[Interaction of the hypnotic lormetazepam with central benzodiazepine receptor subtypes omega 1, omega 2 and omega 3]

Abstract

To clarify the action of lormetazepam, 3-hydroxybenzodiazepine, on the benzodiazepine (BZ) receptor subtypes, effects of lormetazepam on motor performance in the traction test and hexobarbital-induced loss of righting reflex in mice and the binding to BZ receptor subtypes were investigated in comparison with those of other BZ hypnotics. Lormetazepam prolonged the duration of hexobarbital-induced loss of righting reflex. The minimal effective dose (1 mg/kg, p.o.) was higher than that of flunitrazepam, lower than those of diazepam and zopiclone, and the same as those of triazolam and brotizolam. Lormetazepam showed the ataxic effect at 10 mg/kg, p.o., but the separation between its effective doses for the hypnotic and ataxic effects was the largest among the hypnotics tested. In the displacement study on [3H]flumazenil binding to cerebellar and spinal cord membranes, lormetazepam bound with a higher affinity to omega 1 receptor (Ki = 10 nM) than to omega 2 receptor (Ki = 29 nM). The GABA-ratios of lormetazepam to omega 1 and omega 2 receptors were 3.9 and 4.0, respectively; and they were higher and lower than those of flunitrazepam to omega 1 and omega 2 receptors, respectively. In the displacement study on [3H] Ro5-4864 binding to kidney membranes, lormetazepam bound with a lower affinity to the omega 3 receptor (Ki = 213 nM) than flunitrazepam. Thus, lormetazepam was suggested to be a potent hypnotic with weaker ataxic effects than other BZ hypnotics, which may be due to its selective and potent agonistic action on central omega 1 receptors.