Role of unmyelinated fibers in electroacupuncture cardiovascular responses

The afferent fiber type responsible for the transmission of sensory neural traffic to the central nervous system during acupoint stimulation is uncertain. Several early studies evaluating compound action potentials have suggested that myelinated fibers contribute to the afferent input of the autonomic reflex adjustments during electroacupuncture (EA). Our more recent data, employing single unit recordings of somatic afferents, show that both myelinated and unmyelinated fibers are stimulated by EA, although more finely myelinated than unmyelinated fibers are activated by low frequency, low current stimulation. We hypothesized in this study that unmyelinated group VI fibers also contribute significantly to the inhibitory influence of EA on cardiovascular pressor responses. We found that neonatal capsaicin-treated rats depleted of substance P from primary afferents were insensitive to the inhibitory EA effect during gastric distention. Thus, EA at P5–P6 reduced gastric distention-induced pressor responses from 19±3 to 11±2 mmHg in eight untreated rats while capsaicin-treated rats (n=9) were unresponsive to EA. Substance P containing neurons in dorsal root ganglion cells at T1–T5 were significantly decreased in the capsaicin-treated rats from 47±4 to 22±4 cells. Treated compared to untreated rats also demonstrated a significantly (P<0.03) reduced number of group IV fibers identified with single unit recording techniques. This study demonstrates that the inhibitory effect of EA at P5–P6 on cardiovascular autonomic excitatory reflexes involves unmyelinated group IV fibers of the median nerves.