Modulation Of Central Orexinergic System On The Cerebellar Interposed Nucleus And Ventral Pallidum And Its Role In Motor-Emotional Integration

Orexin is a neuropeptide that is restrictedly synthesized in the hypothalamus but widely modulates almost whole brain activity.It has been well known to regulate various nonsomatic activities,such as food intake,sleep,wakefulness,reward and emotion.However,notably,orexin deficiency in humans and animals often results in cataplexy,which is a dangerous motor disorder characterized by a sudden loss of muscle tone.Our previous study demonstrated a direct modulation of orexin and central orexinergic system on somatic motor control.Thus,we speculate that the central orexinergic system may be one of the crucial nodes in the somatic-nonsomatic integration.Intriguingly,cataplexy is often triggered by strong emotions.Therefore,cataplexy is a kind of simultaneous motor and emotional dysfunctions.In fact,physical exercise helps for mood improvement.On the other hand,emotion promotes or constrains motor behaviors.Yet little is known about the neural substrates and mechanisms underlying motor-emotional integration and the exact role of central orexinergic system in motor-emotional integration.The cerebellum and basal ganglia are two important subcortical motor centers,which hold a key position in motor coordination and motor initiation,respectively.Recent studies have also indicated an involvement of these two classical motor structures in regulation of nonsomatic activities including emotion.Considering both the cerebellum and basal ganglia receive abundant orexinergic fibers from the hypothalamus,and express orexin recptors,we propose that the central orexinergic system may modulate motor-emotional integration via its innervations on the cerebellum and basal ganglia.Therefore,in this study,by using retrograde-tracing,immunostaining,patch clamp recording,real time RT-PCR,western blot and behavioral techniques,we investigate the effect of orexin on neuronal activity of the cerebellar interposed nucleus(IN)and the ventral pallidum(VP)in basal ganglia,as well as the underlying receptor and ionic mechanisms,and particularly,the function of orexinergic afferent inputs in the IN and VP-mediated motor-emotional integration.1.Modulation of central orexinergic system on the cerebellar IN and its role in motor-emotional integrationThe cerebellum,the largest subcortical motor structure,plays an important role in the control of balance,regulation of muscle tone,and coordination of voluntary movement.In the cerebellar circuitry,the cerebellar nuclei are not the traditionally considered relay station simply receiving outputs from cerebellar cortical Purkinje cells,but the integration center for the entire cerebellar circuitry.Among the cerebellar nuclei,the IN is one of the final outputs of the spinocerebellum and precisely controls distal muscles of the limbs and digits.Interestingly,it has been well known that the central orexinergic system directly regulates emotional centers in the limbic system and subsequently modulates somatic movement.On the other hand,our previous study demonstrated a direct excitatory effect of orexin on the cerebellar IN,a center for regulation of muscle tone.Considering that the cerebellum may participate in emotion regulation via its connections to the limbic system,in this study,we focus on the circuits via the cerebellum which may mediate the motor-emotional integration of central orexinergic system.We find that:(ⅰ)an abundant density of orexinergic fibers,as well as orexin receptors,are distributed throughout the cerebellar IN and the basolateral amygdala(BLA),and the orexinergic nevurons projecting to the cerebellum are distinct from the subpopulations projecting to the BLA,indicating a parallel innervation of the central orexinergic system on motor and emotional systems;(ii)orexinergic afferent inputs in the cerebellum rather than those in the BLA exert an indispensable role in motor control when an animal is facing motor challenge;(ⅲ)activation of orexinergic inputs in the cerebellar IN significantly ameliorates anxiety(ⅳ)activation of the direct glutamatergic projections from the cerebellar IN to the BLA may mediate the anxiety improvement induced by challenging movements.These results demonstrate a novel mechanism underlying the motor-emotional integration via the cerebellar-amygdala circuit driven by the central orexinergic system.The findings may contribute to understanding the pathophysiology of cataplexy,the neural substrates of motor-emotional integration,and account for why movement improves mood and emotional feelings.2.Orexin produces antidepressant effects by promoting stress resilience via VPVP is a crucial node in the limbic loop of basal ganglia.Although early studies suggested that VP regulates the spontaneous or stimulus-induced motor behaviors via modulation of brainstem motor nuclei,in recent years,growing evidence demonstrated that VP is not only involved in the regulation of movements,but also plays a key role in motivation,addiction and emotion regulation.In clinic,patients with lesions of bilateral VPs show anhedonia to sucrose and social partners.In addition,a smaller baseline VP volume has been reported in depressive patients.Therefore,VP is implicated as a core component of the mesocorticolimbic circuits for reward and depression.Notably,depressive patients exhibit a reduced level of orexin in cerebrospinal fluid.Moreover,patients with narcolepsy-cataplexy also manifest moderate to severe depressive symptoms.These clinical clues indicate an emerging role of the central orexinergic system in the pathophysiology of depression and antidepressant response.Therefore,in this study,we dissect the neural substrates responsible for the antidepressant function of the central orexinergic system,particularly the pathways through the VP.We find that:(ⅰ)hypothalamic orexinergic neurons project directly to the VP;(ⅱ)orexin directly excites GABAergic VP neurons and induces antidepressant-like behavioral effects;(ⅲ)two orexin receptors,OXR1 and OXR2,and their downstream Na+-Ca2+ exchanger and L-type Ca2+ channel co-mediate the effect of orexin;(iv)pharmacological blockade or genetic knockdown of orexin receptors in VP increases depressive-like behaviors in forced swimming test and sucrose preference test;(v)knockdown of orexin receptors in VP has no impact on novel social proximity in social interaction test without acute stress,but notably strengthens social avoidance in a paradigm following an acute psychosocial stress.These results suggest that endogenous orexinergic modulation on VP is critical for antidepressant reactions to acute stressful events.The findings define an indispensable role for the central orexinergic system in preventing depression by promoting stress resilience.The study will not only contributes to clarifying the role of central orexinergic system in motor-emotional integration via the cerebellar and basal ganglia circuitry,but also helps to develop of novel prevention and treatment strategies for emotion disorders including the depression,as well as simultaneous motor and emotional dysfunctions such as cataplexy.