Papers of the Week

The rewarding effect of opiates is mediated through dissociable neural systems in drug naïve and drug-dependent states. Neuroadaptations associated with chronic drug use are similar to those produced by chronic pain, suggesting that opiate reward could also involve distinct mechanisms in chronic pain and pain-naïve states. We tested this hypothesis by examining the effect of dopamine (DA) antagonism on morphine reward in a rat model of neuropathic pain.Neuropathic pain was induced in male Sprague-Dawley rats through chronic constriction (CCI) of the sciatic nerve; reward was assessed in the conditioned place preference (CPP) paradigm in separate groups at early (4-8 days post-surgery) and late (11-15 days post-surgery) phases of neuropathic pain. Minimal effective doses of morphine that produced a CPP in early and late phases of neuropathic pain were 6 mg/kg and 2 mg/kg respectively. The DA D1 receptor antagonist, SCH23390, blocked a morphine CPP in sham, but not CCI, rats at a higher dose (0.5 mg/kg), but had no effect at a lower dose (0.1 mg/kg). The DA D2 receptor antagonist, eticlopride (0.1 and 0.5 mg/kg), had no effect on a morphine CPP in sham or CCI rats, either in early or late phases of neuropathic pain. In the CPP paradigm, morphine reward involves DA D1 mechanisms in pain-naïve but not chronic pain states. This could reflect increased sensitivity to drug effects in pain versus no pain conditions and/or differential mediation of opiate reward in these two states.