ABSTRACT
<p><b>OBJECTIVE</b>To investigate the role of the lateral reticular nucleus (LRN) in descending inhibition of cardiac nociception in rats and the involvement of spinal serotonergic receptors in the descending inhibition.</p><p><b>METHODS</b>Male SD rats were randomly divided into 5 groups, namely bradykinin (BK) group, BK + glutamate group, BK + methysergide group, BK + glutamate + methysergide group, and BK + glutamate + vehicle group. The rats received glutamate microinjection in the LRN combined with the intrathecal injection of serotonergic receptor antagonist methysergide, and the changes in the cardiacsomatic motor reflex induced by intrapericardial BK injection were monitored by observing electromyogram (EMG) responses of the dorsal spinotrapezius muscle; c-Fos expression in the spinal dorsal horn was also tested.</p><p><b>RESULTS</b>Compared with BK group, intra-LRN glutamate administration produced a significant inhibitory effect on intrapericardial BK-induced EMG in a dose-dependent manner, and c-Fos expression was significantly decreased in the spinal dorsal horn (P<0.05). Compared with BK + glutamate group, intrathecal administration of methysergide significantly attenuated the inhibitory effect of chemical stimulation of the LRN on intrapericardial BK-induced EMG and increased c-Fos expression in the spinal dorsal horn (P<0.05). Intrathecal administration of the vehicle did not produce any effect on EMG or c-Fos expression (P>0.05).</p><p><b>CONCLUSION</b>The serotonergic receptors in the spinal cord are involved in LRN-mediated descending inhibition of cardiac nociception in rats.</p>