Dopamine and glutamate receptors are densely expressed in the nucleus accumbens (NAc).Active interactions between these receptors contribute to the development of neuropsychiatric diseases,such as drug addiction and relapse.However,the molecular mechanisms underlying these interactions remain unclear.Our studies focus on the effects of D3Rs on morphine-induced locomotor activity as well as on the regulation of NR2B receptors in the NAc.In our studies,C57BL/6 mice were used.Behavioral experiments were performed between 08:00 and 13:00.We evaluated the basal activities of the mice by measuring their horizontal locomotion on day 0.Within the following 13 d,morphine was injected five times at an interval of 72 h.Similarly,the controls were treated with 0.9%saline(10 mL/kg).Nafadotride(25 μg/kg)was injected into the nafadotride–morphine mice and ifenprodil(5 mg/kg)was injected into the ifenprodil–morphine mice 30 min before each morphine injection.Horizontal trajectories of the mice were video recorded and analyzed to determine their traveled distances in 90 min or per 10-min period.Recording commenced within 2 min to 5 min after each injection.The mice were sacrificed after the last test.The frontal association cortex(FrA),nucleus accumbens(NAc),caudate putamen(CPu),and hippocampus(Hip)of the sacrificed mice were prepared for western blot.For the electrophysiological studies,NAc slices from fresh brain tissue of male C57BL/6 mice were prepared and whole-cell patch clamp were recorded.Excitatory postsynaptic currents(EPSCs)and miniature excitatory synaptic currents(mEPSCs)were recorded.The results indicated that interval morphine administration caused significant locomotor sensitization.Hyperlocomotion and behavioral locomotor sensitization were significantly suppressed when ifenprodil or nafadotride or both ifenprodil and nafadotride were coadministered with morphine.The mice treated alone with saline,ifenprodil,or nafadotride showed constant locomotor activity after each injection.Although ifenprodil and nafadotride did not show behavioral locomotor sensitization effects,they affected morphine-induced behavioral locomotor sensitization.Chronic morphine injection upregulated both pNR2B and NR2B expression in the NAc compared with saline control.In addition,Chronic morphine did not alter pNR2B and NR2B levels in FrA,Hip,and CPu.Nafadotride alone did not alter basal levels of both pNR2B and NR2B expression in the NAc compared with saline control.By contrast,nafadotride blocked the morphine induced increase in both pNR2B and NR2B expression in the NAc compared with the morphine group.Whole-cell NMDA receptor-mediated currents were recorded,the slope of the NMDA receptor-mediated EPSC curves was dramatically greater in the neurons from morphine-dependent mice compared with saline controls.nafadotride can partially downregulate the enhancement of excitatory synaptic transmission in the NAc by chronic morphine administration.mEPSC frequency and amplitude significantly increased in the morphine-sensitized mice compared with saline controls.Nafadotride can significantly reverse the enhancement of mEPSC amplitude and frequency induced by chronic morphine injection.These findings suggest that D3Rs are involved in morphine-induced behavioral locomotor sensitization in mice by regulating the NR2B subunits of NMDA receptors in the NAc.