Effect of intrathecal morphine preconditioning on excitability of substantia gelatinosa neurons in dorsal horn of spinal cord in a rat model of myocardial ischemia-reperfusion

Abstract

Objective To investigate the effect of intrathecal morphine preconditioning(ITMP)on the excitability of substantia gelatinosa(SG)neurons in the dorsal horn of the spinal cord in a rat model of myocardial ischemia-reperfusion(I/R). Methods Thirty-six adult male Sprague-Dawley rats, weighing 200-300 g, in which intrathecal catheters were successfully placed without complications, were randomly divided into 3 groups(n=12 each)using a random number table: sham operation group(group S), group I/R, and group ITMP.Myocardial I/R injury was produced by occlusion of the left anterior descending branch of the coronary artery for 30 min followed by 120 min reperfusion.In group ITMP, the rats received intrathecal morphine 3 μg/kg(10 μl)by three cycles of 5 min infusions interspersed with 5 min infusion-free periods starting from 30 min before ischemia, and the equal volume of normal saline was given instead of morphine in group I/R.At 10 min of reperfusion, 6 rats randomly selected in each group were sacrificed, and the T2-6 segments of the spinal cords were acutely isolated to prepare spinal cord slices.The resting potential, threshold of action potential(APT), peak of action potential(APP)and action potential duration in SG neurons in the dorsal horn of spinal cord slices were determined using the whole-cell patch-clamp technique, and the number of action potentials evoked by currents of 40, 60, 80 and 100 pA was recorded.At 120 min of reperfusion, 6 rats randomly selected in each group were sacrificed, and myocardial specimens were obtained for determination of myocardial infarct size(IS)and area at risk(AAR), and IS/AAR ratio was calculated.The expression of c-fos in the T2-6 dorsal horns of the spinal cords was detected by Western blot. Results Compared with group S, the IS/AAR ratio was significantly increased, the expression of c-fos was up-regulated, the number of action potentials in SG neurons in dorsal horns of spinal cord was increased, APT was decreased, and APP was increased in group I/R(P<0.05). Compared with group I/R, the IS/AAR ratio was significantly decreased, the expression of c-fos was down-regulated, the number of action potentials in SG neurons in dorsal horns of spinal cord was decreased, APT was increased, and APP was decreased in group ITMP(P<0.05). Conclusion The mechanism by which ITMP attenuates myocardial I/R injury is related to decrease in the excitability of SG neurons in the dorsal horn of the spinal cord and reduction of responses to nociceptive stimuli in rats. Key words: Morphine; Ischemic preconditioning; Injection, spinal; Substantia gelatinosa; Myocardial reperfusion injury