Spinal Noradrenergic Sprouting after Nerve Injury
Neuropathic pain remains an unmet medical need, despite recent advances in our understanding of the pathophysiology of central and peripheral sensitization. But nerve injury can also increase the analgesic efficacy of some drugs, including spinally administered 12-adrenoceptor agonists. In exploring the mechanisms underlying this increased efficacy, we have discovered that peripheral nerve injury results in sprouting of noradrenergic fibers in the spinal cord at the dermatomes receiving inputs from injury. Preliminary data suggest that spinal noradrenergic sprouting after peripheral nerve injury is due to brain derived growth factor (BDNF), and Specific Aim 1 will examine mechanisms for bilateral sprouting of spinal noradrenergic fibers at the segments of peripheral nerve injury, with focus on glia as a source of BDNF and TrkB signaling as a cause of sprouting. Few therapies are effective to treat neuropathic pain, and most of these mimic or augment the activity of the release of norepinephrine in the spinal cord. Preliminary data demonstrate that neuropathic pain is associated with a novel action of spinal norepinephrine to activate local spinal cholinergic circuits for analgesia. Specific Aim 2 will examine mechanisms for novel excitatory actions of 12-adrenoceptors on spinal cholinergic neurons after peripheral nerve injury, with focus on 12-adrenoceptor subtype and G protein species activated. These changes in anatomy and circuitry of descending inhibition suggest novel approaches to the treatment of neuropathic pain. We present preliminary data which show that this pathway can be activated by gabapentin. Since norepinephrine also stimulates spinal acetylcholine release for analgesia after nerve injury, this pathway could be augmented by norepinephrine transporter and cholinesterase inhibitors. Exciting new preliminary data in humans suggests that the cholinesterase inhibitor, donepezil (Aricept.) provides analgesia and improves cognition in patients with chronic neuropathic pain. Specific Aim 3 will test in animals and patients the therapeutic consequences of noradrenergic sprouting, using clinically available, oral drugs. These studies will improve our understanding and provide practical guidance for better treatment of neuropathic pain. PUBLIC HEALTH RELEVANCE: Nerve injury, whether from cancer, diabetes, or trauma, can result in chronic pain which is difficult to treat, even with narcotics. These studies in rats and in people with chronic pain will explore new ways to use medicines to help the body's own pain relieving system to work better to relieve pain.