Chronic intrathecal cannulation enhances nociceptive responses in rats

The influence of a chronically implanted spinal cannula on the nociceptive response induced by mechanical, chemical or thermal stimuli was evaluated. The hyperalgesia in response to mechanical stimulation induced by carrageen in or prostaglandin E-2 (PGE(2)) was significantly increased in cannulated (Cn) rats, compared with naive (Nv) or sham-operated (Sh) rats. Only Cn ani mals presented an enhanced nociceptive response in the first phase of the f ormalin test when low doses were used (0.3 and 1%). The withdrawal latency to thermal stimulation of a paw inflamed by carrageenin was significantly r educed in Cn rats but not in Nv or Sh rats. In contrast to Nv and Sh rats, injection in Cn animals of a standard non-steroid anti-inflammatory drug, i ndomethacin, either intraperitoneally or into the spinal cord via an implan ted cannula or by direct puncture of the intrathecal space significantly bl ocked the intensity of the hyperalgesia induced by PGE(2). Cannulated anima ls treated with indomethacin also showed a significant inhibition of second phase formalin-induced paw flinches. Histopathological analysis of the spi nal cord showed an increased frequency of mononuclear inflammatory cells in the Cn groups. Thus, the presence of a chronically implanted cannula seems to cause nociceptive spinal sensitization to mechanical, chemical and ther mal stimulation, which can be blocked by indomethacin, thus suggesting that it may result from the spinal release of prostaglandins due to an ongoing mild inflammation.