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BACKGROUND: Central sensitization, a state of increased excitability of nociceptive neurons in the spinal dorsal h following peripheral tissue injury and/or inflammation, is an important mechanism underlying hyperalgesia and neuropathic pain. Participation of the glutamate-glutamine cycle in central sensitization of the spinal cord remains poorly understood. OBJECTIVE: To determine whether the astrocyte-neuronal glutamate-glutamine cycle is involved in formalin-induced central sensitization in the spinal cord.DESIGN, TIME AND SETTING: A randomized, controlled, animal experiment was performed at the Institute of Orthopedics, Second Hospital, Lanzhou University, China from September 2007 to August 2008. MATERIALS: Methionine sulfoximine (MSO, 0.1 mmol/L), glutamine (0.25 mmol/L), and formalin were used for this study. METHODS: A total of 43 male, Sprague Dawley rats, aged 4 months, were randomly assigned to a sham operation group (n = 6) and a model group (n = 37). Rats in the model group received intrathecal infusion in the spinal cord. 7 days later, 37 model rats were randomly divided into PBS, MSO, glutamine, MSO + glutamine and formalin subcutaneous injection alone groups. The PBS, MSO, glutamine, MSO + glutamine groups were respectively intrathecally injected with PBS, MSO, glutamine, MSO + glutamine (50 μL each), and then infused with 10 μL of saline. Rats from the sham operation group were not subjected to intrathecal infusion in the spinal cord. At 15 minutes after intrathecal injection, a rat model of formalin-induced inflammatory pain was established by subcutaneous injection of 5% formalin (50 μL) in the left hindpaw. MAIN OUTCOME MEASURES: Changes in spontaneous nociceptive behavior (licking/biting or flinching) were observed following formalin injection into the rat hindpaw. RESULTS: Compared with the PBS group, duration of licking/biting was significantly shortened, and flinching frequency was significantly diminished in the MSO group (P < 0.05). Compared with the MSO group, duration of licking/biting was significantly prolonged, and flinching frequency was significantly increased in the MSO + glutamine group (P < 0.05). There was no significant difference in inflammatory pain behaviors among the sham operation, PBS, glutamine, MSO + glutamine, and formalin subcutaneous injection alone groups (P > 0.05). CONCLUSION: The astrocyte-neuronal glutamate-glutamine cycle in the spinal cord was shown to be involved in central sensitization induced by formalin subcutaneous injection into the hindpaw.