Partial agonists of the benzodiazepine receptor: from animal data to results in patients.

The BZR is an allosteric modulatory site of the GABAA receptor, which is unique insofar as it mediates both a positive (facilitatory) and a negative (inhibitory) modulation of the GABA-mediated gating of the chloride channel. Ligands acting as positive modulators are called BZR agonists; ligands producing negative modulation, inverse BZR agonists. The effects of both classes of ligands are blocked by antagonists. A large number of compounds belonging to these three groups of ligands are known. Concepts are presented here that could explain the modulatory function of the BZR, and a number of factors are proposed to determine the pharmacological and therapeutic profile of BZR ligands: intrinsic efficacy at the BZR, density of GABAA-BZR on neurons, functional reserve in GABAA-BZRs, the intensity of the GABAergic input to a neuron, and the excitatory neuronal state. The effect of these factors on the pharmacology of full agonists, partial agonists, antagonists, partial inverse agonists, and inverse agonists is illustrated.