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Cross-tolerance is a phenomenon that occurs when someone who is tolerant to the effects of a certain drug also develops a tolerance to another drug. It often happens between two drugs with similar functions or effects – for example, acting on the same cell receptor or affecting the transmission of certain neurotransmitters. Cross-tolerance has been observed with pharmaceutical drugs such as anti-anxiety agents and illicit substances, and sometimes the two of them together. Often, a person who uses one drug can be tolerant to a drug that has a completely different function. This phenomenon allows one to become tolerant to a drug that they have never even used before.
Drug classifications and cross-tolerance
|Groups of Psychoactive drugs||Drug examples|
|Anxiolytics and sedatives||Benzodiazepines (valium, xanax, klonopin), Z-drugs, barbiturates, ethanol|
|Antipsychotics||Phenothiazines (chlorpromazine), Butyrophenones (haloperidol), Clozapine (Clozaril), Aripiprazole (Abilify)|
|Antidepressants and mood stabilizers||MAO inhibitors, tricyclic antidepressants (imipramine), SSRIs (fluoxetine, sertraline, paroxetine), mood stabilizers (Lithium, sodium valproate, carbamazepine)|
|Opioid analgesics||Morphine, codeine, heroin, endomorphins, enkephalins, dynorphins|
|Psychedelics||Muscarinic (Datura, amanita), Cannabinoidergic (THC), NMDA antagonists (PCP, Ketamine), Serotonergic (LSD, mescaline, psilocybin)|
Anxiolytics and sedatives
Excitation of the GABA receptor produces an influx of negatively-charged chloride ions, which hyperpolarizes the neuron and makes it less likely to give rise to an action potential. In addition to gamma-Aminobutyric acid (GABA) itself, the GABAA receptor can also bind barbiturates and benzodiazepines. Benzodiazepine binding increases the binding of GABA and barbiturates maximize the time the pore is open. Both of these mechanisms allow for influx of chloride ions. When these drugs are taken together, especially with ethanol (drinking alcohol), there is a disproportionate increase in toxicity because the effects of both occur simultaneously and add up since they act on the same receptor at different sites. Convergence upon the GABAA receptor is why tolerance for one drug in the group will most likely cause cross-tolerance for the other drugs in the group. However, the barbiturates are also AMPA receptor blockers, and in addition interact with the nAChR and voltage-gated calcium channels. As a result, somebody who is tolerant to benzodiazepines is more sensitive to barbiturates than vice versa.
These drugs block dopamine receptors and can, in many cases (very common example: chlorpromazine, the very first antipsychotic used in a clinical setting), also block serotonin receptors. Having been on one or more antipsychotics for any appreciable amount of time results in dramatically reduced sensitivity to neuroleptics with a similar, if not identical, mechanism of action. However, an antipsychotic with a substantial disparity in pharmacology (e.g. haloperidol and quetiapine) may retain significant efficacy.
Antidepressants and mood stabilizers
A MAO inhibitor allows for more serotonin release by inhibiting enzymes that break down serotonin. Some of the other drugs block the reuptake transporter which takes some of the neurotransmitters back up, and disallows for the neurotransmitter to reach the next synapse. In stark contrast, several anticonvulsants, such as carbamazepine and lamotrigine are also used for mood disorders. This would demonstrate little to zero cross-tolerance with serotonergic and/or lithium treatment.
These drugs mimic three classes of endorphins, such as endomorphins, enkephalins, and dynorphins. All three of these classes each have their own receptor-mu, kappa, and delta. Opioids will bind to the receptor for the endorphin they are most chemically similar to. Tolerance develops quickly with regular use, with the downregulation of the stimulated receptors to blame.
Amphetamine and cocaine block the reuptake of dopamine and norepinephrine. Amphetamine also helps release dopamine from presynaptic membranes.
Cross-tolerance between drugs of different classifications
Sometimes cross-tolerance occurs between two drugs that do not share mechanisms of action or classification. For example, amphetamine and amphetamine-like stimulants have been shown to exhibit cross-tolerance with caffeine, and it is likely the mechanism of cross-tolerance involves the dopamine receptor D1. Amphetamines also have cross-tolerance with pseudoephedrine, as pseudoephedrine can block dopamine uptake in the same manner that amphetamines do, but less potently.
Alcohol is another substance that often cross-tolerates with other drugs. Findings of cross-tolerance with nicotine in animal models suggest that it is also possible in humans, and may explain why the two drugs are often used together. Numerous studies have also suggested the possibility of cross-tolerance between alcohol and cannabis.
- Kolb, Bryan, and Ian Whishaw. An Introduction to Brain and Behavior. New York: Worth Publishers, 2014. Print.
- The Free Dictionary
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