Individual differences in anhedonic and accumbal dopamine responses to chronic social stress and their link to cocaine self-administration in female rats.

Abstract

Women are twice as likely as men to develop major depressive disorder. Exposure to chronic stress can induce depression in some vulnerable individuals, while others are resistant to depressive-like symptoms after equivalent levels of chronic stress. In female rats, individual differences in saccharin intake during chronic social defeat stress may predict subsequent cocaine self-administration, and may be attributed to alterations in mesolimbic dopamine activity. Female rats were exposed to 21 days of chronic social defeat stress, during which they were evaluated for their anhedonia-like responses in the form of saccharin intake. After chronic social defeat stress, the rats were tested for behavioral cross-sensitization to cocaine and escalated cocaine self-administration in a 24-h "binge." A separate group of animals underwent in vivo microdialysis of the nucleus accumbens (NAc) shell to assess dopamine (DA) in response to acute cocaine challenge. Cluster analysis revealed two phenotypes among the stressed female rats based on their saccharin intake while being exposed to stress, termed stress-resistant (SR, 28 %) and stress-sensitive (SS, 72 %). The amount of cocaine self-administered during the 24-h "binge" was positively correlated with preceding saccharin intake. The NAc DA response to a cocaine challenge was significantly lower in SR rats than in the SS and non-stressed control rats. No other significant differences were observed in behavioral cross-sensitization or cocaine self-administration prior to the "binge." Female rats showed individual differences in their anhedonic-like response to chronic social defeat stress, and these differences were reliably associated with subsequent cocaine-taking behavior.