ANTINOCICEPTION INDUCED BY SIMULTANEOUS INTRATHECAL AND INTRAPERITONEAL ADMINISTRATION OF LOW-DOSES OF MORPHINE

The application of morphine simultaneously into the spinal cord and br ain ventricles produces a supraadditive antinociceptive effect. In thi s study, we attempted to determine whether combined intrathecal (IT) a nd intraperitoneal (IF) administration of small doses of morphine also produces such a synergistic antinociceptive effect. The experiments w ere performed on male Wistar rats. Nociception was measured using the tail immersion test. For IT administration morphine was injected throu gh a catheter implanted in the subarachnoid space. Combined administra tion of small doses of IT (1 mu g) and IP (1 mg/kg) morphine resulted in a strong, highly significant antinociceptive effect. This effect wa s not only much higher than that produced by separate administration o f the same doses of morphine, but also much higher than the expected e ffect of the combination. These results demonstrate that low doses of IT and IP morphine interact in a supraadditive fashion to produce pote nt analgesia.