摘要
正电子发射断层显像(positron emission tomography，PET)利用正电子核素标记示踪分子进行活体显像，它能够在分子水平无创、动态、定量的对活体生理、生化变化过程进行观察，是研究临床疾病发病机制和评价治疗手段的重要方法。但很多医学研究需要首先在动物模型上实施成功后才能应用于临床，临床PET由于其分辨率的限制，不能用于小动物的实验研究，最近小动物PET(small animal PET，MicroPET)的问世，很好的解决了这一问题。
最近，随着MicroPET技术的发展迅速，其空间分辨率大大提高，MicroPET已经成为研究大鼠脑代谢的重要工具。此外，MicroPET能应用于脑代谢改变的急性阶段，这在临床PET上由于显像困难是不易实现的。2-[~(18)F]-fluoro-2-deoxy-D-glucose(~(18)F-FDG)是葡萄糖的同分异构体，是目前应用最广泛的葡萄糖代谢示踪剂，研究表明，脑内葡萄糖代谢可反映神经元的活性。癫痫和脑缺血作为神经内科最常见的两类疾病，都伴有脑内葡萄糖代谢的显著改变。本研究的目的是利用MicroPET技术来评价治疗癫痫和脑缺血的新方法。
PET显像的主要缺点在于PET显像为功能显像，对解剖结构定位比较模糊，这使得结果很难分析，在本研究中，我们将MicroPET图像和MRI图像用软件进行融合，极大地提高了MicroPET图像的分析效果。同时我们对大鼠麻醉方式也做了改进，使其在最大程度上反映大鼠清醒状态时的脑葡萄糖代谢。
第一部分：电刺激丘脑前核对局部脑葡萄糖代谢的调节作用
丘脑前核深部脑刺激(Deep brain stimulation，DBS)是治疗难治性癫痫一种新的手段，但其作用机制目前还不明确。高频电刺激某个脑区与损毁这个脑区的效应有相似之处，以往的研究也发现高频电刺激丘脑前核和毁损丘脑前核在一些癫痫模型上有相似的抗癫痫作用。我们通过建立丘脑前核DBS模型和丘脑前核化学毁损模型，用MicroPET观察处理前后局部葡萄糖代谢的变化。结果发现双侧丘脑前核DBS后标准化葡萄糖代谢率在双侧丘脑前核区、丘脑和海马明显增高，而扣带回和额叶皮层(包括运动皮层)标准化葡萄糖代谢率则明显降低，并且双侧丘脑前核DBS引起上述部位葡萄糖代谢的变化是可逆的。而双侧丘脑前核毁损后丘脑前核区标准化葡萄糖代谢率发生不可逆的下降，双侧丘脑前核毁损后再进行DBS却未见有葡萄糖代谢的改变。因此，双侧丘脑前核DBS可引起靶区以及远隔脑区的可逆性葡萄糖代谢变化，而双侧丘脑前核毁损并不出现相同的葡萄糖代谢改变。我们的研究结果表明，双侧丘脑前核DBS对大鼠脑葡萄糖代谢有抑制和兴奋的双重作用，丘脑前核DBS与毁损丘脑前核很可能有着两种完全不同的抗癫痫机制。
第二部分：重复经颅磁刺激对大鼠局灶性脑缺血的作用及机制研究
重复经颅磁刺激(repetitive transcranial magnetic stimulation，rTMS)的作用具有频率依赖性，高频率刺激(大于1Hz)能兴奋皮层，因此高频rTMS治疗在急性脑缺血中的作用日益受到国内外学者的重视。我们通过建立大鼠局灶性脑缺血模型，1h后应用高频rTMS(20Hz)治疗，以后每隔24h治疗1次，连续治疗7天。结果发现20Hz rTMS早期治疗显著改善了局灶性脑缺血大鼠的神经功能并减少脑梗死体积；MicroPET结果显示，大鼠局灶性脑缺血后7天，梗死半球的葡萄糖代谢明显下降，而经过20Hz rTMS治疗能显著提高梗死半球的葡萄糖代谢。免疫组化结果提示20Hz rTMS治疗能抑制大鼠脑缺血梗死周边区的细胞凋亡，并且使葡萄糖转运蛋白1、3的表达明显增高。我们的研究结果提示在脑缺血早期给予20Hz rTMS治疗对局灶性脑缺血具有神经保护作用，其机制可能与抑制梗死周边区的细胞凋亡和提高梗死半球的葡萄糖代谢有关。
总的来说，我们的研究发现MicroPET能够对大鼠脑葡萄糖代谢改变进行长期的随访观察，已经成为研究癫痫和脑梗塞的重要工具。更为重要的是，由于MicroPET和临床PET研究可以使用相同的示踪药物，使得临床前期研究可以很快应用于临床实践，因而，在不久的将来，MicroPET必将在开发和评估癫痫和脑缺血新的治疗方法上起到十分重要的作用。
Positron emission tomography(PET)is a noninvasive imaging technique thatallows quantitative in vivo determinations of the rates of various physiologic andbiochemical processes with minimal invasiveness.However,PET has not been suitablefor small animal models because of resolution limitations.
Until recently,improvement in scanner resolution has allowed PET to become apotential method to monitor cerebral metabolic pattems in rat brain using small animalpositron emission tomography(MicroPET).Moreover,MicroPET can also be used inthe early stage of acute brain lesions,which cannot be accomplished in the clinical PETstudies because tracer kinetics is unwarranted and image acquisition is difficult in theearly phase of acute stage.The tracer 2-[18F]-fluoro-2-deoxy-D-glucose(~(18)F-FDG)isthe well-known radiotracer that frequently has been used as a marker of metabolicactivity for glucose.The level of glucose metabolism correlates with the degree ofneuronal activity.Epilepsy and cerebral ischemia are common neurological diseaseswhich show significant change of brain glucose metabolism.Ourstudy is aim toevaluation of new treatments for epilepsy and cerebral ischemia using MicroPET.
Although PET images are not always simple to interpret because of the pooranatomical detail that they contain,in this study,we have co-registered a single MRItemplate with PET images,which greatly improves the interpretation of the PET images.
At the same time,we improve the method of anesthesia and ensure the MicroPETimages reflect glucose metabolism of the conscious state.
Part 1:Anterior thalamic nucleus stimulation modulates regional cerebralmetabolism:an FDG-MicroPET study in rats
The mechanism underlying the antiepileptic function of deep brain stimulation(DBS)of the anterior thalamic nucleus(ATN)remains unknown.Experimental studiesrevealed that DBS of the ATN has a similar anticonvulsant action with ATN lesions.We measured the regional normalized cerebral metabolic rate of glucose(nCMRglc)with MicroPET in animals receiving either ATN stimulation or lesioning.Bilateral ATNstimulation reversibly increased nCMRglc in the target region,the thalamus andhippocampus,and decreased nCMRglc in the cingulate cortex and frontal cortex(including motor cortex).However,bilateral ATN lesioning decreased nCMRglc onlyin the target region.Animals with bilateral ATN lesions showed no metabolic changesafter ATN stimulation.Thus,bilateral DBS of the ATN reversibly induces metabolicactivation of the target area and modulates energy metabolism in remote brain regionsvia efferent or afferent fibers in rats.DBS of the ATN may work by a differentmechanism than ATN lesioning.
Part 2:A study on the mechanism of repetitive transcranial magnetic stimulationtreatment after cerebral ischemia in rats
The effect of repetitive transcranial magnetic stimulation(rTMS)hasfrequency-dependent.It is generally assumed that higher frequencies(＞1 Hz)induce anincrease of cortical excitability.Therefore,rTMS may be useful tool to modulate brainactivity for enhance recovery in acute cerebral ischemia.We applied an intraluminalmiddle cerebral artery occlusion model in rats under controlled conditions.20Hz rTMSwere delivered 1 hour after ischemia and interval 24h for 7 days.The resultsdemonstrated that 20Hz rTMS therapy significantly improved the neurological deficitsand reduced infarct volume of brain.Moreover,glucose metabolism in the ischemic area underwent a severe decrease during 7 days after ischemia,while 20Hz rTMStreatment significantly attenuated the decrease in cerebral metabolic rate of glucose inthe ischemic hemisphere.The results of immunohistochemistry demonstrated that20Hz rTMS therapy inhibited neuronal apoptosis and increased the expression ofGLUT-1 and GLUT-3 in the infarct margin.Our results demonstrate that 20Hz rTMStherapy inhibited neuronal apoptosis and maintenance of glucose metabolism in theischemic hemisphere,which might contribute to the the understanding neuroprotectioneffects of 20Hz rTMS therapy on transient cerebral ischemia in rats.
In conclusion,we showed that MicroPET is a useful and provide the possibility toperform serial follow-up studies to investigate the development of functional changes inthe rat brain over time,will become important tools for epilepsy and cerebral ischemiaresearch.More important,an advantage of MicroPET is that the same molecularradiotracers can be used for both animal research and clinical applications facilitating amore rapid progression from preclinical research to clinical practice.In the future,MicroPET may also play a role in the development and evaluation of new treatments forepilepsy and cerebral ischemia.
引文
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