Endomorphin-1 mediates 2 Hz but not 100 Hz electroacupuncture analgesia in the rat.

This work was designed to elucidate the possible involvement of endogenous endomorphin-I (EM1) in analgesia induced by electroacupuncture of low or high frequencies. Taking radiant heat tail flick latency (TFL) as an indication of nociception, rats were subjected to intrathecal (i.t.) injection of 10 microl antiserum against EM1 (EM1-AS) or normal rabbit serum (NRS, as control) and then followed by 2 or 100 Hz electroacupuncture stimulation for 30 min. The analgesia induced by 2 Hz electroacupuncture was attenuated by i.t. injection of EM1-AS at 1:10 and 1:100 but not at 1:1000 dilution. No such suppressive effect was observed for 100 Hz EA analgesia when EM1-AS was injected i.t. at any dilutions. These results indicate that EM1 is involved in 2 Hz but not 100 Hz electroacupuncture analgesia at spinal level.