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Physiological tolerance or drug tolerance is commonly encountered in pharmacology, when a subject's reaction to a specific drug and concentration of the drug is progressively reduced, requiring an increase in concentration to achieve the desired effect. Drug tolerance can involve both psychological drug tolerance and physiological factors. 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. Physiological tolerance also occurs when an organism builds up a resistance to the effects of a substance after repeated exposure. This can occur with environmental substances, such as salt or pesticides. A rapid drug tolerance is termed tachyphylaxis.
Tachyphylaxis is a sudden onset drug tolerance which is not dose dependent.
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, barbiturates, benzodiazapines and opiates.
Pharmacodynamic tolerance (reduced responsiveness) is the response to the substance is decreased by cellular mechanisms. This may be caused by a down regulation of receptor numbers.
Tolerance is a reduced response to repeated administration of the same dose or increase in the dose are required to produce the same magnitude of response.
Morphine as an example
Tolerance to the analgesic effects of morphine is fairly rapid. There are several hypotheses about how tolerance develops, including opioid receptor phosphorylation (which would change the receptor conformation), functional decoupling of receptors from G-proteins (leading to receptor desensitization), mu-opioid receptor internalization and/or receptor down-regulation (reducing the number of available receptors for morphine to act on), and upregulation of the cAMP pathway (a counterregulatory mechanism to opioid effects) (For a review of these processes, see Koch and Hollt.) CCK might mediate some counter-regulatory pathways responsible for opioid tolerance. CCK-antagonist drugs, specifically proglumide, have been shown to slow the development of tolerance to morphine or any other kind of drug, including alcohol.
Significant involvement of the intracellular beta-arrestin-2 protein expression in the agonist-mediated desensitization of G protein-coupled receptors, such as the μ-opioid receptor (MOR), has been elucidated.
It was reported that VTA dopamine neurons in rats remain increased for at least 3 days after a single morphine exposure. Within this limited window of time, the VTA dopamine neurons failed to respond to additional morphine challenge. Indicating a transient morphine tolerance in VTA DA neuron activity in rats was developed with a single dose of morphine treatment. It further demonstrated that this acute morphine tolerance was associated with impairment of opiate receptor-G protein coupling, indicating that down regulation of G-protein activation may contribute to acute morphine tolerance.
- Physical dependence
- Reverse tolerance
- Drug resistance
- Multidrug resistance
- Drug Tolerance at the US National Library of Medicine Medical Subject Headings (MeSH)
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