Pregnancy enhances the antinociceptive effects of extradural lignocaine in the rat.

We have compared the antinociceptive effect of extradural lignocaine on somatic and visceral nociception in pregnant (n = 11) and non-pregnant rats (n = 9). Colorectal distension (CD) threshold and tail flick (TF) latency were measured as visceral and somatic nociception, respectively, for 60 min after extradural injection. On days 19, 20 and 21 of pregnancy, rats received lignocaine 200, 400 or 800 micrograms or normal saline via a chronically implanted lumbar extradural catheter. Extradural lignocaine produced dose-dependent antinociceptive effects on TF latency and CD threshold. Lignocaine 200 or 400 micrograms produced significantly greater peak effects in pregnant than in non-pregnant rats. Although the peak effects with lignocaine 800 micrograms were not different between groups, the duration of the effects were longer in pregnant compared with non-pregnant rats. We conclude that both somatic and visceral antinociceptive effects of extradural lignocaine were potentiated in pregnant rats near term compared with those in non-pregnant rats.