The astrocytic GABA(A)/benzodiazepine-like receptor: the Joker receptor for benzodiazepine-mimetic drugs?

Long-term use of benzodiazepines as hypnotics, anxiolytics, anticonvulsants and muscle relaxing drugs is jeopardized by adverse effects on memory, addictive properties, and development of tolerance. Major efforts have gone into developing 'benzodiazepine-like' drugs that are more selective in their therapeutic effect, have additional uses and/or lack the adverse effects of benzodiazepines. The reviewed prototype patent exemplifies such efforts. Newer drugs are thought to act selectively on one of the two neuronal benzodiazepine receptors, on the astrocytic mitochondrial benzodiazepine receptor and/or on GABA(A)/benzodiazepine receptor complexes displaying specific subunits. It is overlooked that astrocytes also express benzodiazepine receptors that enhance depolarization-mediated entry of Ca(2+) by interacting with membrane-associated GABA(A)-like receptors, mediating depolarization because of a high Cl(-) concentration within astrocytes. The resulting increase in free cytosolic Ca(2+), which stimulates glycogenolysis, is inhibited not only by the 'peripheral-type" benzodiazepine antagonist PK11195 but also by the 'neuronal' antagonist flumazenil. Increasing awareness of the role(s) of astrocytic Ca(2+) homeostasis and energy metabolism for CNS function suggests that activation of this receptor might contribute to both therapeutic and adverse effects of benzodiazepine-like drugs. This receptor should be kept in mind when developing and testing new drugs; in turn these drugs may help elucidating its functional role.