| Objective To investigate the role of metabolic glutamate receptor 5(mGluR5)in the right laterocapsular division of central nucleus of amygdala(CeLC)in the excitatory synaptic transmission of opioid-induced hyperalgesia(OIH)in rats.Methods In experiment 1,male Sprague-Dawley(SD)rats were randomly divided into hyperalgesia group 1(OIH1 group,n = 6)and control group 1(Control1 group,n = 6).The OIH1 group was subcutaneously injected with fentanyl to induce OIH model,with a dosage of 60 μg/kg for four times,time interval was 15 min,and the cumulative dose was 240 μg/kg.Control1 group was subcutaneously injected with equal volume normal saline.Before the first injection and 6.5 h after the last injection,the paw withdrawal mechanical threshold(PWMT)and paw withdrawal thermal latency(PWTL)of the two groups of rats were measured.Subsequently,CeLC tissue was taken and the expression level of mGluR5 protein was detected by western blot.In experiment 2,male SD rats were randomly divided into Vehicle group(n = 6),MTEP(3 μg)group(n = 6),MTEP(7.5 μg)group(n = 6)and MTEP(15 μg)group(n= 6).The right CeLC region was catheterized firstly,after one week recovery,fentanyl was injected to induce OIH model,and after 6.5 h,0.5 μl 10% DMSO(dimethyl sulfoxide),MTEP(inhibitor of metabolic glutamate receptor 5)(3 μg),MTEP(7.5 μg)and MTEP(15 μg)were injected accordingly into CeLC of each group.The changes of PWMT and PWTL were measured before OIH modeling,post-OIH and 0.5 h post-administration of MTEP/ DMSO.After the last behavioral test,rats were sacrificed and CeLC tissues were taken.Western blot was used to detect the expression level of mGluR5 of each group.In experiment 3,male SD rats were randomly divided into OIH2 group(n = 4)and Control2 group(n = 4),OIH2 group was subcutaneously injected with fentanyl to induce OIH model,and Control2 group was injected with normal saline of equal volume subcutaneously.Changes in PWMT and PWTL were measured before modeling and 6.5 h after the last injection.Following the last behavioral test,rats were sacrificed,and the CeLC brain slices were prepared.On the brain slice,the frequency and amplitude of miniature excitatory postsynaptic currents(mEPSCs)in CeLC neurons were recorded before and after MTEP treatment.In experiment 4,male SD rats were randomly divided into OIH3 group(n = 5)and Control3 group(n = 5).The OIH3 group was subcutaneously injected with fentanyl to induce the OIH model and Control3 group was injected with equal volume of normal saline.Changes in PWMT and PWTL were measured before modeling and 6.5 h after the last injection.After the last behavioral test,rats were sacrificed,and the CeLC brain slices were prepared.On the brain slice,the changes of frequency and amplitude of spontaneous excitatory postsynaptic currents(sEPSCs)in CeLC neurons were recorded before and after MTEP administration.In experiment 5,male SD rats were randomly divided into OIH4 group(n = 4)and Control4 group(n = 4),OIH4 group was subcutaneously injected with fentanyl to induce OIH model,and Control4 group was subcutaneously injected with normal saline of equal volume.Changes in PWMT and PWTL were measured before modeling and 6.5h after the last injection.Following the last behavioral test,rats were sacrificed,and brain slices were prepared.On the brain slice,dual-electrode patch clamp recording was performed with the stimulating electrode placed in basolateral amygdala(BLA)region and the recording electrode placed in CeLC region.So the changes in evoked excitatory postsynaptic current(e EPSC)in BLA-CeLC neural pathway were determined before and after MTEP treatment.In experiment 6,male SD rats were randomly divided into OIH5 group(n = 4)and Control5 group(n = 4),OIH5 group was subcutaneously injected with fentanyl to induce OIH model,and Control5 group was subcutaneously injected with equal volume of normal saline.Changes in PWMT and PWTL were measured before modeling and 6.5 h after the last injection.Following the last behavioral test,rats were sacrificed,and brain slices were prepared.On the slice,dual-electrode patch clamp recording was performed with the stimulating electrode placed in parabrachial nucleus(PB)inputting region and the recording electrode placed in CeLC region.So the Changes in e EPSC in PB-CeLC neural pathway were determined before and after MTEP applying.Results1.Compared with Control1 group,the protein expression level of mGluR5 in the right CeLC region of OIH1 group was significantly increased after fentanyl administration(P < 0.05).2.Compared with pre-OIH modeling,the PWMT was reduced and PWTL was shortened in the Vehicle group,MTEP(3 μg)group,MTEP(7.5 μg)group and MTEP(15 μg)group post-OIH modeling(P < 0.05).0.5 h after MTEP treatment,compared with the Vehicle group,the PWMT and PWTL of all MTEP groups were increased/prolonged in a dose-dependent manner(P < 0.05),accordingly the expression of mGluR5 protein in the right CeLC was down-regulated in a same manner(P < 0.05).3.Compared with the Control2/Control3 group,the frequency and amplitude of mEPSCs and sEPSCs of the neurons in the right CeLC region were increased significantly after the OIH model was established in the OIH2 group and OIH3 group(P < 0.05).The amplitude and frequency of mEPSCs and sEPSCs were decreased to normal level following mGluR5 inhibitor MTEP treatment(P < 0.05).4.Compared with Control4/Control5 group,after OIH modeling,the amplitude of e EPSC were increased significantly in BLA-CeLC and PB-CeLC neural pathways in OIH4 group and OIH5 group(P < 0.05).The above increased amplitude of e EPSC were decreased to the normal level following mGluR5 inhibitor MTEP treatment(P <0.05).Conclusions1.The expression of mGluR5 in the right CeLC region of OIH model rats was significantly increased.2.MTEP reversed OIH rats’ hyperalgesia behavior and down-regulated the increase of mGluR5 expression in the right CeLC region in a dose-dependent manner.3.Synaptic plasticity of the right CeLC neurons,and excitatory synaptic transmission of BLA-CeLC and PB-CeLC neural pathways were enhanced in OIH rats,and mGluR5 inhibitor MTEP could reverse these changes.In conclusion,activation of mGluR5 in right CeLC is involved in the modulation of fentanyl-induced hyperalgesia,and its mechanism may be related to enhanced synaptic plasticity of CeLC neurons,and enhanced excitatory synaptic transmission in BLA-CeLC and PB-CeLC neural pathways. |
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