Blockade of dopamine D1 receptors in male rats disrupts morphine reward in pain naive but not in chronic pain states

The rewarding effect of opiates is mediated through dissociable neural systems in drug naive 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-naive 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-naive 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.