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

Abstract

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 rewardpredictive Pavlovian stimuli to activate instrumental responding, but, depending on the time of day of stressor exposure, they did reduce basal instrumental responding.

Acute stressor effects on goal-directed action in rats

“…Results indicated that single exposure of the same multiple stressor as used in the current study reduced the overall lever press rate, but left intact the ability of a reward-predictive stimulus to invigorate instrumental responding, thereby suggesting that the acute stressor did not impair the retrieval of stimulus-reward associations (Pielock et al 2013). Our present results point to the possibility that twofold instead of single multiple stressor exposure could impair this type of stimulus modulation of instrumental performance.…”

“…For instance, male rats exposed to a predator, such as a cat, or to a female receptive rat showed equivalent increases in plasma corticosterone levels, but only cat-exposed rats exhibited working memory deficits (Woodson et al 2003). Together, these findings provide further evidence to the notion that plasma corticosterone levels are not always a sensitive index for stressor-induced behavioral effects (e.g., Natelson et al 1987;Pielock et al 2013).…”

“…Of note, plasma corticosterone levels after single stressor exposure in the light cycle (Experiment 2) and multiple stressor exposure in the dark cycle (Experiment 3) were increased significantly, but only multiple stressor exposure produced a bias toward habitual responding. Importantly, acute stressor-induced plasma corticosterone elevations reported in a number of previous studies were not or only partially related to the light -dark cycle (Pielock et al 2013), stressor intensity, and the magnitude of the stressor-induced cognitive impairments (Woodson et al 2003). For instance, male rats exposed to a predator, such as a cat, or to a female receptive rat showed equivalent increases in plasma corticosterone levels, but only cat-exposed rats exhibited working memory deficits (Woodson et al 2003).…”

“…Consistent with this notion, a selective devaluation effect was also detected in rats exposed to the pharmacological (Experiment 1) or single stressor (Experiment 2), though baseline responding was low. In an earlier study we showed that instrumental responding was altered by the single and multiple stressor exposure in the dark but not in the light period, suggesting that their efficacy is determined in particular by the time of day of administration (Pielock et al 2013). Furthermore, the multiple stressor, in behavioral terms, was moderately more effective in the dark period than the single stressor (Pielock et al 2013); therefore, we used the former stressor in the current study.…”

“…In an earlier study we showed that instrumental responding was altered by the single and multiple stressor exposure in the dark but not in the light period, suggesting that their efficacy is determined in particular by the time of day of administration (Pielock et al 2013). Furthermore, the multiple stressor, in behavioral terms, was moderately more effective in the dark period than the single stressor (Pielock et al 2013); therefore, we used the former stressor in the current study. However, it is conceivable that the single stressor if applied in dark period could also induce a bias toward habitual responding, a possibility that we did not test in the current study.…”