Precise Neuronal Mechanisms of Spinal Dynorphin in Chronic Pain

Abstract

Opioid analgesics are highly effective for acute pain; however, tolerance and addiction liability limit their long-term usefulness. Although novel, non-addictive, approaches for addressing chronic pain have been developed, most acutely combat chronic pain symptomologies or engage targets within discrete areas in the pain circuitry such as peripheral ganglia, which limits their efficacy. Thus, there is a critical need to identify targets that can effectively combat or reverse the multifaceted pathophysiology of chronic pain. Dynorphin A (1-17) (DynA17), and its degradation products, are novel targets by which therapeutics could produce immediate and long-term chronic pain relief. DynA17 protein upregulation in the spinal dorsal horn (SDH) and limbic areas results in sensitization of nociceptive pathways and negative affect, respectively, through kappa opioid receptor (KOR) and non-opioid receptor mechanisms. We hypothesize that peripheral injury-induced upregulation of DynA17 in the spinal dorsal horn (SDH) results in temporo-spatial enhancement of excitatory neuronal activity and synaptic drive contributing to allodynia, and that blocking DynA17 and fragments will reverse neuronal pathophysiology and associated pain behaviors. We utilize a combination of targeted DynA17 inhibition with commercially available antibodies, intrathecal DynA17 peptide injection or in vitro application, patch-clamp electrophysiology, immunohistochemistry, and genetic mouse models. We demonstrate that injection of anti-DynA antibody alleviates mechanical allodynia in the spared nerve injury (SNI) model of neuropathic pain in mice at 4 weeks post-injury. Intrathecal DynA17 peptide injection to mice elicits chronic pain, but only following a ‘double hit' strategy. In addition, current clamp recordings reveal that in vitro pre-treatment of dorsal root ganglia (DRG) neurons with DynA17 peptide increases the prevalence of spontaneous firing. This work will provide new insights into the discrete spinal cellular and molecular mechanisms by which DynA17 maintains chronic pain.