Abstract
Neuropathic pain results from damage to the peripheral sensory nervous system, which may have a number of causes. The calcium channel subunit alpha(2)delta-1 is upregulated in dorsal root ganglion (DRG) neurons in several animal models of neuropathic pain, and this is causally related to the onset of allodynia, in which a non-noxious stimulus becomes painful. The therapeutic drugs gabapentin and pregabalin (PGB), which are both alpha(2)delta ligands, have antiallodynic effects, but their mechanism of action has remained elusive. To investigate this, we used an in vivo rat model of neuropathy, unilateral lumbar spinal nerve ligation (SNL), to characterize the distribution of alpha(2)delta-1 in DRG neurons, both at the light- and electron-microscopic level. We found that, on the side of the ligation, alpha(2)delta-1 was increased in the endoplasmic reticulum of DRG somata, in intracellular vesicular structures within their axons, and in the plasma membrane of their presynaptic terminals in superficial layers of the dorsal horn. Chronic PGB treatment of SNL animals, at a dose that alleviated allodynia, markedly reduced the elevation of alpha(2)delta-1 in the spinal cord and ascending axon tracts. In contrast, it had no effect on the upregulation of alpha(2)delta-1 mRNA and protein in DRGs. In vitro, PGB reduced plasma membrane expression of alpha(2)delta-1 without affecting endocytosis. We conclude that the antiallodynic effect of PGB in vivo is associated with impaired anterograde trafficking of alpha(2)delta-1, resulting in its decrease in presynaptic terminals, which would reduce neurotransmitter release and spinal sensitization, an important factor in the maintenance of neuropathic pain.