[Mechanism of action of the analgesic flupirtine].

To answer the questions of mode and site of action partly supplementary, partly new investigations with flupirtine (Katadolon) were carried out which are described below. The investigation for opiate receptor affinity of flupirtine in rat brain homogenate did not show any reduction in 3He-etorphine binding up to the highest concentration of flupirtine of 10(-5) mol/1. This result suggests that flupirtine either has a very low opiate receptor affinity or lacks it fully. Therefore the analgesic activity of flupirtine is not based on opiate mechanism. The intracerebroventricular and intrathecal administration of flupirtine and the other analgesics tested showed dose dependent analgesic activity in doses which, when applied systemically, did not cause any analgesia in rats. Thus these substances show cerebral or spinal analgesic activity. In relation to the effective doses (ED50 in micrograms/rat) flupirtine was of the same efficacy in both kinds of administration. Pethidine tested comparatively was found to be less potent by intrathecal than by intracerebroventricular application. On the other hand, morphine was weaker by intracerebroventricular than by intrathecal application. As in the experiments by oral administration, naloxone did not show any effect on the analgesic activity of flupirtine, neither by intracerebroventricular nor by intrathecal application. On the other hand, the analgesic effects of pethidine and morphine were completely suppressed by naloxone. These results demonstrate that the analgesic activity of flupirtine is not caused by the opiate mechanism.(ABSTRACT TRUNCATED AT 250 WORDS)