Chronic constriction injury of sciatic nerve induces the up-regulation of descending inhibitory noradrenergic innervation to the lumbar dorsal horn of mice.

Peripheral nerve injury in rodents results in hypersensitivity to mechanical and thermal stimuli accompanied by reduced antinociceptive efficacy of opioids and, in some models, sensitivity to sympathetic blockade. alpha2-Adrenergic receptor agonists increase in potency and efficacy after nerve injury in rodents and effectively relieve neuropathic pain in humans who do not get pain relief from opioids. However, the underlying mechanisms are unclear. It has been well known that the major noradrenergic innervation of the spinal dorsal horn originates from the locus coeruleus nucleus (LC) in the brainstem. Therefore, the aim of this study is to examine whether peripheral nerve injury that causes neuropathic pain modulates the noradrenergic innervation to the lumbar dorsal horn, in order to determine the possible anatomical substrates underlying increased potency and efficacy of noradrenergic receptor agonists in alleviating neuropathic pain. At 2 weeks after chronic constriction injury (CCI) of the sciatic nerve, a remarkable increase in tyrosine-hydroxylase (TH) and dopamine beta-hydroxylase (DbetaH) immunoreactive (IR) axonal terminals was observed in the ipsilateral L4-L6 dorsal horn. Consistently, greater numbers of both TH- and DbetaH-IR neurons were detected in the ipsilateral LC. Interestingly, in the lower lumbar and upper sacral spinal dorsal horn, numerous TH-IR neurons were observed in the superficial dorsal horn (primarily lamina I). CCI of the sciatic nerve did not change the number of these TH-IR cells. These findings suggest that augmented descending inhibitory noradrenergic innervation to the dorsal horn could be one of the mechanisms underlying the increased effectiveness in the anti-allodynic effect elicited by alpha2-adrenergic receptor agonists.