TOP ＞ シンポジウム

シンポジウム 神経化学の歴史を知って未来を拓く～大発見はこうして行われた～ 1S1-1 GABA as an inhibitory neurotransmitter in the mammalian central nervous system Obata Kunihiko1 1Nationa Institute for Physiological Sciences，2National Institute for Physiological Sciences In the early 1960’s any neurotransmitter substances had not been identified in the mammalian central nervous system except for acetylcholine in the motoneuron axon collaterals. γ-Aminobutyric acid（GABA）was demonstrated commonly in the nervous tissue but its inhibitory action was not considered identical to the synaptic transmission. Synaptic inhibition is mediated by membrane hyperpolarization but GABA did not induce it in the spinal cord. Then, we started identifying an inhibitory neurotransmitter of the cerebellar Purkinje cells which form inhibitory synapses upon neurons of the lateral vestibular nucleus（Deiters）and deep cerebellar nuclei in the cat. Following criteria were investigated by electrophysiological and neurochemical experiments：1. mimicry of synaptic transmission and pharmacological properties, 2. selective distribution in the synapses and 3. release during synaptic activation. Among several candidates, only GABA satisfied all items of the criteria. In those days no histological demonstration was available and high GABA content was shown in Purkinje cells and their terminals by the sensitive enzymic assay. These studies on Purkinje cells led a concept of GABA as a principal inhibitory neurotransmitter in the mammalian brain.In the 1990’s gene targeting was introduced in neuroscience. We produced knockout mice for two isoforms of GABA-synthesizing enzyme（GAD）65 and 67 and disclosed several roles of GABA in development and behavior. 1S1-2 Identification and characterization of the muscarinic acetylcholine receptor and the high-affinity choline transporter Haga Tatsuya Neurochem.. Med., Univ. of Tokyo Acetylcholine（ACh）is the first substance which was identified as a neurotransmitter. ACh is synthesized from acetyl-CoA and choline by choline acetyltransferase. The rate of ACh synthesis is limited by the rate of choline uptake from the extracellular space. In 1973, we identified the high-affinity choline uptake activity in rat brain synaptosomes, which was shown to be dependent on the presence of Na+ ion and to be sensitive to inhibition by hemicholinium-3（Haga ＆ Noda）. In 2000, we identified the molecular entity of the high-affinity choline uptake activity as a transmembrane protein, which was named as CHT1（Okuda et al.）. A single nucleotide polymorphism（SNP）of CHT1 with the activity of 50-60% of that of wild-type CHT1 was identified. The frequency of the SNP was high among Asians including Japanese（13.0%）than among Caucasians（4.1%）or Africans（1.2%）. Supply of choline in the diet might be important for those with the SNP, particularly when they suffered from Alzheimer or other diseases with the lower cholinergic activity. In 1985, we purified muscarinic acetylcholine receptors（mAChRs）by using the affinity chromatography system（Haga ＆ Haga）. In 1986, we cloned subtypes 1 and 2 of mAChRs by using partial amino acid sequences of purified mAChRs and identified the function of mAChR as G protein activator by using reconstitution system of purified mAChRs and G proteins in collaboration with groups of Numa, Matsuo and Ui. Thus studies on mAChRs, together with those on rhodopsin and beta adrenergic receptors, served to establish the concept of G protein-coupled receptors（GPCRs）. In 1992-1994, we demonstrated mAChRs to be phosphorylated by G protein-coupled receptor kinases in an agonist-dependent manner and to be internalized in a phosphorylation-dependent manner, which provides explanation in terms of molecular interactions, at least partly, for stimulus-dependent desensitization in cholinergic systems. In 2012, we determined a tertiary structure of mAChR subtype 2 by X-ray analysis in collaboration with groups of Kobilka and Kobayashi, which is expected to contribute to the theoretical design of drugs acting on mAChRs. Molecular properties of mAChRs and CHT1 were recently reviewed in Proc. Japan Acad. 9, 226（2013）and J. Biochem. 156, 181（2014）, respectively. 1S1-3 How to create exciting trends in Neurochemistry! Mikoshiba Katsuhiko RIKEN, BSI Japanese Society for Neurochemistry（JSN）is the oldest and biggest society in the International Society for Neurochemistry.“Neurochemistry”is a science to understand the mechanism of the brain function at the molecular level in order to correlate behavior, morphology and molecules. After deepeningour understanding to correlate molecules and behavior, it is possible to manipulate the molecules to modify behavior and morphology. We have been actively working in JSN in these decades. I here describe some of the examples of the activity of neurochemistry taking the examples of the research going on in my laboratory. To understand the complex structure of the brain which exerts variety of functions including learning ＆ memory and behavior, it is necessary to introduce variety of strategies such as biochemistry, molecular biology, biophysics, structural biology etc. It is sometimes necessary to introduce the developmental aspects and comparative analysis of the abnormal diseased brain with the control one. Combination and fusion of differentresearch areas gives us unexpected ideas to solve the unknown mechanism of mysterious brain function and structure. I will describe here 1）the mechanism of myelination taking the example of introducing shiverer and mld mutations Nature 299 357-359（1982）, Annual Rev. Neurosci. 14 201-17（1991）. 2）the mechanism of neuronal positioning in the cortical layers in the brain taking an example to revealing the molecular mechanism by introducing reeler and yotari mutations Neuron 14899-912（1995）Nature 385 70-74（1997）Nature 389 730-733（1997）. 3）the mechanism of IP3 receptor/calcium signaling whichwediscovered from the analysis of the P400-protein deficient mice by introducing pcd, nervous mutant deficient of Purkinje neurons and also staggerer mutant in the cerebellum Nature 342 32-38（1989）Science 257 251-255（1992）Cell 73 555-570（1993）Science 292 920-923（2001）Nature 379 168-171（1996）. All these unexpectedway of doing research has given us glorious results which are so important for revealing the mechanism of the function of the brain. These may make newtrends in neurochemistry.