Multiple sites of action of neomycin, Mg2+ and spermine on the NMDA receptors of rat hippocampal CA1 pyramidal neurones.
1. The effects of neomycin on NMDA-evoked currents in isolated CA1 hippocampal pyramidal neurones were investigated and single channel activity was examined in outside-out patches taken from cultured hippocampal neurones. The effects of neomycin on two combinations of NMDA receptor subunits (NR1a-NR2A and NR1a-NR2B) expressed in human embryonic kidney (HEK293) cells were also studied. 2. Neomycin (0. 01-1 mM) caused a potentiation of NMDA-activated currents in all neurones examined. No evidence of a voltage-dependent depression was observed in whole-cell recordings. 3. In outside-out patch recordings relatively low concentrations (30 and 100 microM) of neomycin caused a voltage-dependent reduction in single channel current amplitude as well as a large increase in the frequency of channel opening. 4. In saturating concentrations of glycine, neomycin enhanced NMDA-activated currents and this glycine-independent enhancement was confirmed using recombinant NR1a-NR2B receptors. Neomycin substantially increased the potency of glycine for the receptor by reducing the rate of dissociation of glycine from the receptor. Neomycin demonstrated a glycine-dependent enhancement of currents mediated by the NR1a-NR2A combination of subunits but a paradoxical depression was observed in saturating concentrations of glycine. 5. Neomycin increased the rate of deactivation of glutamate-activated currents consistent with neomycin causing a reduction in the affinity of the receptor for agonist. 6. These results indicate that neomycin has multiple and complex effects on NMDA receptors.
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Department of Pharmacology, Faculty of Medicine, University of Toronto, 1 King's College Circle, Toronto, Ontario, Canada M5S 1A8.
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Orser
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MacDonald
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D000818:Animals; D001665:Binding Sites; D002118:Calcium / Q000494:pharmacology; D002460:Cell Line; D002478:Cells, Cultured; D005071:Evoked Potentials / Q000187:drug effects; D018698:Glutamic Acid / Q000494:pharmacology; D005998:Glycine / Q000494:pharmacology; D006624:Hippocampus / Q000166:cytology / Q000502:physiology*; D006801:Humans; D008274:Magnesium / Q000378:metabolism / Q000494:pharmacology*; D016202:N-Methylaspartate / Q000494:pharmacology; D009355:Neomycin / Q000493:pharmacokinetics / Q000494:pharmacology*; D017966:Pyramidal Cells / Q000166:cytology / Q000187:drug effects / Q000502:physiology*; D051381:Rats; D016194:Receptors, N-Methyl-D-Aspartate / Q000737:chemistry / Q000187:drug effects / Q000502:physiology*; D013096:Spermine / Q000493:pharmacokinetics / Q000494:pharmacology*; D014162:Transfection
D002118:Calcium; D018698:Glutamic Acid; D005998:Glycine; D008274:Magnesium; D016202:N-Methylaspartate; D009355:Neomycin; D016194:Receptors, N-Methyl-D-Aspartate; D013096:Spermine