Neural mechanism of depressor responses of arterial pressure elicited by acupuncture-like stimulation to a hindlimb in anesthetized rats.

The effects of acupuncture-like stimulation of a hindlimb on renal sympathetic nerve activity (RNA) as well as mean arterial blood pressure (MAP) were examined in anesthetized rats. An acupuncture needle (diameter of 160 microns) was inserted into the skin of a hindlimb and underlying muscles to a depth of 5 mm and was twisted at about 1 Hz. Under deep anesthetic condition, in about 70% of trials, acupuncture-like stimulation for 60 s induced a decrease in MAP which was accompanied by a decrease in RNA. Acupuncture-like stimulation applied to the muscles alone, but not to the skin alone, induced inhibition of RNA and MAP. Transection of sciatic and femoral nerves ipsilateral to the hindlimb stimulation completely abolished the responses of RNA and MAP. The hindlimb stimulation excited the femoral and common peroneal afferent nerves. In spinalized animals, the hindlimb stimulation did not produce any changes in RNA and MAP. The results indicate that the decrease in MAP induced by acupuncture-like stimulation of a hindlimb is a reflex response. The afferent pathway is composed of hindlimb muscle afferents while the efferent pathway is composed of sympathetic vasoconstrictors including the renal nerves. Endogenous opioids may not be involved in the present reflex, because an intravenous injection of naloxone, an antagonist of the opioid receptors, did not influence the reflex.