Neuroscience Letlers, 121 ( 1991 ) 133 135 133 Elsevier Scientific Publishers Ireland Ltd. NSL 07410 Inhibition of high affinity L-glutamic acid uptake into rat co synaptosomes by the conformationally restricted analogue of g acid, cis- 1-aminocyclobutane- 1,3-dicarboxylic acid Elizabeth J. Fletcher, Kenneth N. Mewett, Colleen A. Drew, Robin D. Allan and Graham A.R. Johnston Department of'Pharmacology, The Universi O, o['Sydnev. Sydney. N.S. W. (Australia) (Received 27 August 1990; Accepted 10 September 1990) Key words" Glulamic acid: Uptake inhibition: Cyclobutane derivative The action of two cyclobutane derivatives of L-glutamic acid on the high affinity uptake of L-glutamic acid was investigated using a prep of synaptosomes from rat cerebral cortex, cis-l-Aminocyclobutane-l,3-dicarboxylic acid (also known as trans-2,4-methanoglutamic acid) potently inhibited L-glutamic acid uptake (ICs0 30 ~tM), whereas trans-l-aminocyclobutane-l,3-dicarboxylic acid (also known as cis-2,4-methanoglutamic acid), a potent N-methyl-D-aspartate (NMDA) agonist, was inactive. Analysis of the kinetics of L-glutamic acid uptake in the presence and a of cis-l-aminocyclobutane-l,3-dicarboxylic acid (CACB) suggests that it may act as a competitive inhibitor (K~ 8 ,uM). CACB may be sub for the L-glutamic acid high-affinity uptake carrier since preincubation of CACB with the synaptosomal preparation increased its potency in in ing c-glutamic acid uptake. The conformationally restricted structure of CACB may be indicative of the conformations of c-glutamic acid that act with the high affinity uptake carrier. L-Glutamic and L-aspartic acids play specific roles as excitatory neurotransmitters in the CNS and termina- tion of their excitatory actions must come under tight control in order to preventexcessive depolarisation which has been shown to contribute to cell death [13]. In contrast with some other neurotransmitters (e.g. acetyl- choline, noradrenaline), the major route by which the ac- tion of glutamic and aspartic acids is terminated is via uptake from the synaptic cleft rather than degradation [10]. A number of studies have suggested that there are two uptake sites: one of high affinity but low capacity, possibly localised in the pre- and postsynaptic neuronal membrane and one of low affinity but high capacity localised in extraneuronal membranes (such sites includ- ing glial cells). High-affinity sites have, however, also beenidentified in the latter cells [4]. Compounds that interact with the excitatory amino acid neurotransmitter system were originally developed by simple manipula- tions of the glutamic or aspartic acid structure. Such transformations resulted in the production of agonists and antagonists for subtypes of the glutamic acid/aspar- tic acid receptor subtypes[15] which have greatly increased our understanding of excitatory neurotrans- Corre67Yondence: G.A.R. Johnston, Department of Pharmacology, The University of Sydney, Sydney, N.S.W. 2006, Australia. mitter systems. By contrast, relatively little interest been centred on compounds that influence glutamic aci transport systems and few uptake blockers of any nota ble potency exist [2, 3, 10, 12]. Recently is has been s gested that modification of glutamic acid uptake coul play a role in processes underlying synaptic plasticity the CNS [4] thus creating more interest in the deve ment of investigative tools. The most potent competitive inhibitor of high affinity glutamic acid uptake known is threo-3-hydroxy-L-aspartic acid (ICs0 3/tM) [3]. Studies on cyclobutane analogues ofglutamic acid hav shown that trans-l-aminocyclobutane-l,3-dicarboxylic acid (TACB, also known as eis-2,4-methanoglutamic acid) is a potent and selective N-methyl-D-aspartate (NMDA) receptor agonist [1, 11] being some 20 times more potent than NMDA. CACB was first isolated from the seeds of the Costa Rican legume Ateleiaherbert smithii Pittier in 1980 by Bell et al. [6], who determine its structure by X-ray crystallography and discussed i possiblebiologicalrole in deterring insectpredators from eating the seeds. The chemical nomenclature of these cyclobutane derivatives is confused in the lite ture; the natural product was given the trivial name 2, methanoglutamic acid by Bell et al. [6] and the systema ic name cis-l-aminocyclobutane-1,3-dicarboxylic acid by Chemical Abstracts (registry number (73550-55-7)) fo lowing IUPAC rules pertaining to the stereochemistry o 0304-3940/91 $ 03 50 ¢7:1991 Elsevier Scientific Publishers lreland Ltd.