6). the rate of recurrence of miniature EPSCs in dorsal horn neurons of FK506-treated rats. In addition, intrathecal injection of an NMDAR antagonist or systemic administration of memantine effectively reversed nociceptive and mechanical hypersensitivity in FK506-treated rats. Our findings show that calcineurin inhibition increases glutamate-mediated nociceptive input by potentiating presynaptic and postsynaptic NMDAR activity in spinal cords. NMDAR antagonists may symbolize a new therapeutic option for the treatment SJFδ of CIPS. Key points We developed an animal model Cd69 to study the mechanisms underlying the pain syndrome commonly seen in organ transplant patients receiving calcineurin inhibitors. Systemic treatment with the calcineurin inhibitor induces long-lasting pain hypersensitivity and increases glutamate receptor activity in the spinal cord. Blocking glutamate receptor activity at the spinal cord level effectively reduces pain hypersensitivity induced by the calcineurin inhibitor. This information improvements our understanding of the molecular basis of pain caused by calcineurin inhibitors and identifies new strategies for treating such pain syndrome in transplant patients. Introduction Calcineurin inhibitors, such as cyclosporin A and tacrolimus (FK506), are the most commonly used immunosuppressive drugs to prevent rejection of transplanted organs and tissues. However, these drugs can cause unexplained severe pain and pain hypersensitivity, often referred to as calcineurin inhibitor-induced pain syndrome (CIPS; Grotz or whether increased NMDAR activity contributes to pain hypersensitivity associated with CIPS. In this study, we developed a rat model of CIPS and tested the hypothesis that chronic treatment of the calcineurin inhibitor causes pain hypersensitivity by increasing synaptic NMDAR activity and glutamatergic input to spinal dorsal horn neurons. Our study provides new information about the important contribution of spinal NMDARs to CIPS and suggests NMDARs as a potential new target for treating CIPS. Methods Animal model Male SpragueCDawley rats (6C7 weeks aged) were purchased from Harlan Laboratories (Indianapolis, IN, USA). A total of 97 rats were used for the entire study. FK506, a SJFδ highly specific calcineurin inhibitor (Liu test to compare the statistical differences in puff NMDAR currents and the ratio of NMDAR-EPSCs to AMPAR-EPSCs between the vehicle- and FK506-treated groups. One-way analysis of variance (with Dunnetts or Tukeys test) was used to compare the effects of AP5 on sEPSCs and mEPSCs and the behavioural data. The effects of NMDAR antagonists on paw withdrawal thresholds were determined by ANOVA followed by Dunnetts test. The level of significance was set at 0.05. Results Chronic systemic administration of FK506 causes mechanical hypersensitivity Systemic injection of FK506 (1.5 mg kg?1, i.p.) once a day for 7 days caused a gradual decrease in the baseline tactile and pressure withdrawal thresholds in 11 rats (Fig. SJFδ 1. Notably, the FK506-induced reduction in the paw withdrawal thresholds persisted for at least another 10 days after cessation of FK506 treatment. Both the tactile and pressure withdrawal thresholds returned to pretreatment levels 2 weeks after the discontinuing FK506 administration. In another 9 rats, systemic treatment with the vehicle (DMSO, i.p.) for 7 days did not alter the paw withdrawal thresholds, measured with von Frey filaments and the pressure stimulus (Fig. 1). Open in a separate window Physique 1 Chronic systemic administration of FK506 induces pain hypersensitivity in ratsand 0.05 compared with the baseline value. FK506 treatment potentiates postsynaptic NMDAR activity of spinal dorsal horn neurons To determine changes in NMDAR activity of spinal dorsal horn neurons in CIPS, the spinal cord slices were obtained from vehicle- and FK506-treated rats at 3C5 days after the last treatment. We recorded monosynaptic NMDAR-EPSCs and AMPAR-EPSCs in lamina II neurons evoked by electrical activation of the dorsal root. Monosynaptic EPSCs were identified on the basis of the constant latency of evoked EPSCs and the absence of conduction failure of evoked EPSCs in response to a brief 20 Hz electrical.