1 Institute of Neurobiology, Institutes of Brain Science and State Key Laboratory of Medical Neurobiology, Fudan University, Shanghai 200032, China
2 College of Medicine, Jiaxing University, Jiaxing, Zhejiang 314001, China

Abstract 

Objective
The present study aimed to investigate the potential roles of spinal microglia and downstream molecules in the induction of spinal long-term potentiation (LTP) and mechanical allodynia by tetanic stimulation of the sciatic nerve (TSS).
Methods
Spinal LTP was induced in adult male Sprague-Dawley rats by tetanic stimulation of the sciatic nerve (0.5 ms, 100 Hz, 40 V, 10 trains of 2-s duration at 10-s intervals). Mechanical allodynia was determined using von Frey hairs. Immunohistochemical staining and Western blot were used to detect changes in glial expression of interleukin-18 (IL-18) and IL-18 receptor (IL-18R).
Results
TSS induced LTP of C-fiber-evoked field potentials in the spinal cord. Intrathecal administration of the microglial inhibitor minocycline (200 μg/20 μL) 1 h before TSS completely blocked the induction of spinal LTP. Furthermore, after intrathecal injection of minocycline (200 μg/20 μL) by lumbar puncture 1 h before TSS, administration of minocycline for 7 consecutive days (once per day) partly inhibited bilateral allodynia. Immunohistochemistry showed that minocycline inhibited the sequential activation of microglia and astrocytes, and IL-18 was predominantly colocalized with the microglial marker Iba-1 in the spinal superficial dorsal horn. Western blot revealed that repeated intrathecal injection of minocycline significantly inhibited the increased expression of IL-18 and IL-18Rs in microglia induced by TSS.
Conclusion
The IL-18 signaling pathway in microglia is involved in TSS-induced spinal LTP and mechanical allodynia.

Keywords

long-term potentiation; allodynia; glia; interleukin-18

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