Uridine Release In The Striatum Induced By Morphine

Opioid drugs are addictive drugs most commonly used in China.Although researchers have made great progress on the mechanisms of opioid addiction,including the classic neurotransmitter receptor regulation,gene expression,synaptic plasticity changes,the mechanism of addiction is quite complex,and is not yet fully understood.Uridine,a pyrimidine nucleoside,plays a crucial role in synthesis of RNA,glycogen,and biomembrane.There has been a growing interest in the neuronal functions of uridine apart from its role in pyrimidine metabolism,such as regulation of arousal and sleep,epilepsy,temperature,cognition and memory.Recently,uridine,a neuromodulator in the central nervous system(CNS),has been proposed to participate in drug addiction.In addition,our previous study has shown that acute morphine induced a significant decrease of extracellular levels of uracil,a metabolite of uridine,in mice striatum.Thus,the present study aimed to characterize the effect of morphine-induced uridine release process,and to determine the role of opioid-receptor,dopamine receptor,the nucleoside transporter and the enzyme of uridine in morphine-induced uridine release by using in vivo microdialysis,neurobehavioral assay and western blotting.The effect of exogenous uridine on morphine-induced hyperlocomotion and behavioral sensitization were also investigated.Firstly,to investigate whether basal extracellular uridine was derived from neuronal release,we applied three criteria:sensitivity to high K+ depolarization,TTX(a voltage sensitive sodium channel blocker)and Ca2+-free perfusion solution.To determine the mechanism of morphine-induced uridine release,uridine extracellular levels were detected in successive experiments following the administration of naloxone(an opioid receptor antagonist),TTX(a voltage sensitive sodium channel blocker)and EGTA(a calcium chelator).Our results demonstrated that the basal extracellular uridine levels were mainly derived from non-vesicular sources.In addition,morphine increased the spontaneous uridine release from mice striatum in a opioid receptor-dependent manner,which was independent of TTX and external Ca2+.Extracellular uridine levels are regulated by nucleoside transporter(NT)and related metabolic enzyme.This study further examined the effects of concentrative nucleoside transporters(CNT)inhibitor phlorizin,equilibrative nucleoside transporters(ENT)inhibitor NBTI and extracellular-5'nucleotidase(e5NT)inhibitor APCP on the release of uridine caused by acute morphine.The effects of morphine on the protein expression of transporter and the metabolic enzyme were studied.The results showed that,CNT,ENT and e5NT inhibitors could inhibit morphine-induced uridine release.In addition,morphine and PMA(a PKC agonist)could up-regulate the protein expression of e5NT.These results suggested that acute morphine might inhibit nucleoside transporter function,and could activate e5NT function through the PKC,leading to increased striatal extracellular uridine level.The effects of the drugs acted on dopaminergic receptors on morphine-induce uridine release from mice striatum were studied by using brain microdialysis technique coupled with HPLC with UV detection.The results revealed that the release of DA and uridine induced by morphine possessed a direct proportional linear correlation.Local infusion of apomorphine,a dopamine D1/D2 agonist increased uridine release in striatum.Sulpiride,a dopamine D2 receptor antagonist,could enhanced morphine-induced uridine release in mice striatum.But Sch23390,a dopamine D1 receptor antagonist,antagonized morphine effect.These results suggested that morphine-induced uridine release was associated with presynaptic dopamine release.In addition,dopamine Dl,D2 receptors may play different roles in morphine-induced striatal urndine release.As mentioned above,morphine can cause brain extracellular uridine content changes,indicating that the uridine as a neuromodulator in the CNS involved in morphine central action.This paper intends to study the effects of exogenous uridine on morphine-induced behavior changes and neurotransmitter release,clarifying uridine as a neuromodulator role in drug addiction.The results showed that systemic administration of uridine,inhibit the acute morphine induced locomotor activity and increased the release of DA.The DPD inhibitor CDHP antagonized the inhibitory effect,and uridine metabolite uracil and beta alanine could also inhibit the morphine induced locomotor activity.In addition,Bicuculline,a GABA receptor antagonist,reversed uridine's effect.The results showed that the effect of uridine may be through its metabolic product beta alanine acting on GABA transporter.Chronic administration of uridine could inhibit the chronic morphine-induced behavioral sensitization.In conclusion,the current study demonstrates an interaction between morphine and uridine,which could extend previous studies by identification of the neuromodulator role of uridine.Investigating the targets of morphine-induced uridine release,it will highlight a novel viewpoint for understanding the mechanism of opioid-addiction,and will contribute to the development of a new type of anti-opioid addiction drug.