Bretazenil has a more broad spectrum of action than traditional benzodiazepines as it has been shown to have high affinity binding to alpha 4 and alpha 6 GABAA receptors in addition to acting on alpha1,2,3 and 5 subunits which traditional benzodiazepine drugs work on. The partial agonist imidazenil does not, however, act at these subunits. The approximate equivalent dose of bretazenil is 0.5 mg of bretazenil is equivalent to 10 mg of diazepam. The elimination half-life of bretazenil is 2.5 hours and is rapidly absorbed after oral administration. Contrary to the results found in animals, no indications for a dissociation of the sedative and anxiolytic effects of bretazenil were found in man. In fact. bretazenil has a more pronounced psychomotor-impairing effect than diazepam. Bretazenil produces marked sedative-hypnotic effects when taken alone and when combined with alcohol. This human study also indicates that Bretazenil is possibly more sedative than Diazepam. The reason is unknown, but the study suggests the possibility that a full-agonist metabolite may be generated in humans but not animals previously tested or else that there are significant differences in benzodiazepine receptor population in animals and humans.
In a study of monkeys bretazenil has been found to antagonise the effects of full agonist benzodiazepines. However, bretazenil has been found to enhance the effects of neurosteroids acting on the neurosteroid binding site of the GABAA receptor. Another study found that bretazenil acted as an antagonist provoking withdrawal symptoms in monkeys who were physically dependent on the full agonist benzodiazepine triazolam.
Partial agonists of benzodiazepine receptors have been proposed as a possible alternative to full agonists of the benzodiazepine site to overcome the problems of tolerance, dependence and withdrawal which limits the role of benzodiazepines in the treatment of anxiety, insomnia and epilepsy. Such adverse effects appear to be less problematic with bretazenil than full agonists. Bretazenil, also has been found to have less abuse potential than benzodiazepine full agonists such as diazepam and alprazolam, long-term use of bretazenil would still be expected to result in dependence and addiction.
Bretazenil alters the sleep EEG profile and causes a reduction in cortisol secretion and increases significantly the release of prolactin. Bretazenil has effective hypnotic properties but impairs cognitive ability in humans. Bretazenil causes a reduction in the number of movements between sleep stages and delays movement into REM sleep. At a dosage of 0.5 mg of bretazenil REM sleep is decreased and stage 2 sleep is lengthened.
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