Anti-hyperalgesic effects of intrathecally administered neuropeptide W-23, and neuropeptide B, in tests of inflammatory pain in rats

Neuropeptide W-23 (NPW23) is an endogenous ligand of both GPR7 and GPR8, and neuropeptide B (NPB) is an endogenous ligand of GPR7. GPR7 mRNA has been detected in regions of the cortex, the hippocampus, the hypothalamus, and the spinal cord in the rat, but GPR8 has not been found in rodents. GPR7 and GPR8 receptors have structural features in common with both opioid and somatostatin receptors. The effects of intrathecal (i.t.) application of NPW23 and NPB were tested in two inflammatory pain models (plantar injection of formalin or carrageenan) and two thermal nociceptive tests (52.5 °C and 50.5 °C hot plates) and one mechanical nociceptive test in the rat. I.t. injection of either NPW23 or NPB decreased the number of agitation behaviors induced by paw formalin injection and attenuated the level of mechanical allodynia, but not the level of thermal hyperalgesia, induced by paw carrageenan injection in a dose-dependent manner at a dose between 0.1 and 10 μg, significantly. The effects of either 10 μg of NPW23 or 10 μg of NPB were not antagonized by 10 μg of naloxone. I.t. injection of either NPW23 or NPB had no effect in both the 52.5 °C hot plate test or in the 50.5 °C hot plate tests at a dose between 1 and 100 μg. I.t. injection of either 10 μg of NPW23 or 10 μg of NPB had no effect in the mechanical nociceptive test. I.t. injection of either 10 μg of NPW23 or 10 μg of NPB significantly suppressed the expression of Fos-like immunoreactivity of the L4-5 spinal dorsal horn induced by paw formalin injection. These data suggest that both spinally-applied NPW23 and NPB suppressed the input of nociceptive information to the spinal dorsal horn, produced an analgesic effect in the formalin test, and attenuated the level of mechanical allodynia in the carrageenan test, and that either spinally applied NPW23 or spinally applied NPB had no effect in the physiological condition.