Tuesday, May 28, 2013

"Itch molecule" discovery a big step forward

All over the media last week was the news that two scientists at the National Institutes of Health in Bethesda, MD had discovered “the molecule responsible for itch.”

This molecule, “Nppb,” relays signals from certain neurons that detect itch in the skin to other neurons that carry the signals up the spinal cord to the brain. The scientists, Santosh Mishra and Mark Hoon, engineered mice in which the gene for Nppb had been turned off. The mice could not, apparently, feel itch.

The media hype is evident. Nppb is not THE molecule responsible for itch. Several molecules are known to be involved in detecting itch in the first place, and we know many others must be involved in the signaling pathway.

What is remarkable, though, is that the scientists were able to define a model for how itch gets from the skin to the spinal cord.
Mishra and Hoon's model of how neurons carry the itch signal. (Fig 4G from their Science paper.)

We now know that there are at least two pinch points: the synapse across which Nppb carries the signal, and a second downstream synapse across which another molecule, GRP, sends the information to the next stage of neurons.

Blocking the receptors for Nppb or GRP would seem to be a prime candidate for an anti-itch therapy. 

But, of course, there are complications. Nppb was originally known because it is important in the heart, where it controls blood pressure. GRP controls digestion. The genetically engineered Nppb-free mice died early. (The scientists said so in their media interviews.) 

So you can’t just take a pill that blocks Nppb receptors everywhere. That would be a disaster.

But this kind of restriction on where a drug can act is well-known in pharmacology. That’s why, e.g., I can use the anti-pain Voltaren gel (diclofenac) safely by rubbing it into my joints, but diclofenac is known to be pretty toxic if you swallow it.

You can’t design an Nppb receptor-blocking topical cream, because the important synapses are in the spinal cord. A cream would only be effective on the surface.

But it might be possible to take a pill that blocks Nppb only in the spinal cord. I’m not sure how, but that’s what major pharma companies are paying their scientists the big bucks to find out. Maybe the receptors in the spinal cord are subtly different than those elsewhere in the body.

This is very exciting stuff. The massive question is whether the work applies to humans. I would expect it did. Mice and human immune systems are quite different, but our nervous systems are not. We most likely have an analog of Nppb that carries our itch signals.

Just to put this in context—the new work tells us substantially more about itch signaling than previous work in the field. I’d been aware of studies that had identified a class of itch neurons, or certain molecules important in detecting itch in the skin, but this research builds on those foundations in a big way.

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