Midazolam and
morphine are often used in pediatric intensive care unit (ICU) for
analgesia and sedation. However, how these two drugs interact behaviorally remains unclear. Here, we examined whether (1) co-administration of
midazolam with
morphine would exacerbate
morphine tolerance and
morphine-induced hyperactive behaviors, and (2)
protein kinase C (PKC) would contribute to these behavioral changes. Male rats of 3-4 weeks old were exposed to a hindpaw
burn injury. In Experiment 1,
burn-injured young rats received once daily saline or
morphine (10mg/kg, subcutaneous, s.c.), followed 30min later by either saline or
midazolam (2mg/kg, intraperitoneal, i.p.), for 14 days beginning 3 days after
burn injury. In Experiment 2, young rats with
burn injury were administered with
morphine (10mg/kg, s.c.),
midazolam (2mg/kg, i.p.), and
chelerythrine chloride (a non-specific PKC inhibitor, 10nmol, intrathecal) for 14 days. For both experiments, cumulative
morphine anti-nociceptive dose-response (ED50) was tested and hyperactive behaviors such as jumping and scratching were recorded. Following 2 weeks of each treatment, ED50 dose was significantly increased in rats receiving
morphine alone as compared with rats receiving saline or
midazolam alone. The ED50 dose was further increased in rats receiving both
morphine and
midazolam. Co-administration of
morphine and
midazolam also exacerbated
morphine-induced hyperactive behaviors. Expression of the NR1 subunit of the
N-methyl-d-aspartate (
NMDA) receptor and PKCĪ³ in the spinal cord dorsal horn (immunohistochemistry; Western blot) was upregulated in
burn-injured young rats receiving
morphine alone or in combination with
midazolam, and
chelerythrine prevented the development of
morphine tolerance. These results indicate that
midazolam exacerbated
morphine tolerance through a spinal
NMDA/PKC-mediated mechanism.