Electroacupuncture modifies the expression of c-fos in the spinal cord induced by noxious stimulation

The present study was designed to investigate the effect of 4 Hz vs. 100 Hz electroacupuncture (EA) on c-fos expression in the spinal cord induced by noxious stimulation (NS). A second objective was to evaluate the sensitivity of these two different frequencies of EA stimulation to the opiate antagonist, naloxone. Mechanical NS was applied to the right hindpaw following 30 min of either 4 Hz or 100 Hz EA treatment and the resulting c-fos expression in the spinal cord dorsal horn was compared to that obtained in rats exposed only to the noxious stimulation. The involvement of endogenous opioids in the EA response to 4 Hz or 100 Hz stimulation frequencies was evaluated by pretreating rats with naloxone (5 mg/kg, i.p.) 10 min prior to EA. Both 4 Hz and 100 Hz EA reduced the number of c-fos-immunoreactive neurons in the spinal dorsal horn induced by noxious stimulation by 58% and 50%, respectively. The suppression of c-fos expression induced by 4 Hz EA was completely reversed by prior treatment with naloxone. On the other hand, the suppression of c-fos induced by 100 Hz EA was only partially blocked by this opiate antagonist. These data indicate that both high- and low-frequency EA are capable of inhibiting the expression of c-fos in the dorsal horn induced by NS. Low-frequency EA appears to be mediated primarily by endogenous opioid systems, while non-opioid mechanisms may be involved in mediating the analgesic effect of high frequency EA. These results support the hypothesis that EA has a direct inhibitory effect on spinal cord dorsal horn neurons and extend the results of previous studies which indicate low frequency EA is mediated by opiate sensitive circuitry, while high frequency EA is predominantly mediated by non-opioid neurotransmitters.