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J Neurophysiol (January 1, 2003). 10.1152/jn.00371.2002
Submitted on Submitted 16 May 2002; accepted in final form 20 September 2002
Department of Cell Biology, Neurobiology and Anatomy, Medical College of Wisconsin, Milwaukee, Wisconsin 53226-0509
Dirajlal, Sahera,
Laura E. Pauers, and
Cheryl L. Stucky.
Differential Response Properties of IB4-Positive and
-Negative Unmyelinated Sensory Neurons to Protons and Capsaicin. J. Neurophysiol. 89: 513-524, 2003. Activation of unmyelinated (C-fiber) nociceptors by noxious
chemicals plays a critical role in the initiation and maintenance of
injury-induced pain. C-fiber nociceptors can be divided into two groups
in which one class depends on nerve growth factor during postnatal
development and contains neuropeptides, and the second class depends on
glial cell line-derived neurotrophic factor during postnatal
development and contains few neuropeptides but binds isolectin
B4 (IB4). We determined the
sensitivity of these two populations to protons and capsaicin using
whole cell recordings of dorsal root ganglion neurons from adult mouse.
IB4-negative unmyelinated neurons were
significantly more responsive to protons than
IB4-positive neurons in a concentration-dependent
manner. Approximately 86% of IB4-negative
neurons responded to pH 5.0 with an inward current compared with only
33% of IB4-positive neurons. The subtypes of
proton-evoked currents in IB4-negative unmyelinated neurons were also more diverse. Many
IB4-negative neurons exhibited transient, rapidly
inactivating proton currents as well as sustained proton currents. In
contrast, IB4-positive neurons never displayed
transient proton currents and responded to protons only with sustained,
slowly inactivating inward currents. The two classes of neurons also
responded differently to capsaicin. Twice as many naïve
IB4-negative unmyelinated neurons responded to 1 µM capsaicin as IB4-positive neurons, and the
capsaicin-evoked currents in IB4-negative neurons
were approximately fourfold larger than those in
IB4-positive neurons. Interestingly, proton
exposure altered the capsaicin responsiveness of the two classes of
neurons in opposite ways. Brief preexposure to protons increased the
number of capsaicin-responsive IB4-positive
neurons by twofold and increased the capsaicin-evoked currents by
threefold. Conversely, proton exposure decreased the number of
capsaicin-responsive IB4-negative neurons by
50%. These data suggest that IB4-negative
unmyelinated nociceptors are initially the primary responders to both
protons and capsaicin, but IB4-positive
nociceptors have a unique capacity to be sensitized by protons to
capsaicin-receptor agonists.
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