Antinociceptive effect of spinally delivered prostaglandin E receptor antagonists in the formalin test on the rat

The antinociceptive effect of spinally administered prostaglandin E2 receptor antagonists, SC-51089 and SC-51234A, which are selective for EP1 receptors, was examined in rats using the formalin test. Intrathecal injection of SC-51089 (30-300 micrograms) or SC-51234A (30-300 micrograms) resulted in a significant, dose-dependent, suppression of the second phase (10-60 min), but not the first phase (0-9 min), flinching behavior evoked by formalin injection into the paw. ED25 values and 95% confidence intervals for the second phase were 120 (70-200) micrograms for SC-51089 and 80 (50-140) micrograms for SC-51234A. These data demonstrate that specific nociceptive behaviors are attenuated by spinal prostaglandin E2 receptor antagonists and suggest that prostaglandin E2 is involved in facilitated processing at the spinal level.