Physostigmine is equipotent to flumazenil only in excessive doses, while naloxone is ineffective in reversing midazolam anesthesia.

As fast-acting, water-soluble compounds, like midazolam (MID) are increasingly used in anesthetic practice, the need for quick and effective reversal of benzodiazepine actions in accidental overdoses is obvious. Quite a few drugs have been used in both animal models and clinically towards this aim, including: physostigmine (PHYS), naloxone (NAL), aminophylline, doxapram, in the past and, more recently, flumazenil (FLU) or Ro-15-1788. In the present study we assessed the relative potency, safety and efficacy of the antagonistic actions of PHYS (0.06 and 0.6 mg/kg), NAL (2 mg/kg) and FLU (2.5 mg/kg) on the state of sleep induced by MID (2.5 mg/kg) in 50 male, drug-naive rats. Time of induction of anesthesia and duration of sleep were measured with digital chronometer. Level of consciousness was determined by pain reactivity, spontaneous motor activity and computer-aided analysis of cumulative EEG patterns. Times obtained were integrated as mean values and statistical analysis was made using the t-test (Student's criterion). Rats were given MID followed 30 min later by an i.p. injection of either vehicle (VEH = water for injection = control group) or low-dose PHYS or high-dose PHYS or NAL or FLU (n = 10 in each group). We measured the duration of anesthesia following the injection of each antagonist. The following results were obtained: [table: see text] Thus, the relative potency scale of MID antagonism in this series of experiments is: FLU = High-dose PHYS greater than low-dose PHYS much greater than NAL VEH. Although FLU has a strong duration and rapid awakening action, this action is of short duration and the animals are resedated after 33.3 +/- 7.14 min. PHYS does not appear to show this resedation effect, while in high doses is equipotent to FLU. Nevertheless, in these excessive doses, the animals become restless and excited, showing phenomena of an acute, severe nicotinic crisis. In conclusion, FLU appears to have much more specific and reliable effects upon reversing MID-induced anesthesia in comparison to either PHYS or NAL. In fact, PHYS must reach toxic levels to counteract MID effects, while FLU is very safe. The existence of the resedation effect is due to the short t1/2 of FLU and can be overcome by continuous i.v. infusions or repeated bolus injections.