How are the mole rats able to feel aches if they lack Substance P?

[Q_Goest]

I’m trying to understand some research about naked mole rats. Apparently they don’t feel pain.

The blind, furless creatures that live underground in colonies lack a body chemical called Substance P, a neurotransmitter normally in the skin that sends pain signals to the central nervous system. The rats feel no immediate pain when cut, scraped or subjected to heat stimuli. They only feel some aches.

Ref: Science Daily, http://www.sciencedaily.com/releases/2003/11/031117073925.htm

Can anyone give me some background here? How is substance P manufactured by the body? In what cells is it made? How is it used to transmit the ‘pain’ signal?

I would have imagined that nerve cells relay a signal to the brain and that signal is interpreted as pain because it is received at a very specific location in the brain. Sounds like this model is overly simplified at best, and perhaps totally wrong. What’s the correct model here?

There’s also a journal article online about the experiments done here:
http://biology.plosjournals.org/perlserv/?request=get-document&doi=10.1371%2Fjournal.pbio.0060013&ct=1

 

[Q_Goest]

Exploiting a cold sore virus as a genetic ferry of sorts, the researchers tweaked the germ to carry a bit of DNA into the naked mole rats that would cause their nerves to produce substance P.

Ref: http://abcnews.go.com/Health/PainManagement/Story?id=4211942&page=2

Substance P is produced by nerve cells. I take it there is a gene in DNA which encodes the instructions for making substance P. Is that correct?

 

[Moonbear]

Yes, substance P is produced in neurons, and there's a gene encoding it's production (just as there's a gene for the production of all substances in cells). I'm not sure if I would assume that a lack of substance P would eliminate ALL pain perception though. It is ONE of the neurotransmitters involved in pain perception, but not by far the only one. And, indeed, the actual scientific article does not say they experience no pain, but only lack responses to SOME painful stimuli. They also lack TWO peptides associated with pain perception...SubP and CGRP.

And, to be most technically correct, we have to say they lack a response to pain. We cannot know what they are feeling to know if they feel the pain, only that they don't react in an overt, measurable way to it. This could mean they don't feel it, or it could mean they feel it but don't show that they feel it...this would be a good adaptive response for a prey animal if they don't appear injured when they are so predators don't pick them out so quickly.

 

[Math Is Hard]

Yes, substance P is produced in neurons, and there's a gene encoding it's production (just as there's a gene for the production of all substances in cells). I'm not sure if I would assume that a lack of substance P would eliminate ALL pain perception though. It is ONE of the neurotransmitters involved in pain perception, but not by far the only one. And, indeed, the actual scientific article does not say they experience no pain, but only lack responses to SOME painful stimuli. They also lack TWO peptides associated with pain perception...SubP and CGRP.

I have read about studies of special "knockout" mice that lack either the gene for the precursor to substance P or the gene for the substance P receptor. The interesting thing about these mice is that they are unresponsive to certain kinds of intense pain, but still respond to mildly painful stimuli.

 

[Q_Goest]

Thanks Moonbear,

I'm not sure if I would assume that a lack of substance P would eliminate ALL pain perception though.

You're right, they do feel pain. The title of the thread sensationalizes the fact they don't sense a certain type of pain. So I should make sure readers understand that it is only acid, capsaicin and heat (please verify) they seem to be immune to.

From the paper:

Naked mole-rats show a unique and remarkable lack of pain-related behaviors to two potent algogens, acid and capsaicin. Furthermore, when exposed to inflammatory insults or known mediators, naked mole-rats do not display thermal hyperalgesia.

Ref: http://biology.plosjournals.org/perlserv/?request=get-document&doi=10.1371%2Fjournal.pbio.0060013&ct=1

And, to be most technically correct, we have to say they lack a response to pain. We cannot know what they are feeling to know if they feel the pain, only that they don't react in an overt, measurable way to it. This could mean they don't feel it, or it could mean they feel it but don't show that they feel it...this would be a good adaptive response for a prey animal if they don't appear injured when they are so predators don't pick them out so quickly.

Interesting point. I don't think it's applicable in this case however. It seems they genuinely don't feel the affect of acid and capsaicin. Seems to be compelling circumstantial evidence that pain is not experienced, namely that they behave as if feeling pain when Substance P is produced by the nerve cells (after the virus tweaks the DNA in the nerve cells). Is the DNA in the nerve cells actually being modified then? By introducing this virus, are some cells in the rat's bodies actually being altered to encode the gene to produce substance P?

They also lack TWO peptides associated with pain perception...SubP and CGRP.

Can you elaborate? I assume SubP is an abreviation for substance P but what is CGRP and what does it do?

Also, it seems from reading the article that the 'hardware' is available in these rats to sense pain. If I'm not mistaken, they talk about C-fibers not being connected to the spinal cord the same way they are connected in other mammals. I assume "C-fibers" to be bundles of nerve cells (ie: neurons) that connect sensing nerve cells to the spinal column, but these bundles don't connect to some location in the spinal column as 'normal'. Not sure if that's correct or not.

 

[Moonbear]

Interesting point. I don't think it's applicable in this case however. It seems they genuinely don't feel the affect of acid and capsaicin. Seems to be compelling circumstantial evidence that pain is not experienced, namely that they behave as if feeling pain when Substance P is produced by the nerve cells (after the virus tweaks the DNA in the nerve cells).

The reason I point it out is that it's possible it is modifying the behavioral response, not the perception. But, as the mole rats showed a normal response to other stimuli (i.e., in the formalin test), you're right, it shows it's not a behavioral/motor deficit, but more likely a sensory deficit.

Is the DNA in the nerve cells actually being modified then? By introducing this virus, are some cells in the rat's bodies actually being altered to encode the gene to produce substance P?

Yes, that's what the virus is doing, it's inserting the gene for the precursor for Substance P so they make it. There is a flaw in this, because the gene can only encode the precursor, not the mature peptide. Pre-protachykinin can be alternatively spliced to also produce Neurokinin A as well as Substance P, so one cannot completely rule out an effect of Neurokinin A in this model...it could potentially be compensatory for the improperly functioning Substance P system, and they will hopefully follow up on this (unless I missed something in the article that rules out Neurokinin A).

Can you elaborate? I assume SubP is an abreviation for substance P but what is CGRP and what does it do?

Yes, SubP is substance P. CGRP is calcitonin gene-related peptide. I picked that up from the introduction to the article. I don't know much else about it. They mention it in the same sentence as Substance P and don't elaborate on the differences in their functions, just that both are involved in cutaneous sensations.

Also, it seems from reading the article that the 'hardware' is available in these rats to sense pain. If I'm not mistaken, they talk about C-fibers not being connected to the spinal cord the same way they are connected in other mammals. I assume "C-fibers" to be bundles of nerve cells (ie: neurons) that connect sensing nerve cells to the spinal column, but these bundles don't connect to some location in the spinal column as 'normal'. Not sure if that's correct or not.

C-fibers are unmyelinated, sensory nerve fibers, and they are involved in the sensation of things like burns. The other cutaneous fiber types are A fibers (two subtypes), which are myelinated. But, in the article, they are saying C-fibers are not as prevalent as in other mammals, so the "hard wiring" may NOT be present.

In isolated C-nerve fibers, they did find receptor firing rates similar to those in those isolated from mice, so that indicates that the fibers themselves CAN respond to capsaicin, but somehow, this is not transmitted beyond the sensory nerves in the whole animal.

I.E.:

Considering the naked mole-rats' behavioral insensitivity to capsaicin, we were surprised to find that approximately 40% of CMH and CM fibers respond robustly to 1 mM capsaicin (Figure 4A–4C), the same concentration of capsaicin used in the behavioral experiments. The rates of nociceptor firing following excitation by capsaicin were in the range of those evoked by noxious thermal and mechanical stimuli (Figure 2).

On the other hand, the response to acid was a deficit in neuronal response...they got no firing activity in response to acid in the isolated fibers.

So, they explored the capsaicin response further by looking at the anatomy of the C-fibers, rather than just their phyisiological/pharmacological responses. That's where they found anatomical differences in where these fibers form functional connections with the spinal cord, with more functional connections with the deep dorsal horn in the mole rats than in mice (they are using mice for the comparison throughout). So, while the lack of response to acid may be physiological, the lack of response to capsaicin may be more anatomical.

 

[Q_Goest]

Hi Moonbear. Thanks for the help on this. I have no background in biology to speak of so these questions may seem fairly obvious but hope you can bear with me.

Yes, that's what the virus is doing, it's inserting the gene for the precursor for Substance P so they make it. There is a flaw in this, because the gene can only encode the precursor, not the mature peptide. Pre-protachykinin can be alternatively spliced to also produce Neurokinin A as well as Substance P, so one cannot completely rule out an effect of Neurokinin A in this model...it could potentially be compensatory for the improperly functioning Substance P system, and they will hopefully follow up on this (unless I missed something in the article that rules out Neurokinin A).

In laymen’s terms, I think you’re saying that the virus delivered a gene which does not directly encode SubP. Instead, the delivered gene creates a molecule that is used in the production of SubP. That molecule is called pre-protachykinin, and it is also catagorized as a “precursor” as it is used to make subP. But this molecule can also be used to make Neurokinin A. Did I get that correct?

I’m having a very hard time reading through the PLoS article, but from what I gather, I believe they verified that SubP was being manufactured after at least 1 week after virus injection. I also understand the virus had 'migrated' up the various nerve cells to the spinal column after about a week, but had not spread the SubP gene to other limbs, so only one limb was affected. If the rat cells also manufactured Neurokinin A, I think what you’re saying is that potentially it is this other molecule, Neurokinin A, that is allowing for the nerve cells to signal pain, and not SubP. I guess you’re suggesting they should have verified Neurokinin A was not being manufactured also or that it didn’t have an affect, is that right?

C-fibers are unmyelinated, sensory nerve fibers, and they are involved in the sensation of things like burns. The other cutaneous fiber types are A fibers (two subtypes), which are myelinated.

Why are they unmyelinated? Does this allow for better sensitivity or something? Are the myelinated fibers only transmitting the signal and not sensing then? Sorry for being such a novice here.

So, they explored the capsaicin response further by looking at the anatomy of the C-fibers, rather than just their phyisiological/pharmacological responses. That's where they found anatomical differences in where these fibers form functional connections with the spinal cord, with more functional connections with the deep dorsal horn in the mole rats than in mice (they are using mice for the comparison throughout). So, while the lack of response to acid may be physiological, the lack of response to capsaicin may be more anatomical.

Good. This is what I wanted to eventually get to. The anatomical differences. I assume “deep dorsal horn” is part of the spinal column, or attached to it, and it is at this deep dorsal horn that the signals from the nerves end up. Is that right? When you say the lack of capsaicin response may be more anatomical, does that mean that there is a signal produced by the nerve cells (sensory cells if that’s the correct term) that simply doesn’t go to the brain (through the deep dorsal horn) in some way which is ‘normal’ to other mammals? Perhaps these nerve cells are connected differently to the deep dorsal horn in other mammals. If that's true, I wonder how one can even tell and where these signals are going when they get to the brain. Or is there no signal produced because there is no subP, but everything else in the animal is functionally identical to other rats? What exactly is the anatomical difference? I'm so confused!