Excessive activation of
NMDA receptor (NMDAR) signaling within the spinal dorsal horn contributes to central sensitization and the induction and maintenance of pathological
pain states. However, direct antagonism of NMDARs produces undesirable side effects which limit their clinical use. NMDAR activation produces central sensitization, in part, by initiating a signaling cascade that activates the
enzyme neuronal nitric oxide synthase (nNOS) and generates the signaling molecule
nitric oxide. NMDAR-mediated activation of nNOS requires a scaffolding
protein,
postsynaptic density protein 95kDa (PSD95), which tethers nNOS to NMDARs. Thus, disrupting the
protein-
protein interaction between PSD95 and nNOS may inhibit pro-nociceptive signaling mechanisms downstream of NMDARs and suppress central sensitization while sparing unwanted side effects associated with NMDAR antagonists. We examined the impact of small molecule PSD95-nNOS
protein-
protein interaction inhibitors (ZL006,
IC87201) on both nociceptive behavior and
formalin-evoked Fos
protein expression within the lumbar spinal cord of rats. Comparisons were made with ZL007, an inactive analog of ZL006, and the NMDAR antagonist
MK-801.
IC87201 and ZL006, but not ZL007, suppressed phase 2 of
formalin-evoked
pain behavior and decreased the number of
formalin-induced Fos-like immunoreactive cells in spinal dorsal horn regions associated with nociceptive processing.
MK-801 suppressed Fos
protein expression in both dorsal and ventral horns.
MK-801 produced
motor ataxia in the rotarod test whereas
IC87201 and ZL006 failed to do so. ZL006 but not ZL007 suppressed
paclitaxel-induced mechanical and cold
allodynia in a model of
chemotherapy-induced
neuropathic pain. Co-immunoprecipitation experiments revealed the presence of the PSD95-nNOS complex in lumbar spinal cord of
paclitaxel-treated rats, although ZL006 did not reliably disrupt the complex in all subjects. The present findings validate use of putative small molecule PSD95-nNOS
protein-
protein interaction inhibitors as novel
analgesics and demonstrate, for the first time, that these inhibitors suppress
inflammation-evoked neuronal activation at the level of the spinal dorsal horn.