the involvement, in fear conditioning, of afferents to the lateral nucleus of the amygdala originating from the auditory thalamus, auditory cortex, and perirhinal area. The acoustic startle reflex was used as the behavioral index of conditioning because it is reliably enhanced in the presence of a conditioned stimulus (CS) previously paired with a footshock. Auditory and visual CSs were used to assess the modality specificity of lesions to the above brain areas. Pre- or posttraining lesions of the entire auditory thalamus including the ventral, dorsal, and medial divisions of the medial geniculate body, the posterior intralaminar nucleus, and the suprageniculate nucleus, completely blocked fear-potentiated startle to the auditory CS, but had no effect on fear-potentiated startle to the visual CS. Posttraining lesions mostly restricted to the ventral and dorsal divisions of the medial geniculate body specifically disrupted fear-potentiated startle to the auditory CS. However, retraining in rats sustaining ventral and dorsal medial geniculate body lesions led to reliable fear-potentiated startle to the auditory CS. Posttraining lesions mostly restricted to the medial division of the medial geniculate body, posterior intralaminar, and suprageniculate nuclei did not disrupt fear-potentiated startle. These results indicate that the auditory thalamus is specifically involved, through either its direct or indirect amygdaloid afferents, in fear conditioning to auditory CSs. Pre- or posttraining lesions mostly restricted to the primary auditory cortex did not reliably attenuate fear-potentiated startle to the auditory or visual CSs. Extensive posttraining lesions of the perirhinal area (including secondary auditory cortices), but not its rostral aspect alone, blocked fear-potentiated startle to both CSs. However, reliable potentiated startle was observed to both CSs following similarly extensive pretraining lesions of the perirhinal area. Because post- but not pretraining lesions of the perirhinal area blocked fear-potentiated startle nonspecifically, at least with regard to auditory and visual CSs, the results are consistent with the involvement of the perirhinal area in general memory functions such as information storage or retrieval. Alternatively, the secondary auditory and/or perirhinal cortices might function as multimodal sensory relays to the amygdala.