Figure 9.

Serum corticosterone responses in Experiment 4. On Day 33 of Experiment 4, rats were given yohimbine (1 mg/kg, i.p.) or vehicle. The psychosocial stress and yohimbine-treated groups exhibited greater serum corticosterone concentrations 30 min post-injection (60 min time point) than the no psychosocial stress and vehicle-treated groups, respectively. Corticosterone levels were increased in all groups following 20 min of restraint (indicated by the dark black line between 60–80 min); the increases were greater in groups that had been psychosocially stressed and/or pre-treated with yohimbine. Psychosocial stress and yohimbine treatment had an additive effect on corticosterone levels at each of the first two time points: the group that had been psychosocially stressed and pre-treated with yohimbine (S-YOH) exhibited the greatest corticosterone levels at both time points. While corticosterone levels of the two vehicle-treated groups (S-VEH, NS-VEH) significantly declined 60 min following the cessation of immobilization, those of the two yohimbine-treated groups (S-YOH, NS-YOH) remained elevated. *p < 0.05 versus the no psychosocial stress-vehicle group; **p < 0.05 versus psychosocial stress-vehicle and no psychosocial stress-vehicle groups; ***p < 0.05 versus all other groups. Data are group means ± SEM.