Glycine transporter inhibitors as a potential therapeutic strategy for chronic pain with memory impairment

Background: Impaired excitatory and inhibitory balance in the spinal dorsal horn has a crucial role in the pathophysiology of chronic pain. The authors addressed the therapeutic impact of increasing spinal glycine applied exogenously or via blockade of glycine transporter 1 using its selective inhibitors sarcosine and N-[3-(4'-fluorophenyl)-3-(4'-phenylphenoxy)propyl]sarcosine on neuropathic and inflammatory pain in mice. Methods: Mice with thermal and mechanical hypersensitivity after partial ligation of the sciatic nerve (Seltzer model) or mice with mechanical hypersensitivity after streptozotocin injection received intrathecal injection of glycine, sarcosine, and N-[3-(4'-fluorophenyl)-3-(4'-phenylphenoxy)propyl]sarcosine. These drugs were also intrathecally injected in mice to assess their effects on formalin-evoked nociceptive behaviors. The supraspinal effect of blockade of glycine transporter 1 was studied on tetanus-induced long-term potentiation of the Schaffer-collateral synapses in hippocampal slices prepared from Seltzer model mice. Results: Glycine, sarcosine, and N-[3-(4'-fluorophenyl)-3-(4'-phenylphenoxy)propyl]sarcosine ameliorated thermal and mechanical hypersensitivity in Seltzer model mice, and reduced mechanical hypersensitivity in streptozotocin-injected diabetic mice. Moreover, they selectively inhibited the second phase of formalin-evoked licking/biting behavior. In hippocampal slices prepared from Seltzer model mice, long-term potentiation was maintained at a significantly lower level than that in sham-treated mice. Such impairment of long-term potentiation was never observed when it was induced in the presence of N-[3-(4'-fluorophenyl)-3-(4'-phenylphenoxy)propyl]sarcosine. Conclusions: An increase in endogenous glycine via glycine transporter 1 blockade not only results in a net inhibitory influence on pain transmission at the spinal level but also supraspinally relieves decreased synaptic efficacy presumably related to cognitive disturbance often described in patients with chronic pain.