On the mechanism of anticonvulsant effect of tramadol in mice.

The present study was conducted to examine the effects of tramadol, an atypical opioid on convulsive behaviour in maximal electroshock (MES) seizure test on mice. Moreover, an attempt was also made to investigate the role of possible receptor mechanisms involved. MES seizures were induced via transauricular electrodes (60 mA, 0.2 sec). Seizure severity was determined by (1) the duration of tonic hindlimb extensor (THE) phase and by (2) mortality due to electroconvulsions. Intraperitoneal (i.p.) administration of tramadol dose-dependently (10-50 mg/kg) decreased the duration of THE phase of MES. The anticonvulsant effect of tramadol was antagonized by the opioid antagonists, naloxone in high dose, and MR2266, a selective kappa antagonist but not by naltrindole, a delta opioid antagonist. Coadministration of either gamma-aminobutyric acid (GABA)-ergic drugs (diazepam, GABA, muscimol and baclofen) or N-methyl-D-aspartate (NMDA) receptor antagonist, MK801 with tramadol augmented the anticonvulsant effect of the latter drug. By contrast, flumazenil, a central benzodiazepine (BZD) receptor antagonist, counteracted the diazepam-induced facilitation of anti-MES effect of tramadol. Similarly, delta-aminovaleric acid (DAVA), a GABAB receptor antagonist, abolished the facilitatory effect of baclofen, a GABAB agonist on anti-MES action of tramadol. These BZD-GABAergic antagonists, flumazenil or DAVA, on their own also antagonized the anti-MES effect of tramadol administered alone. No significant effect on mortality was observed in any of the studied groups. Taken together, the current results have demonstrated a possible role for multitude of important neurotransmitter systems, i.e., opioid (kappa), GABAA-BZD receptors system, GABAB receptors and NMDA channel involvement in the antielectroshock effect of tramadol in mice.