Axotomy depletes intracellular calcium stores in primary sensory neurons.
BACKGROUND
The cellular mechanisms of neuropathic pain are inadequately understood. Previous investigations have revealed disrupted Ca signaling in primary sensory neurons after injury. The authors examined the effect of injury on intracellular Ca stores of the endoplasmic reticulum, which critically regulate the Ca signal and neuronal function.


METHODS
Intracellular Ca levels were measured with Fura-2 or mag-Fura-2 microfluorometry in axotomized fifth lumbar (L5) dorsal root ganglion neurons and adjacent L4 neurons isolated from hyperalgesic rats after L5 spinal nerve ligation, compared to neurons from control animals.


RESULTS
Endoplasmic reticulum Ca stores released by the ryanodine-receptor agonist caffeine decreased by 46% in axotomized small neurons. This effect persisted in Ca-free bath solution, which removes the contribution of store-operated membrane Ca channels, and after blockade of the mitochondrial, sarco-endoplasmic Ca-ATPase and the plasma membrane Ca ATPase pathways. Ca released by the sarco-endoplasmic Ca-ATPase blocker thapsigargin and by the Ca-ionophore ionomycin was also diminished by 25% and 41%, respectively. In contrast to control neurons, Ca stores in axotomized neurons were not expanded by neuronal activation by K depolarization, and the proportionate rate of refilling by sarco-endoplasmic Ca-ATPase was normal. Luminal Ca concentration was also reduced by 38% in axotomized neurons in permeabilized neurons. The adjacent neurons of the L4 dorsal root ganglia showed modest and inconsistent changes after L5 spinal nerve ligation.


CONCLUSIONS
Painful nerve injury leads to diminished releasable endoplasmic reticulum Ca stores and a reduced luminal Ca concentration. Depletion of Ca stores may contribute to the pathogenesis of neuropathic pain.
Rigaud
Marcel
M
Department of Anesthesiology, Medical College of Wisconsin, Milwaukee, Wisconsin, USA.
Gemes
Geza
G
Weyker
Paul D
PD
Cruikshank
James M
JM
Kawano
Takashi
T
Wu
Hsiang-En
HE
Hogan
Quinn H
QH
D000818:Animals; D001369:Axons / Q000473:pathology; D019771:Axotomy; D002110:Caffeine / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism*; D020013:Calcium Signaling / Q000502:physiology*; D002211:Capsaicin / Q000494:pharmacology; D002478:Cells, Cultured; D000697:Central Nervous System Stimulants / Q000494:pharmacology; D004721:Endoplasmic Reticulum / Q000502:physiology; D006930:Hyperalgesia / Q000473:pathology; D015759:Ionomycin / Q000494:pharmacology; D008026:Ligation; D008297:Male; D009410:Nerve Degeneration / Q000473:pathology; D010147:Pain Measurement; D051381:Rats; D017207:Rats, Sprague-Dawley; D019837:Ryanodine Receptor Calcium Release Channel / Q000187:drug effects; D012584:Sciatic Nerve / Q000293:injuries; D011984:Sensory Receptor Cells / Q000378:metabolism* / Q000502:physiology*; D013127:Spinal Nerves / Q000293:injuries; D019284:Thapsigargin / Q000494:pharmacology
D000697:Central Nervous System Stimulants; D019837:Ryanodine Receptor Calcium Release Channel; D002110:Caffeine; D015759:Ionomycin; D019284:Thapsigargin; D002211:Capsaicin; D002118:Calcium