Carisoprodol produced a substantial and dose-dependent loss of motor coordination as measured by the loss-of-righting reflex [F(4,99)=103.606, p<.001], as shown in . Tolerance to this effect of carisoprodol occurred over the 4 days, as evidenced by a decrease in the loss-of-righting score over time [F(6,594)=38.915, p<.001]. There was a dose by time interaction [F(24,594)=8.561, p<.001], as the decrease in loss-of-righting reflex occurred in the groups receiving 200, 300 and 500 mg/kg, but not the groups receiving vehicle or 100 mg/kg. The loss-of-righting score increased in the 500 mg/kg group between the morning and evening sessions on the first day.
There was a significant main effect of bemegride on withdrawal signs 15 min after administration [F(1,36)=28.733, p<.001]. As shown in , this outcome reflected the significantly higher withdrawal scores in the 100 and 500 mg/kg carisoprodol-treated mice receiving bemegride when compared to their matching vehicle-injected controls. Although there was no main effect of carisoprodol dose [F(3,36)=0.939, p=0.432], there was a significant interaction effect [F(3,36)=20.870, p=0.013], which suggests that the degree of withdrawal increased as dose of carisoprodol increased. The interaction may also have been due to the small drop in withdrawal signs in the vehicle group, although this further adds to the appearance of a dose-dependent effect. Withdrawal signs remained elevated 30 min after administration [F(1,38)=48.03, p<.001], as evidenced by significant differences from treatment-matched control in the mice that had received bemegride, for the 100, 300 and 500 mg/kg carisoprodol groups. There was no main effect of carisoprodol dose [F(3,36)=1.995, p=0.132], and no interaction effect [F(3,36)=0.908, p=0.447].
Precipitated Withdrawal Scores Following Bemegride or Flumazenil
There was a significant main effect of flumazenil on withdrawal signs 15 min after administration [F(1,39)=8.682, p=0.005]. This outcome was driven mainly by the significantly higher withdrawal scores in the flumazenil-injected versus vehicle-injected mice of the 500 mg/kg carisoprodol goup (). There was no main effect of carisoprodol dose [F(3,39)=2.752, p=0.055], and no interaction effect [F(3,39)=1.563, p=0.214] at the 15-min time period. At 30 min after administration, there was also a significant effect of flumazenil on withdrawal signs [F(1,39)=20.263, p<0.001], and there were significant differences from treatment-matched control in the number of withdrawal signs after flumazenil for the mice receiving the 100, 300 and 500 mg/kg carisoprodol doses. There was a main effect of carisoprodol dose [F(3,39)=4.272, p<0.011] which reflected the observation that flumazenil-associated withdrawal scores increased with the dose of carisoprodol. However, there was no interaction effect [F(3,39)=1.946, p<0.138].
In general, the bemegride- and flumazenil-associated withdrawal scores did not differ qualitatively and involved caudal posture, tremor, and enhanced startle response. In contrast, mice receiving the same regimens of carisoprodol showed very few signs when tested for spontaneous withdrawal from 6 to 24 hours following the last dose (). While there was indeed a statistically significant main effect of carisoprodol dose [F(3,33)=4.024, p=0.013], this outcome reflected a time- and dose-independent trend involving relatively small differences between carisoprodol (100 and 500 mg/kg) and vehicle-treated mice. The increase in withdrawal signs at the 100 and 500 mg/kg doses could not be confirmed by comparisons with control at the individual time points. There was also neither a main effect of time [F(2,66)=0.270, p=0.765] nor a dose by time interaction [F(6,66)=0.570, p=0.753]. Weights did not change over the course of the experiment [F(4,132)=1.941, p=0.107].