In order to better understand the relationship between behavioral traits and addiction vulnerability, we first characterized SHRs to discern different aspects of impulsivity relevant to human substance abuse. Since SHRs are genetically derived from Wistar-Kyoto rats (WKYs), these animals were used as controls. Animals performed a temporal discounting task (intolerance-to-delay, ITD) that measured choice between an immediate small reward (1 food pellet) and a delayed large reward (5 food pellets) (Supplementary Fig. 1). The delay length was gradually increased during the test phase and impulsive choice was reflected by relative preference for the immediate reward (Supplementary Fig. 1c). Both SHRs and WKYs decreased preference for the large reward as delay increased (F6,174=53.76, p<0.001). This relative shift in preference towards the smaller, immediate reward was, however, significantly faster in impulsive rats (F6,174=3.05, p=0.007) confirming that SHRs exhibit elevated impulsive choice (Fig. 1a and Supplementary Fig. 2a). Lever presses were also recorded during the delay period, after pressing L5 but before receiving food. Although intra-delay presses did not result in additional food-delivery, shifting responses to L1 during this period reflected impulsive action
