Effects Of Dopamine Receptors In M1 On Thalamostriatal Pathway In Rats With Parkinson’s Disease

Parkinson’s disease(PD)is a progressive neurodegenerative disorder primarily characterized by dyskinesia,rigidity,resting tremors,and gait disturbances.Based on the 1.7% prevalence rate among the group over 65 in China and its large population,the Chinese PD patients are expected to reach 5 million by 2030,accounting for almost half of the world’s total.With the progression of PD,the patients’ quality of life could be seriously affected,and their families and society could suffer heavy medical burdens.The main pathological feature of PD is the degeneration and loss of dopaminergic neurons in the substantia nigra of the midbrain.The striatum(STR)is the primary projection target nuclei of dopaminergic neurons in the midbrain.As the main output nuclei in the basal ganglia,STR has extensive and close fiber connections with the primary motor cortex(M1)and thalamus,thus playing a major role in regulating movement and posture.According to the autopsy of PD patients,the thalamic centromedian-parafascicular complex(CM/PF,PF in rodents)is the main site of non-dopaminergic neuronal degeneration in PD patients,which has also been demonstrated in animal models of PD where PF showed substantial neuronal degeneration and loss as well as abnormalities in electrophysiological activity.Retrograde tracking revealed that over two-thirds of the fibers in CM/PF projected to STR and a small amount diffusely projected to M1.Studies have shown different abnormal discharge patterns and abnormal spectral distributions in the PD-related cortico-striatal-thalamo-cortical(CSTC)pathway.Deep brain stimulation(DBS)on some sites of the CSTC circuit could alleviate symptoms such as tremors and cognitive disorders.Thus,the CSTC circuit has received increasing attention in PD research.Since brain functions are essentially achieved through electrical signals,analyzing the cellular electrophysiological signal collected by microelectrodes implanted in the target nuclei could reveal the characteristics of brain electrical activities in real-time and reflect the functional state of the brain with higher temporal and spatial accuracy.Thus,neuroelectrophysiological records and their analysis could offer crucial assistance in researching the pathophysiological changes and therapeutic mechanisms of many diseases.PD studies have found abnormal frequency or pattern of electrical discharge activities in the CSTC circuit as well as abnormal correlations between different nuclei.However,little is known about the key nodes and roles of the CSTC circuit in PD.In addition to affecting STR and PF,DA damage also causes abnormalities in M1 metabolism and neuronal activity via substantia nigra-cortex projections.However,no study has reported how DA abnormalities modulate downstream thalamic-striatal pathways via M1.In particular,little is known about how DA losses in M1 link to dyskinesia in PD and the abnormal electrophysiological activity in the thalamic-striatal pathway.This study demonstrated the role of the DA receptors within M1 in the motor behavior regulation of PD rats with animal behavioral experiments and elucidated the role of DA receptors within M1 in the regulation of the M1-PF-STR pathway using the conscious animal in vivo brain neuron extracellular discharge technique,immunohistochemistry,and juxtacellular labeling.Methods:(1)Behavioral experiments: PD rat models were established by unilateral injection of 6-hydroxydopamine(6-OHDA)in the medial forebrain bundle(MFB);dopamine D1 receptor agonist SKF82958 and dopamine D2 receptor agonist quinpirole were infused into the M1 of 6-OHDA-lesioned rats,respectively.Threshold doses and the optimal time of DA receptor agonists were identified through behavioral experiments.(2)Electrophysiological experiment: PD rat models were established by unilateral injection of 6-OHDA in MFB,and two electrode arrays of 16 channel nickel-chromium alloy wire microelectrodes were implanted in the STR and PF of the rats,respectively.Firstly,the threshold dose of quinpirole,a DA D2 receptor agonist,was administered in M1.Secondly,in vivo multichannel recording technique was used to collect the spike and local field potential of extracellular single-cell discharge before and after quinpirole intervention in the four groups of rats.Finally,Offline Sorter,Neuroexplorer,and Matlab were used to analyze the electrophysiological data and investigate the effect of dopamine receptors in M1 on the thalamicstriatal pathway in PD rats.Results:(1)At threshold doses,both dopamine D1 receptor agonist and dopamine D2 receptor agonist facilitated the motor behavior of PD rats.However,agonist doses above the thresholds inhibited the motor behavior.More importantly,when half threshold doses were co-administered,rats displayed synergistic facilitation of motor behaviors than either agonist.(2)In 6-OHDA-lesioned rats,the dopamine D2 receptor agonist injection in M1 was more effective in facilitating animal behavior than the dopamine D1 receptor agonist injection.(3)Juxtacellular labeling and immunofluorescence histochemistry revealed two subtypes of neurons in PF,i.e.,glutamatergic(GLU)neurons and γ-aminobutyric(GABA)neurons,and two subtypes of interneurons in STR,i.e.,parvalbumin(PV)interneurons and cholinergic(CIN)interneurons.(4)In the PF nuclei,administration of quinpirole in M1 reversed the pathological changes of the glutamatergic neuron subtype and LFP activities but had no effects on those of γ-aminobutyric neurons.(5)In the STR nuclei,administration of quinpirole in M1 reversed the pathological changes of medium spiny neurons(MSN)and LFP activities during resting periods while showing no therapeutic effects on the abnormalities of MSN,fast spiking interneurons(FSI),and LFP activities during catching periods.(6)While administration of quinpirole in M1 reversed the pathological changes in the LFP coherence between the PF and STR nuclei and PF-Ⅱ spike-STR-LFP coherence between the two nuclei in PD rats during conscious resting periods,it did not affect the LFP coherence and PF-Ⅰspike-STR-LFP coherence during catching periods.Conclusions:Microinjected dopamine receptor agonist in M1 can alleviate motor behavioral disorders in PD rats by modulating abnormal electrical activities in the thalamic-striatal pathway.The effect of dopamine receptors in M1 on PF and STR is achieved through the projection neurons and is more significant on PF than STR.Significance:Based on PD rat models,this study explored and revealed the correlation between the PDrelated M1-PF-STR signaling pathways and the key role of each nucleus on the PD-related M1-PF-STR circuit at different levels,including intracerebral nucleus neuron single-cell spiking activity,the coherence of synchronous activities of LFP spectra and nuclei,and behavioral experiments.The findings could provide a theoretical basis for thoroughly clarifying the mechanism of PD and a theoretical foundation for furthering the clinical practice and targeted treatment of PD.