DISTINCT REGIONS OF THE PERIAQUEDUCTAL GRAY ARE INVOLVED IN THE ACQUISITION AND EXPRESSION OF DEFENSIVE RESPONSES

In fear conditioning, a rat is placed in a distinct environment and de livered footshock. The response to the footshock itself is called an a ctivity burst and includes running, jumping, and vocalization. The fea r conditioned to the distinct environment by the footshock elicits com plete immobility termed freezing. Lesions of the ventral periaqueducta l gray (vPAG) strongly attenuate freezing. However, lesions of the dor solateral periaqueductal gray (dlPAG) increase the amount of freezing seen to conditional fear cues acquired under conditions in which intac t rats do not demonstrate much fear conditioning. To examine the neces sity of these regions in the acquisition and expression of fear, we pe rformed five experiments that examined the effects of electrolytic les ions of the dlPAG and the vPAG in learned and unlearned fear. In exper iment 1, lesions of the vPAG strongly attenuated, whereas lesions of t he dlPAG enhanced, unconditional freezing to a cat. In experiment 2, l esions of the dlPAG made before but not after training enhanced the am ount of freezing shown to conditional fear cues acquired via immediate footshock delivery. In experiment 3, vPAG lesions made either before or after training with footshock decreased the level of freezing to co nditional fear cues, Neither dlPAG lesions nor vPAG lesions affected f ootshock sensitivity (experiment 4) or consumption on a conditioned ta ste aversion test that does not elicit antipredator responses (experim ent 5). On the basis of these results, it is proposed that activation of the dlPAG produces inhibition of the vPAG and forebrain structures involved with defense. In contrast, the vPAG seems to be necessary for postencounter freezing defensive behavior.