Antinociceptive effect of systemic and intrathecal morphine in spinally transected rats.

The antinociceptive effect of morphine on the tail withdrawal reflex was examined in spinally transected rats. The efficacy of systemically administered morphine was significantly reduced within 24 h after transection, and continued to decline during the first three posttransection weeks. In contrast to the diminished effect of systemic morphine, the efficacy of intrathecal morphine was not reduced during the first three weeks after a spinal transection. These data demonstrate a significant difference in the functional effect of systemic and spinal morphine in spinally transected rats. The results indicate that the direct antinociceptive effect of morphine on the spinal cord is not reduced after spinal transection.