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γ-氨基丁酸(gamma-aminobutyric acid, GABA)是中枢神经系统最主要的抑制性神经递质,可产生镇静催眠、抗惊厥、抗焦虑等作用,与癫、焦虑症、植物状态、成瘾等诸多神经精神疾病密切相关。利用PET研究γ-氨基丁酸A型-苯二氮(gamma-aminobutyric acid type A-benzodiazepine, GABAA-BZ)受体,成功实现了无创性活体功能显像,在临床诊断、预后评估、疗效评价方面的价值无可替代。
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GABA在大脑皮质浅层和小脑皮质浦肯野细胞(Purkinje cell)层含量较高,同时也存在于新纹状体GABA能中间神经元[1]。
GABA受体可分为GABAA受体、B型GABA受体以及C型GABA受体,其中,GABAA受体和C型GABA受体为配体门控促离子型受体,B型GABA受体为G蛋白耦联促代谢型受体。GABAA受体广泛分布于中枢神经系统,为配体门控Cl-通道,介导哺乳动物脑内的大部分快速抑制性神经的传递。当GABAA受体与GABA结合后,其激动效应使细胞膜上的Cl-通道开放,Cl-大量进入细胞,产生快速抑制性突触后电位(inhibitory postsynaptic potential),引起膜超极化,使神经兴奋性降低。GABAA受体还可与BZ类药物结合,因此又被称为GABAA-BZ受体,属于中枢型BZ受体(central benzodiazepine receptor)。BZ可促进GABA与GABAA-BZ受体的结合,也可通过增加Cl-通道开放的频率增强GABA的中枢抑制效应[2]。
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GABAA-BZ受体是嵌于神经细胞膜上的异质性多肽五聚体,由多种亚单位组合成不同的受体亚型,发挥各自药理作用。体外克隆实验证实,GABAA-BZ受体亚单位根据氨基酸序列相似程度可分为8族(表 1),包括α1-6、β1-3、γ1-3、δ、ε、θ、π和ρ1-3(部分学者将ρ受体归类于C型GABA受体),它们在体内分别由各自的独立基因和mRNA编码、转录[3]。
亚单位 分布 药理作用 α1 全脑 镇静,催眠,抗惊厥,与顺性遗忘有关 α2 副嗅球、齿状回分子层、海马、杏仁核、隔核、纹状体、横核、下丘脑 抗焦虑,肌肉松弛,与快速动眼睡眠相中地西泮诱发的θ脑电波有关 α3 嗅球、大脑皮质内层、梨状内核、杏仁核、外侧隔核、屏状核、上丘 抗焦虑,肌肉松弛,与精神分裂症有关 α4 丘脑、尾状壳核、伏核、嗅结节、海马 增加惊厥易感性,降低苯二氮敏感性,与酒精依赖有关 α5 嗅球、大脑皮质内层、梨状内核、菌丝层、海马 介导记忆损害 α6 小脑、耳蜗神经核 与酒精依赖有关 β1 大脑皮质、小脑分子层、海马CA2区 不详 β2 大脑皮质、小脑颗粒细胞层、苍白球、丘脑核团(网状核除外)、中间神经元 参与介导依托咪酯的催眠效应 β3 大脑皮质、小脑颗粒细胞层、新纹状体、海马主细胞、海马CA1区与CA3区 与生长发育有关,参与介导依托咪酯和丙泊酚的催眠效应等 γ1 苍白球、黑质、隔核、杏仁核、终纹床核 不详 γ2 嗅球、大脑皮质、海马、杏仁核、隔核、基底前脑、苍白球、下丘脑 与生长发育有关 γ3 大脑皮质、内侧膝状体核 不详 δ 小脑颗粒细胞、丘脑、齿状分子层、菌丝层、大脑皮质、纹状体 增加神经活性类固醇敏感性 ε 隔区、视前区、下丘脑核团、杏仁核、丘脑 不详 π 海马、颞叶皮质、子宫 不详 θ 下丘脑、杏仁核、海马、黑质、中缝背核、蓝斑 不详 ρ1-ρ3 视网膜、上丘、背外侧膝状体、小脑浦肯野细胞 不详 表 1 γ鄄氨基丁酸A型鄄苯二氮受体亚单位及其分布与药理作用
目前普遍认为,多数天然亚型的五聚体GABAA-BZ受体由α亚单位、β或θ亚单位、以及γ、δ或ε亚单位以2:2:1的比例组成。借助免疫沉淀法与免疫亲和层析法,研究人员掌握了各种亚型的数量和分布信息。在已知的所有天然亚型中,α1βxγ2亚型所占比例最高,其次为α2βxγ2、α3βxγ2、α4βxγ2、α5βxγ2、α6βxγ2、α4βxδ、α6βxδ等常见亚型。值得注意的是,剩余少见亚型所占比例虽小,但由于脑内GABA能系统的规模庞大,其数量仍与去甲肾上腺素、多巴胺、5-羟色胺、肽类受体等水平相当。
虽然通过多种手段逐渐对各亚单位的作用有了一定认识,但是,仍有大量实验结果表明,许多亚型结合位点的结构特征异常复杂,药理作用、药物亲和力的多样性并不仅仅取决于组分中单一亚单位的贡献,而似乎是亚型内众亚单位相互作用的结果[4-6],其具体机制远未明确,有待今后更深入的研究。
目前,亚单位及亚型的功能主要通过药理学、生理学、动物基因敲除或突变等方法研究,而它们的分布则主要通过特异性抗体免疫组化等方法的观察来确定[7]。
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α亚单位共有6种。目前普遍认为,GABAA-BZ受体的药理学特性大部分由组分中的α亚单位所决定。
α1亚单位丰度最高,分布几乎遍及全脑,含α1亚单位的GABAA-BZ受体介导地西泮等BZ类药物产生镇静催眠和抗惊厥作用。此外,α1亚单位还与顺性遗忘有关。有研究表明,长期服用BZ类药物者,若突然撤药,将导致其皮质和海马区域α1亚单位表达下调,提示这种调节机制可能参与BZ类药物的耐药性形成和戒断反应[8]。
α2亚单位主要分布于杏仁核和海马,在低浓度地西泮的作用下即可介导抗焦虑和肌肉松弛效应。α2亚单位还与快速动眼睡眠相中地西泮诱发的θ脑电波有关。
α3亚单位通常在多巴胺能、5-羟色胺能和胆碱能神经元中表达,调节情感并产生抗焦虑作用。α3在高浓度BZ类药物作用下亦介导肌肉松弛效应。有研究表明,GABA能系统对多巴胺能系统的抑制性调节主要通过含α3亚单位的受体亚型完成,提示α3选择性激动剂可能有助于改善精神分裂症的症状[9]。
α4亚单位对多数BZ类药物亲和力较低,因此,药理学实验难以观察到α4亚单位的特性。有生理学实验发现,孕酮产物3α, 5α-四氢孕酮水平下降能使α4亚单位基因转录得到增强,随之产生的惊厥易感性增加及BZ敏感性降低提示,α4亚单位可能与经前期综合征关系密切[10]。还有研究发现,长期酒精暴露可使小鼠α4亚单位mRNA表达上调,提示α4亚单位的适应性改变与酒精成瘾之间存在联系[11]。
α5亚单位与学习和记忆存在复杂的联系。Collinson等[12]观察发现,α5基因突变小鼠与服用α5选择性反向激动剂的小鼠在水迷宫实验中均表现出空间学习能力的显著提高,提示α5亚单位可能参与介导BZ类药物引起的记忆损害。
α6亚单位对多数BZ类药物的亲和力低,因此,其药理学特性亦不十分明确。体外实验发现,呋塞米对α6亚单位的选择性极强,但是它难以穿透血脑屏障,无法用于体内GABAA-BZ受体的研究。此外,与δ亚单位共表达的特性使α6亚单位的自身特性也难以界定。有人发现,在对酒精敏感与不敏感的小鼠间,α6亚单位存在氨基酸序列差异,且α6亚单位能弱化GABAA-BZ受体激动剂引起的运动损害[13]。
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3种β亚单位广泛分布于脑内,并呈现一定的互补性,例如,在苍白球,以β2亚单位为主,而在新纹状体则以β3亚单位为主;β1亚单位在海马CA2区的浓度比海马CA1区和海马CA3区更高,而β3亚单位则恰好相反。
通常情况下,β亚单位无直接药理学特性。然而有研究发现,依托咪酯的催眠效应由β2和β3亚单位参与介导,β3亚单位同时还参与介导丙泊酚的制动、催眠与呼吸抑制作用,但与其减慢心率、降低体温等作用无关[14-17]。另有研究表明,β3亚单位基因敲除的小鼠出现畸形且新生死亡率高,并易出现癫、过敏等症状,提示该亚单位在生长发育过程中不可或缺[18]。
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γ1与γ3亚单位含量较少,前者分布于苍白球、黑质、隔核和杏仁核,且在雄性动物中的表达高于雌性动物,后者则弥漫分布于全脑。γ2亚单位在γ亚单位族中表达最丰富,在除丘脑外的其他区域广泛分布。γ2基因敲除小鼠出现生长迟缓、运动感觉及行为功能失调,并且寿命显著缩短,提示其在生长发育过程中的重要性[19]。
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δ亚单位常与α4亚单位在丘脑、纹状体、齿状细胞层和大脑皮质等处共同组装成α4βxδ受体亚型,而在小脑则与α6亚单位共同组装成α6βxδ受体亚型。δ亚单位基因敲除小鼠对神经活性类固醇敏感性下降,并表现出多种与酒精反应相关的行为异常[20]。
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ε亚单位分布于小鼠的隔区、视前区、下丘脑核群、杏仁核及丘脑等区域,存在于胆碱能、多巴胺能、5-羟色胺能及去甲肾上腺素能等系统中。
π亚单位除海马和颞叶皮质外还分布于周围组织中,其在子宫中的分布相当丰富。
θ亚单位需要与α、β、γ亚单位一起组装成一个功能受体,它表达于下丘脑、杏仁核等脑区,与ε亚单位分布的重叠度高。
ρ亚单位表达于视网膜,其mRNA则在上丘、背外侧膝状体、小脑浦肯野细胞中表达。
γ-氨基丁酸A型-苯二氮受体显像剂在神经系统疾病中的应用
Application of gamma-aminobutyric acid type A-benzodiazepine receptor imaging for study of neu-ropsychiatric disorders
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摘要: γ-氨基丁酸A型-苯二氮(GABAA-BZ)受体广泛分布于中枢神经系统,是嵌于神经细胞膜上的异质性多肽五聚体,不同的亚单位组合发挥不同的神经抑制性药理作用,如镇静催眠、抗惊厥、抗焦虑等。PET可用于活体内受体结合的研究。GABAA-BZ受体PET显像剂分为拮抗剂、激动剂、反向激动剂3类,其中以拮抗剂显像剂11C-氟马西尼最为成熟,在癫癎、焦虑症、抑郁症、植物状态、成瘾等神经精神疾病中广泛应用。
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关键词:
- 神经系统疾病 /
- 受体 /
- GABA-A /
- 正电子发射断层显像术
Abstract: Gamma-aminobutyric acid type A-benzodiazepine receptors are heterogeneous polypeptide pentamers widely spread in the central nervous system on the neuron membrane. Different subunit combinations educe various neuro-inhibitory pharmacological effects such as sedative, hypnosis, anticonvulsion and anxiolysis. PET can be utilized to study the binding of the receptors in vivo. PET radioligands of gamma-aminobutyric acid type A-benzodiazepine receptors can be classified into 3 types: antagonists, agonists and reverse agonists, of which antagonist radiotracer 11C-flumazenil is the most commonly applied in epilepsy, anxiety disorders, depression, vegetative state, addiction and other neuro-psychiatric disorders.-
Key words:
- Nervous system diseases /
- Receptors /
- GABA-A /
- Positron-emission tomography
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表 1 γ鄄氨基丁酸A型鄄苯二氮受体亚单位及其分布与药理作用
亚单位 分布 药理作用 α1 全脑 镇静,催眠,抗惊厥,与顺性遗忘有关 α2 副嗅球、齿状回分子层、海马、杏仁核、隔核、纹状体、横核、下丘脑 抗焦虑,肌肉松弛,与快速动眼睡眠相中地西泮诱发的θ脑电波有关 α3 嗅球、大脑皮质内层、梨状内核、杏仁核、外侧隔核、屏状核、上丘 抗焦虑,肌肉松弛,与精神分裂症有关 α4 丘脑、尾状壳核、伏核、嗅结节、海马 增加惊厥易感性,降低苯二氮敏感性,与酒精依赖有关 α5 嗅球、大脑皮质内层、梨状内核、菌丝层、海马 介导记忆损害 α6 小脑、耳蜗神经核 与酒精依赖有关 β1 大脑皮质、小脑分子层、海马CA2区 不详 β2 大脑皮质、小脑颗粒细胞层、苍白球、丘脑核团(网状核除外)、中间神经元 参与介导依托咪酯的催眠效应 β3 大脑皮质、小脑颗粒细胞层、新纹状体、海马主细胞、海马CA1区与CA3区 与生长发育有关,参与介导依托咪酯和丙泊酚的催眠效应等 γ1 苍白球、黑质、隔核、杏仁核、终纹床核 不详 γ2 嗅球、大脑皮质、海马、杏仁核、隔核、基底前脑、苍白球、下丘脑 与生长发育有关 γ3 大脑皮质、内侧膝状体核 不详 δ 小脑颗粒细胞、丘脑、齿状分子层、菌丝层、大脑皮质、纹状体 增加神经活性类固醇敏感性 ε 隔区、视前区、下丘脑核团、杏仁核、丘脑 不详 π 海马、颞叶皮质、子宫 不详 θ 下丘脑、杏仁核、海马、黑质、中缝背核、蓝斑 不详 ρ1-ρ3 视网膜、上丘、背外侧膝状体、小脑浦肯野细胞 不详 -
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