Effects of the Uncompetitive NMDA Receptor Antagonist Memantine on Hippocampal Long‐term Potentiation, Short‐term Exploratory Modulation and Spatial Memory in Awake, Freely Moving Rats
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Chronic treatment of adult male F-344 rats (9-12 months old) with therapeutically relevant doses of memantine (30 mg/kg/day in chow for > 8 weeks) increased the maintenance of long-term potentiation of field excitatory postsynaptic potentials from perforant path-granule cell hippocampal synapses recorded in the fascia dentata in vivo. In contrast, there was no effect of memantine on baseline synaptic responses or population spikes. Likewise, short-term exploratory modulation of these hippocampal evoked responses was not different between memantine-treated and control rats. Both groups of rats were able to learn the spatial version of the Morris water task equally well, but the memantine-treated group showed a strong tendency to show more selective spatial search patterns in the training quadrant of the water pool during a final probe trial. As such, these studies provide the first electrophysiological evidence that memantine can increase the durability of synaptic plasticity and provide preclinical confirmation of the cognitive improvement seen with memantine in the treatment of demented patients.Keywords:
Memantine
Memantine and ketamine are clinically useful NMDA receptor (NMDAR) open channel blockers that inhibit NMDARs with similar potency and kinetics, but display vastly different clinical profiles. This discrepancy has been hypothesized to result from inhibition by memantine and ketamine of overlapping but distinct NMDAR subpopulations. For example, memantine but not ketamine may inhibit extrasynaptic NMDARs more effectively than synaptic NMDARs. However, the basis for preferential NMDAR inhibition depending on subcellular location has not been investigated systematically. We integrated recordings from heterologously expressed single NMDAR subtypes, kinetic modeling, and recordings of synaptically evoked NMDAR responses in acute brain slices to investigate mechanisms by which channel blockers may distinguish NMDAR subpopulations. We found that memantine and ketamine differentially alter NMDAR desensitization and that memantine stabilizes a Ca 2+ -dependent desensitized state. As a result, inhibition by memantine of GluN1/2A receptors in tsA201 cells and of native synaptic NMDARs in cortical pyramidal neurons from mice of either sex increased in conditions that enhanced intracellular Ca 2+ accumulation. Therefore, differential inhibition by memantine and ketamine based on NMDAR location is likely to result from location dependence of the intensity and duration of NMDAR activation. Modulation of Ca 2+ -dependent NMDAR desensitization is an unexplored mechanism of inhibitory action with the potential to endow drugs with NMDAR selectivity that leads to superior clinical profiles. Our results suggest that designing compounds to target specific receptor states, rather than specific receptor types, may be a viable strategy for future drug development. SIGNIFICANCE STATEMENT Memantine and ketamine are NMDA receptor (NMDAR) channel-blocking drugs with divergent clinical effects. Understanding mechanistically their differential actions may advance our understanding of nervous system disorders and suggest strategies for the design of more effective drugs. Here, we show that memantine and ketamine have contrasting effects on NMDAR desensitization. Ketamine binding decreases occupancy of desensitized states of the GluN1/2B NMDAR subtype. In contrast, memantine binding increases occupancy of GluN1/2A and native NMDAR desensitized states entered after accumulation of intracellular Ca 2+ , a novel inhibitory mechanism. These properties may contribute to inhibition of distinct NMDAR subpopulations by memantine and ketamine and help to explain their differential clinical effects. Our results suggest stabilization of Ca 2+ -dependent desensitized states as a new strategy for pharmaceutical neuroprotection.
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The Ts65Dn mouse is the best-studied animal model for Down syndrome. In the experiments described here, NMDA-mediated or mGluR-mediated LTD was induced in the CA1 region of hippocampal slices from Ts65Dn and euploid control mice by bath application of 20 µM NMDA for 3 min and 50 µM DHPG for 5 min, respectively. We found that Ts65Dn mice display exaggerated NMDA-induced, but not mGluR-induced, LTD in the CA1 region of the hippocampus compared with euploid control animals. In addition, this abnormal level of LTD can be pharmacologically rescued by the NMDA receptor antagonist memantine.
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We examined the effect of 1-amino-3,5-dimethyladamantane (memantine) and (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine (MK-801) on the inhibition of [3H]MK-801 binding to crude synaptic membranes of rat forebrains in the absence or presence of Ca2+. Ca2+ decreased the potency of memantine to inhibit [3H]MK-801 binding. The effect of Ca2+ was apparently competitive with memantine and was not annulled by the addition of Mg2+. Ca2+ slightly enhanced [3H]MK-801 binding, but showed no effect on the displacement of [3H]MK-801 binding by MK-801. The Ca2+-sensitive interaction of memantine with N-methyl-D-aspartate (NMDA) receptor-gated ion channels may provide a clue to understanding its voltage-dependent and clinically tolerated character.
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Pharmacological inhibition of NMDA receptor activity by ketamine is accompanied by pyschotomimetic side-effects; however, the Alzheimer's disease therapeutic memantine blocks NMDA receptor activity without debilitating side-effects. This dissertation provides electrophysiological and behavioral characterizations of these two NMDA receptor antagonists in an attempt to understand the unique therapeutic utility of memantine. The following work explores memantine and ketamine inhibition at NMDA receptors, their main site of action, with a focus on the mechanism of inhibition and receptor subtype selectivity in physiologically relevant conditions. This research shows NMDA receptors possess a second binding site at which memantine, but not ketamine, can inhibit activity. The research also shows the dramatic effect physiological concentrations of magnesium has on the ability of these drugs to inhibit NMDA receptor activity. Behavioral and cognitive effects of memantine and ketamine are also assessed and compared directly in rat. The effects of memantine and ketamine in rat were found to be similar at the low doses tested and more divergent as dose increased. Furthermore, memantine's effects appeared to be more pronounced and longer-lasting than those of ketamine. These findings demonstrate the importance of considering the physiological environment in which a drug acts, as well as the principles of drug action, when examining the effects of a drug on central nervous system activity.
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