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