[Frequency dependence of substance P release by electroacupuncture in rat spinal cord]

Previous studies in our laboratory have shown that electroacupuncture (EA) using different frequencies produced differential opioid peptides release in the spinal cord of rats and human beings. In the present study we observed the frequency dependence of substance P (SP) release from rat spinal cord, with the frequencies of EA set at 2, 4, 8, 15, 30 and 100 Hz. The spinal perfusate was collected in 3 periods of 30 min before, during and after EA, and the immunoreactive SP (SP-ir) was measured by radioimmunoassay (RIA). The effectiveness of EA-induced analgesia was assessed by tail flick latency (TFL). Rats showing an increase of TFL over 40% was considered as EA responder. The results showed that in the responders, SP-ir in spinal perfusate showed a moderate decrease during 2 Hz EA, (P < 0.01 compared with baseline level), no change in the 4 Hz EA group, and a marked increase during 8, 15, 30 and 100 Hz EA (P < 0.01), with maximal increase occurring at 15 Hz (P < 0.001). The above results suggest that EA may induce upward or downward modulation in SP-ir release depending on the frequency of EA. However in the non-responder rats no change in spinal fluid SP-ir content was observed. This suggests that changes in SP-ir release have same causal relation with the analgesia induced by EA stimulation.