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 degrees C and 50.5 degrees 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 microg, significantly. The effects of either 10 microg of NPW23 or 10 microg of
NPB were not antagonized by 10 microg of
naloxone. I.t. injection of either NPW23 or
NPB had no effect in both the 52.5 degrees C hot plate test or in the 50.5 degrees C hot plate tests at a dose between 1 and 100 microg. I.t. injection of either 10 microg of NPW23 or 10 microg of
NPB had no effect in the mechanical nociceptive test. I.t. injection of either 10 microg of NPW23 or 10 microg 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.