The Role of NMDA and NK1 Receptors in Cardiac Nociceptive Information Transmissions in the Spinal Cords of Rats

Objective To determine the effects of NMDA and NK1 receptor agonist and antagonist on the EMG and the synaptic mechanism of nociceptive information transmissions in the spinal cords. Methods Male SD rats were randomly divided into seven groups, with intrathecal injection of the following chemicals respectively:control group (10 μL saline), NMDA group (0.147 μg/10 μL NMDA), MK801 group (6.8 μg/10 μL MK801), MK801+NMDA group (6.8 μg/10 μL MK801+0.147 μg/10 μL NMDA), Sar-SP group (1.4 μg/10 μL Sar-SP), CP-96345 group (5 μg/10 μL CP-96345), and CP-96345+Sar-SP group (1.4 μg/10 μL Sar-SP+5 μg/10 μL CP-96345). A cardiac pain model in rats through intrapericardial injection of capsaicin was established. Intrapericardial injection of capsaicin was given to the rats 10 min after intrathecal injection of the tested chemicals. The spinotrapezius electromyography (EMG) activities as an index of cardiac-somatic motor reflex were recorded simultaneously. Results Compared with the pre-test controls (100%), saline did not make a significant change to the capsaicin-evoked EMG response (96.9%±12.5%, P>0.05); NMDA agonist increased the capsaicin-evoked EMG response (185.2%±24.4%) significantly (P<0.05); neither MK801 nor a combined administration of MK801 and NMDA made a significant change to the capsaicin-evoked EMG response (106.6%±10.2%, P>0.05); Sar-SP increased the capsaicin-evoked EMG response (145.6%±10.1%) significantly (P<0.05); whereas neither CP-96345 nor a combined administration of CP-96345 and Sar-SP made a significant change to the capsaicin-evoked EMG response (102.2%±8.4%, P>0.05). Conclusion NMDA and NK1 receptors may have participated in the transmissions of cardiac nociception information in the spinal cords of rats.