The NMDA receptor. Glutamate and glycine are required for NMDA receptor activation. Activation involves the opening of a channel allowing calcium and sodium ions to flow into the neuron. Recent attempts to translate NMDA pharmacology into the clinic have focussed on the glycine site.
Twenty years ago it all looked so promising. The model was as follows: Learning and memory were clearly being driven by activity at the glutamate NMDA receptor. Boost the NMDA receptor by pharmacological means, and perhaps intellectual performance could be improved above baseline. The hope was that an NMDA enhancer might work in schizophrenia, which many had come to regard as a disorder of cognition. Yet the story has not played out as anticipated. The latest generation of NMDA enhancers, like their predecessors, has failed in schizophrenia [link]. And it is looking increasingly likely that the basic model [boost NMDA -> boost intellectual functioning] was overtly simplistic.
Long Term Potentiation (LTP) is induced by NMDA receptor activation. The mechanism of early-phase LTP involves the enhancement of AMPA receptor conductances and insertion of new AMPA receptors into the post-synaptic membrane.
An recent review article by Collingridge and colleagues is worthy of study. Back in 1983, Collingridge had shown that activation of the glutamate NMDA receptor was the initial catalyst for the process of LTP (long-term-potentiation). At that time glutamate was only just gaining entry to the neurotransmitter club, whereas LTP [a process in which excitatory synapses become and remain stronger] had achieved fame ten years earlier as a likely substrate for learning and memory in nervous systems.
The discovery of NMDA-dependent LTP, as the phenomena came to be known, was the stimulus for an enormous, worldwide research effort into glutamate neurobiology. Since then, our knowledge of NMDA receptors has advanced, to the point where the complexity can be overwhelming [figure below]. But the medicines have not materialised. The biology appears to be several orders more complex than the model. Is that why the drugs have failed? In any case, the model [boost NMDA -> boost intellectual functioning] can now be safely abandoned with little risk of missing a major therapeutic breakthrough.
Sites of intracellular modulation of NMDARs. Schematic representation of the distribution of selected posttranslational regulatory sites on the intracellular C-terminal domains of NMDAR subunits. Properties such as channel gating, receptor desensitisation and receptor shuttling are modulated by phosphorylation at key residues. Collingridge et al 2013
POSTSCRIPT
Recently the NIMH (National Institute of Mental Health], the main funder of mental health research in the world, announced that they would no longer support clinical trials of new drugs unless there was a clear mechanistic advance at the same time:
The NMDA receptor story calls the logic of this approach into question. That story is the archetypal case in which a mechanism was clearly defined, and well supported after decades of preclinical research. Indeed the mechanism [the model] had become so appealing that many were reluctant to abandon it, even as it was becoming obvious that the therapeutics were not going to work. An overhaul of drug discovery in psychiatry is needed, but it will require to be more realistic than solving mechanism and efficacy problems concurrently. Pulling back the bureaucracy, the inflated costs and the micromanagement could be a more fruitful intervention.