Benzodiazepine - Pharmacology - Mechanism of Action

Mechanism of Action

Benzodiazepines work by increasing the efficiency of a natural brain chemical, GABA, to decrease the excitability of neurons. This reduces the communication between neurons and, therefore, has a calming effect on many of the functions of the brain.

GABA controls the excitability of neurons by binding to the GABA_A receptor. The GABA_A receptor is a protein complex located in the synapses of neurons. All GABA_A receptors contain an ion channel that conducts chloride ions across neuronal cell membranes and two binding sites for the neurotransmitter gamma-aminobutyric acid (GABA), while a subset of GABA_A receptor complexes also contain a single binding site for benzodiazepines. Binding of benzodiazepines to this receptor complex promotes binding of GABA, which in turn increases the conduction of chloride ions across the neuronal cell membrane. This increased chloride ion conductance hyperpolarizes the neuron's membrane potential. As a result, the difference between resting potential and threshold potential is increased and firing is less likely. Different GABA_A receptor subtypes have varying distributions within different regions of the brain and, therefore, control distinct neuronal circuits. Hence, activation of different GABA_A receptor subtypes by benzodiazepines may result in distinct pharmacological actions. In terms of the mechanism of action of benzodiazepines, their similarities are too great to separate them into individual categories such as anxiolytic or hypnotic. For example, a hypnotic administered in low doses will produce anxiety-relieving effects, whereas a benzodiazepine marketed as an anti-anxiety drug will at higher doses induce sleep.

The subset of GABA_A receptors that also bind benzodiazepines are referred to as benzodiazepine receptors (BzR). The GABA_A receptor is a heteromer composed of five subunits, the most common ones being two αs, two βs, and one γ (α₂β₂γ). For each subunit, many subtypes exist (α_1-6, β_1-3, and γ_1-3). GABA_A receptors that are made up of different combinations of subunit subtypes have different properties, different distributions in the brain and different activities relative to pharmacological and clinical effects. Benzodiazepines bind at the interface of the α and γ subunits on the GABA_A receptor. Binding also requires that alpha subunits contain a histidine amino acid residue, (i.e., α₁, α₂, α₃, and α₅ containing GABA_A receptors). For this reason, benzodiazepines show no affinity for GABA_A receptors containing α₄ and α₆ subunits with an arginine instead of a histidine residue.

Once bound to the benzodiazepine receptor, the benzodiazepine ligand locks the benzodiazepine receptor into a conformation in which it has a greater affinity for the GABA neurotransmitter. This increases the frequency of the opening of the associated chloride ion channel and hyperpolarizes the membrane of the associated neuron. The inhibitory effect of the available GABA is potentiated, leading to sedatory and anxiolytic effects. Furthermore, different benzodiazepines can have different affinities for BzRs made up of different collection of subunits. For instance, those with high activity at the α₁ are associated with stronger hypnotic effects, whereas those with higher affinity for GABA_A receptors containing α₂ and/or α₃ subunits have good anti-anxiety activity.

The benzodiazepine class of drugs also interact with peripheral benzodiazepine receptors. Peripheral benzodiazepine receptors are present in peripheral nervous system tissues, glial cells, and to a lesser extent the central nervous system. These peripheral receptors are not structurally related nor coupled to GABA_A receptors. They modulate the immune system and are involved in the body response to injury. Benzodiazepines also function as weak adenosine reuptake inhibitors. It has been suggested that some of their anticonvulsant, anxiolytic and muscle relaxant effects may be in part mediated by this action.