Roles of the anterior basolateral amygdalar nucleus during exposure to a live predator and to a predator-associated context.

The basolateral amygdala complex, which includes the lateral, basolateral and basomedial nuclei, has been implicated in innate and contextual fear responses to predator threats. In the basolateral complex, the lateral and posterior basomedial nuclei are able to process predator odor information, and they project to the predator-responsive hypothalamic circuit; lesions in these amygdalar sites reduce innate responses and practically abolish contextual fear responses to predatory threats. In contrast to the lateral and posterior basomedial nuclei, the basolateral nucleus does not receive direct information from predator olfactory cues and has no direct link to the predator-responsive hypothalamic circuit. No attempt has previously been made to determine the specific role of the basolateral nucleus in fear responses to predatory threats, and we currently addressed this question by making bilateral N-methyl-D-aspartate lesions in the anterior basolateral nucleus of the amygdala (BLAa), which is often regarded as being contiguous with the lateral amygdalar nucleus, and tested both innate and contextual fear in response to cat exposure. Accordingly, BLAa lesions decreased both innate and contextual fear responses to predator exposure. Considering the targets of the BLAa, the nucleus accumbens appears to be a potential candidate to influence innate defensive responses to predator threats. The present findings also suggest that the BLAa has a role in fear memory of predator threat. The BLAa is likely involved in memory consolidation, which could potentially engage BLAa projection targets, opening interesting possibilities in the investigation of how these targets could be involved in the consolidation of predator-related fear memory.

Lack of functional link in the tadpole morphology induced by predators.

Most studies of predator-induced plasticity have focused on documenting how prey species respond to predators by modifying phenotypic traits and how traits correlate with fitness. We have previously shown that Pleurodema thaul tadpoles exposed to the dragonfly Rhionaeschna variegata responded strongly by showing morphological changes, less activity, and better survival than non-exposed tadpoles. Here, we tested whether there is a functional link between morphological plasticity and increased survival in the presence of predators. Tadpoles that experienced predation risk were smaller, less developed, and much less active than tadpoles without this experience. Burst speed did not correlate significantly with morphological changes and predator-induced deeper tails did not act as a lure to divert predator strikes away from the head. Although we have previously found that tadpoles with predator-induced morphology survive better under a direct predator threat, our results on the functional link between morphology and fitness are not conclusive. Our results suggest that in P. thaul tadpoles (1) burst speed is not important to evade predators, (2) those exposed to predators reduce their activity, and (3) morphological changes do not divert predator attacks away from areas that compromise tadpole survivalEE. Our results show that morphological changes in P. thaul tadpoles do not explain burst speed or lure attraction, although there was a clear reduction of activity, which itself reduces predation. We propose that changes in tadpole activity could be further analyzed from another perspective, with morphological change as an indirect product of behavior mediated by physiological mechanisms.

What ethologically based models have taught us about the neural systems underlying fear and anxiety

Classical Pavlovian fear conditioning to painful stimuli has provided the generally accepted view of a core system centered in the central amygdala to organize fear responses. Ethologically based models using other sources of threat likely to be expected in a natural environment, such as predators or aggressive dominant conspecifics, have challenged this concept of a unitary core circuit for fear processing. We discuss here what the ethologically based models have told us about the neural systems organizing fear responses. We explored the concept that parallel paths process different classes of threats, and that these different paths influence distinct regions in the periaqueductal gray - a critical element for the organization of all kinds of fear responses. Despite this parallel processing of different kinds of threats, we have discussed an interesting emerging view that common cortical-hippocampal-amygdalar paths seem to be engaged in fear conditioning to painful stimuli, to predators and, perhaps, to aggressive dominant conspecifics as well. Overall, the aim of this review is to bring into focus a more global and comprehensive view of the systems organizing fear responses.