3. Results3.1. Effect of stressor controllability on extinction of cocaine seekingIn order to evaluate the impact of stressor controllability onextinction of cocaine-seeking behavior, stress treatment was given24 h after cocaine self-administration training so that any effects ofcontrollability on extinction could not be attributed to modulationof acquisition (Fig. 1A). Rats were trained to nose poke for cocaine(0.5 mg/kg/infusion) under an FR1 schedule of reinforcement.Analysis of cocaine self-administration behavior prior to stressgroup assignment showed no differences in the number of cocaineinfusions across the daily 3-h sessions (group session interaction:F12,168 ¼ 0.604, p ¼ 0.837; data not shown) and total cocaine intake(group: F2,28 ¼ 0.786, p ¼ 0.466; data not shown). Stress treatmentconsisted of 80 trials of ES, IS, or HC.One week following stress exposure, animals were returned totheir self-administration chambers for daily 3-h extinction sessionsand continued until the extinction criterion was reached (numberof previously reinforced responses in a given session was <20% ofextinction day 1 levels). Extinction to a criterion was used, ratherthan a fixed number of extinction sessions, so that facilitation orinterference effects of stress could more readily be detected. Priorexposure to ES significantly reduced the number of sessionsnecessary for extinction, whereas prior IS increased the number ofsessions (days to extinction: ES, 6.8 ± 0.969; HC, 14.5 ± 2.00; IS,23.1 ± 2.83; group: F2,28 ¼ 13.510, p < 0.001; Fig. 1B,C) relative toHC. All three groups were significantly different from each other,with the ES group exhibiting a fewer number of sessions to reachextinction compared to HC (p < 0.05) and IS (p < 0.001) groups.Analysis of within-session responding during the first extinctionsession yielded no group differences (group: F2,28 ¼ 0.075,p ¼ 0.787; group time interaction: F10,140 ¼ 0.831, p ¼ 0.600;Fig. 1D). Similarly, a repeated measures ANOVA revealed thatgroups did not differ across the initial three extinction sessions(when all subjects were represented) for total number of nosepokes in the active port (group: F2,28 ¼ 0.325, p ¼ 0.573;group session interaction: F4,56 ¼ 1.14, p ¼ 0.345; Fig. 1E), furtherdemonstrating that the ES-induced acceleration of extinction wasnot due to a difference in initial levels of responding.
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