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 Ca2+ stores of the endoplasmic reticulum, which critically regulate the Ca2+ signal and neuronal function. Methods: Intraceflular Ca2+ 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 Ca2+ stores released by die ryanodine-receptor agonist caffeine decreased by 46% in axotomized small neurons. This effect persisted in Ca2+-free bath solution, which removes the contribution of store-operated membrane Ca2+ channels, and after blockade of the mitochondrial, sarco-endoplasmic Ca2+-ATPase and the plasma membrane Ca2+-ATPase pathways. Ca2+ released by the sarco-endoplasmic Ca2+-ATPase blocker thapsigargin and by the Ca2+-ionophore ionomycin was also diminished by 25% and 41%, respectively. In contrast to control neurons, Ca2+ stores in axotomized neurons were not expanded by neuronal activation by K+ depolarization, and the proportionate rate of refilling by sarco-endoplasmic Ca2+-ATPase was normal. Luminal Ca2+ 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 Ca2+ stores and a reduced lutninal Ca" concentration. Depletion of Ca2+ stores may contribute to the pathogenesis of neuropathic pain.