Fear conditioning provides an ideal model for investigations of learning and memory processes. In addition, it has been used to model aspects of emotion dysregulation related to anxiety disorders. Fear conditioned freezing behavior is typically proportional to the number and magnitude of the footshocks used during conditioning. However, exposure to a significant stressor prior to fear conditioning can produce a disproportionately enhanced fear response (stress-enhanced fear learning, SEFL). In adult rats, exposure to an initial stressor (15 unsignalled footshocks) dramatically increases subsequent fear conditioning to a novel context or discrete tone CS. SEFL has been shown to persist for at least 3 months in adult rats, though little is known regarding the establishment and persistence of SEFL in rats exposed to the initial stress early in life. The present series of experiments assessed this by exposing juvenile rats to 0 or 15 footshocks in one context (stress exposure) on postnatal day 17 (PND17). Subsequently, these rats were given 0 or 1 footshock(s) in a second, novel context (fear conditioning) on postnatal day 18, 24, or 90. Rats were tested for fear of both contexts over the next two consecutive days. Results indicate enhanced fear conditioning to the second context in rats previously exposed to 15 footshocks, regardless of age. Interestingly, rats that received 1-footshock fear conditioning on PND18 and no prior stress exposure show no context fear conditioning. By contrast, rats fear conditioned on PND24 or PND90 with no prior stress exposure show significant 1-footshock fear conditioning. Taken together, these data suggest that footshock stress exposure enhances subsequent fear learning, even in very young rats that do not normally show fear conditioning.
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