OPPOSITE RESPONSES OF MESOLIMBIC DOPAMINE SYSTEM TO CONTROLLABLE AND UNCONTROLLABLE AVERSIVE EXPERIENCES

It has been previously shown that rodents exposed to stressful experie nces show a biphasic response of the mesolimbic dopamine (DA) system, that is, initial increase of DA release followed by a decrease below c ontrol levels (Puglisi-Allegra et al., 1991). Evidence is now presente d showing that mice exposed to a series of foot shocks show an increas e of DA release in the nucleus accumbens septi (NAS) if they are allow ed to control the shock experience (shocked condition) and a decrease of DA release in this brain area if they are not allowed to exert any control (yoked condition). These results indicate that escapable/contr ollable and inescapable/uncontrollable aversive experiences elicit opp osite responses from the mesolimbic DA system. Mice exposed to the app aratus without receiving shock (sham condition) show a time-dependent biphasic evolution mesolimbic DA release in line with previous reports indicating that confinement in an unknown environment represents a st ressful experience for mice. Moreover, exposure to the sham condition for a time comparable to the duration of shock and yoked exposure indu ces a mesolimbic DA response only quantitatively different from the re sponse of the yoked group but qualitatively different from the respons e of the shocked mice. These results suggest that in environmental con ditions that allow behavioral control, enhanced mesolimbic DA release is maintained regardless of the intensity of the aversive stimuli. On the other hand, evaluation of changes in acid DA metabolites levels in the frontal cortex (FC) of mice exposed to the shocked, yoked, and sh am conditions suggests that stressful experiences characterized by a d ifferent intensity of the aversive stimuli could elicit graded respons es in the FC DA system. Taken together, the present results may indica te that the initial activation of the mesolimbic DA system induced by stressful experiences accompanies the organism's attempts to cope beha viorally with such a pressure while the subsequent inhibition takes ov er when, despite the efforts made by the organism, no behavioral copin g is possible.