Afferent mechanisms underlying stimulation modality-related modulation of acupuncture-related cardiovascular responses

Despite the use of acupuncture to treat a number of heart diseases, little is known about the mechanisms that underlie its actions. Therefore, we examined the influence of acupuncture on sympathoexcitatory cardiovascular responses to gastric distension in anesthetized Sprague-Dawley rats. Thirty minutes of low-current, low-frequency, (0.3-0.5 mA, 2 Hz) electroacupuncture (EA), at P 5-6, S 36-37, and H 6-7 overlying the median, deep peroneal, and ulnar nerves significantly decreased reflex pressor responses by 40, 39, and 44%, respectively. In contrast, sham acupuncture involving needle insertion without stimulation at P 5-6 or 30 min of EA at LI 6-7 acupoints overlying the superficial radial nerve did not attenuate the reflex. Similarly, EA at P 5-6 using 40- or 100-Hz stimulation frequencies did not inhibit the reflex. Compared with EA at P 5-6, EA at two sets of acupoints, including P 5-6 and S 36-37, did not lead to larger inhibition of the reflex. Two minutes of manual acupuncture (MA; 2 Hz) at P 5-6 every 10 min for 30 min inhibited the reflex cardiovascular pressor response by 33%, a value not significantly different from 2-Hz EA at P 5-6. Single-unit afferent activity was not different between electrical stimulation (ES) and manual stimulation. However, 2-Hz ES activated more somatic afferents than 10- or 20-Hz ES. These data suggest that, although the location of acupoint stimulation and the frequency of stimulation determine the extent of influence of EA, there is little difference between low-frequency EA and MA at P 5-6. Furthermore, simultaneous stimulation using two acupoints that independently exert strong effects did not lead to an additive or a facilitative interaction. The similarity of the responses to EA and MA and the lack of cardiovascular response to high-frequency EA appear to be largely a function of somatic afferent responses.