Novel intracellular mechanisms of NMDA receptor-dependentudspinal nociceptive plasticity

摘要

Prolonged activation of spinal NMDA receptors, after peripheral inflammation orudnerve damage, can activate intracellular signalling cascades leading to plasticudchanges in synaptic transmission. This results in central sensitization of dorsal hornudsensory neurones and manifests in patients as increased sensitivity to painful stimuliud(hyperalgesia), and pain resulting from normally non-painful tactile stimuli (allodynia).udTherefore, targeting NMDA-mediated intracellular signalling pathways could be audsuccessful analgesic strategy, potentially devoid of side-effects associated withudreceptor blockade. NMDA receptors bind to the intracellular scaffold protein PSD-95,udwhich couples the receptor to cytoplasmic effector pathways. The role of thisudcoupling in spinal sensory transmission and nociceptive plasticity was investigatedudusing biochemical, electrophysiological and behavioural methods. Disruption ofudbinding between PSD-95 and NR2B subunits of NMDA receptors was achievedudthrough the use of a decoy mimetic peptide, Tat-NR2B9c. I show that Tat-NR2B9cudselectively reduces wind-up of dorsal horn wide dynamic range neurones andudprevents both neuronal and behavioural measures of formalin-induced centraludsensitization. In the spinal nerve ligation model of chronic pain, Tat-NR2B9c reducedudneuronal responses to mechanical and thermal stimulation and was able to reverseudbehavioural mechanical and cold hypersensitivity, clinical signs of neuropathic pain.udIn addition, the roles of two kinases, atypical PKCζ/PKMζ and PI3K, known to beudinvolved in hippocampal LTP, were investigated using biochemical,udimmunohistochemical, electrophysiological and behavioural measures. I found thatudactivation of spinal PKCζ/PKMζ is dependent on coupling between NR2B-subtypeudreceptors and PSD-95, and contributes to central sensitization of dorsal hornudneurones. PI3K was also found to be active in the NMDA-dependent formalin modeludand regulates various intracellular mechanisms in central sensitization. Finally, Iudinvestigated the role of DDAH-1, an enzyme which is involved in the regulation ofudnNOS, in spinal nociceptive plasticity. DDAH-1 inhibition reduced neuronal wind-upudand both neuronal and behavioural measures of formalin-induced centraludsensitization. These findings further our understanding of NMDA-dependent spinaludnociceptive plasticity. Disrupting the interaction between NR2B-containing NMDAudreceptors and PSD-95 or inhibition of downstream intracellular signalling pathwaysudmay be successful analgesic strategies for the treatment of chronic pain.