In search of mediators of skin vasodilation induced by transcutaneous nerve stimulation: II. Serotonin implicated

Previous studies have shown that brain serotonin is increased and noradrenaline decreased in acupuncture and transcutaneous nerve stimulation (TNS). Increases in available brain serotonin and decreases in noradrenaline enhance pain suppression. The present study tests the possibility that the widespread and prolonged cutaneous vasodilation which can be produced by low-frequency TNS in patients with peripheral circulatory insufficiency is similarly dependent on a central serotonergic pathway leading to sympatho-inhibition. The serotonin receptor antagonist cyproheptadine was given to 4 patients with either Raynaud's phenomenon or diabetic polyneuropathy, who all prior to drug administration responded to TNS with marked and prolonged cutaneous vasodilation in the ischaemic limbs. Cyproheptadine almost completely blocked the vascular response. Contrary to endorphin-serotonin mediated pain inhibition, vasoconstrictor inhibition is not antagonized by conventional, low doses of naloxone (Kaada, 1982a). However, the involvement of more naloxone-resistant opioid receptors in the vascular response cannot be excluded.