Abstract
The periaqueductal gray matter (PAG) is an essential structure involved in the elaboration of defensive responses, such as when facing predators and conspecific aggressors. Using a prey vs predator paradigm, we aimed to evaluate the PAG activation pattern evoked by unconditioned and conditioned fear situations. Adult male guinea pigs were confronted either by a Boa constrictor constrictor wild snake or by the aversive experimental context. After the behavioral test, the rodents were euthanized and the brain prepared for immunohistochemistry for Fos protein identification in different PAG columns. Although Fos-protein-labeled neurons were found in different PAG columns after both unconditioned and conditioned fear situations at the caudal level of the PAG, we found greater activation of the lateral column compared to the ventrolateral and dorsomedial columns after predator exposure. Moreover, the lateral column of the PAG showed higher Fos-labeled cells at the caudal level compared to the same area at the rostral level. The present results suggested that there are different activation patterns of PAG columns during unconditioned and conditioned fear in guinea pigs. It is possible to hypothesize that the recruitment of specific PAG columns depended on the nature of the threatening stimulus.
Highlights
The triggering of defensive behaviors involves the activation of a complex neural system responsible for the recognition and evaluation of the aversive stimuli and, for appropriate motor activity to exert the most appropriate defensive response
At the caudal level, we found that exposure of Cavia porcellus to the predator caused a higher increase in Fos protein immunoreactivity (Fos-IR) in the lPAG compared to vlPAG and dmPAG
Previous studies suggest that the lPAG is associated with the modulation of fear responses to proximal danger situations, such as flight and fight behaviors [35], whereas the vlPAG is associated with inhibitory behavior [36]
Summary
The triggering of defensive behaviors involves the activation of a complex neural system responsible for the recognition and evaluation of the aversive stimuli and, for appropriate motor activity to exert the most appropriate defensive response. It has been shown that aversive unconditioned stimuli produce a significant increase in the activity of limbic and paralimbic structures, such as the PAG, hypothalamus, amygdaloid complex, and corpora quadrigemina, which elicit defensive behavior that allow flight or attack [4,5,6]. The organization of defensive responses seems to be hierarchically modulated, as responses induced by amygdaloid complex or hypothalamus activation are abolished after electrolytic lesions in the PAG [7]. Numerous studies confirm a PAG activation during unconditioned and conditioned fear critical role played by the PAG on autonomic responses [11], nociception [1,12,13], and behaviors such as flight, vocalization, and tonic immobility [1,14,15]
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