Claudia Jatzke

Potency, voltage-dependency, agonist concentration-dependency, blocking kinetics and partial untrapping of the uncompetitive N-methyl-D-aspartate (NMDA) channel blocker memantine at human NMDA (GluN1/GluN2A) receptors.

Both the clinical tolerability and the symptomatic effects of memantine in the treatment of Alzheimers disease have been attributed to its moderate affinity (IC(50) around 1 microM at -70 mV) for NMDA receptor channels and associated fast, double exponential blocking/unblocking kinetics and strong voltage-dependency. Most of these biophysical data have been obtained from rodent receptors. Some substances show large species-specific differences, so using human rather than rodent receptors and tissue may highlight important differences in the effects of drugs. In the present study we compared the potency of memantine, ketamine and (+)MK-801 in binding to NMDA receptors in post-mortem human cortical tissue and to antagonize intracellular Ca(2+) responses of human GluN1/GluN2A receptors expressed in HEK-293 cells. In addition, the biophysical properties of memantine and ketamine were compared using patch clamp recordings from these cells. Memantine was confirmed to be a moderate affinity (IC(50) at -70 mV of 0.79+/-0.02 microM, Hill=0.92+/-0.02), strongly voltage-dependent (delta=0.90+/-0.09) uncompetitive antagonist of human GluN1/GluN2A receptors. Moreover, the rapid double exponential blocking kinetics (e.g. at 10 microM - onset tau(fast)=273+/-25 ms (weight 69%), onset tau(slow)=2756+/-296 ms, offset tau(fast)=415+/-82 ms (weight 38%) offset tau(slow)=5107+/-1204 ms) and partial untrapping (around 20%) previously reported for memantine on rodent receptors were confirmed for human receptors. Ketamine showed similar potency (IC(50) at -70 mV of 0.71+/-0.03 microM, Hill=0.84+/-0.02) but somewhat less pronounced voltage-dependency (delta=0.79+/-0.04), slower, single exponential kinetics (ketamine: k(on)=0.15+/-0.05 x 10(6)M(-1)s(-1), k(off)=0.22+/-0.05 s(-1)c.f. memantine following normalization k(on)=0.32+/-0.11 x 10(6)M(-1)s(-1), k(off)=0.53+/-0.10s(-1)) and was fully trapped. The present data closely match previously reported data from studies in rodent receptors and suggest that the proposed mechanism of action of memantine in Alzheimers disease as a fast, voltage-dependent open-channel blocker of NMDA receptors can be confirmed for human NMDA receptors.

Positive and Negative Modulation of Group I Metabotropic Glutamate Receptors

A discriminating pharmacophore model for noncompetitive metabotropic glutamate receptor antagonists of subtype 1 (mGluR1) was developed that facilitated the discovery of moderately active mGluR1 antagonists. One scaffold was selected for the design of several focused libraries where different substitution patterns were introduced. This approach facilitated the discovery of potent mGluR1 antagonists, as well as positive and negative mGluR5 modulators, because both receptor subtypes share similar binding pockets. For mGluR1 antagonists, a homology model of the mGlu1 receptor was established, and a putative binding mode within the receptors transmembrane domain was visualized.

Using cholinergic M1 receptor positive allosteric modulators to improve memory via enhancement of brain cholinergic communication

Benzylquinolone carboxylic acid (BQCA) is a recently described cholinergic muscarinic M(1) receptor positive allosteric modulator having potential as cognitive enhancer in dementia. The present study focused on the characterisation of BQCAs mode of action in relation to positive effects on memory and side-effects in an animal model. To get insight into this mode of action, in vitro receptor potency/left shift experiments in cells stably expressing the rats M(1) receptor were performed. They revealed an inflection point value of BQCA corresponding to 306nM, and potentiation of the agonist response up to 47-fold in presence of 10μM of BQCA. In vivo, brain microdialysis showed a maximal brain level of 270nM, 40min after i.p. administration at 10mg/kg. Based on in vitro data obtained with this dose, it can be concluded that BQCA reaches brain levels which should potentiate the agonist response about 4-fold. Behavioural data confirmed that BQCA used at 10mg/kg attenuated scopolamine-induced memory deficit in a spontaneous alternation task. Moreover, BQCA showed no side effect at 10mg/kg and above in spontaneous locomotion and salivation tests. The profile of BQCA observed in the present study displays a clear advantage over the M(1)-M(3) agonist cevimeline. The present data show the therapeutic potential of the M(1) receptor positive allosteric modulator BQCA for the treatment of memory deficits observed in Alzheimers disease.

Effect of 5-HT5A antagonists in animal models of schizophrenia, anxiety and depression.

The few available data on the pharmacological effect of 5-HT5A receptors suggest that antagonists may have anxiolytic, antidepressant and antipsychotic activity. The aim of our study was to verify these suggestions in relevant animal models. Two 5-HT5A antagonist ligands, SB-699551-A (N-[2-(dimethylamino)ethyl]-N-[[4′-[[(2-phenylethyl)amino]methyl][1,1′-biphenyl]-4-yl]methyl]cyclopentanepropanamide dihydrochloride) (3–60 mg/kg, intraperitoneally) and A-843277 (N-(2,6-dimethoxybenzyl)-N′[4-(4-fluorophenyl)thiazol-2-yl]guanidine) (3–30 mg/kg, intraperitoneally), were examined in the open-field test, in a foot-shock-induced ultrasonic vocalization test, in the forced swim test (FST) and in the amphetamine-induced and phencyclidine-induced hyperlocomotion tests to examine their effect on general behavioural patterns, and their anxiolytic-like, antidepressant-like and antipsychotic-like properties, respectively. In the open-field test, SB-699551-A induced sedation and A-843277 induced writhing. In the ultrasonic vocalization test, SB-699551-A reduced vocalizations, whereas A-843277 was ineffective. In the FST, SB-699551-A was ineffective and A-843277 reduced immobility, but only at the highest dose. In the amphetamine-induced and phencyclidine-induced hyperlocomotion test, both compounds were ineffective. SB-699551-A showed an anxiolytic-like property in the ultrasonic vocalization test; however, this compound has a sedative effect. A-843277 showed an antidepressant-like property in the FST, but its immobility-reducing effect may also be a consequence of abdominal irritation. Consequently, further investigations are required to define the therapeutic potential of 5-HT5A receptor ligands in anxiety, depression and schizophrenia models.

Memantine does not show intracellular block of the NMDA receptor channel

Mg2+ is known to gain access to the NMDA receptor channel from both the extra- and intracellular compartments. Memantine, being an amphiphilic substance, reaches intracellular concentrations of approximately 30 microM, which are much higher than therapeutic extracellular concentrations ( approximately 1 microM). We therefore investigated whether memantine can also block the NMDA receptor channel from the intracellular compartment. NR1a/NR2A receptors were expressed in Xenopus oocytes and in classical two electrode voltage-clamp recordings, voltage-ramps from -100 to +100 mV confirmed moderate inward rectification of NR1a/NR2A receptor control responses at positive membrane potentials above +40 mV. Patch clamp recordings from these same cells (applying 100 microM glutamate and 1 mM Mg2+) revealed similar rectification at positive potentials in cell-attached mode which disappeared after pulling an inside-out patch. Application of 1 mM Mg2+ to the intracellular side of the receptor re-introduced the rectification seen in cell-attached mode, and 5 mM Mg2+ produced much more pronounced block. In contrast, 30 microM memantine was completely unable to block the NMDA receptor from the intracellular compartment. In conclusion, intracellular block of the NMDA receptor, as reported for Mg2+, is not of significance for the therapeutic effects of memantine.

A novel class of amino-alkylcyclohexanes as uncompetitive, fast, voltage-dependent, N-methyl-D-aspartate (NMDA) receptor antagonists – in vitro characterization

SummaryThe fact that potent NMDA receptor channel blockers produce phencyclidine-like psychotropic symptoms in man and rodents implies that uncompetitive antagonism of NMDA receptors may not be a promising therapeutic approach. However, recent data indicate that agents with moderate affinity such as memantine and neramexane (MRZ 2/579) are useful therapeutics due to their strong voltage-dependency and rapid unblocking kinetics. Merz has developed a series of novel uncompetitive NMDA receptor antagonists based on an amino-alkylcyclohexane structure. These compounds displaced [3H]-MK-801 binding to rat cortical membranes with Ki values between 1 and 100 µM and inward current responses of cultured hippocampal neurons to NMDA were antagonized in a strongly voltage-dependent manner with rapid blocking/unblocking kinetics. Three of these compounds, with similar biophysical properties to memantine, were chosen for development. MRZ 2/759 (1-ethenyl-3,3,5,5-tetramethyl-cyclohexylamine), 2/1010 (1,3,3,5-tetramethyl-6-azabicyclo[3.2.1]octane) and 2/1013 (8,8,10,10-tetramethyl-1-azaspiro[5.5] undecane) displaced [3H]-MK-801 binding with Ki values of 1.18, 2.59 and 3.64 µM, respectively. They were similarly potent against NMDA-induced currents in hippocampal neurons – IC50 values of 1.51, 3.06 and 2.20 µM, respectively. In line with their moderate affinity, all were voltage-dependent (δ = 0.86, 0.96 and 0.89, respectively) and fast, open-channel blockers (kon 7.90, 1.70 and 2.60 × 104 M−1 sec−1, koff 0.13, 0.12 and 0.24 sec−1, respectively). These compounds are also NMDA receptor antagonists in the CNS following systemic administration and have good therapeutic indices in a variety of in vivo behavioural models where glutamate is known to play a pivotal role. In view of their relatively low affinity and associated rapid kinetics, they should prove to be useful therapeutics in a wide range of CNS disorders.

Blocking kinetics of memantine on NR1a/2A receptors recorded in inside-out and outside-out patches from Xenopus oocytes

Previous experiments on primary cultures of hippocampal/cortical neurones revealed that the block and unblock of N-Methyl-d-Aspartate (NMDA) receptor channels by memantine showed double exponential kinetics and that the offset kinetics following a voltage-step were much faster than following a concentration jump. There are, however, two major problems when using such cultured primary neurones for these experiments (1) the almost certain expression of heterogeneous NMDA receptor subunits which could underlie double exponential kinetics due to different potencies at receptor subtypes and (2) slow space- and concentration-clamp due to neuronal morphology which could mask even faster kinetics. Therefore, we performed similar experiments with Xenopus oocytes exclusively expressing one NMDA receptor type (NR1a/2A) at high levels which allowed recordings from membrane patches with large currents. The use of inside-out patches for voltage-step and outside-out patches in combination with a piezo driven fast application system largely negated potential space- and concentration-clamp problems. Block and unblock of the NMDA receptor by memantine after both voltage jump and concentration jumps showed triple exponential kinetics. The fast onset kinetics of NMDA receptor channel block following both concentration-clamp and voltage jumps from +70 to −70 mV were similar. In contrast, offset kinetics after a voltage-step from −70 to +70 mV were much faster than following a concentration jump at the holding potential of −70 mV. These results provide further support for the hypothesis that rapid relief of block via strong synaptic membrane depolarisation underlies the good therapeutic profile of memantine.