Modulations Of Central Histaminergic And Orexinergic Systems On Subthalamic Neurons And Their Roles In Parkinsonian Motor Dysfunctions

Parkinson’s disease(PD)is a highly prevalent neurodegenerative disorder manifests not only motor but also nonmotor symptoms.Its motor symptoms are characterized by bradykinesia,rigidity,and rest tremor,whereas the nonmotor symptoms mainly include sleep and emotional disorders.The parkinsonian motor dysfuntions is mainly due to an imbalance of direct and indirect pathway induced by degeneration of dopaminergic neurons in the substantia nigra pars compacta.The subthalamic nucleus(STN),the only excitatory glutamatergic nucleus in the basal ganglia circuitry,is not only a key node in the classical indirect pathway and the so-called "hyperdirect" pathway directly connecting with the cortex,but also even considered as a pacemaker for activity of whole basal ganglia.In addition,the STN is an optimal target for deep brain stimulation(DBS)for clinical treatment for PD.Intriguingly,the central histaminergic and orexinergic systems,uniquely originating from the hypothalamus,have direct innervation on the basal ganglia.Recent clinical observations indicate that the central histaminergic and orexinergic systems may be closely related to PD.In PD patients,the concentration of histamine in their blood and basal ganglia including the substantia nigra,putamen,and globus pallidus,are significantly increased,and the density of histaminergic fibers and histamine receptors in the substantia nigra are elevated.On the other hand,the orexinergic neurons degenerate with the progression in PD and the level of orexin in the cerebrospinal fluid remarkably reduced.However,roles of central histaminergic and orexinergic systems in basal ganglia motor contral and parkinsonian motor deficits remains largely unknown.Therefore,the present study focuses on the modulations and mechanisms of central histaminergic and orexinergic systems on STN,a key structure in the basal ganglia.Combining molecular,electrophysiological,immunohistochemical,and behavioral techniques,we study the effects of histamine and orexin on STN neurons and the underlying receptor and ionic mechanisms,particularly the roles of histaminergic and oreixnergic afferent inputs in the STN in motor function of basal ganglia and their relationship to parkinsonian motor dysfunction.The results will not only contribute to comprehensive understanding functions of central histamine and orexin on motor contral,but also provide a novel insight into optimizing clinical strategies for treatment of PD.1.Modulation of central histaminergic system on STN neurons and its role in parkinsonian motor dysfunctionThe STN holds a key position in the basal ganglia circuitry,and is an optimal clinical target for DBS for treatment of motor symptoms in PD.Intriguingly,histamine level in the basal ganglia significantly changes in PD progression.Yet little is known about role of central histaminergic system in the basal ganglia motor function,as well as the neuronal mechanism underlying DBS of STN.Here,we report that firing patterns of STN are more crucial than firing rates for maintenance of normal basal ganglia motor function and amelioration of parkinsonian motor dysfunction.We show that both histamine and high K+ directly increase firing rates of STN neurons,but histamine rather than high K+ ameliorates the parkinsonian motor dysfunction.Regularizing firing patterns of STN neurons via the hyperpolarization-activated cyclic nucleotide-gated(HCN)channel coupled to H2 receptor accounts for the unexpected amelioration effect of histamine.Furthermore,upregulation of HCN2 channel,as well as DBS,regularizes firing patterns of STN neurons under parkinsonian condition.Our results demonstrate that the regularity of STN neuronal firing patterns,modulated by histamine or DBS,is essential for basal ganglia motor function.The findings suggest an indispensable role for histaminergic system in regularizing STN neuronal firing patterns and provide a novel insight into the mechanisms underlying DBS treatment of parkinsonian motor dysfunction.2.Modulation of central orexinergic system on STN neurons and its role in parkinsonian motor dysfunctionThe central orexinergic system,originating from the lateral hypothalamus/perifornical area(LHA/PFA),holds a key position in regulation of various basice physiological functions.Orexin deficiency results in narcolepsy,which is characterized by excessive daytime sleepiness,in both humans and animals.Interestingly,in PD patients,motor symptoms are often accompanied by various sleep disorders such as REM sleep behavior disorder,narcolepsy-like excessive daytime sleepiness,and insomnia.In addition,orexinergic neurons are progressively lost with the development of PD,and the levels of orexin in the cerebrospinal fluid of parkinsonian patients and rodents significantly decrease.The evidence suggests that sleep disorders in PD may be closely related to the degeneration of central orexinergic neurons.Here,we show that the orexinergic neurons in the LHA project directly to the STN,and two orexin receptors,OX1R and OX2R,are coexpressed on STN neurons.Orexin directly excites STN neurons via activation of postsynaptic OX1R and OX2R and their coupled Na+-Ca2+ exchangers and inward rectifier K+ channels.Different from histamine,microinjection of orexin into the STN significantly decreases motor performances of normal rats and deteriorates motor deficits in PD rats.Blockage of the endogenous orexinergic inputs ameliorates the motor performances of parkinsonian rats on day 14.However,the amelioration decrease or even disappear on day 21 and 28.These results strongly suggest that endogenous orexinergic inputs to the STN decreases with the progression of PD.Anterograde tracings and immunofluorescence histochemistry show a heterogeneity of two classes of orexinergic neurons projecting to the STN and the locus caeruleus(LC),an arousal-related nucleus,and a selective degeneration of orexinergic neurons projecting to the STN rather than to the LC in the parkinsonian rats.These results demonstrate that the degeneration of central orexinergic neurons may be closely related to parkinsonian motor dysfunctions rather than sleep disorders.In conclusion,the central histaminergic and orexinergic system originating from the hypothalamus directly participate in motor function of basal ganglia circuitry via their innervation on STN,and play an important role in parkinsonian motor dysfunction.Although both histamine and orexin excite the STN neurons,histamine rather than orexin ameliorates parkinsonian motor dysfunctions via regularizing STN neuronal firing patterns.The present study reveals a direct motor function of central histaminergic and orexinergic systems,and provides an insight into understanding and developing new strategies for treatment of parkinsonian motor dysfunction.