These individual differences have become apparent in rodent models selectively bred for specific traits. The Lewis and Fischer 344 rats

are rodents with heightened (Fischer 344) or attenuated (Lewis) HPA-axis reactivity, and have been shown to differ in a wide range of HPA-axis-related behavioral and physiological traits (Sternberg et al., 1992). Stohr and colleagues showed that PNS had differential effects in the Lewis and Fischer 344 rats. In Lewis rats, PNS improved acquisition of active avoidance, decreased immobility in the forced swim test, and reduced novelty-induced locomotion, whereas in Fischer 344 rats PNS had no effect in the active avoidance or forced swim test, and increased novelty-induced AUY-922 purchase locomotion (Stohr et al., 1998). Studies in rats selectively bred for High and Low anxiety traits suggest that PNS has opposite effects in anxious versus non-anxious rats. Rats bred for high anxiety traits became less anxious after PNS, whereas rats bred for low anxiety traits became more anxious (Bosch et al., 2006). In a similar fashion, rats selectively bred for low novelty seeking behavior were reported to show less anxiety than their controls, whereas those rats selectively bred for high novelty seeking behavior were not affected by PNS (Clinton et al., 2008). Taken together these studies

suggest that PNS may have opposite effects dependent on the genetic background ZD1839 of the individual. In addition to the differences in anxiety traits or HPA-axis responsivity, the way a stressor is perceived may play an important role in effects of PNS. The stress-coping style of an individual PAK6 determines the behavioral and physiological response of an organism to stress. Two clear stress-coping phenotypes can be distinguished, the proactive and passive stress-coping styles. Behaviorally, proactive stress-copers are characterized

by active responses to stressors; they will attempt to modulate the environment to reduce the stress (Koolhaas et al., 1999). This proactive stress response is illustrated in rodents during a defensive burying test. In this test proactively coping rats will bury an electrified prod that is placed in their cage with saw dust in order to avoid a shock. In contrast, passive stress-copers respond to stress in a more inhibited manner. In the defensive burying test, passive rodents will sit as far away from the prod as possible to avoid being shocked (de Boer and Koolhaas, 2003). These stress-coping phenotypes are highly correlated with other behavioral responses. Proactive stress-coping individuals tend to show more aggression and impulsivity and are less behaviorally flexible than passive stress-copers (Coppens et al., 2010).