Acute stress impairs trace eyeblink conditioning in females without altering the unconditioned response

Exposure to an acute stressor of inescapable swimming or intermittent tailshocks impairs classical eyeblink conditioning 24 h later in female rats (Wood, Beylin, & Shors, 2001). This effect is often attributed to a deficit in "learning," but since stress has been shown to induce analgesia (Jackson, Maier, & Coon, 1979), an alternative explanation is that stressor exposure reduces conditioning by lessening the perceived intensity of the unconditioned stimulus (US). To address this possibility we examined the amplitude of the unconditioned response (UR) during training and found that although exposure to the stressor impaired trace conditioning, there was no difference in the UR amplitude. We also found that eyeblink responses to different US intensities (4-12 V) in the absence of training were unaffected by stressor exposure. Taken together, these experiments indicate that the stress-induced impairment of conditioning in females is not due to a decreased perception of US strength. (C) 2004 Elsevier Inc. All rights reserved.