Effects And Mechanisms Of Apelin-13 In The Pain Modulation In Mice

APJ is a G protein-coupled receptor that was originally isolated from human genomic DNA, subsequently cloned in mice and rat. It shares the closest identity to the angiotensin II type 1 (AT1) receptor ranging from 40% to 50% in the hydrophobic transmembrane regions, but it does not bind to angiotensin II. The endogenous ligand for the APJ receptor was first isolated in 1998 and named apelin (APJ endogenous ligand). It deuced from a 77 amino acid precursor, preproapelin, which can be cleaved into several molecular forms including apelin-36 and apelin-13 in different tissues. Apelin-13 is involved in modulating a variety of biological actions, such as thermal, cardiovascular, food and water intake, immune response and hormone modulation. Apelin-13 and apelin-13(F13A) were synthesized by solid phase. The present study is designed to study the effect of apelin-13 on the pain modulation.We used the tail immersion test to study the effect and the mechanism in the acute pain. Intracerebroventricular (i.c.v.) administration of apelin-13 (0.3, 0.5, 0.8 and 3 jig/mouse) produced dose- and time-related antinociceptive effect. The dose-response curve of apelin-13 induced analgesic behavioral response at 15 min after i.c.v. injection was "U"-shaped. This effect was significantly antagonized by the APJ receptor antagonist apelin-13(F13A), indicating an APJ receptor-mediated mechanism. Furthermore, naloxone,β-funaltrexamine and naloxonazine, could reverse the analgesic effect. However, naltrindole or nor-binaltorphimine could not reverse the effect, suggesting thatμopioid receptor (primarilyμ₁ opioid receptor subtype) is involved in the analgesic response evoked by apelin-13. Moreover, i.c.v. administration of apelin-13 potentiated the analgesic effect induced by morphine (i.c.v., 5μg/kg) and this potentiated effect can be also reversed by naloxone.In the formalin test, intracerebroventricular (i.c.v.) administration of apelin-13 dose-related potentiated the licking duriation of the affected paw in the second phase of the formalin test. Also the dose—response curve of apelin-13 induced hyperalgesic behavioral response at 15 min after i.c.v. injection was "U"-shaped. The effect was significantly reversed by the APJ receptor antagonist apelin-13(F13A), indicating an APJ receptor-mediated mechanism. Unlike apelin-13-induced analgesia in the tail immersion test, apelin-13-induced hyperalgesia in the formalin test was not reversed by the opioid antagonist naloxone, suggesting an independent mechanism of action. It is striking that bicuculline (a competitive antagonist at GABAa receptors) and picrotoxin (a chloride channel blocker), effectively blocked the hyperalgesia induced by apelin-13, implying that the hyperalgesic effect is mediated through the GABAergic neurons.