Stress enhances excitatory trace eyeblink conditioning and opposes acquisition of inhibitory conditioning

Exposure to a brief, stressful event is reported to facilitate classical ey eblink conditioning in the male rat (Rattus norvegicus) by use of a delay p aradigm in which the conditioned stimulus (CS) and unconditioned stimulus ( US) overlap and coterminate. This study examined the effects of stress on t race conditioning, a task in which the CS and US were separated by 500 ms. Experiment 1 showed that exposure to brief (1 s), low-intensity (1 mA) tail shocks facilitated acquisition 24 hr later. Experiment 2 showed that stress or exposure did not affect retention or extinction of trace conditioning in rats that were stressed after acquisition. Experiment 3 showed that exposu re to the same stressor opposed acquisition of inhibitory conditioning. The se results suggest that exposure to a stressful event persistently facilita tes acquisition of trace conditioning and enhances a bias toward acquiring positive versus negative associations.