Apr 08, 2014
Excerpt from Harvard Mental Health Letter
Operant conditioning accounts for repeated use when a drug is available. Classical or Pavlovian conditioning is said to explain the relapses that often occur after long periods of abstinence. When a neutral stimulus or cue is consistently associated with a stimulus that causes an unconditioned response (food or an addictive drug, for example), the neutral stimulus eventually brings about a conditioning response on its own. A person is more likely to return to using a drug when exposed to cues that have been reliably associated with its use or its effects in the past. Many types of external and internal cues or conditioned stimuli may cause an addict to relapse: the sight of the bar where he used to drink, a needle of the type used for injection, even the onset of a mood in which she was accustomed to taking the drug.
Classical conditioning also helps to determine patterns of tolerance and withdrawal symptoms. Tolerance originally develops because the brain because adapted to the new chemical environment created by the drug and no longer responds to it with the same intensity. Presumably neurotransmitters are depleted, or the number or sensitivity of nerve receptors for the drug decreases. This homeostatic or compensatory reaction is the body's way of returning to a relatively normal state. Almost all addictive drugs produce some tolerance and a person or animal that is tolerant to one drug will also be tolerant to others in the same class--sedative, stimulant, or opioid.
Withdrawal symptoms are another form of compensatory response. Drugs that produce a certain intense withdrawal syndrome, especially sedatives and opioids, are sometimes said to cause physical dependence. But the physical intensity of the withdrawal reaction is rarely one of the most important reasons why a person goes on using a drug. Many researchers believe that the brain mechanisms producing these reactions are unrelated to the reward system. People who take large doses of morphine for the relief of pain in a hospital, even when they suffer a withdrawal reaction afterward, are unlikely to look for opiates on the street.
Simple unconditioned compensatory responses to the immediate presence or absence of a drug are not the only source of tolerance and withdrawal symptoms. By way of classical conditioning environmental cues can stimulate similar responses long after drug use has stopped and the chemical is no longer exerting direct effects on the body and brain. An addict may start to develop mild withdrawal symptoms on returning to the old neighborhood or simply anticipating an injection. Formally addicted rats are most likely to survive a high dose of heroin or cocaine if they are kept in the cage where they were originally addicted, apparently because the familiar environment excites a compensatory tolerance. If the experimenter substitutes a placebo for the drug in that environment, the conditioned tolerance will eventually by eliminated (extinguished).
Patterns of conditioned tolerance and withdrawal are much more complicated than any straightforward automatic response to an internal state of the body or brain. A rat that presses a level to inject heroin will develop more tolerance and more severe abstinence symptoms than another rat that is simply attached to the same machinery and absorbs the drug passively. Rabbits develop tolerance to the effect of alcohol on motor activity and coordination faster if they take it before rather than after they are required to work for food. Rats become tolerant to the appetite reducing effects of amphetamine more quickly if they are given plenty of sweetened milk at the same time. The presence of food or the need to work apparently provides the cue for a compensatory reaction.
The language of behavioral conditioning does have words for desires, urges, and cravings, or even for purpose of intention. Behaviorists regard these concepts as scientifically valueless, because they refer to subjective states that are not consistently or clearly associate with distinct physical symptoms or patterns of drug use. In fact, addicts usually do not say they relapse because of cravings or urge; instead they talk about moods and situations that provoke drug use. But there is another way to think about craving. Repetition of any activity tends to make it automatic, a uniform response to a uniform stimulus. Urgings and cravings can be seen as the result of complicated nonautomatic processes that are activated in the brain either to oppose the automatic process or to overcome an obstacle to it (such as unavailability of the drug). In other words, craving is a sign that the addict is either trying to obtain a drug that is temporarily unavailable or trying to resist the temptation to use a drug that is available. This view of craving is compatible with the DSM-III-R definition, which describes unsuccessful attempts to stop as one of the symptoms of drug dependence.