SOCIAL DEFEAT STRESS SELECTIVELY ALTERS MESOCORTICOLIMBIC DOPAMINE RELEASE - AN IN-VIVO MICRODIALYSIS STUDY

Exposure to various aversive stimuli ('stresors') as well as positivel y-reinforcing stimuli has been shown to increase extracellular dopamin e concentrations in terminal areas of the mesocorticolimbic dopamine s ystem. The magnitude and site specificity of the dopaminergic response may depend on the nature of the aversive stimulus. in the present stu dy, in vivo microdialysis was used to examine the effects of an etholo gically relevant stressor, namely threat of social defeat, on dopamine concentrations in nucleus accumbens, striatum, and prefrontal cortex of freely-moving male Long-Evans rats. During the test session, dialys ate and video recording samples were collected from previously-defeate d ''intruder' rats in consecutive phases, while (1) in the home cage, (2) when paced in the empty, soiled cage of a resident rat which had p reviously defeated them, (3) when exposed to threat of defeat by the r esident, and (4) when returned to their home cages. Control animals we re not defeated; in this group of rats video recording and dialysate s amples were obtained when they were placed into an empty, clean novel cage and later returned to their home cage. The results indicated that levels of dopamine were elevated to approximately 130% of baseline in nucleus accumbeus and prefrontal cortex when rats were placed into ei ther the resident or novel cage. In defeated intruders, extracellular dopamine levels in accumbens and prefrontal cortex were increased furt her (approximately 160% of baseline), during social threat; these bioc hemical changes were synchronous with high levels of orienting toward the resident but not with heightened motor activity. Extracellular dop amine levels in lateral striatum were not affected by either manipulat ion. These results suggest that altered accumbens and cortical extrace llular dopamine concentrations during social threat are not secondary to motor activation but instead reflect increased attention to the pro vocative stimulus or attempts by the intruder to 'cope' with the stimu lus.