Effects And Mechanisms Of CX3CL1 On Addictive Behaviors Induced By Cocaine In Mice

Part Ⅰ The role of CX3CL1 in cocaine-induced addiction in miceObjective: Chemokines served as a mediator in the neuron-microglia crosstalk under physiological and pathological condition. CX3CL1 signaling played a critical role in the regulation of microglial properties, synaptic activity, synaptic pruning, adult hippocampal neurogenesis, as well as learning and memory. CX3CL1 reduced spontaneous glutamate release and post-synaptic glutamate currents. The study explored whether CX3CL1 regulated cocaine addictive behavior.Method: The m RNA level of CX3CL1 in the nucleus accumbens(NAc) of mouse was evaluated using q PCR after chronic cocaine exposure and withdrwal. The protein level of CX3CL1 was evaluated using western blotting after acute, chronic cocaine exposure and withdrwal, respectively. Behavioral sensitization was conducted to investigate the behavioral effects of cocaine following intra-accumbal injections of the exogenous CX3CL1.Results: q PCR analysis showed that the m RNA level of CX3CL1 in NAc did not change 24 h after chronic cocaine exposure(COC:99.14 ± 6.07%, n = 8, p = 0.23) and withdrawal(COC:92.59 ± 4.86%, n = 6, p = 0.44). Western blotting analysis revealed that the protein expression of CX3CL1 did not change in NAc after acute cocaine exposure(COC:107.8 ± 6.06%, n = 6, p =0.87). However, the protein expression of CX3CL1 was significantly reduced after chronic cocaine exposure(79.33 ± 5.57%, n = 7, p < 0.01) and withdrawal(73.47 ± 7.06%, n = 6, p < 0.01). Intra-accumbal infusion of exogenous CX3CL1 attenuated behavioral sensitization(WD + CX3CL1: 16380 ± 818 cm, n = 8, p < 0.05).Conclusion: Chronic cocaine exposure and withdrawal significantly reduced the expression of CX3CL1 in the NAc of mice. Intra-accumbal infusion of exogenous CX3CL1 attenuated behavioral sensitization.Part Ⅱ The mechanisms of CX3CL1 inhibiting cocaine-induced addictive behaviors in miceObjective: Depressing synaptic strength in the NAc reduced behavioral sensitization to cocaine. Several lines of evidence indicated that CX3CL1 modulated AMPA receptors at active glutamatergic synapses and depressed synaptic transmission. Here, we tested if CX3CL1 could active the microglia and inhibit the surface expression of AMPA receptors.Methods: The m RNA level of CX3CR1 was evaluated in the NAc of mice using q PCR. The protein level of CX3CR1 was evaluated in the NAc of mice using western blotting. The activity of microglia was evaluated in the NAc of mice using Immunofluorescence. Surface expression of Glu A1/2 was assayed using the membrane impermeable crosslinking reagent bis(sulfosuccinimidyl) suberate(BS3).Results: q PCR analysis showed that the m RNA level of CX3CR1 in NAc did not change 24 h after chronic cocaine exposure(WD:91.97 ± 4.24%, n = 9, p = 0.153) and withdrawal(WD:94.94 ± 6.37%, n = 9, p = 0.58). However, the protein expression of CX3CR1 significantly was reduced after chronic cocaine exposure(WD:75.74 ± 5.04%, n = 8, p < 0.001)and withdrawal(COC:69.36 ± 4.63%, n = 7, p < 0.001). Western blotting analysis revealed a selective increase in CD11 b protein 24 h after withdrawal from repeated cocaine(WD:124.3 ± 9.92%, n = 7, p < 0.05). Staining of NAc slices for microglia showed Iba1 intensity was increased after withdrawal(WD:130.3 ± 3.16%, n = 3, p < 0.01).Intra-accumbal infusion of exogenous CX3CL1 increased the expression of CD11b(WD + CX3CL1:197.6 ± 32.63%, n = 4, p < 0.05) and Iba1 intensity(125.2% ± 13.79%, n = 3, p < 0.01). Intra-accumbal infusion of exogenous CX3CL1 reduced the surface expression of Glu A1(WD + CX3CL1:74.48 ± 9.80%, n = 9, p < 0.05).Conclusion: Our data identified that CX3CL1 activated the microglia and reduced the surface expression of Glu A1. CX3CL1 may inhibit behavioral sensitization by depressing NAc glutamatergic synapses transmission.