PSYC62H3 Chapter Notes -Drug Tolerance, Tachyphylaxis, Pharmacodynamics

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24 Apr 2012
Chapter Six - Page 1 of 2
Chapter Six: Tolerance, Sensitization, Dependence, and Addiction
Tolerance and Sensitization
Drug tolerance occurs when there’s a reduction in the potency or efficacy of a drug with
repeated administration
o Increasingly larger doses of the drug are required to induce the same behavioural effect,
although in some cases, tolerance can be so dramatic that no amount of drug is capable
of inducing its original effects
o Tolerance due to drug exposure is different from genetic or dispositional tolerance
Although tolerance generally requires several drug exposures before it`s evidenced, there`s a
special case of tolerance in which a noticeable decrease in the organism`s sensitivity to the drug
occurs over a very short period of time tachyphylaxis or acute tolerance
o the same amount of drug administered on two separate occasions a couple hours apart
may induce greater effects with the first dose than with the second
o also evident when the behavioural/physiological effects of a given dose of a drug
dissipate at a faster rate than the rate at which the drug is eliminated from the brain
Cross-tolerance refers to the phenomenon in which the development of tolerance to one type
of drug results in decreased sensitivity to the effects of another type of drug
o Alcohol with barbiturate
Tolerance to the effects of drugs doesn’t develop normally; some effects may show profound
tolerance, whereas others may show little/no tolerance
The beneficial or recreational effects of the drugs may show considerable tolerance, but little
tolerance develops to the lethal effects of these drugs
With this type of tolerance, the therapeutic index for these drugs gets smaller and smaller
With other drugs, tolerance to the desirable effects of a drug may occur while the person
actually becomes more sensitive to the side effects of the drug
The ability of amphetamine to induce euphoria decreases with regular use, but the ability for it
to induce psychotic-like effects may actually increase with regular use of large doses
Tolerance is of concern because many of the mechanisms resulting in tolerance contribute to a
person’s compulsion to take a drug with loss of control over drug intake – that is, drug addiction
Drug sensitization reverse tolerance increase in potency/efficacy with repeated
administration. This means that the drug produces greater effects at the same dose with
repeated administration, and that the same effect can be produced by lower doses. It could be a
critical aspect of drug seeking, taking, and related processes
Tolerance and Sensitization Mechanisms
Three types of drug tolerance and sensitization: pharmacokinetic, pharmacodynamic, and
context specific
Pharmacokinetic and pharmacodynamic forms of tolerance are produced by exposure to high
concentrations of a drug, generally with a certain minimum amount of exposure time required,
and are not affected by environmental or behavioural manipulations of the organism
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