Examination of the afferent fiber responsible for the suppression of jaw-opening reflex in heat, cold, and manual acupuncture stimulation in rats.

The possible afferent fibers that participate in the inhibition of jaw-opening reflex (JOR) were examined using selective conduction blockade by topically applied capsaicin. Blood pressure, heart rate, and rectal temperature were monitored, and bilateral femoral nerves were denervated in thiamylal anesthetized Wistar rats. The sciatic nerves were exposed bilaterally and two cotton balls, one soaked with 1.5% capsaicin and the other with saline, were directly applied to the nerve trunk on the respective sides. We verified the conduction blockade of the compound action potentials A delta and C fibers by 1.5% capsaicin. The evoked activity of the digastric electromyography elicited by electrical stimulation of the tongue (1.5 x T, duration 200 microseconds, interval 2 ms, twin pulse, 0.2 Hz) was monitored. On the saline-treated side, the JOR was gradually inhibited by manual acupuncture stimulation of the ipsilateral hindpaw (80 s), and the effect continued after the cessation of the stimulus. This response was not obtained with the manual acupuncture stimulation on the capsaicin-treated side. Immersion of the hindpaw in hot water (53 degrees C, 40 s) induced a rapid and potent inhibition of the JOR on the saline-treated side, but not on the capsaicin-treated side. Cold water immersion (10 degrees C, 40 s) had no apparent suppressive effect on either side, but it had a rather facilitative effect on the JOR on the saline-treated side. The inhibition of the JOR was elicited by manual acupuncture stimulation of various segmental areas such as the nose, auricle, forepaw, abdomen, hindleg and hindpaw. These results suggest that the capsaicin-sensitive thin afferent fibers (A delta and C afferent fibers) mediated by receptors such as polymodal receptors are activated by manual acupuncture stimulation and that they participate in the peripheral processes that of inhibition of the JOR by diffuse noxious inhibitory controls.