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 gabaa 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 gabaa 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. © 2013 bentham science publishers.