Drug tolerance

Addiction and dependence glossary[1][2][3]
• addiction – a state characterized by compulsive engagement in rewarding stimuli despite adverse consequences
• addictive behavior – a behavior that is both rewarding and reinforcing
• addictive drug – a drug that is both rewarding and reinforcing
• dependence – an adaptive state associated with a withdrawal syndrome upon cessation of repeated exposure to a stimulus (e.g., drug intake)
• drug sensitization or reverse tolerance – the escalating effect of a drug resulting from repeated administration at a given dose
• drug withdrawal – symptoms that occur upon cessation of repeated drug use
• physical dependence – dependence that involves persistent physical–somatic withdrawal symptoms (e.g., fatigue and delirium tremens)
• psychological dependence – dependence that involves emotional–motivational withdrawal symptoms (e.g., dysphoria and anhedonia)
• reinforcing stimuli – stimuli that increase the probability of repeating behaviors paired with them
• rewarding stimuli – stimuli that the brain interprets as intrinsically positive or as something to be approached
• sensitization – an amplified response to a stimulus resulting from repeated exposure to it
• tolerance – the diminishing effect of a drug resulting from repeated administration at a given dose

Drug tolerance is a pharmacology concept where a subject's reaction to a specific drug and concentration of the drug is reduced followed repeated use, requiring an increase in concentration to achieve the desired effect.^[4]

The following are characteristics of drug tolerance: it is reversible, the rate depends on the particular drug, dosage and frequency of use, differential development occurs for different effects of the same drug.

Tachyphylaxis

Tachyphylaxis is a sudden onset drug tolerance which is not dose dependent.

Pharmacodynamic tolerance

Pharmacodynamic tolerance occurs when the cellular response to a substance is reduced with repeated use. A common cause of pharmacodynamic tolerance is high concentrations of a substance constantly binding with the receptor, desensitizing it through constant interaction.^[5] Other possibilities include a reduction in receptor density (usually associated with receptor agonists), or other mechanisms leading to changes in action potential firing rate.^[6] Pharmacodynamic tolerance to a receptor antagonist involves the reverse, i.e., increased receptor firing rate, an increase in receptor density, or other mechanisms.

While most occurrences of pharmacodynamic tolerance occur after sustained exposure to a drug, instances of acute or instant tolerance can occur.^[7]

Pharmacokinetic (metabolic) tolerance

Pharmacokinetics refers to the absorption, distribution, metabolism, and excretion of drugs. All psychoactive drugs are first absorbed into the bloodstream, carried in the blood to various parts of the body including the site of action (distribution), broken down in some fashion (metabolism), and ultimately removed from the body (excretion). All of these factors are very important determinants of crucial pharmacological properties of a drug, including its potency, side effects, and duration of action.

Pharmacokinetic tolerance (dispositional tolerance) occurs because of a decreased quantity of the substance reaching the site it affects. This may be caused by an increase in induction of the enzymes required for degradation of the drug e.g. CYP450 enzymes. This is most commonly seen with substances such as ethanol.

This type of tolerance is most evident with oral ingestion, because other routes of drug administration bypass first-pass metabolism. Enzyme induction is partly responsible for the phenomenon of tolerance, in which repeated use of a drug leads to a reduction of the drug’s effect. However, it is only one of several mechanisms of tolerance

Behavioral tolerance

Behavioral tolerance occurs with the use of certain psychoactive drugs, where tolerance to a behavioral effect of a drug, such as increased motor activity by methamphetamine, occurs with repeated use; it may occur through drug-independent learning or as a form of pharmacodynamics tolerance in the brain; the latter mechanism of behavioral tolerance occurs when people learn how to actively overcome drug-induced impairments through practice. Behavioral tolerance is often context dependent, meaning tolerance depends on the environment in which the drug is administered, and not the drug itself.^[8] Behavioral sensitization describes the opposite phenomenon.

See also

• Addiction
• Drug dependence
• Rebound effect
• Sensitization
• Desensitization
• Cross-tolerance
• Cross-sensitization

References

1. Malenka RC, Nestler EJ, Hyman SE (2009). "Chapter 15: Reinforcement and Addictive Disorders". In Sydor A, Brown RY. Molecular Neuropharmacology: A Foundation for Clinical Neuroscience (2nd ed.). New York: McGraw-Hill Medical. pp. 364–375. ISBN 9780071481274.
2. Nestler EJ (December 2013). "Cellular basis of memory for addiction". Dialogues Clin. Neurosci. 15 (4): 431–443. PMC 3898681. PMID 24459410.
3. "Glossary of Terms". Mount Sinai School of Medicine. Department of Neuroscience. Retrieved 9 February 2015.
4. Drug Tolerance at the US National Library of Medicine Medical Subject Headings (MeSH)
5. Bespalov, Anton; Müller, Reinhold; Relo, Ana-Lucia; Hudzik, Thomas (2016-05-01). "Drug Tolerance: A Known Unknown in Translational Neuroscience". Trends in Pharmacological Sciences 37 (5): 364–378. doi:10.1016/j.tips.2016.01.008. ISSN 1873-3735. PMID 26935643.
6. Klaassen, Curtis D. (2001-07-27). Casarett & Doull's Toxicology: The Basic Science of Poisons (6th ed.). McGraw-Hill Professional. p. 17. ISBN 0-07-134721-6.
7. Swanson, James; Gupta, Suneel; Guinta, Diane; Flynn, Daniel; Agler, Dave; Lerner, Marc; Williams, Lillie; Shoulson, Ira; Wigal, Sharon (1999-10-01). "Acute tolerance to methylphenidate in the treatment of attention deficit hyperactivity disorder in children*". Clinical Pharmacology &#38 Therapeutics 66 (3). doi:10.1016/S0009-9236(99)70038-X. ISSN 0009-9236.
8. Wolgin, D. L (2000-05-01). "Contingent tolerance to amphetamine hypophagia: new insights into the role of environmental context in the expression of stereotypy". Neuroscience & Biobehavioral Reviews 24 (3): 279–294. doi:10.1016/S0149-7634(99)00070-6.