The effects of acute stress on Pavlovian-instrumental transfer in rats.

Pavlovian stimuli invigorate ongoing instrumental action, a phenomenon termed the Pavlovian-instrumental transfer (PIT) effect. Acute stressors can markedly enhance the release of corticotropin-releasing factor (CRF), and CRF injection into the nucleus accumbens increases the PIT effect. However, it is unknown whether acute stressors by themselves would amplify the PIT effect. Here, we examined the effects of acute stressors on PIT. Rats first received Pavlovian and instrumental training, and then the impact of the Pavlovian stimuli on instrumental responding was analyzed in the subsequent PIT test. Acute stressors were applied prior to the PIT test. Because the effects of acute stressors critically depend on stressor type and time of day, we used two acute stressors that involved one or several distinct stressors (denoted here as "single" vs. "multiple" stressors) applied either in the light or the dark period of the light:dark cycle. The results revealed that single and multiple stressors applied in the light period did not alter the PIT effect--that is, the ability of an appetitive Pavlovian stimulus to enhance leverpressing--or the basal leverpress rate. When applied in the dark period, single and multiple stressors also did not alter the PIT effect, but they did markedly reduce the basal leverpress rate. Diazepam pretreatment did not counteract the declines in basal instrumental responding in the PIT test that were induced by either a single or multiple stressors. Our findings suggest that acute stressors were unable to amplify the incentive salience of reward-predictive Pavlovian stimuli to activate instrumental responding, but, depending on the time of day of stressor exposure, they did reduce basal instrumental responding.