In general, the behavioral changes in the intruder rats after repeated social defeat

stress model some of the symptoms expressed in clinically depressed patients, including

impairment in social or occupational functioning and loss of interest (DSM IV, 1994).

Also, the elevated basal CORT concentrations after daily social defeat stress during the

nadir of the daily cycle confirm previous findings from our laboratory (Lopes and

Devine, 2004) and mimic the augmented circulating hormones found in clinically

depressed patients (Linkowski et al., 1985; Pfohl et al., 1985).

 Porsolt Swim Test

 Based on the combined results from the daily and the intermittent stress regimens, a

daily social defeat stress regimen was used to for the Porsolt study to optimize the

potential to produce behavioral despair. The fact that acute social defeat stress did not

produce as much immobility as repeated stress indicates that the neuronal plasticity

caused by repeated exposure to social stress (demonstrated through hormonal and

behavioral plasticity) is a determining factor in the expression of behavioral despair.

 A 5-min swim test is commonly used for detection of behavioral despair (Gavioli et

al., 2003; Hinojosa et al., 2006; Porsolt et al., 1978; Rygula et al., 2005). Interestingly,

when analyzed in 5-min bins, the data suggest that in all the bins the repeatedly stressed

rats exhibited more immobility than the controls did, and in one bin (5-10 min) the

acutely defeated rats also exhibited more immobility than the controls did. Therefore, a

10-min session may be necessary to closely examine the subtle differences between

groups that have a less severe stress history and non-stressed control rats.

 Overall, these results model the behavioral despair, or low mood and anhedonia,

found in clinically depressed patients (for review, see Harrison, 2002). Also, the

significantly elevated immobility times for the repeatedly stressed rats confirm and