Aspects of benzodiazepine receptor structure and function with relevance to drug tolerance and dependence

Potential development of tolerance to and dependence on benzodiazepine tranquilizers often limit their use for long-term treatment of epilepsy, anxiety and insomnia. Current developments in benzodiazepine receptor pharmacology, i.e. the advent of partial agonists and receptor subtype specific agonists (Fig. 1), however, might eventually overcome these limitations, thus greatly improving therapeutic prospects. The present study demonstrates that subchronic administration of alprazolam (a high-efficacy agonist) results in strong withdrawal reactions upon injection of a benzodiazepine receptor antagonist in mice and monkeys. The partial agonist bretazenil, as well as the benzodiazepine receptor type 1-preferring agonist zolpidem, however, are much less prone to producing such reactions. Neurochemical studies showed that subchronic infusion of lorazepam (a high-efficacy agonist), in contrast to betrazenil, led to benzodiazepine receptor downregulation in vivo and reduced potentiation of gamma-aminobutyric acid (GABA)-stimulated chloride flux by diazepam ex vivo. These findings indicate that partial and receptor subtype 1-selective agonists differ from full, non-selective agonists in their liability to induce drug dependence and tolerance upon chronic administration. It is hypothesized that the neurochemical basis of these adaptive phenomena may be receptor downregulation and/or reduced coupling between GABAA receptor/chloride channel gating and benzodiazepine receptor binding.