Experimental Studies Of Noradrenergic Pathway Mediates Pain In A Rat Model Of Parkinson's Disease

Objective:To establish the rat model of Parkinson's disease?PD?–associated pain,investigate the change of thermal and mechanical hypersensitivity after injuring or restoring noradrenergic?NE?pathway in PD model,and explore its'mechanism.Methods:Firstly,rat model of PD was established by stereotaxically injection of6-hydroxydopamine?6-OHDA,10?g/5?l each side?in the bilateral caudate putamen.Virifying the success of a PD rat model by behavior test,immunofluorescence and western blotting.Applying the high performance liquid chromatography?HPLC?to test the change of NE,5-hydroxytryptamine?5-HT?and dopamine?DA?cotent.Using Hargreaves test and VFF test to compare the change of thermal and mechanical hypersensitivity,respectively,in 6-OHDA-lesioned and SHAM rats before and after surgery.Secondly,in SD rats or PD model rats,respectively,investigating the change of pain hypersensitivity after injecting N-?2-chloroethyl?-N-ethyl-2-bromobenzyla-mine?DSP-4,100?g/4?l,each side?and5,7-dihydroxytryptamine?5,7-DHT,100?g/4?l,each side?to impaire noradrenergic and serotonergic neurons.Lastly,exploring the effect of droxidopa,clonidine and pramipexole ec al.on thermal and mechanical hypersensitivity in 6-OHDA-lesioned rats at the 5^th week after surgery.Results:?1?Comparing to the SHAM group,immunofluorescence analysis revealed that the relative level of TH-immunoreactive staining intensity in SN in the6-OHDA-lesioned group was decreased by about 80%,and western blotting analysis showed that the protein level of TH in 6-OHDA-lesioned group significantly decreased in the CPu by 45%.?2?HPLC analysis showed that NE content,but not 5-HT and DA,significantly decreased in lumbar spinal cord in PD rats?P<0.05?.?3?Thermal?P<0.001?and mechanical?P<0.05?hypersensitivity were developed at the 4^th week after 6-OHDA injection in rats.Additionally impaired noradrenergic neurons aggravated and accelerated the development of pain hypersensitivity in 6-OHDA-lesioned rats.The thermal?P<0.05?and mechanical?P<0.05?hypersensitivity occurred earlier at the 2^nd week after surgery.?4?Furthermore,at the 5^th week after 6-OHDA injection,systemic treatment with pharmacologically inactive NE precursor droxidopa?10,20 mg/kg,i.g.?,alpha-2adrenoceptor agonist clonidine?200?g/kg,i.p.?,and the NE and 5-HT reuptake inhibitors duloxetine?10 mg/kg,i.p.?,increased the nociceptive threshold in varying degrees(thermal:F_droxidopa?6,54?=5.867,P<0.0001;F_clonidine?6,36?=18.89,P<0.0001;F_duloxetine?5,50?=4.506,P=0018.mechanical:F_droxidopa?6,54?=5.867,P<0.0001;F_clonidine?6,36?=6.125,P=0.0002;F_duloxetine?5,50?=3.365,P=0.0107).Systemic administration of madopar or the D2/D3 agonist pramipexole did not inhibited the mechanical?P>0.05?hypersensiti-vity in PD rats,but only at the 90 min after injecting PRA attenuates the thermal hyp-ersensitivity.Conclusions:Stereotaxic infusion of 6-OHDA into the bilateral CPu in rats showed thermal and mechanical hypersensitivity.Impairment of descending noradrenergic pathway may play a key role in PD-associated pain hypersensitivity and restoring spinal noradrenergic inhibitory tone may serve as a novel strategy to manage PD-associated pain.