A single exposure to novelty differentially affects the accumbal dopaminergic system of apomorphine-susceptible and apomorphine-unsusceptible rats

Individual differences in responses to mild, acute stressors in laboratory animals have commonly been observed in behavioural tests and at the level of hypothalamic-pituitary-adrenal axis responses. These differences are associated with dopamine transmission in the nucleus accumbens. Although the effect of mild stressors on dopamine transmission has been studied with microdialysis, it has not been studied at the level of the catecholaminergic network in the nucleus accumbens. In this study we have used microdialysis to measure extracellular concentrations of dopamine in vivo and immunocytochemistry for the enzyme tyrosine hydroxylase to assess the effect of a single exposure to novelty on the neurochemistry of the nucleus acc umbens in apomorphine-susceptible and apomorphine-unsusceptible rats. These rats are a valid animal model for studying individual differences in responses to environmental stressors and drugs of abuse. We demonstrated that a mild stressor like novelty increased the extracellular concentration of dopamine in the nucleus accumbens in apomorphine-susceptible rats to a larger and longer-lasting degree than in apomorphine-unsusceptible rats. Furthermore we demonstrated that novelty increased the tyrosine hydroxylase-immunoreactive fibre network in the nucleus accumbens shell of apomorphine-susceptible rats, which are rats that are particularly reactive to stressors, but not in the shell of apomorphine-unsusceptible rats, which are rats that are relatively stress-resistant. In conclusion, we have shown that the accumbal dopaminergic system of apomorphine-susceptible rats is more sensitive to an environmental stressor than that of apomorphine-unsusceptible rats. Combined with the fact that these animals also differ in their sensitivity to drugs of abuse, which are known to affect the dopaminergic system, these data provide a solid basis for further studying the differences in the dopaminergic responsiveness to drugs of abuse between apomorphine-susceptible and apomorphine-unsusceptible rats.