Contribution of Calcium Channel Subtypes to the Intracellular Calcium Signal in Sensory Neurons: The Effect of Injury

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Background:Although the activation-induced intracellular Ca2+ signal is disrupted by sensory neuron injury, the contribution of specific Ca2+ channel subtypes is unknown.Methods:Transients in dissociated rat dorsal root ganglion neurons were recorded using fura-2 microfluorometry. Neurons from control rats and from neuropathic animals after spinal nerve ligation were activated either by elevated bath K+ or by field stimulation. Transients were compared before and after application of selective blockers of voltage-activated Ca2+ channel subtypes.Results:Transient amplitude and area were decreased by blockade of the L-type channel, particularly during sustained K+ stimulation. Significant contributions to the Ca2+ transient are attributable to the N-, P/Q-, and R-type channels, especially in small neurons. Results for T-type blockade varied widely between cells. After injury, transients lost sensitivity to N-type and R-type blockers in axotomized small neurons, whereas adjacent small neurons showed decreased responses to blockers of R-type channels. Axotomized large neurons were less sensitive to blockade of N- and P/Q-type channels. After injury, neurons adjacent to axotomy show decreased sensitivity of K+-induced transients to L-type blockade but increased sensitivity during field stimulation.Conclusions:All high-voltage–activated Ca2+ current subtypes contribute to Ca2+ transients in sensory neurons, although the L-type channel contributes predominantly during prolonged activation. Injury shifts the relative contribution of various Ca2+ channel subtypes to the intracellular Ca2+ transient induced by neuronal activation. Because this effect is cell-size specific, selective therapies might potentially be devised to differentially alter excitability of nociceptive and low-threshold sensory neurons.

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