The Role Of Projections From TH~+ Neurons In Locus Coeruleus To Hippocampus In Cocaine-induced Conditioned Place Preference Of Mice

Objective:Tyrosine hydroxylase(TH)is an enzyme that produces noradrenaline(NA)and dopamine(DA).Locus coeruleus(LC)is located at the bottom of the fourth ventricle,Immunofluorescence detection shows that there are a large number of TH~+neurons in the LC.It was found that LC had a lot of fibers projected to the dorsal hippocampus(dHPC),and released NA and DA directly in CA1,CA3 and DG,thus promoting to consolidate the hippocampal dependent long-term memory.Recent studies have found that,in addition to the ventral tegmental area and substantia nigra,the LC is the third most dopamine-derived brain area,and dopaminergic neurons in LC have more projections to dHPC compared with ventral tegmental area.Photoactivation of dopaminergic neurons in LC will produce a phenomenon similar to new environment promoting memory.Cocaine addiction is a kind of strong pathological memory,which is closely related to the dysfunction of dopamine system.However,the role of projections from TH~+neurons in LC to dHPC in cocaine addiction is not clear.Methods:After acute cocaine administration or conditioned place preference(CPP)training,the activity of TH~+neurons in the LC were tested by in vivo calcium signal optical fiber recording technology.Chemical genetics was used to activate or inhibit projections from TH~+neurons in LC to dHPC,and then detected the effect on cocaine-induced CPP.Optogenetics was used to activate or inhibit projections from TH~+neurons in LC to CA3,CA1 or DG,and then detected the effect on cocaine-induced CPP.Immunofluorescence was used to determine the cell types of CA3 that received input from TH~+neurons in LC.Results:(1)In the CPP model,the CPP score of the saline group mice has no significant difference after training(pre-test:8.06±19.46;test:-8.21±22.52,n=8),while the CPP score of the cocaine group mice increased from 10.80±22.85 to 159.98±51.18(n=8,P<0.01),indicating that the cocaine training could induce the expression of CPP.(2)Using in vivo calcium signal optical fiber recording technology,it was found that the activity of TH~+neurons in LC decreased by about 10%after acute administration of cocaine,indicating that cocaine could inhibit the activity of TH~+neurons in LC.(3)After cocaine-induced CPP training,the mice were put into the same environment,and it was found that the activity of the TH~+neurons in LC decreased(n=2)when the mice entered into cocaine-paired chamber,indicating that the TH~+neurons in LC were involved in the addiction process.(4)Chemical genetics was used to inhibit projections from TH~+neurons in LC to dHPC,after cocaine-induced CPP training,the CPP score of hM4Di group mice was significantly higher than that of mCherry group mice(mCherry:17.81±26.21,n=7;hM4Di:147.58±47.11,n=8,P<0.05),indicating that inhibition of projections from TH~+neurons in LC to dHPC could promote the expression of cocaine-induced CPP.(5)Projections from TH~+neurons in LC to dHPC were activated by chemical genetics technology,after CPP training,the CPP score of hM3Dq group mice was significantly lower than that of mCherry group mice(mCherry:256.04±38.01,n=7;hM3Dq:132.82±18.10,n=8,P<0.01),indicating that activation of projections from TH~+neurons in LC to dHPC could inhibit the expression of cocaine-induced CPP.(6)Optogenetics was used to activate projections from TH~+neurons in LC to CA3,after CPP training,the CPP score of ChR2group mice was significantly lower than that of mCherry group mice(mCherry:250.30±24.48,n=15;ChR2:118.00±51.62,n=13,P<0.05),indicating that activation of projections from TH~+neurons in LC to CA3 could inhibit the expression of cocaine-induced CPP.(7)Optogenetic technique was used to inhibit projections from TH~+neurons in LC to CA3,after CPP training,the CPP score of NpHR group mice were significantly higher than that of EYFP group mice(EYFP:229.33±47.75,n=6;NpHR:421.43±50.79,n=7,P<0.05),indicating the inhibition of projections from TH~+neurons in LC to CA3 could promote the expression of cocaine-induced CPP.(8)Projections from TH~+neurons in LC to DG were activated by optogenetics,after CPP training,there was no difference between the CPP scores of ChR2 group mice and the mCherry group mice(mCherry:229.16±24.03,n=5;ChR2:240.40±41.40,n=4),indicating that activation of projections from TH~+neurons in LC to to DG had no effect on the expression of cocaine-induced CPP.(9)Projections from TH~+neurons in LC to CA1 were activated by optogenetics,after CPP training,there was no difference between the CPP scores of ChR2 group mice and the mCherry group mice(mCherry:188.07±15.57,n=4;ChR2:196.27±16.05,n=3),indicating that activation of projections from TH~+neurons in LC to CA1 had no effect on the expression of cocaine-induced CPP.(10)Through immunofluorescence co-localization detection,it was found that TH~+neurons in LC projected to excitatory neurons in CA3 subarea.Conclusion:After acute administration of cocaine,the activity of TH~+neurons in LC was decreased,and the activity of LC was also decreased during the memory retrieval caused by cocaine-related environment,indicating that the TH~+neurons of LC were involved in the addiction process.Activation of projections from TH~+neurons in LC to dHPC could inhibit the expression of cocaine-induced CPP.Inhibition of projections from TH~+neurons in LC to dHPC could promote the expression of cocaine-induced CPP,It indicating that projections from TH~+neurons in LC to dHPC plays an important role in cocaine addiction.