BACKGROUND:It has been previously shown that intrathecal administration of either ouabain or neostigmine can produce antinociceptive effeets.Moreover,ouabain and neostigmine are differently associated with acetylcholine.OBJECTIVE:It has been hypothesized that intrathecal administration of ouabain,in combination with neostigmine,can produce antinociceptive synergistic effects.Atropine,as a competitive antagomst,was pre-injected to verify the mechanisms of action.DESIGN,TIME AND SETTING:This study was a randomized,controlled,animal experiment,performed at the State Key Laboratory of Oneology in Southern China between May 2006 and Febrnary 2007.MATERIALS:A total of 102 healthy.adult,Sprague Dawley rats were included.Ouabain and neostigrnine (Sigma,USA),as well as atropine(Tanabe Seiyaku,Japan),were also used.METHODS:Varied doses of ouabain,neostigmine,and a combination of the two were intrathecally injected into rats.Six rats were allotted for each dose group.Intrathecal pretreatment with atropine was tested 10 minutes prior to intrathecal administration of neostjgmine or the combination of ouabain and neostigmine.MAIN OUTCOME MEASURES:Tail-flick tests were performed to measure tail-flick latency(seconds)prior to and after administration.The response in the tail-flick test was expressed as the percentage of maximum possible effect(% MPE),where% MPE=[tail-flick latency after administration(seconds)-mean baseline value for tail-flick latency]/[10 seconds-the mean baseline value for tail-flick latency(seconds)]×100%.RESULTS:Rat spinal intrathecal administration of either ouabain or neostigminc alone produced antinociceptive effects in a dose-dependent manner.Intrathecally administration of neostigmine(0.05,0.1,0.3 μg)in combination with ouabain(1 μg)produced enhanced antinociceptive effects,with a % MPE of 29%,78%,and 95%,respectively(P