Effects of morphine on conditioned place preference and pain are independent of uptake-2

Morphine changes neurotransmitter release, including norepinephrine, dopamine, and serotonin. Decynium-22 (D22) inhibits an alternative neurotransmitter removal pathway, namely uptake-2. Uptake-2 includes plasma membrane monoamine transporter (PMAT) and organic cation transporters that have a low affinity, but high capacity for uptake of various monoamines such as norepinephrine, dopamine, and serotonin. This study was done to assess the effect of uptake-2 inhibition on morphine-induced conditioned place preference (CPP) and analgesia. In this study, the effects of morphine and/or D22 on CPP were evaluated following intraperitoneal injection in mice. Afterward, changes in motor activity were evaluated by the open field test. Using the tail-flick model, the effects of D22 and/or morphine were evaluated on the pain threshold. The results showed that 20 mg/kg of morphine induced a place preference response. D22, at the dose of 0.03 mg/kg, caused place avoidance, while at the dose of 0.3 mg/kg, it produced a notable place preference response. Co-administration of D22 and morphine showed that morphine reversed the CPP aversion induced by D22 at the lowest dose. Motor activity did not alter. In the tail-flick test, morphine, at the dose of 3 mg/kg but not 1 mg/kg, increased the pain threshold. D22 induced significant analgesic responses. Co-administration of D22 and morphine caused considerable analgesic effects. The findings revealed that D22 induced both conditioned aversion and preference depending on the dose while morphine induced CPP. Both drugs produced analgesia.