Biochemical adaptations in the mesolimbic dopamine system in response to repeated stress

We have demonstrated previously that chronic administration of morphine, cocaine, or ethanol produces some common biochemical adaptations in the ventral tegmental area (VTA) and nucleus accumbens (NAc), components of the mesolimbic dopamine system implicated in the reinforcing and locomotor activating properties of these drugs of abuse. Because this neural pathway is also regulated by stress, and because stress has been shown to influence an animal's behavioral responses to drugs of abuse, it was of interest to determine whether repeated exposure to stress results in similar biochemical adaptations. By use of immunoblot analysis, we show here that a course of chronic "unpredictable" stress, like chronic drug exposure, increased levels of immunoreactivity of tyrosine hydroxylase and glial fibrillary acidic protein and decreased levels of immunoreactivity of neurofilament proteins in the VTA. Chronic unpredictable stress also increased levels of cyclic AMP-dependent protein kinase activity and decreased levels of immunoreactivity of the G protein subunit, Gi alpha, in the NAc. These effects required long-term exposure to stress and were in most cases not seen in the substantia nigra and caudate-putamen, components of the nigrostriatal dopamine system studied for comparison. The biochemical effects of chronic stress in the VTA and NAc differed among three strains of rat studied. Fischer 344 rats were the most responsive in that they exhibited all of the aforementioned adaptations, whereas Lewis rats were the least responsive in that they exhibited none of these adaptations; Sprague-Dawley rats exhibited an intermediate number of responses. Taken together, the results of the present study demonstrate that chronic exposure to stress results in biochemical adaptations in the mesolimbic dopamine system that resemble the chronic actions of several drugs of abuse. These adaptations could contribute to the convergent behavioral effects induced by treatments that are mediated via the VTA-NAc pathway.