ACUTE STRESS INDUCES TIME-DEPENDENT RESPONSES IN DOPAMINE MESOLIMBIC SYSTEM

Exposure to either restraint or footshock (3-60 min) induced similar biphasic alterations of 3-methoxytyramine (3-MT) concentrations (initial increase followed by decrease below control levels) in the nucleus accumbens septi (NAS) of mice, as revealed by tissue analysis. The only difference between the two stressors was the earlier onset of the decrease phase in the restrained mice. In both stressful conditions acid metabolites, 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) increased throughout stress, while no significant changes in dopamine (DA) concentrations occurred. These data suggest biphasic alteration of DA release during prolonged stress exposure. The analysis of release in restrained conscious rats by in vivo microdialysis (10-240 min) showed a similar biphasic DA evolution (initial increase followed by decrease below baseline levels) in the NAS. The only difference from the previous experiment was the delayed onset of the decrease phase. Similar changes in DOPAC and HVA were also evident. Moreover, freed rats showed an immediate increase of DA release over baseline levels, also indicating that depletion of the neurotransmitter cannot account for the reduction of released DA. Taken together, these results support the hypothesis that biphasic alteration of DA transmission in the mesolimbic system is a general response to stress and suggest that the initial increase of DA release represents an arousal response while the subsequent decrease in DA release may be related to coping failure.