roles of the nucleus accumbens (shell) in the acquisition and expression of morphine-induced conditioned behavior in freely moving rats

Background: The nucleus accumbens (NAc) is a part of the rewarding cortico-mesolimbic dopamine (DA) pathway. This is a heterogeneous structure divided in two sub regions termed core and shell. DA function in the NAc is critical for goal-oriented behaviors, including those motivated by drug and brain stimulation reward. In the conditioned-place preference (CPP) paradigm, a test assessing animal′s ability to associate drug-induced effects with environmental cause to quantify drug reward for example morphine. Methods: In the present study, we investigated the influence of electrical stimulation with different current intensities on (25 and 100 µA) with and without an effective dose of morphine (0.5 and 5 mg/kg) on CPP. Results: Subcutaneous administration of morphine 5 mg/kg produced significant CPP in comparison with saline group. Our findings also showed that electrical stimulation of NAc (100 µA) significantly (P < 0.01) suppressed morphine-induced CPP that reveals impaired learning and memory formation in the process of conditioning. We found that morphine-induced CPP can be successfully suppressed by current intensity (100 µA). It was probably due to decreasing of dopamine contents and its metabolites in the NAc. Current intensity (100 µA) in combination with ineffective dose of morphine (0.5 mg/kg) increased morphine-induced CPP probability via the prove reward system. Conclusions: Since stimulation of dopaminergic neurons increases tendency to dependence to morphine, therefore in the present study, the stimulation of the NAc suppressed morphine-induced CPP that this shows impairment of learning and memory formation.