Role of spinal P2X4 receptor in remifentanil-induced postoperative hyperalgesia

Abstract

To explore the role of P2X4 receptor in opioid-induced hyperalgesia (OIH). Methods: A total of 30 Sprague-Dawley (SD) male rats were randomly divided into 5 groups: a saline (N0) group, a remifentanil at 0.5 μg/(kg.min) (R1) group, a remifentanil at 1.0 μg/(kg.min) (R2) group, a remifentanil at 1.5 μg/(kg.min) (R3) group, and a remifentanil at 5.0 μg/(kg.min) (R4) group. The paw withdrawal mechanical threshold (PWMT) and paw withdrawal thermal latency (PWTL) were measured at follow time points to optimize the dosages: the day before treatment (T1), 30 min after tail intravenous catheterization (T2), and 30 min (T3), 1 h (T4), 2 h (T5), 24 h (T6) after withdrawal from remifentanil. Then, the rats were randomly divided into 2 groups: a saline group (N group), a remifentanil at 1.0 μg/(kg.min) group (R group). The PWMT and PWTL were measured at follow time points: T1, T2, and T4. The lumbar enlargement of spine was selected at 1 h after withdrawal from remifentanil, and the expression of P2X4 receptor mRNA and protein was examined in OIH. Additional male rats were selected and randomly divided into 2 groups: a plantar incision surgery followed by saline treatment group (I+N group), a plantar incision surgery followed by remifentanil treatment group (I+R group). The PWMT and PWTL were measured at follow time points: T1, T2, T3, T4, T5, T6, 48 h (T7) and 72 h (T8) after withdrawal from remifentanil. The lumbar enlargement of spine was selected at 1 h after withdrawal from remifentanil, the expression of P2X4 receptor mRNA and protein was examined by PCR and Western blotting, and the microglial activation in spine 1 h after withdrawal from remifentanil were assessed by immunofluorescence. Results: The pain thresholds including PWMT and PWTL in different groups were as follows: R4 group<R3 group<R2 group<R1 group at T3 and T5 (all P<0.05). At T4, the PWMT and PWTL in the R4 group were lower than those in the R2 group, while in the R4 group they were lower than those in the R3 group (both P<0.05). Meanwhile, the PWMT and PWTL in the R2 group, the R3 group and the R4 group at T4 were lower than those in the other time points (all P<0.05). Compared with the N group, the rats in the R group had a lower PWMT and PWTL at T4 (both P<0.05), but there was no significant difference in P2X4 receptor protein and mRNA expression in the lumbar enlargement of spine between the R group and the N group. Compared with the I+N group, the pain threshold including PWMT and PWTL at T4, T5, and T6 in the I+R group was significantly decreased (all P<0.05). Meanwhile, compared with the I+N group, the expressions of P2X4 receptor mRNA and protein in rat spinal cord in the I+R group were significantly up-regulated, and the microglia was activated (all P<0.05). Conclusion: Remifentanil pumped via tail intravenous can result in the hyperalgesia, but the spinal P2X4 receptor might be not relevant to remifentanil-induced hyperalgesia. Remifentanil incision pain can cause significant hyperalgesia in rats, and the spinal P2X4 receptors and microglial activation is involved in the remifentanil-induced postoperative hyperalgesia.