Systemic morphine in combination with TENS produces an increased antihyperalgesia in rats with acute inflammation

Given that transcutaneous electrical nerve stimulation (TENS) achieves its anti-hyperalgesia through endogenous opioid receptors, this study was under taken to assess if TENS in combination with morphine was more effective at reducing primary hyperalgesia. Acute inflammation was induced by subcutaneo us injection of 3% carrageenan into the rat's hindpaw. The withdrawal laten cy to heat and the mechanical withdrawal threshold were assessed before and after inflammation and after treatment with TENS (high- or low-frequency). Animals were divided into the following groups: Group 1, saline, no TENS; Group 2, high- or low-frequency TENS plus saline; Group 3, morphine (0.3, 1 , and 3 mg/kg, intraperitoneal [IP]), no TENS; and Group 4, high- or low-fr equency TENS plus morphine (0.3, 1, and 3 mg/kg, IP). There was an increase in inhibition of primary heat but not mechanical hyperalgesia after treatm ent with either high- or low-frequency TENS in combination with morphine. I n combination with morphine, low-frequency TENS produced a similar reductio n in mechanical hyperalgesia when compared with morphine alone. High-freque ncy TENS in combination with morphine produced a similar reduction in mecha nical hyperalgesia when compared with the effects of high-frequency TENS al one. Thus, a lower dose of morphine could be used in combination with TENS to decrease the side effects of systemic morphine and achieve the same degr ee of analgesia.