Tolerance is defined as a decrease in pharmacologic response following repeated or prolonged drug administration. Tolerance can be separated into two main classifications: innate or acquired. Innate tolerance is a predisposition to exhibit drug sensitivity or insensitivity due to pharmacogenetic makeup. In most situations, innate tolerance is determined upon administration of the initial dose. In contrast, acquired tolerance is a consequence of repeated drug exposure, and can be subdivided into three general types based on the prevailing mechanism: pharmacokinetic, pharmacodynamic, or learned. Pharmacokinetic tolerance occurs when drug disposition or metabolism is altered as a function of time, often a consequence of the drug being an inducer or inhibitor of a specific metabolic enzyme or transporter system, resulting in a time-dependent decrease in presentation of the active moiety to the receptor biophase. Pharmacodynamic tolerance occurs when the intrinsic responsivity of the receptor system diminishes over time.
Acute tolerance is mediated predominantly by pharmacodynamic mechanisms, manifested as a decreased response following a single administration of the agent or during repeat-dosing but over a short time frame. This phenomenon is exemplified by nasally-administered cocaine. Initially, the relationship between cocaine-associated euphoria and blood cocaine concentrations is proportionate. However, at later points in time, the euphoric response decreases despite continued, or even increased, circulating concentration (3). In contrast to acute tolerance, chronic tolerance can be mediated through either pharmacokinetic or pharmacodynamic mechanisms, with an end result of a long-term decrease in drug response in the face of constant systemic exposure. In cases in which chronic tolerance develops, cross-tolerance within the pharmacologic class also may occur. Replacing the initial drug with a comparable agent will result in a lower pharmacologic response compared to that experienced by a drug-naive individual. Cross-tolerance is the principle underlying methadone substitution in the treatment of heroin addicts. Although methadone produces opioid effects in heroin-addicted subjects, euphoria and side effects are minimized by the tolerance produced by long-term heroin exposure. The significance of pharmacokinetic and pharmacodynamic mechanisms in relation to tolerance development will be examined in depth in subsequent sections of this review.
The final class of acquired tolerance is attributed to learning, either behavioral or conditioned. Behavioral tolerance occurs when an individual learns to function despite repeat exposure to a drug. Chronic alcohol abusers, for example, may not exhibit an outward appearance of motor impairment as a consequence of intoxication because of learned motor function adaptations and awareness of their impairment. Conditioned tolerance follows Pavlovian principles in which situational cues are associated with drug administration. Removal of these environmental cues will result in an enhancement in pharmacologic effect. For example, when morphine-tolerant rats are placed in a novel environment and challenged with morphine, antinociceptive tolerance is reduced (4). Analysis of behavioral tolerance has been reviewed in detail elsewhere