intruder rat was then placed into the cage of the resident (direct interaction). The session

was then closely monitored for submissive, supine posture expressed by the intruder.

Each time the intruder exhibited a supine posture with the resident physically contacting

the intruder for 2 sec or more, one defeat was counted. After three defeats or a total of

5 min elapsed (whichever came first), the intruder was taken out of the resident' s cage

momentarily and quickly placed into a 10 x 10 x 15 cm double-walled protective wire

mesh cage. The intruder (within the wire mesh cage) was then placed back into the

resident' s cage for the remainder of the 10-min session (indirect interaction). The two

intruder rats in each housing pair were run in simultaneous defeat sessions in the cages of

two resident males that were placed side by side. Each pair of cage-mates received the

same treatment. Each social defeat session was run between 8:00a.m. and 10:00a.m., and

was videotaped for further analysis.

Social Defeat Stress Regimen: Experiment la

 Thirty-six naive male LE rats were assigned to six experimental groups (Table 2-1).

The control rats from Group 1 (n = 6) remained in their original cages throughout the

experiment and were rapidly decapitated between 8:00a.m. and 10:00a.m. on the final

experimental day. These rats were exposed to no chronic and no acute stress (NC/NA).

The intruders from Group 2 (n = 6) were exposed to social defeat stress once every 24 h

across six experimental days and were killed by rapid decapitation between 8:00a.m. and

10:00a.m., 24 h after their final social defeat session. These rats were exposed to chronic,

but not acute stress (C/NA). The intruders from Group 3 (n = 6) were only exposed to

social defeat stress on the final experimental day and were then immediately decapitated,

10 min after the start of the social defeat session (t = 10 min). These rats were exposed to

no chronic, just acute stress (NC/A-10). The intruders from Group 4 (n = 6) were only