Intrathecal Apelin-13 Produced Different Actions in Formalin Test and Tail-flick Test in Mice

Apelin was identified as natural ligand for APJ, a G protein-coupled receptor. APJ is expressed in spinal cord and dorsal root ganglion. This study was designed to investigate the effects and mechanisms of intrathecally (i.t.) administered apelin-13 on nociceptive response in formalin test and tail-flick test. In formalin test, i.t. injection of apelin-13 (0.3 3 nmol/mouse) had no effect on the nociceptive response in either acute phase (0 10 min) or interphase (10 20 min), but significantly produced hyperalgesic effect in late phase (20 30 min) at the dose of 3 nmol/mouse. The APJ receptor antagonist apelin-13(F13A) and GABA(A) receptor antagonist bicuculline methiodide, but not opioid receptor antagonist naloxone, significantly blocked the hyperalgesia caused by apelin-13 in late phase, indicating that i.t. apelin-13-induced hyperalgesia was mediated by APJ and GABA(A) receptor, rather than opioid receptor. However, in tail-flick test, i.t. injected apelin-13 (1 and 3 nmol/mouse) induced a significant antinociceptive effect, which was significantly antagonized by apelin-13(F13A) and naloxone, suggesting APJ and opioid receptor were involved in the antinociception of spinal apelin-13.