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

Abstract

Exposure to an acute stressor of inescapable swimming or intermittent tail shocks impairs classical eye blink conditioning 24h later in female rats. This effect is often attributed to a deficit in "learning," but since stress has been shown to induce analgesia, 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 eye blink 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.

Fig. 1

The figure depicts the EMG response to the US. The artifact from the 10 ms, 10 V, square-wave pulse US is underlined in black, whereas the EMG of the UR is underlined in black and white hatched marks. The opening of the eyelid after the US can be observed and analyzed under these conditions.

Fig. 2

(A) Stressor exposure significantly impaired conditioning in female rats compared to unstressed controls, as measured by the number of CRs across 4 blocks of 100 trials each. (B) Stressor exposure did not affect the amplitude of the UR (measured in arbitrary units), as measured during 10 US alone test trials per block.

Fig. 3

(A) In both stressed and unstressed animals, the number of blinks increased with US intensity and there was no difference in the number of blinks at each voltage between stressed and unstressed animals. (B) Blink amplitude also increased with US intensity in both stressed and unstressed animals and there was no difference in the blink amplitude between groups.