Objective: To observe the effect of 2 Hz electroacupuncture (EA) on long-term depression (LTD) of synaptic transmission in the spinal dorsal horn in rats with neuropathic pain, so as to explore the central mechanisms of the antinociceptive effects of 2 Hz electroacupuncture on neuropathic pain.
Methods: The neuropathic pain models were produced by tight ligation of the L5/L6 spinal nerves in Sprague-Dawley rats. The C-fiber-evoked field potentials in the spinal dorsal horn were recorded with extracellular recording techniques. The parameters of the electroacupuncture were as follows: frequency of 2 Hz, wavelength of 0.6 ms, intensity of 1, 2, 3 mA lasting 10 min for each intensity, stimulation time of 30 min. The positive stimulating electrode was placed in acupoint "sanyinjiao" and the negative electrode in "zusanli".
Results: (1) 2 Hz electroacupuncture significantly decreased the amplitudes of C-fiber-evoked field potentials in the spinal dorsal horn in rats with neuropathic pain to (49.4 +/- 0.6)% of the control, compared with that (100.1 +/- 1.2)% of the control before EA (unpaired t test, P < 0.001, n = 6), which lasted for at least 3 hours. (2) This EA suppression on long-term depression of C-fiber-evoked field potentials in the spinal dorsal horn in rats with neuropathic pain could be blocked significantly by intravenous application of NMDA receptor antagonist MK-801 (0.5 mg.kg-1), or by opioid receptor antagonist naloxone (1 mg.kg-1).
Conclusion: EA of 2 Hz could induce long-term depression of the nociceptive synaptic transmission in the spinal dorsal horn in rats with neuropathic pain. This kind of 2 Hz EA-suppression on LTD was NMDA receptor-dependent and the endogenous opioid system may be involved in it. These data indicate that the NMDA receptor-dependent LTD at the nociceptive sensory synapses in the spinal dorsal horn in rats with neuropathic pain induced by activating the endogenous opioid system may play the primary role in the antinociceptive effects of 2 Hz EA on neuropathic pain.