〔Abstract〕 Objective To investigate the effect of morphine preconditioning in rostral ventrolateral medullar(RVLM) on myocardial ischemia-reperfusion(I/R) injury in rats and the changes of β-endorphin(β-EP) contents in T2-5 spinal cord and left ventricular muscle during the process. Methods 72 adult male SD rats were randomly divided into 4 groups(18 in each group):sham-operated group(Sham group), ischemia-reperfusion group(IR group), ischemic preconditioning group(IPC group), and RVLM morphine preconditioning group(MPC group). The ischemia reperfusion(I/R) model was treated with coronary artery left anterior descending branch ligation for 30 min and then reperfusion for 120 min. The ischemic preconditioning(IPC) was myocardial ischemia for 5 min, reperfusion for 5 min, and repeated the above operations for 3 times. Sham group, IR group and IPC group were injected 0.9% saline through the catheter. Then I/R was performed in the IR group, and IPC was performed before I/R in the IPC group. In the MPC group,morphine was injected before I/R. The numbers of ventricular arrhythmias [premature ventricular contraction( PVCs) and ventricular tachycardia/ventricular fibrillation( VT/VF) ]were recorded within 30 min of reinfusion by electrocardiogram monitoring. Cardiac specimens were stained with TTC.Left ventricle( LV),right ventricle( RV),infarct area( IS),ischemic risk area( AAR) volume were measured,and the IS/AAR ratio was calculated. Western blot assay was used for RVLM nucleus c-fos protein expression detection. ELISA assay and immunohistochemical staining assay were used for quantitative and qualitative detection the levels of β-EP expression in T2-5 spinal cord and left ventricular muscle,respectively. Results Compared with the Sham group,the IS volume,IS/AAR,the numbers of PVCs,VT/VF,and expression of c-Fos in RVLM and β-EP in spinal cord and myocardium were increased in IR group( P<0. 01). Compared with. IR group,the IS volume,IS/AAR,the numbers of PVCs,VT/VF,and the levels of c-Fos expression were significantly decreased( P<0. 01),but the levels of β-EP expression in myocardium and spinal cord were significantly increased in IPC group and MPC group( P<0. 01). Conclusion Morphine preconditioning in RVLM can alleviate myocardial I/R injury in rats,which may be related to the decrease of c-Fos expression in RVLM nucleus after I/R． β-endorphin may be involved as a transmitter in the myocardial protective effect of morphine preconditioning in RVLM.