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 microL saline), NMDA group (0.147 microg/10 pL NMDA), MK801 group (6.8 microg/10 microL MK801), MK801+NMDA group (6.8 microg/10 pL MK801+0. 147 microg/10 pL NMDA), Sar-SP group (1.4 pg/10 microL Sar-SP), CP-96345 group (5 microg/10 pL CP-96345), and CP-96345+Sar-SP group (1.4 micro/10 microL Sar-SP+5 microg/10 microL 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.