Mechanism
Tolerance and physical dependence
Tolerance develops rapidly to the sleep-inducing effects of benzodiazepines. The anticonvulsant and muscle-relaxant effects last for a few weeks before tolerance develops in most individuals. Tolerance results in a desensitization of GABA receptors and an increased sensitization of the excitatory neurotransmitter system, such as NMDA glutamate receptors. These changes occur as a result of the body trying to overcome the drugs effects. Other changes that occur are the reduction of the number of GABA receptors (downregulation) as well as possibly long-term changes in gene transcription coding of brain cells. The differing speed at which tolerance occurs to the therapeutic effects of benzodiazepines can be explained by the speed of changes in the range of neurotransmitter systems and subsystems that are altered by chronic benzodiazepine use. The various neurotransmitter systems and subsystems may reverse tolerance at different speeds, thus explaining the prolonged nature of some withdrawal symptoms. As a result of a physical dependence that develops due to tolerance, a characteristic benzodiazepine withdrawal syndrome often occurs after removal of the drug or a reduction in dosage. Changes in the expression of neuropeptides such as corticotropin-releasing hormone and neuropeptide Y may play a role in benzodiazepine dependence. Individuals taking daily benzodiazepine drugs have a reduced sensitivity to further additional doses of benzodiazepines. Tolerance to benzodiazepines can be demonstrated by injecting diazepam into long-term users.
In normal subjects, increases in growth hormone occurs, whereas, in benzodiazepine-tolerant individuals, this effect is blunted.Animal studies have shown that repeated withdrawal from benzodiazepines leads to increasingly severe withdrawal symptoms, including an increased risk of seizures; this phenomenon is known as kindling. Kindling phenomena are well established for repeated ethanol (alcohol) withdrawal; alcohol has a very similar mechanism of tolerance and withdrawal to benzodiazepines, involving the GABAA, NMDA, and AMPA receptors.The shift of benzodiazepine receptors to an inverse agonist state after chronic treatment leads the brain to be more sensitive to excitatory drugs or stimuli. Excessive glutamate activity can result in excitotoxicity, which may result in neurodegeneration. The glutamate receptor subtype NMDA is well known for its role in causing excito-neurotoxicity. The glutamate receptor subtype AMPA is believed to play an important role in neuronal kindling as well as excitotoxicity during withdrawal from alcohol as well as benzodiazepines. It is highly possible that NMDA receptors are involved in the tolerance to some effects of benzodiazepines.Animal studies have found that glutamergic changes as a result of benzodiazepine use are responsible for a delayed withdrawal syndrome, which in mice peaks 3 days after cessation of benzodiazepines. This was demonstrated by the ability to avoid the withdrawal syndrome by the administration of AMPA antagonists. It is believed that different glutamate subreceptors, e.g., NMDA and AMPA, are responsible for different stages/time points of the withdrawal syndrome. NMDA receptors are upregulated in the brain as a result of benzodiazepine tolerance. AMPA receptors are also involved in benzodiazepine tolerance and withdrawal.