Tag Archives: NMDA receptor

Why NMDA drugs keep failing in schizophrenia.

Posted on by
nmda receptor

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

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.

Intracellular modulation of NMDA receptors

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:

“a positive result will require not only that an intervention ameliorated a symptom, but that it had a demonstrable effect on a target, such as a neural pathway implicated in the disorder or a key cognitive operation.”

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.

Ketamine for resistant depression: Outstanding promise, outstanding issues.

Posted on by

Outstanding Promise.

Ketamine has been around for many years, firstly as a dissociative anaesthetic and then as a psychedelic drug. But it might become best known for it's powerful antidepressant properties (Berman et al 2000; Zarate et al 2006). Compared to existing antidepressants, which take around 2 weeks to work, ketamine exerts a large antidepressant effect on the first day of treatment.

depression ketamine murrough

Figure 1: The antidepressant effect of ketamine over 6 treatment sessions. The improvement on day 1 (measured using the MADRAS scale) was predictive of the response achieved following the sixth treatment session.

The robust antidepressant effect of ketamine also occurs in patients who have not found relief with existing drugs or with ECT. In the latest study to be reported, 24 patients with treatment-resistant depression underwent up to 6 sessions of intravenous ketamine (0.5mg/Kg in 40 mins) over ~2 weeks. Over 70% of patients responded to ketamine, and the overall reduction in depression was large and rapid (Murrough et al 2013) (Figure 1).

Outstanding Issues.

To date a major issue has been the lack of persistence of the antidepressant effect. In previous studies, involving a single ketamine treatment, depression returned within one week of the session or less. In the study by Murrough et al, this was extended to an average of 18 days. This is an improvement, but further work will be needed to solve the problem of the relatively short-lived antidepressant effect of ketamine.

An understanding of the mechanism by which ketamine alleviates depression may be necessary if we are to extend the duration of it's beneficial effects. Pre-clinical work suggests that ketamine boosts the health and integrity of synapses and neuronal networks. Much of the action is believed to take place within dendritic spines, and involves local protein synthesis (Duman et al 2012) (Figure2).

ketamine mechanism

Figure 2: The antidepressant effects of ketamine may depend upon activation of mTOR and local protein synthesis in dendritic spines.

Two molecules of relevance are mTOR and GSK-3. Ketamine enhances local protein synthesis by activating mTOR and by inhibiting GSK-3. [GSK-3 inhibits mTOR]. A drug, such as lithium, which inhibits GSK-3 might enhance the antidepressant effect of ketamine. This has now been demonstrated in pre-clinical studies (Liu et al 2013). The clinical question, which will now be addressed in trials is whether lithium treatment extends and enhances the antidepressant effects of ketamine. Lithium has been used for treatment-resistant depression for many years, and has a good evidence base (Bauer et al 2010) so that the combination of ketamine and lithium presents as an interesting and relatively straightforward strategy for stubborn depression.

However it is somewhat odd that the proposed mechanism for ketamine involves new protein synthesis and synaptogenesis (which take time, and are sustained) whereas the clinical effects of ketamine are very rapid (and transient). Other mechanisms may have more explanatory power. For instance a recent fMRI study showed that ketamine decreased the connectivity of limbic and prefrontal regions which are known to be overactive in depression (Scheidegger et al 2012). More provocatively, it appears that the antidepressant effect of ketamine depends upon the extent of the acute psychological reaction produced by the drug. Although the dissociative/psychedelic properties of ketamine are sometimes regarded as unwanted “side-effects”, a recent paper showed that the acute psychedelic and subsequent antidepressant effects are related (Sos et al 2013).

Psychosis Research. Where have we been & where are we going?

Posted on by
 
phenotype and genotype

The Institute of Psychiatry at The Maudsley is the largest centre for psychiatric research in Europe. Recently a group of leading researchers were tasked with summarising an area of research as it pertains to psychosis and psychopharmacology.

The outcome was a series of short lectures, delivered to a lively audience of psychiatrists, mental health workers and psychologists at The Maudsley. The lecture slides and audio are now available below and constitute a unique training resource for those who treat patients.

1. Sir Robin Murray,
“Psychosis research: Deconstructing the dogma“
2. David Taylor,
“Current Psychopharmacology: Facts & Fiction“
3. Oliver Howes,
“How can we Treat psychosis better?“
4. Marta DiForti,
“An idiot's guide to psychiatric genetics“
5. Sameer Jauhar,
“Ten psychosis papers to read before you die!”
6. Paul Morrison,
“Future antipsychotics“

 

NMDA receptor encephalitis: An acute organic psychosis.

Posted on by

Mental health clinicians should be mindful that numerous physical illnesses can present with psychiatric symptoms. A case in point is a recently described autoimmune disorder in which antibodies target glutamate NMDA receptors within the brain. Acute psychosis and cognitive dysfunction are so prominent in this condition, Anti-NMDA receptor encephalitis, that many patients are initially referred to psychiatry. Swift and accurate diagnosis is essential, as the appropriate therapy is immunological rather than psychiatric.

The antibody targets the extra-cellular portion of the NMDA receptor. Initially, there is an increase of NMDA mediated currents. But hypofunction emerges, the receptor appears to be internalised and vital functions such as long-term potentiation (LTP), which are essential for cognition, are lost. [see link]

Anti-NMDA receptor encephalitis

Symptoms and signs

Anti-NMDA receptor encephalitis was first described in young women with underlying ovarian tumours. But cases in males, in children and non-tumour cases are well documented. In about 20% of presentations, neuropsychiatric symptoms are preceded by a flu-like illness. Early symptoms in adults are psychosis (hallucinations, delusions and bizarre behaviour), cognitive impairment (confusion, memory dysfunction, dysphasia), and seizures. Over days to weeks additional neuropsychiatric features emerge; movement disorder (choreoathetoid, myoclonus, parkinsonism, rigidity), autonomic instability (tachy/bradycardia, labile BP, hypersalivation, central hypoventilation) and reduced levels of consciousness [full paper].

Investigations

In terms of investigation, CSF lymphocytosis, CSF oligoclonal bands, EEG slowing and epileptiform potentials can be found. The MRI scan is usually normal. The diagnosis is clinched by the presence of CSF IgG antibodies against the NR1 subunit of the NMDA receptor.

Treatment

The treatment of choice is immunotherapy (IV steroids, IV immunoglobulin, plasma exchange) – as well as tumour resection. A good outcome is associated with a decrease in NMDA receptor antibody levels. In some patients the recovery is prolonged, and 2nd line immunotherapies are required. Interestingly, many patients have also responded well to modified ECT.

NMDA receptor autoantibodies & Schizophrenia?

There has been recent interest in the possibility that many cases of diagnosed schizophrenia may actually be alternative forms of anti-NMDA receptor encephalitis. But the evidence for this is not convincing. In a Spanish study of 80 patients, no cases were positive for anti-NMDA receptor IgG antibodies. In a larger study from Germany (approx 450 acute patients), there was an excess of anti-NMDA receptor antibodies in acute schizophrenia (10%) versus major depression (3%). But these were not the IgG antibodies against the NR1 subunit, which is the defining feature of NMDA receptor encephalitis. Instead there were IgA and IgM antibodies against the NR1 and NR2 subunits. The significance of these antibodies is not entirely clear, especially as they were also found in healthy controls (0.4%). Are they a marker of a prior insult against the NMDA receptor or an incidental finding? – A question which will now attract much research.