Abstract: Objective To observe the descending modulation of cardiac nociception by the rostral ventromedial medulla (RVM)
in rats. Methods A rat model of cardiosomatic motor reflex (CMR) was established by injecting capsaicin into the pericardial
sac to induce cardiac nociception, and the electromyogram (EMG) response of the dorsal spinotrapezius muscle was studied.
The RVM was electricaly stimulated (25, 75 and 100 μA) or destroyed to examine whether RVM exerted descending
modulation on cardiac nociception. Results Electrical stimulation of the RVM at 8 sites produced intensity-dependent
inhibition of EMG responses to noxious cardiac stimulus (F[2,21] =43.188, P=0.001). Electrical stimulation at 3 sites caused
facilitated EMG responses, but the increased magnitude of the EMG was not dependent on stimulation intensity (F[2,6]=0.884, P=
0.461). Stimulation at 11 sites produced biphasic effects: at a low intensity (25 μA), the elicited EMG magnitude was
significantly larger than baseline (P<0.05), and at greater intensities (75/100 μA), the stimulation caused suppression of the
EMG magnitude to a level significantly lower than the baseline (P<0.05). Electrolytic lesion of the RVM resulted in significantly
increased EMG responses compared with the baseline and sham lesion group. Conclusion Cardiac nociception evoked by
capsaicin stimulation is subjected to descending biphasic modulation by the RVM, which produces predominantly descending
inhibition on heart pain.