Objective To investigate the relationship between the
antinociception of
intrathecal tramadol and Oh -adrenoceptor effect
of the spinal cord in rats. Methods Forty adult Wistar rats of both sexes weighing 280-320 g wen- anesthetized with ketamine 40 mg . kg-1 and pentobarbital 20 mg. kg-1 ip and PE-10 polyethylene catheter was placed in subarachnoid space 4-6 days before experiment. Tbe animals were randomly divided into live groups of 8 animals : (1) control group received normal saline; (2) group T tramadol 10 μg; (3) group Y yohimbine 10μg;group Y + T yohimbine 10 μg + tramacol 10 μg and (5) group Y + N + T received yohimbine 10 μg + naloxone 10μg + tramadol 10 μg. The above-mentioned drugs including normal saline were injected intrathecally. 2.5% formalin 50ul was injected subcutaneously into the plantar region of left hindpaw. 15 min later the above-mentioned drugs were injected intrathecally. In group 4 and 5 yohimbine and naloxone were given 5 min before tramadol. The animals were placed in a 30 cm × 30 cm× 30 cm glass box for evaluation of pain behavior, which was graded from 0 ( no pain) to 3 ( severe Pain) according to Dobuisson and Dennis. Another sixteen animals were decapitated and lumbar spinal cord was rapidly removed and made into crude membrane suspension. H-yohimbine was used as radioligand for specific binding, yohimbin HCl for nonspecific binding and tramadol as competitor. Drug displacement studies for specific H-yohimbine binding were carried out under standard incubation conditions with 0.21 nmol.L-1 to 2. 1 nmol.L-1 (10 concentrations) of tramadol. Dissociation constant (Kd) and inhibition constant ( Ki) were obtained by special computerized parameter estimation method program. Results
intrathecal tramadol 10 μg could produce time-dependent
antinociception. Yohimbine 10μg alone did not produce antinociception ( P > 0.05), but pretreatment with yohimbine 10 ug significantly reduced the antinociceptive effect of tramadol ( 10ug) at 35 min and 40 min and the nociception score increased by 56% and 41 % respectively ( P <0.01). Addition of naloxone increased the nociception score by 200% and 75% but the score was still lower than that in control group, but the difference was not statistically significant ( P > 0.05). Scatchard analysis of the saturation isotherms showed that H-yohimbine was bound to a single binding site with a Kd value of 1.79 nM. The competition curve of tramadol was sigmoidal with a Ki value of 34.14 uM and an IC50 value of 68.25 uM. Tramadol was 19 000-fold less potent for binding to a2-adrenoceptor of the spinal cord as compared to H-yahimbine. Conclusion Intrathecal tramadol produces time-dependent antinociception. Tramadol has very low affinity with a2-adrenoceptor of the spinal cord. A part of its intrathecal antinociceptive effect was related to indirect a2-adrenoceptor effect of the spinal cord.