Ethanol-induced effects on the dopamine and serotonin systems in adult Wistar rats are dependent on early-life experiences.

Some individuals control their ethanol consumption throughout life, but others escalate their intake to levels that increase the risk for addiction. The early environment influences the individual response to ethanol and affects the underlying physiological processes that lead to a transition from a voluntary to a compulsive use of ethanol. However, the neurobiological substrates for these processes are not understood. The present study aimed to test the hypothesis that early environmental experiences affect the neurobiological effects that are induced by voluntary ethanol consumption. Rat pups were subjected to three different rearing environments: conventional animal facility rearing or separation from dam and littermates for either 15 or 360min. In adulthood, the rats were exposed to a two-bottle free choice between ethanol and water for seven weeks. Tissue levels of dopamine, 5-hydroxytryptamine (5-HT) and their metabolites were measured in brain areas that have been implicated in reward and addiction processes. Differences in ethanol-induced effects were noted in 5-HT-related measurements in the nucleus accumbens and ventral tegmental area and in dopamine-related measurements in the dorsal raphe nucleus (DRN). These results provided evidence of an early environmental impact on interactive neuronal circuits between the DRN and reward pathways. The amygdala, a key area in addiction processes, was particularly sensitive to early-life conditions. The animals that experienced the longest separation differed from the others; they had low basal 5-HT levels and responded with an increase in 5-HT after ethanol. These altered responses to initial ethanol consumption as a result of early environmental factors may affect the transition from habitual to compulsive drinking and contribute to individual vulnerability or resilience to addiction.