Temperature sensitivity of CNS axons

T
Temperature sensitivity of central axons


Classical work in squid axon reports resting membrane potential is independent of temperature, but our findings suggest that this is not the case for axons in mammalian optic nerve. Refractory period duration changes over 10 times between 37 °C and room temperature, and afterpotential polarity is also acutely temperature sensitive, inconsistent with changes in temperature impacting nerve function only through altered rates of ion channel gating kinetics. Our evidence suggests that the membrane potential is enhanced by warming, an effect reduced by exposure to ouabain. The temperature dependence can be explained if axonal Na+/K+ ATPase continuously expels Na+ ions that enter axons largely electroneutrally, thereby adding a substantial electrogenic component to the membrane potential. Block of the Na+ transporter NKCC1 with bumetanide increases refractoriness, like depolarization, indicating that this is a probable route by which Na+ enters, raising the expectation that the rate of electroneutral Na+ influx increases with temperature and suggesting a temperature-dependent transmembrane Na+ cycle that contributes to membrane potential.

BACKGROUND:
MS symptoms can commonly become worse with small increases in core body
temperature, and the mechanism of these effects (known as Uhthoff’s phenomena)
has not been investigated for some years. Also, the temperature dependence of
normal central nerve fibres is not well described, although understanding the
effects of changing temperature is important because cooling can protect the
brain following trauma or during surgery (eg applied therapeutic hypothermia)
by mechanisms that are incompletely understood.

METHODS:The
studies were carried out on isolated rodent optic nerve in an ex-vivo nerve
bath, for some experiments using a computer-controlled constant-current
stimulator to measure excitability. Other experiments estimated changes in
resting membrane potential in the optic nerve fibres with changing temperature.
Drugs could be applied to the nerves by adding them to the constantly perfusing
buffer solution to either block Na+ entry into or exit from the
axons.

RESULTS:
The amount of time taken to recover from a single impulse is much longer in a
nerve at room temperature than in one at 30 °C (P<0.017) suggesting that the Na+ channels underlying
the action potential take much longer to return to their resting state. This
would be consistent with the cool nerve having a less negative membrane
potential. Estimates of membrane potential change indicate that -40 mV can be
added to the membrane potential by warming from room temperature towards 37 °C
over a period of 1.5 to 2 minutes (P=0.018), and this membrane potential change
can be inhibited by applying ouabain, a drug that blocks the Na+-pump,
where the Na+-pump is the mechanism that normally extrudes Na+
from the axons and makes the membrane potential more negative as it does so.
The Na+-pump is energy hungry. Most metabolic energy used by the
brain goes on moving ions, and importantly Na+ ions, around, so the
activity of this pump is important in understanding the energy requirements of
brain tissue. The drug bumetanide consistently appeared to make the membrane
potential less negative in both rat and mouse warm axons (P=0.016 and
P<0.05, respectively), and we suggest that the ion transporter NKCC1 (that
is blocked by bumetanide), is normally constantly bringing Na+ into
central axons in a manner that is temperature dependent, and that increases
with warming.

CONCLUSION:
Normal mammalian central axon function is acutely sensitive to changes in
temperature, and we propose that it is the way the axons handle movements of Na+
that explains the property. Raising the temperature increases the rate at which
Na+ is transported into axons, and is then subsequently pumped out.
Our estimate of the effect of raising temperature on the resting membrane
potential is a hyperpolarization of around 4 millivolts per °C. Hyperpolarizing
the membrane potential would be expected to make the axons more difficult to
excite, helping to explain why conduction can fail in a temperature dependent
way in MS damaged axons, where impulse conduction may already be compromised by
the disease. The function of the brain NKCC1 transporter may be a useful
target for reducing energy expenditure in central nerve fibres, and so
potentially reducing symptoms associated with conduction failure, and also
protecting them.



CoI: I am the author of this paper

About the author

Mark Baker

11 comments

  • I thought plagiarism was a no-no in academia. This paper was by Mark Baker. In the Mark D Baker an attempt to sound like J K Rowling. Your paper is no Harry Potter. I suppose you have to produce something as Prof G will soon return and will be asking "what the hell have you lot been doing for the last six months?". Only MD2 can hold his / her head up high.

    • The paper is by Mark Baker, a valuable member of Team G and no relation to the esteemed Mouse Doctor of this parish. believe me Prof G is more than happy with our progress over the last 6 months, as you will realise when we post what we've been up to here. it's not only me that can hold their head up high 😉

    • Sorry, I thought Mark D Baker was a pen name for Prof Mouse. Good to see that nepotism is alive and kicking in the was end. Does Mark where a ponytail like his big brother? How many days until the return of the Italian Stallion? I hope he brings you a gift from all the countries he has visited. I also hope that you can convince him that you have done something productive in his absence i.e. more than lunchtime drinks down the local and listening to Big Mouse on his bass guitar (Level 42 tracks). If you are listening Prof G, MD2 has carried the load during you sabbatical. Star in the making. My nomination for MS Researcher of the year (UK MS Society awards). Young Baker also showing promise. 5 days until research day!

    • Thanks for the kind words Anon 5:40. Look forward to meeting you on Saturday. If me and MD had got our act together we could have treated you all to a selection of rock classics (no Level 42), him on bass, me on guitar. Maybe next time 😉

    • Hello. I am one of the authors of this paper (an undergraduate student who did some work with Dr M Baker over the summer).

      With this research we were trying to better understand how axons (the long projections between neurons) in the brain work and how conduction of impulses along these projections is affected by changes in temperature. It is really basic science research, quite far removed from the development of new treatments for MS, but nonetheless the important insights it provides into how axons work may help us develop new treatments for MS in the future.

      For example, bumetanide, the drug we used to block the NKCC1 Na+ transporter in our experiments, appeared to relieve some of the ‘work’ the over-stressed nerve fibres had to do. This drug is already licenced for use as a diuretic (a water tablet) but, as of yet, no-one appears to have investigated its potential to protect neurons from damage in neurological conditions such as MS. ‘Repurposing’ a drug which has already come to market and been proven to be safe would be a way to allow patients to access a drug actually slow the progression of MS quickly and relatively cheaply (as has already been discussed on this blog: http://multiple-sclerosis-research.blogspot.com/search?q=repurpose).

      In addition, we already know that cooling can relieve the symptoms of MS but this research has thrown up some new ideas as to why this may be the case.

      However, it is important to be realistic about all this. We are certainly not (yet) claiming that bumetanide has neuroprotective properties, just suggesting that it might be an interesting avenue to explore. There are still many more experiments to do before this sort of research can be ‘translated’ into something which would help patients.

      I hope this gives you a bit of an idea why this sort of basic science research is important (and may be relevant for patients with MS) and that I have not exaggerated the implications of our findings (Dr Baker)!

    • Nice reply Joseph. Rest assured bumetanide will be investigated for it's neuroprotective potential in our model asap. You certainly haven't exaggerated the potential implications!

  • So is this is a problem related to too great an intake of sodium? If so, can it potentially be neutralised by adding a bit of pepper?

By Mark Baker

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