STRESS, DEPRESSION AND THE MESOLIMBIC DOPAMINE SYSTEM

The present review was aimed at re-evaluating results obtained from an imal models of depression based on experimental stressors in the light of the most recent data on the effects of stress on mesolimbic dopami ne (DA) functioning. The data reviewed reveal that the effects of stre ssful experiences on behaviour and on mesoaccumbens DA functioning can be very different or even opposite depending on the behavioural contr ollability of the situation, the genetic background of the organism an d its life history. Exposure to a single unavoidable/uncontrollable av ersive experience leads to inhibition of DA release in the accumbens a s well as to impaired responding to rewarding and aversive stimuli. Mo reover, the data reviewed indicate a strong relationship between these neurochemical and behavioural effects and suggest that they could mod el stress-induced expression and exacerbation of some depressive sympt oms such as anhedonia and feeling of helplessness caused by life event s as well as syndromal depression provoked by traumatic experiences in humans. Repeated and chronic stressful experiences can reduce the abi lity of stressors to disrupt behaviour, induce behavioural sensitisati on to psychostimulants and promote adaptive changes of mesolimbic DA f unctioning. Opposite neural and behavioural changes, however, can be p romoted in specific environmental conditions (repeated variable stress ful experiences) or in genetically predisposed individuals. Thus, depr essive symptoms may not represent the necessary outcome of stress expe riences but be promoted by specific environmental conditions and by a genetically determined susceptibility.