Muscle rigidity induced by the opioid analgesic tramadol, but not by the non-opioid flupirtine.

The influence of high doses of two analgesic drugs, tramadol and flupirtine on the electromyographic activity in the gastrocnemius soleus muscles was examined. Tramadol (100-200 mg/kg po) dose-dependently induced a tonic electromyographic activity, which is generally accepted as a model of the opiate-induced muscle rigidity. That effect was antagonized by intraperitoneal injection of naloxone (0.8 mg/kg ip). On the other hand, flupirtine even in the high doses (100-200 mg/kg po) did not induce any tonic electromyographic activity. The obtained results confirm an opiate-like action of tramadol, but not that of flupirtine, on the muscle tension.