Christopher Graham Raphael Parsons

NMDA receptors as targets for drug action in neuropathic pain

Hyperalgesia and allodynia following peripheral tissue or nerve injury are not only due to an increase in the sensitivity of primary afferent nociceptors at the site of injury but also depend on NMDA receptor-mediated central changes in synaptic excitability. Functional inhibition of NMDA receptors can be achieved through actions at different recognition sites such as the primary transmitter site (competitive), strychnine-insensitive glycine site (glycine(B)), polyamine site (NR2B selective) and phencyclidine site located inside the cationic channel. Unfortunately, most agents which completely block NMDA receptors cause numerous side effects such as memory impairment, psychotomimetic effects, ataxia and motor incoordination. There is now, however, considerable evidence that moderate affinity channel blockers, glycine(B) and NR2B selective antagonists show a much better profile in animal models than high affinity channel blockers and competitive NMDA receptor antagonists. These therapeutically safe NMDA receptor antagonists are also able to slow or prevent the development of opioid tolerance, indicating the utility of their combination with opioids in the treatment of chronic pain. The antinociceptive effects of NMDA receptor antagonists and opioids could be predicted to be synergistic and the presence of an NMDA receptor antagonist should block both the development of chronic pain states and inhibit the development of tolerance to the analgesic effects of morphine. Peripheral NMDA receptors offer a very attractive target for NMDA receptor antagonists that do not cross the blood brain barrier in inflammatory and visceral pain. Such agents might be predicted to be devoid of CNS side effects at doses producing powerful antinociception at peripheral NMDA receptors.

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

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.

Effects of memantine and MK-801 on NMDA-induced currents in cultured neurones and on synaptic transmission and LTP in area CA1 of rat hippocampal slices.

1 The effects of the uncompetitive N‐methyl‐D‐aspartate (NMDA) receptor antagonists, memantine (1‐amino‐3,5‐dimethyladamantane) and MK‐801 ((+)‐5‐methyl‐10,11‐dihydro‐5H‐dibenzocyclo‐hepten‐5,10‐imine maleate) were compared on synaptic transmission and long‐term potentiation (LTP) in hippocampal slices and on NMDA‐induced currents in cultured superior collicular neurones. 2 Memantine (10–100 μm) reversibly reduced, but did not abolish, NMDA receptor‐mediated secondary population spikes recorded in area CA1 of hippocampal slices bathed in Mg2+‐free artificial cerebrospinal fluid. 3 Memantine (100 μm) antagonized NMDA receptor‐mediated excitatory postsynaptic currents recorded in area CA1 in a strongly voltage‐dependent manner i.e. depressed to 11±4% of control at −35 mV and 95±5% of control at +40 mV (n=9), with no apparent effect on response kinetics. 4 The effects of MK‐801 and memantine on the induction of LTP were assessed after prolonged pre‐incubations with these antagonists. When present for 6.6±0.4 h prior to tetanic stimulation, memantine blocked the induction of LTP with an IC50 of 11.6±0.53 μm. By comparison, similar long pre‐incubations with MK‐801 (6.4±0.4 h) blocked the induction of LTP with an IC50 of 0.13±0.02 μm. 5 Memantine and MK‐801 reduced NMDA‐induced currents in cultured superior colliculus neurones recorded at −70 mV with IC50S of 2.2±0.2μm and 0.14±0.04 μm respectively. The effects of memantine were highly voltage‐dependent and behaved as though the affinity decreased ∍ fold per 50 mV of depolarization (apparent δ=0.71). In contrast, under the conditions used, MK‐801 appeared to be much less voltage‐dependent i.e. affinity decreased ∍ fold per 329 mV of depolarization (apparent δ=0.15). 6 Depolarizing steps from −70 mV to +50 mV in the continuous presence of memantine (10 μm) caused a rapid relief of blockade of NMDA‐induced currents from 83.7±1.9% to 21.8±1.8% (n=5). This relief was best fitted by a double exponential function (17.2±11.7 and 698±204 ms), the faster component of which was most pronounced. 7 In conclusion, whereas MK‐801 is equipotent in blocking NMDA‐induced currents (at −70 mV) and the induction of LTP, memantine is relatively less potent in blocking the induction of LTP. This is due to its rapid relief of blockade upon depolarization; a property which might explain its promising clinical profile in the treatment of chronic neurodegenerative diseases.

Learning deficits induced by chronic intraventricular infusion of quinolinic acid — protection by MK-801 and memantine

The NMDA receptor agonist quinolinic acid (9 mM) was infused i.c.v. via ALZET osmotic minipumps for 2 weeks. This treatment produced a persistent, short-term memory deficit in the T-maze. Autoradiography revealed a decrease in the density of choline uptake sites in the hippocampus. Parallel s.c. infusion by another minipump of the uncompetitive NMDA receptor antagonist memantine (1-amino-3,5-dimethyladamantane, 20 mg/kg per day) or (+)-5-methyl-10,11-dihydro-5H-dibenzocyclohepten-5,10-imine maleate ((+)-MK-801, 0.31 mg/kg day) prevented the learning deterioration induced by quinolinic acid. The treatment with memantine resulted in steady-state serum levels of 1.2 mu M which, based on in vitro data, should assure inhibition of NMDA receptors and are similar to levels seen in the serum of demented patients treated with this agent. In naive animals this treatment had no effect on either learning or on ex vivo induction of long-term potentiation, indicating that under chronic conditions it is possible to obtain neuroprotective effects with NMDA receptor antagonists without negative effects on memory processes. This contrasts to some acute insults (e.g. ischaemia) where high doses of NMDA receptor antagonists that produce side effects are required.

Memantine and the amino-alkyl-cyclohexane MRZ 2/579 are moderate affinity uncompetitive NMDA receptor antagonists –in vitro characterisation

Summary. There is general agreement that moderate affinity uncompetitive NMDA receptor antagonists combine good efficacy and tolerability in animal models of disturbances in glutamatergic transmission. There are several theories on which properties are important for this profile including 1, rapid access to the channel at the start of pathological overactivity 2, rapid, voltage-dependent relief of blockade during physiological synaptic activation and 3, partial untrapping. Merz has developed a series of novel uncompetitive NMDA receptor antagonists based on the cyclohexane structure. In cultured hippocampal neurones MRZ 2/579 (1-amino-1,3,3,5,5-pentamethyl-cyclohexane) shows similar blocking kinetics to memantine (Kon 10.7 * 104 M−1 sec−1, Koff 0.20 sec−1 at −70 mV) and binds at the same depth in the NMDA receptor channel (δ = 0.8). The potency of MRZ 2/579 assessed as Kd = Koff/Kon = 1.87 μM agrees well with the IC50 of 1.29 μM against steady-state currents in cultured hippocampal neurones (at −70 mV) and with the Ki in [3H]-MK-801 binding of 0.65 μM. MRZ 2/579 protected cultured cortical neurones against glutamate toxicity with an IC50 of 2.16 μM and was also effective in protecting hippocampal slices against hypoxia / hypoglycaemia-induced reduction of fEPSP amplitude in CA1 with an EC50 of 7.01 μM. MRZ 2/579 has similar potency and bio-availability to memantine in vivo assessed using microdialysis, microiontophoresis and MES-induced seizures. Initial characterization in animal models provides strong support for the assump-tion that MRZ 2/579 could be a useful therapeutic in morphine/alcohol dependence, inhibition of morphine tolerance, chronic pain and as a neuroprotective agent.

NMDA channel blockers: memantine and amino-aklylcyclohexanes –In vivo characterization

Summary. The previous overviews provided the basis for better therapeutic efficacy/tolerability of low to moderate affinity NMDA channel blockers. This prediction finds support in in vitro studies comparing protective and plasticity impairing effects (therapeutic vs. side-effect) of memantine and (+)MK-801. In fact it turned out that memantine had a far better therapeutic index. But can it be confirmed in the in vivo situation?

GlycineB antagonists as potential therapeutic agents

SummaryIt is not clear what therapeutic application is most likely for agents blocking glycine site of the NMDA receptors (glycineB). Majority of the studies to date used either glycineB antagonists with doubtful brain penetration or partial agonists. Following systemic administration to rats of our newly developed glycmeB antagonists (MRZ 2/570; 2/571 and 2/576) and L-701,324 (MSD) as a reference agent the following behavioural effects were observed: weak (if any) antiparkinsonian-like effects, lack of anxiolytic activity, inhibition of physical and motivational aspects of morphine dependence and neuroprotective activity in global ischaemia. The side effects include: sedation, ataxia, and myorelaxation. We detected neither vacuolisation in the cingulate cortex nor impairment of pre-pulse inhibition indicating lack of psychotomimetic potential.