Nerve injury results in chronic pain following surgery, such as amputation, thoracotomy and mastectomy, as well as in cancer, degenerative conditions and metabolic diseases. The pain is typically intense, persistent and poorly responsive to currently available therapies. Significant progress has been made in understanding the pathophysiology of neuropathic pain, but there has been minimal direct examination of cell membrane and ion channel mechanisms. Increased excitability of neuronal somata of primary afferent neurons is a component of the pain generating process following nerve injury. Intracellular Ca2+ is the dominant second messenger regulating neural activity including electrogenesis, synaptic transmission, gene expression, and cell growth and death, yet no studies of membrane Ca2+ current (ICa) and intracellular Ca2+ levels following nerve injury have been reported. Using tissue from animals showing neuropathic pain behavior following nerve trauma, our novel preliminary findings from whole-cell patch clamp experiments reveal decreased membrane ICa in dorsal root ganglion (DRG) neurons with axons projecting to a sciatic nerve injury site. We have also confirmed in intact DRGs that decreased ICa substantially elevates neuronal excitability.
The aim of this proposal is to examine cellular mechanisms of neuropathic pain by determining the effects of nerve injury on ICa and intracellular Ca2+ in primary afferent neurons that may mediate hyperalgesia. We will employ a clinically relevant model of pain following nerve injury to characterize altered calcium channel function in sensory neurons, identify the channel subtype affected by injury, describe the changes in calcium channel expression with immunocytochemistry, examine intracellular Ca2+ regulation in spatial and temporal detail using Ca2+ microfluorimetry, and demonstrate the effect of decreased Ca2+ flux on membrane excitability in dissociated cells and intact tissue. The proposed studies will test the hypothesis that, in a subgroup of DRG neurons, axonal injury decreases inward Ca2+ current, which in turn decreases intracellular Ca2+ concentration both directly and through diminished Ca2+-induced Ca2+ release. The decrease in intracellular Ca2+ diminishes the Ca 2+-activated K+ current, thereby decreasing membrane afterhyperpolarization and ultimately producing elevated primary afferent excitability. Decreased ICa has not previously been explored as a mechanism of sensory change following nerve injury. This translational research will be valuable in identifying pharmacologically and anatomically specific sites for application of agents to treat neuropathic pain while preserving desired sensory and motor function.
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