Purpose: Spinal wide dynamic range (WDR) neurons are well studied in pain models and they play critical roles in regulating nociception. Evidence has started to accumulate that acupuncture produces a good analgesic effect via activating different primary fibers with distinct intensities. The purpose of the present study was to compare the distinct intensities of pre-electroacupuncture (pre-EA) at local muscular receptive fields (RFs), adjacent or contralateral non-RFs regulating the nociceptive discharges of spinal WDR neurons evoked by hypertonic saline (HS).
Materials and Methods: Spinal segments of electrophysiological recording were identified by neural tracers applied at the left gastrocnemius muscle. The thresholds of Aβ (T_Aβ), Aδ (T_Aδ) and C (T_C) components of WDR neurons were measured to determine the intensity of pre-EA by extracellular recording. The discharges of WDR neurons induced by distinct intensities of pre-EA and 200 μL HS (6%) injection in left gastrocnemius muscle of rats were observed by extracellular recording.
Results: The spinal segments of WDR neurons were confirmed in lumbar (L)5– 6 area according to the projective segments of dorsal root ganglion. T_Aβ, T_Aδ and T_C of WDR neurons was determined to be 0.5, 1, and 2 mA, respectively. The pre-EA with intensities of T_Aβ (P < 0.05), T_Aδ (P < 0.05), T_C (P < 0.05) or 2T_C (P < 0.01) at ipsilateral adjacent non-RFs significantly reduced the discharges of WDR neurons, while at local RFs only pre-EA of T_Aδ (P < 0.05), T_C (P < 0.05) and 2T_C (P < 0.01) could inhibit the nociceptive discharges. In addition, intensity of pre-EA at contralateral non-RFs should reach at least T_C to effectively inhibit the firing rates of WDR neurons (P < 0.01).
Conclusion: Pre-EA could suppress nociceptive discharges of WDR neurons and the inhibitory effects were dependent on the distinct intensities and locations of stimulation.
Keywords: pre-electroacupuncture, wide dynamic range neurons, muscular receptive fields, intensity, hypertonic saline