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The pattern of rise and fall of intracellular Ca2+ levels ([Ca2+]c) initiated by depolarization-induced Ca2+ influx (ICa) is regulated by complex interactions of plasmalemmal Ca2+ influx, intracellular buffering, uptake, release from intracellular stores, and extrusion mechanisms.1 The resulting Ca2+ transient regulates neuronal function, including cell development, excitability, neurotransmitter release, genetic expression, and apoptotic cell death.2 Because the rise in [Ca2+]c induced by neuronal activity lasts seconds compared with the few-millisecond duration of individual action potentials, the Ca2+ transient provides a time-integrated signal encoding neuronal activity history.3 Through these mechanisms, altered Ca2+ dynamics play a central role in pathologic processes in neurologic disease. Critical changes in the intracellular Ca2+ signal of primary sensory neurons contribute to hyperalgesia after peripheral nerve injury. Specifically, influx of Ca2+ through high- and low-voltage–activated Ca2+ channels is reduced after chronic constriction of the sciatic nerve4 or ligation of the fifth lumbar (L5) spinal nerve.5 Further, resting [Ca2+]c is decreased6 and the activity-induced Ca2+ transient is disrupted7 after experimental peripheral nerve injury accompanied by hyperalgesic behavior in rats. Primary sensory neurons of the dorsal root ganglia (DRGs) express a variety of voltage-gated Ca2+ channel subtypes that can be distinguished according to their pharmacologic and electrophysiologic properties.8–10 The various channels that conduct L, N, P/Q, R, and T subtypes of ICa are variably expressed in DRG neurons of different sizes.11,12 In addition, the subtypes are functionally diverse and linked to specific Ca2+ regulated processes, partly on the basis of subcellular colocalization.13 We expect that the different subtypes likewise do not contribute equally to the generation of the activity-induced intracellular Ca2+ transient, although this has not been examined in sensory neurons. Further, developing treatments for neuropathic pain would be aided by understanding how injury affects individual Ca2+ channel subtypes in generating the Ca2+ signal. We therefore examined the sensitivity to subtype-specific Ca2+ channel blockers of activity-induced Ca2+ transients. Using digital microfluorometry, dissociated DRG neurons from control animals were compared with neurons from animals demonstrating hyperalgesic behavior after peripheral nerve trauma by spinal nerve ligation (SNL). This model allows separate study of L5 DRG neurons that are injured by axotomy versus L4 neurons exposed to inflammatory mediators induced by wallerian degeneration of adjacent L5 fibers in the shared sciatic nerve.14,15 |
