Physiological mechanisms and potentiative effects of pregnancy-induced analgesia by various types of calcium ion channel antagonists at the level of the spinal cord.

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This is one of series of studies to examine the physiology and pharmacology of pregnancy-induced analgesia at the level of spinal cord in rats.(1)We investigated the effect of L-type calcium channel blocker, verapamil, on pregnancy-induced analgesia at the level of the spinal cord in rats. Using time mated, pregnant rats, antinociceptive effects of verapamil (50mug, 100mug, 200mug) was assessed by tail-flick test. In pregnant animals, a significant increase in tail-flick latency was demonstrated on late in pregnancy. Intrathecally administered verapamil produced a dose-dependent prolongation of tail flick latency on late pregnancy. However, this potensiative effect in tail-flck test was not observed on early gestation and post-partum period. The results suggest a synergistic antinocieptive effect of verapamil due to an interaction with an endogenous opiate system that is only activated late in pregnancy. (Anesthesiology 1997 : 87 : A653).(2)This study was designed to evaluate the anti nociceptive and hemodynamic interactions of alpha-2 adrenergic agonist, tizanidine and clonidine with lidocaine. Using Male SD rats chronically implanted with lumbar intrathecal catheters, the tail-flick test was used to assess the thermal nociceptive threshold. The ability of intrathecal tizanidine, clonidine, lidocaine, or the combinations of alpha-2 adrenergic agonist and lidocaine to alter the tail-flick latency was examined. Intrathccal tizanidine, clonidine, or the combinations increased the tail-flick latency in dose-, and time-dependent fashion without affecting motor function. With isobolographic analysis, tizanidine and clonidine with lidocaine showed significantly synergistic antinociceptive interaction. Theses results may be useful in clinical practice without circulatory side effects. (Anesthesiology 1997 : 87 : 436-448).