Questions And Answer Session

DR. MEYERHOFF: All right. Thank you very much. I am pleased to share my time with my esteemed colleague from Walter Reed, Dr. Debra Yourick, who is going to describe some of our in vitro experiments, and then I will describe some in vivo experiments on spinal injury.

While Dr. Yourick is setting up, I wanted to comment on one of Dr. Namboodiri's questions to the previous speaker, with respect to mechanism. The last speaker cited an experiment by Gadge et al., using a rodent ALS variant that is an SOD transgene.

In neurons cultured from that species, there was neuro protection achieved with 2PMPA. Dr. Slusher was the co-author on that paper by Gadge. They tested the mechanism by administering the mGluR3 antagonist ethylglutamate, eGlu, and that did not reverse the PMPA amelioration and increase survival of these neurons in culture with the SOD defect.

They also applied antibodies to transforming growth factor Beta, and that also did not reverse the ameliorative effect of 2PMPA, which is very interesting in terms of the ongoing discussions about whether the primary therapeutic effect is at the presynaptic mGluR3 or whether it is on glutamate levels or whether it is at the NMDA receptor, and it may be that these things are somewhat model specific. But I just wanted to mention that, because if Dr. Slusher had been able to be here, I imagine she might have brought it up in terms of mechanisms, a very interesting aspect of that experiment.

DR. WEINBERGER: Has there actually been any pharmacologic characterization of NAAG as a partial agonist per se? Has it been actually characterized as a partial agonist?

DR. YOURICK: Yes, but not a partial agonist.

DR. COYLE: I think we have to be careful. I mean, we have just been working like for five years on this Schaffer collateral CA1 synapse, and done it five ways from Sunday. At the concentrations used at that synapse, it is a noncompetitive glycine reversible antagonist.

Now that is not seen at the granule cells in the cerebellum, and in the whatchamacallit, in the hippocampus -- no, not CA1, the area you all looked. But that is a gyrus. LTD in the dentate gyrus, the mGluR3 effect predominates. But, actually, the work done in 1986 on primary cultures of motor neurons -- there it was shown to have a partial agonist effect or a weak agonist effect.

One problem with tissue cultures in trying to tease this out is the influence of high levels of glycine in the culture medium, if it works by a glycine reversible effect. So it may be very dependent on the NR2 subunit and other factors in terms of how it behaves at different sites.

DR. LIEBERMAN: That was my question, is whether or not it had been characterized as the NMDA receptor there versus the NMDA receptors in other places where you don't see any NAAG effects on that receptor.

DR. WROBLEWSKA: There was some work done on the NMDA receptors -- but in our hands, NAAG is -- it is not an antagonist NMDA receptor of the granular type -but it has some other effects in very high concentrations.

DR. COYLE: Right. The CA1 Schaffer collateral synapse, as far as we can see, has no influence of mGluR3 receptors.

DR. WROBLEWSKA: I have a question about the ALS. I mean, protective effect in the ALS are 2-PMPA receptor. Right?

DR. MEYERHOFF: If you mean in the in vitro model by Gadge, et al., on which Barbara Slusher was a co-author, they just suggested it wasn't acting through the presynaptic mGluR3. But that leaves the possibility that the drug could have simply reduced synaptic glutamate availability through inhibition of the enzyme or that it could have had an effect through the NMDA receptor. But the paper, as I recall, doesn't really address what the mechanism is, except to say what it doesn't appear to be. That was just in the 2-PMPA paper that was done in cultured rat neurons that had the transgene.

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