Peripheral modulation of pain in conscions guinea pigs: effect of morphine nalorphine and clonidine

Morphine and nalorphine were shown to have a peripheral effect on conscious guinea pigs, producing a decrease in the vocalization response to noxious electrical stimulation. Naloxone antagonized the effect of morphine and nalorphine. Locally administered clonidine had a peripheral effect on conscious guinea pigs, producing a decrease in the vocalization response to noxious electrical stimulation. The peripheral analgesia of clonidine was antagonized by yohimbine and naloxone. The analgesic effect clonidine was 250-300 times more potent than that of lidocaine. It is suggested that alfa2-adrenergic receptor agonists may active enkephalin-like substances released at the peripheral level