Comparison of the Modulating Affects of Acute and Chronic Pain States on Morphine Reward in C57Bl6 Mice

The purpose of our study was to compare the modulating affects of pain states on morphine reward in two sets of experiments: (i) previous exposure to different, acute pain states, thermal versus chemical nociception; and, (ii) existing acute versus chronic pain states on morphine conditioned reward. C57Bl6 mice were exposed to different pain states such as the acetic‐acid induced abdominal constriction (chemical), hot‐plate (thermal), and paclitaxel (chemotherapeutic agent)‐induced chronic neuropathic pain. Morphine reward was assessed using the conditioned place preference (CPP) paradigm. In the first set of experiments, mice were either pretreated with 0.4% acetic acid or had hot‐plate exposure 4 h prior to saline or morphine conditioning on alternative days for 6 days. In the second set of experiments, mice expressed acetic‐acid induced writhing or paclitaxel‐induced neuropathic pain during saline or morphine conditioning. The time spent in the morphine‐paired side was calculated on the test day. Results demonstrated that while morphine CPP was significantly reduced in the acetic acid treated mice, morphine CPP was enhanced in the mice with hot plate exposure or neuropathic pain. In conclusion, pain states can differentially modulate the conditioned rewarding effects of morphine and should be an important consideration in pain clinics for patients and their healthcare providers.