Strain differences in the rewarding and dopamine-releasing effects of morphine in rats

Studies examining differential sensitivity to psychoactive drugs in mice suggest that genotype may play a critical role. Furthermore, an involvement of genotype in mediating individual differences in sensitivity to the rewarding effects of several drugs of abuse has also been postulated. The aim of this study was to examine the conditioned rewarding and dopamine-releasing effects of morphine in two outbred rat strains commonly used in addiction research. Additionally, the behavioural and neuroendocrine responses of these strains to the stress of novelty were also examined. Basal locomotor activity was higher in Wistar rats than Sprague-Dawley following exposure to a novel environment. In contrast, elevations in plasma corticosteroid levels following novelty exposure did not differ between the two strains. In a counterbalanced place preference conditioning procedure, increasing doses of morphine (1.0-10.0 mg/kg SC) produced significant conditioned place preferences (CPP) in both Wistar and Sprague-Dawley strains. However, Wistar rats required a significantly larger dose of morphine (5.0 mg/kg) to produce a significant CPP than the Sprague-Dawley rats. In the latter strain, CPP occurred with doses of 3.0 mg/kg and greater. In parallel microdialysis experiments, both strains showed significant dose-related increases in dopamine release in the nucleus accumbens following acute morphine challenge (1.0-10.0 mg/kg SC). Again in Wistar rats, a larger dose of morphine was necessary to produce a significant increase in comparison to Sprague-Dawley rats. These results show that genetically distinct rat strains can show differential sensitivity to opioids, more specifically to drug-seeking responses.