We briefly review our recent studies on the neural mechanisms of the reflex effects of acupuncture-like stimulation on heart rate in rats. In pentobarbital anesthetized rats, acupuncture-like stimulation of one of various segmental areas of the body (forelimb, chest, abdomen, hindlimb) invariably induces a decrease in heart rate. In the case of the hindlimb, the effect can be produced by stimulation of the muscles alone but not of skin alone, and is abolished by severance of the hindlimb somatic nerves. Electrical stimulation of groups III and IV nerve fibers (in the tibial nerve) decreases heart rate. Decrease in heart rate by acupuncture-like stimulation of a hindlimb is accompanied by a decrease in cardiac sympathetic nerve activity, and is abolished by cardiac sympathectomy but not by vagotomy. High spinal cord transection or infusion of the GABA(A) receptors antagonist, bicuculline, into the cisterna magna is effective in disrupting the reflex bradycardia. Opioid receptor blockade does not disrupt the reflex arc. We conclude that the reflex pathway involved in the decrease of heart rate by acupuncture-like stimulation comprises groups III and IV muscle afferent nerves whose activation stimulates GABAergic neurons in the brainstem and inhibits sympathetic outflow to the heart. When the sympathetic tone is high due to hypercapnia, the induced reduction in both cardiac sympathetic nerve activity and heart rate is not augmented, suggesting that the magnitude of sympatho-inhibitory response to acupuncture-like stimulation does not depend on pre-existing sympathetic tone.
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