The antinociceptive effects of intrathecally administered levonantradol and desacetyllevonantradol in the rat.

Levonantradol and its desacetylated metabolite (desacetyllevonantradol) produced a dose-dependent increase in the hot plate and tail flick response latencies following intrathecal administration in a dose range from 4 to 40 micrograms. No difference in potency between the two drugs was observed, as defined by the ED50 values obtained in either test. The duration of the effect of either drug was also dose dependent ranging from 30 to 120 minutes. No effect on placing, stepping or righting reflexes or the ability of the rats to negotiate a 60 degrees inclined plane was observed at these doses. At the highest doses, however, an exaggerated myoclonic response to abrupt muscle stretch was observed. None of those effects was antagonized by naloxone (2 mg/kg) administered intraperitoneally before or after levonantradol.