Stress facilitates acquisition of the classically conditioned eyeblink response at both long and short interstimulus intervals

Exposure to inescapable stress facilitates acquisition of the classically conditioned eyeblink response in freely moving rats. Here, we determined whether facilitated acquisition of the eyeblink conditioned response (CR) depended on the interstimulus interval (ISI). Rats were either stressed (40 2-mA tailshocks delivered over a single 1-h session) or served as nonstressed controls. Paired training was accomplished with a 300-, 500-, 700-. or 1200-ms ISI. Separate groups of stressed and nonstressed rats were exposed to explicitly unpaired stimuli. Consistent with eyeblink conditioning using humans or rabbits as subjects, acquisition depended on the ISI: a "U-shaped" curve was described for nonstressed rats. Acquisition was fastest at the 700-ms ISI: the order was 700 > 500 > 300 > 1200 ms. The unconditioned response decreased with training. Except for the 700-ms ISI, exposure to stress facilitated acquisition. Exposure to stress did not affect the amplitude of the unconditioned response. Facilitated acquisition of the eyeblink CR after exposure to inescapable stressors is independent of nonassociative changes in reactivity to the conditioned stimulus or unconditioned stimulus. (C) 2001 Academic Press.