Lithium chloride produces illness-induced analgesia

The analgesic effects of lithium chloride (LiCl) and their interaction with the analgesic effects of morphine were studied in rats. After recording rats’ baseline pain sensitivity in the hot-plate test, we injected half the rats with LiCl (127 mg/kg i.p.) and half with saline. A second hot-plate latency measurement was taken 100 min later. Following this measurement, half the LiCl- and half the saline-injected rats were injected with morphine (10 mg/kg s.c), and the others with saline. A third hot-plate latency measurement was taken 20 min later. LiCl produced strong analgesic effects that were comparable to those produced by morphine. The analgesic effects in the group that received both LiCl and morphine was approximately equal to the sum of the analgesic effects measured in the groups that received each of these drugs with saline. The results support our hypothesis that drugs that produce gastrointestinal illness and nausea also produce analgesia.