Pregnancy enhances the antinociceptive effects of extradural lignocaine in the rat

We have compared the antinociceptive effect of extradural lignocaineon somatic and visceral noci-ception in pregnant (n=11) andnon-pregnant rats (n=9). Colorectal distension (CD) thresholdand tail flick (TF) latency were measured as visceral and somaticnociception. respectively, for 60 min after extradural injection.On days 19, 20 and 21 of pregnancy, rats received lignocaine200, 400 or 800 µg or normal saline via a chronicallyimplanted lumbar extradural catheter. Extradural lignocaineproduced dose-dependent antinociceptive effects on TF latencyand CD threshold. Lignocaine 200 or 400 µg produced significantlygreater peak effects in pregnant than in non-pregnant rats.Although the peak effects with lignocaine 800 µg werenot different between groups, the duration of the effects werelonger in pregnant compared with non-pregnant rats. We concludethat both somatic and visceral antinociceptive effects of extradurallignocaine were potentiated in pregnant rats near term comparedwith those in non-pregnant rats.