Nicotinic Receptor & Hippocampal Synaptic Function

描述（申请人提供）：在海马，认知功能整合的中心，烟碱受体（nAChRs）分别调节由主要兴奋性和抑制性递质-谷氨酸和GABA介导的突触传递。然而，海马功能也依赖于张力激活的谷氨酸能和gaba能受体。事实上，临床上用于治疗癫痫发作的一些苯二氮卓类药物更多地选择性地影响强直性（稳定状态）而非相性（突触）gaba能活动。然而，烟碱配体对海马体补益活动的影响尚不清楚。在最近的一项研究中，我们已经证明了色氨酸代谢物kynurenic acid (KYNA)，它主要由大脑中的星形胶质细胞产生和释放，调节大鼠海马中的尼古丁胆碱能活性。在生理相关浓度下，KYNA比NMDA受体（曾被认为是KYNA在大脑中的主要靶点）更有效地抑制a7 nAChR，并调节a4B2 nAChR的表达。其他证据也表明，KYNA和尼古丁胆碱能系统之间可能存在功能上的交叉对话：(i)尼古丁调节大脑中内源性KYNA的水平；（ii）在精神分裂症、阿尔茨海默病和帕金森病（AD， PD）等神经系统疾病中，KYNA水平的变化往往与尼古丁胆碱能系统的变化平行。该提案旨在使用融合的多学科方法，包括电生理、生化和分子生物学分析，来解决海马中nAChR活性的大小调节强直性和阶段性谷氨酸能或gaba能活性的程度，并由内源性KYNA调节的中心假设。具体目的是：(i)研究nAChR配体对海马紧张性和相位性兴奋或抑制的影响；（ii）研究KYNA与烟碱能系统之间的相互作用如何调节海马中gaba能和谷氨酸能活动的强性或相位形式，以及（iii）确定KYNA对7种nachr的作用位点，分析KYNA和变构增强配体加兰他明（最近被引入用于治疗AD患者）对nachr的作用位点是同一位点的可能性。编码a7 nAChR亚基或脑内ICYNA合成关键酶的基因无突变的敲除小鼠，犬尿氨酸氨基转移酶11 (KATII)，重组a7 nAChR（突变体和嵌合体）和不同的制剂（急性切片和异种系统）将用于解决这些基本问题。这些研究的结果将是深远的；它们将使我们更全面地了解尼古丁配体调节神经元活动的方式，并为未来开发治疗策略提供机制基础，这些治疗策略旨在通过犬尿氨酸途径补偿AD、PD和精神分裂症等疾病中尼古丁功能的缺陷。

DESCRIPTION (provided by applicant): In the hippocampus, the center for integration of cognitive functions, nicotinic receptors (nAChRs) modulate synaptic transmission mediated by the major excitatory and inhibitory transmitters - glutamate and GABA, respectively. However, hippocampal function relies also on tonically activated glutamatergic and GABAergic receptors. In fact7 some benzodiazepines that are clinically used for treatment of epileptic seizures affect more selectively tonic (steady state) than phasic (synaptic) GABAergic activity. Yet, the effects of nicotinic ligands on tonic activities in the hippocampus are unknown. In a recent study, we have demonstrated that the tryptophan metabolite kynurenic acid (KYNA), which is largely produced and released by astrocytes in the brain, modulates nicotinic cholinergic activity in the rat hippocampus. At physiologically relevant concentrations, KYNA inhibits more potently a7 nAChRs than NMDA receptors (once believed to be the major targets for KYNA in the brain) and regulates a4B2 nAChR expression. Other lines of evidence also indicate that there might be functional cross-talk between KYNA and the nicotinic cholinergic system: (i) nicotine regulates endogenous levels of KYNA in the brain and (ii) changes in KYNA levels often parallel alterations in the nicotinic cholinergic system in such neurological disorders as schizophrenia, Alzheimer's and Parkinson's diseases (AD, PD). This proposal is designed to use convergent, multidisciplinary approaches, including electrophysiological, biochemical and molecular biological assays, to address the central hypothesis that in the hippocampus the magnitude of nAChR activity modulates the degree o tonic and phasic glutamatergic or GABAergic activities and is regulated by endogenous KYNA. The specific aims are: (i) to investigate the effects of nAChR ligands on tonic and phasic forms of excitation or inhibition in the hippocampus; (ii) to examine how interactions between KYNA and the nicotinic cholinergic system modulate the tonic or phasic forms of GABAergic and glutamatergic activities in the hippocampus, and (iii) to determine the site of action of KYNA on o7 nAChRs, analyzing the possibility that the site for KYNA and for the allosteric potentiating ligand galantamine (that has been recent/y introduced for treatment of AD patients) on the nAChRs is one and the same. Knockout mice with null mutations in the gene that encodes the a7 nAChR subunit or the enzyme critical for ICYNA synthesis in the brain, kynurenine aminotransferase 11 (KATII), recombinant a7 nAChRs (mutants and chimeras) and different preparations (acute slices and heterologous systems) will be used to address these fundamental issues. The results of these studies win be far reaching; they will lead to a more comprehensive understanding of the means by which neuronal activity is regulated by nicotinic ligands, and provide the mechanistic basis for future development of therapeutic strategies designed to compensate, via the kynurenine pathway, for deficits in nicotinic function in disorders such as AD, PD and schizophrenia.