| Apelin and APJ receptor are widely distributed in both central nervous system and peripheral tissues. In peripheral tissues, the apelin and APJ mRNA has been found at many organs including spleen, thymus, heart, breast, the stomach, small intestine and colon. In the brain, the expression of the APJ mRNA has been detected in the hypothalamus, thalamus, brain cortex and pituitary, which is closely related with gastrointestinal motility. Recent reports showed that apelin had an important effect on physiological and pathological functions, including cardiovascular system, immune function, neuroendocrine, fluid balance and food intake. Many reports show that food intake and gastrointestinal motility share a close relationship. However, the role of apelin in regulating gastrointestinal motility is still unclear.The present study was designed to investigate the actions of intracerebroventricularly administered apelin-13 on colonic transit as well as the actions of apelin-13 on the contraction of isolated distal colon in vitro. Intracerebroventricular (i.c.v.) injection of apelin-13 (0.3,0.5,1 and 3 ug/mouse) dose-dependently inhibited fecal pellet output and bead expulsion, which demonstrated that central apelin-13 play a role in modulating gastrointestinal motility. In order to investigate the mechanism of this effect, we selected the APJ receptor antagonist apelin-13 (F13A), which significantly antagonized the inhibitory effect of apelin-13, indicating an APJ receptor-mediated mechanism. To perform a further investigation on the mechanism, we choose the classical opioid receptor antagonist naloxone, which could also reverse the inhibitory effect of apelin-13 on fecal pellet output and bead expulsion, suggesting the involvement of opioid receptors in the regulation of apelin-13 on distal colon transit. However, apelin-13 (10-8-10-6 M) did not affect distal colonic contractions in vitro.
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