Painful nerve injury shortens the intracellular Ca2+ signal in axotomized sensory neurons of rats.

Neuropathic pain is a maladaptive, persistent pain syndrome that results from damage to the peripheral or central nervous system.1 It accompanies diverse diseases such as diabetes mellitus, herpes zoster, intervertebral disc herniation, or trauma and is difficult to treat.2 A hallmark of neuropathic pain is hyperexcitability of peripheral and central nociceptive pathways. Patients report spontaneous pain, allodynia (pain from stimuli at intensities that normally do not produce pain), or hyperalgesia (greater pain from a normally painful stimulus), which result from complex pathologic processes at the site of injury, the dorsal root ganglion (DRG), the spinal cord, and the brain. Substantial membrane modifications have been identified in primary sensory neurons in different animal models of neuropathic pain. Loss of current through potassium channels,3 altered expression of various sodium channel isoforms,4 and diminished influx through plasmalemmal voltage-gated calcium channels5 produce electrical abnormalities of injured sensory neurons that contribute to hyperexcitability and pain.6,7 The level of cytoplasmic calcium ([Ca2+]c) is closely regulated by a complex system of buffers, pumps, and release mechanisms that shape the pattern of rise and fall of [Ca2+]c (the Ca2+ transient) initiated by depolarization-induced calcium influx. The duration of such [Ca2+]c events are considerably prolonged compared with membrane depolarization, with durations of the transient spanning many seconds compared with the few-millisecond duration of an action potential (AP).8 In addition, the amplitude and duration of the Ca2+ transient expand in response to repetitive neuronal AP generation. Therefore, the [Ca2+]c signal provides a cellular integration and memory function. Calcium transients critically regulate diverse neuronal functions, including excitability, kinase activity, neurotransmitter release, genetic expression, and apoptotic cell death.9 Our previous data have identified reduced Ca2+ influx through high- and low-voltage-activated Ca2+ channels of primary sensory neurons after painful peripheral nerve injury.5,10,11 We have further observed that resting Ca2+ levels are lower in sensory neurons after injury.12 The influence of these alterations on depolarization-induced Ca2+ transients is unknown. We hypothesized that the documented injury-induced loss of inward Ca2+ flux across the plasmalemma will lead to diminished amplitude and duration Ca2+ transients in sensory neurons. We therefore used digital microfluorometry to examine Ca2+ transients in dissociated DRG neurons from control animals and animals with hyperalgesia after peripheral nerve trauma by spinal nerve ligation (SNL). This model allows separate examination of neurons that are injured by axotomy in the fifth lumbar (L5) ganglion versus L4 neurons exposed to inflammatory mediators induced by wallerian degeneration13,14 of L5 fibers in the distal nerve. Both field stimulation and depolarization by K+ elevation were used to trigger Ca2+ transients, and neurons were categorized by size and sensitivity to capsaicin as a means of segregating predominantly nociceptive and nonnociceptive neurons.