Biochemistry Of Ligand Gated Ion Channels

尼古丁受体是尼古丁作用的配基门控离子通道，在尼古丁滥用中发挥重要作用，了解其神经生物学可能是开发更合理的尼古丁滥用治疗方法的核心。受体-配体相互作用的体外表征是理解配体在体内的作用(即解释受体功能研究结果)的重要一步。 最近，3-(2(S)-氮杂二乙基甲氧基)吡啶(A-85380)的三个放射性卤代类似物被成功地用于正电子发射计算机断层扫描显示人脑中的α4β2*烟碱受体。此前，我们报道了其中一个放射性配体5-[I-125]碘-3-(2(S)-氮杂二甲氧基)吡啶的性质。我们对其中的第二个放射性配体6-[F-18]氟-3-(2(S)-氮杂二甲氧基)吡啶(6[F-18]氟-A-85380)进行了体外表征。在人死后皮质组织中，6-[F-18]F-A-85380可逆性地与单个位点结合(37℃时Kd=59 pm，Bmax=0.7pmol/g组织)。该结合是完全可逆的，并在37℃下由T？对于2.2min(配基浓度为39 pm)和T？解离时间为3.6min。6-Fluoro-A-85380对α4β2*亚型的选择性明显高于其他主要哺乳动物尼古丁受体亚型：α7、α3β4和肌肉型。这些结果表明，6[F-18]F-A-85380是一种很有前途的放射性配体，可用于脑内α4β2烟碱受体的活体显像。 在对吸烟精神分裂症患者脑组织的研究中发现，烟碱型乙酰胆碱受体的上调程度低于精神健康吸烟者的脑组织。由于精神分裂症患者经常长期服用典型或非典型的抗精神病药物，我们在大鼠身上测试了这些药物对尼古丁上调α4β2*尼古丁受体的影响。文献报道表明，典型的抗精神病药物氟哌啶醇没有任何效果。我们用5-[I-125]碘-A-85380体外测定了非典型抗精神病药物利培酮和奥氮平对尼古丁上调大鼠丘脑、纹状体和海马区这些受体的影响。两种药物都不能减弱尼古丁的作用，尼古丁使这些区域的尼古丁受体密度分别增加50%、80%和90%；然而，不含尼古丁的利培酮使纹状体尼古丁受体密度略有增加(25%)。 大量轶事证据表明，尼古丁可能是非法药物使用的门户药物。因此，我们利用药物奖赏的动物行为模型，条件性位置偏爱(CPP)来检验尼古丁对不同类别的滥用药物，特别是鸦片类药物、兴奋剂和大麻产生行为交叉敏感化的假设。在大鼠身上的这些实验表明，尼古丁预处理在尼古丁停止后至少3-5天内增强苯丙胺的奖赏作用，这种作用在19天内消失。其潜在的机制涉及α4beta2 nAChRs，因为竞争性的alpha4beta2拮抗剂二氢β乙酸乙二胺有效地阻断了尼古丁诱导的交叉敏化的发展。有趣的是，α7尼古丁拮抗剂甲基柳叶乌头碱在不阻止大鼠尼古丁自身给药的剂量下也能拮抗交叉敏化。这项研究和发表的报告清楚地表明，尼古丁对用CPP测量的鸦片类药物和精神刺激剂的奖励作用产生交叉敏化。尼古丁与α4β2和α7 nAChRs的相互作用不同于主要由α4β2受体介导的尼古丁自身给药。

Nicotinic acetylcholine receptors, the ligand gated ion channels at which nicotine acts, play an important role in nicotine abuse and understanding its neurobiology may be central to developing more rational approaches for nicotine abuse treatments. The in vitro characterization of the receptor-ligand interaction is an important step in understanding the action of a ligand in vivo (i.e., for interpretation of the results from the functional studies of receptors). Recently, three radiohalogenated analogs of 3-(2(S)-azetidinylmethoxy)pyridine (A-85380) have been used successfully for the in vivo visualization of alpha4beta2* nicotinic receptors in the human brain with PET/SPECT. Previously we reported the characterization of one of these radioligands 5-[I-125]iodo-3-(2(S)-azetidinylmethoxy) pyridine. We performed an in vitro characterization of the second of these radioligands, 6-[F-18]fluoro-3-(2(S)-azetidinylmethoxy) pyridine (6 [F-18]fluoro-A-85380. In human postmortem cortical tissue, 6-[F-18]fluoro-A-85380 reversibly binds with high affinity to a single population of sites (Kd =59 pM at 37?C, Bmax =0.7 pmol/g tissue). The binding is fully reversible and is characterized at 37 degrees C by a T? for the association of 2.2 min (at a ligand concentration of 39 pM) and by a T? for the dissociation of 3.6 min. 6-Fluoro-A-85380 exhibits clear selectivity for the alpha4beta2* subtype over the other major mammalian nicotinic receptor subtypes: alpha7, alpha3beta4, and muscle-type. These results suggest that 6 [F-18]fluoro-A-85380 is a promising radioligand for in vivo imaging of brain alpha4beta2* nicotinic receptors. Less upregulation of nicotinic acetylcholine receptors is found in studies of brain tissue from patients with schizophrenia who smoke than in brain tissue from mentally healthy smokers. As schizophrenic patients often take typical or atypical neuroleptics chronically, the impact of these drugs on the upregulation of alpha4beta2* nicotinic receptors by nicotine was tested in rats. Literature reports suggest that the typical neuroleptic, haloperidol, has no effect. We tested the effect of the atypical neuroleptics, risperidone and olanzapine on the upregulation of these receptors by nicotine in the thalamus, striatum and hippocampus of rats using in vitro assays with 5-[I-125]iodo-A-85380. Neither drug attenuated the effect of nicotine, which increased the density of the receptors in these areas by 50, 80 and 90%, respectively; however, risperidone given without nicotine produced a modest increase (25%) in the density of the nicotinic receptors in the striatum. Substantial anecdotal evidence suggests that nicotine may function as a gateway drug to illicit drug use. As such we utilized an animal behavorial model of drug rewards, conditioned place preference, (CPP) to test the hypothesis that nicotine produces behavioral cross-sensitization to different classes of abused drugs, specifically opiates, stimulants and marihuana. These experiments in rats demonstrated that nicotine pretreatment enhances the rewarding effects of amphetamine for at least 3 to 5 days following the cessation of nicotine, with this effect dissipating within 19 days. The underlying mechanism involves alpha4beta2 nAChRs as the competitive alpha4beta2 antagonist dihydro beta erythroidine effectively blocked the development of nicotine-induced cross sensitization. Interestingly, the alpha7 nicotinic antagonist methyllycaconitine also antagonized cross-sensitization at doses that do not block nicotine self-administration in rats. This study and published report have clearly demonstrated that nicotine produces cross-sensitization to the rewarding effects of both opiates and psychostimulants measured with CPP. The development of cross-sensitization to the rewarding effects of these drugs involves the interaction of nicotine with both alpha4beta2 and alpha7 nAChRs, which differs from niccotine self-adminiistratiiion, which is primarily mediated by alpha4beta2 receptors.