Current Drug Targets - Volume 2, Issue 3, 2001

The NMDA receptor is an important target for drug development, with agents from many different classes acting on this receptor. While the severe side effects associated with complete NMDA receptor blockade have limited clinical usefulness of most antagonists, the understanding of the multiple forms of NMDA receptors provides an opportunity for development of subtype specific agents with potentially fewer side effects. Different NMDA receptor subtypes are assembled from combinations of NR1 and NR2 subunits with each subunit conveying distinct properties. The NR1 subunit is the glycine binding subunit and exists as 8 splice variants of a single gene. The glutamate binding subunit is the NR2 subunit, which is generated as the product of four distinct genes, and provides most of the structural basis for heterogeneity in NMDA receptors. Pharmacological heterogeneity results from differences in the structure of ligand binding regions, as well as structural differences between subtypes in a modulatory region called the LIVBP-like domain. This region in NR1 and NR2B controls the action of NR2B-selective drugs like ifenprodil, while this domain in receptors containing the NR2A subunit controls the action of NR2A-selective drugs such as zinc. This suggests that NMDA receptor subtype selective drugs can be created, and further understanding of subtype specific mechanisms ultimately may allow successful use of NMDA receptor antagonists as therapeutic agents.

NMDA receptors are a subclass of excitatory, ionotropic L-glutamate neurotransmitter receptors. They are.heteromeric, integral membrane proteins being formed by the assembly of the obligatory NR1 subunit together with modulatory NR2 subunits of which four different types, NR2A - NR2D, have been described. This results in a heterogenous population of receptor proteins with distinct pharmacological and biophysical properties thus yielding potential for the development of NMDA receptor subtype-selective therapeutic agents. Anti-NMDA receptor subunit antibodies have been generated and used in immunoprecipitation or immunoaffinity purification studies to determine the in vivo subunit complements of NMDA receptors. This article summarizes knowledge on the subunit compositions of NMDA receptors based on these approaches together with the current status of NMDA receptor subunit stoichiometry and hence quaternary structure. of native NMDA receptors.

Because of adverse reactions, early efforts to introduce high affinity competitive or use-dependent NMDA receptor antagonists into patients suffering from stroke, head trauma or epilepsy met with failure. Later it was discovered that both low affinity use-dependent NMDA receptor antagonists and compounds with selective affinity for the NR2B receptor subunit met the criteria for safe administration into patients. Furthermore, these low affinity antagonists exhibit significant mechanistic differences from their higher affinity counterparts. Success of the latter is attested to the ability of the following low affinity compounds to be marketed: 1) Cough suppressant - dextromethorphan (available for decades) 2) Parkinsons disease - amantadine, memantine and budipine 3) Dementia - memantine and 4) Epilepsy - felbamate. Moreover, Phase III clinical trials are ongoing with remacemide for epilepsy and Huntingtons disease and head trauma for HU-211. A host of compounds are or were under evaluation for the possible treatment of stroke, head trauma, hyperalgesia and various neurodegenerative disorders. Despite the fact that other drugs with associated NMDA receptor mechanisms have reached clinical status, this review focuses only on those competitive and use-dependent NMDA receptor antagonists that reached clinical trails. The ensuing discussions link the in vivo pharmacological investigations that led to the success / mistakes / failures for eventual testing of promising compounds in the clinic.

In the first part of this review studies are considered in which pre- or post-training peripheral or intracerebroventricular administrations of competitive or noncompetitive N-methyl-D-aspartate (NMDA) receptor antagonists were carried out in a variety of animal species tested in different experimental conditions, in order to investigate the effects of these drugs on acquisition and memory processes. In particular, post-training treatments, which are known to affect memory consolidation, ruling out the possible aspecific effects linked to the pre-training administrations, show that the NMDA receptor antagonists impair memory in animals tested in various tasks. Memory impairments are also evident when the NMDA antagonists (in particular AP5) are injected into different brain structures, including amygdala and hippocampus.In a second part of this review some recent studies are considered showing the existence of: a) cholinergic-glutamatergic interactions; b) interactions between NMDA receptors and opioid system, and c) interactions between NMDA receptor antagonists (MK-801) and cocaine, in the modulation of memory processes of laboratory animals.The results of some studies showing the involvement of glutamatergic mechanisms in Alzheimers disease are finally reported, and the therapeutic efficacy of glutamatergic drugs in the treatment of this disease is considered.

Ifenprodil is a novel N-methyl-D-aspartate (NMDA) receptor antagonist that selectively inhibits receptors containing the NR2B subunit. As such, it has become widely used as a tool to study subtypes of NMDA receptors both in vitro and in vivo, and as a tool for molecular studies of the properties and regulation of NMDA receptors. Ifenprodil has an unusual form of activity-dependence and its mechanism of action may involve an increase in proton inhibition of NMDA receptors. These properties are shared by analogs or derivatives of ifenprodil, some of which may be lead compounds for therapeutically useful NMDA antagonists. Such antagonists have potential as neuroprotectants, anticonvulsants, analgesics, and for the treatment of Parkinsons disease and other disorders of the nervous system. The location of the ifenprodil binding site on NMDA receptors and the structural and mechanistic basis of its effects are still unknown. Recent work suggests that at least part of the ifenprodil binding site is located in the R1 / R2 domain of the NR1 subunit. This region, like the S1 / S2 agonist binding domain, shares homology with bacterial periplasmic binding proteins.

Synaptic plasticity, or long-term potentiation (LTP), of excitatory synapses in the hippocampus contributes to learning and the establishment of spatial memories. In the CA1 region, induction of LTP enhances the function of postsynaptic alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate receptors (AMPARs) because of the Ca2+-calmodulin kinase II (CaMKII)-dependent phosphorylation of this subtype of glutamate receptor. Entry of Ca2+, via N-methyl-d-aspartate receptors (NMDARs), during strong synaptic stimulation provides the stimulus to trigger phosphorylation of AMPARs. However, this induction also requires activation of a protein kinase C (PKC)-dependent tyrosine kinase signal cascade and a concomitant upregulation of NMDARs. This review focuses upon NMDARs as potential targets of PKC and / or of the PKC-dependent tyrosine kinase cascade. PKC, acting via the CAK beta / Src tyrosine kinase cascade, enhances NMDAR activation and may increase the number of receptors expressed in synapses. In contrast, direct phosphorylation of NMDARs by PKC increases the sensitivity of NMDA channel inactivation to intracellular Ca2+. In CA1 neurons, PKC provides a point of convergence of control of NMDARs and synaptic plasticity for a wide variety of G-protein coupled and growth factor receptors.

The three members of the conantokin peptide family identified to date are conantokin(con)-G, -T and -R. Their defining attributes include a high relative content of gama-carboxyglutamic acid (Gla), N-terminal sequence identity, as well as considerable overall sequence homology, and antagonism of the N-methyl-D-aspartate receptor (NMDAR). As promising templates for the design of neuroprotective agents, a thorough evaluation of structure-function relationships in these peptides will be invaluable in aiding rational drug modeling. To this end, a comprehensive assessment of the contributions of individual residues to conantokin structure and function is required. The current review summarizes recent efforts in this area, and also includes the effects of peptide length, as well as structural-stabilization and -destabilization on the structural and inhibitory profiles of an extensive panel of conantokin derivatives.

The involvement of the glutamate-glycine activated ion channels of the NMDA receptor in various neurophysiological processes has made this ion channel the focus of intense research. The excessive release of glutamate in a variety of neuronal hypoxic conditions implicates the NMDA receptor in a number of neuropatholological states, such as stroke, chronic pain, Parkinsons disease, Alzheimers disease, ALS, and epilepsy, among others, thus making this receptor a prime drug target candidate. A variety of agents are known to be effective in opening and closing of the ion channels of this receptor, among the latter group of agents is the peptidic conantokins. Through the use of electrophysiological measurements with a number of cell types containing natural and recombinant subunits of the NMDA receptor, much knowledge is evolving regarding the mechanism of action of activators and inhibitors of the NMDA receptor ion channels. In addition, structure-function studies of the conantokins in these systems have been revealing in terms of their complimentary sites on the NMDA receptor. These relationships serve as the main focus of this review.

According to a recent World Health Organization survey, there are over four hundred million people worldwide suffering from mental and neurological disorders schizophrenia affects some forty-five million people, and unipolar major depression ranked fifth in major causes of disability and death. Clearly it is of the utmost importance to develop new, effective, and safe neuro-pharmaceuticals with this increasing global burden of disease. To this end, we have developed a strategy of generating monoclonal antibodies that act as modulators of the cell-surface central nervous system receptor-ion channel complexes.In this review we will focus on the generation and characterization of a monoclonal antibody that acts as a partial agonist to the N-methyl-D-aspartate receptor. The creation of peptide mimetics, derived from this monoclonal antibody, that may be useful as cognitive enhancers and protect neurons hypoxic and ischemic insults caused by stroke, will also be discussed.