Inhibition of System A-mediated glycine transport in cortical synaptosomes by therapeutic concentrations of clozapine: implications for mechanisms of action

• Published in: Molecular psychiatry Vol. 10; no. 3; pp. 275 - 287
• Main Authors: JAVITT, D. C, DUNCAN, L, BALLA, A, SERSHEN, H
• Format: Journal Article
• Language: English
• Published: Basingstoke Nature Publishing Group 01-03-2005
• Subjects: Adult and adolescent clinical studies; Amino Acid Transport System A - metabolism; Amino acids; Amino Acids - metabolism; Animals; Antipsychotic Agents - pharmacology; Antipsychotics; Behavioral psychophysiology; beta-Alanine - analogs & derivatives; beta-Alanine - pharmacology; Biological and medical sciences; Brain research; Cell Line, Tumor; Cerebral Cortex - metabolism; Clozapine; Clozapine - pharmacology; Dizocilpine Maleate - metabolism; Dizocilpine Maleate - pharmacology; Excitatory Amino Acid Antagonists - metabolism; Excitatory Amino Acid Antagonists - pharmacology; Fundamental and applied biological sciences. Psychology; Glioma; Glutamic acid receptors; Glutamic acid receptors (ionotropic); Glycine - metabolism; Hippocampus - metabolism; Ketamine; Medical sciences; Mental disorders; Metabolites; N-Methyl-D-aspartic acid receptors; Neurosciences; Neurotransmission; Neurotransmission and behavior; Polyamines; Potentiation; Psychiatric research; Psychology. Psychoanalysis. Psychiatry; Psychology. Psychophysiology; Psychopathology. Psychiatry; Psychoses; Psychotropic drugs; Radioligand Assay; Rats; Rats, Sprague-Dawley; Receptors, N-Methyl-D-Aspartate - metabolism; Sarcosine; Sarcosine - analogs & derivatives; Sarcosine - pharmacology; Schizophrenia; Synaptosomes; Synaptosomes - drug effects; Synaptosomes - metabolism; Tritium; Animal model; Rat; Neuroleptic; Psychotropic; Rodentia; Schizophrenia; Glutamate receptor; Clozapine; Glycine; Uptake; Psychosis; Vertebrata; Synaptosome; Mammalia; Aminoacid; Animal; NMDA; NMDA receptor; glutamate
• ISSN: 1359-4184; 1476-5578
• DOI: 10.1038/sj.mp.4001552

Clozapine is an atypical antipsychotic with particular efficacy in schizophrenia, possibly related to potentiation of brain N-methyl-D-aspartate receptor (NMDAR) -mediated neurotransmission. NMDARs are regulated in vivo by glycine, which is regulated in turn by glycine transporters. The present study investigates transport processes regulating glycine uptake into rat brain synaptosomes, along with effects of clozapine on synaptosomal glycine transport. Amino-acid uptake of amino acids was assessed in rat brain P2 synaptosomal preparations using a radiotransport assay. Synaptosomal glycine transport was inhibited by a series of amino acids and by the selective System A antagonist MeAIB (2-methyl-aminoisobutyric acid). Clozapine inhibited transport of both glycine and MeAIB, but not other amino acids, at concentrations associated with preferential clinical response (0.5-1 microg/ml). By contrast, other antipsychotics studied were ineffective. The novel glycine transport inhibitor N[3-(4'-fluorophenyl)-3-(4'-phenylphenoxy)propyl]sarcosine (NFPS) produced biphasic inhibition of [(3)H]glycine transport, with IC(50) values of approximately 25 nM and 25 microM, respectively. NFPS inhibition of [(3)H]MeAIB was monophasic with a single IC(50) value of 31 microM. Clozapine significantly inhibited [(3)H]glycine binding even in the presence of 100 nM NFPS. In conclusion, this study suggests first that System A transporters, or a subset thereof, may play a critical role in regulation of synaptic glycine levels and by extension of NMDA receptor regulation, and second that System A antagonism may contribute to the differential clinical efficacy of clozapine compared with other typical or atypical antipsychotics.