Cocaine is believed to work by blocking the dopamine transporter (DAT) and thereby increasing the availability of free dopamine within the brain. Although this concept is central to current cocaine research and to treatment development, a direct relationship between DAT blockade and the subjective effects of cocaine has not been demonstrated in humans. We have used positron emission tomography to determine what level of DAT occupancy is required to produce a subjective 'high' in human volunteers who regularly abuse cocaine. We report here that intravenous cocaine at doses commonly abused by humans (0.3-0.6 mg kg(-1)) blocked between 60 and 77% of DAT sites in these subjects. The magnitude of the self-reported high was correlated with the degree of DAT occupancy, and at least 47% of the transporters had to be blocked for subjects to perceive cocaine's effects. Furthermore, the time course for the high paralleled that of cocaine concentration within the striatum, a brain region implicated in the control of motivation and reward. This is the first demonstration in humans that the doses used by cocaine abusers lead to significant blockade of DAT, and that this blockade is associated with the subjective effects of cocaine. Although these findings provide justification to target the DAT for medication development they suggest that for drugs to be effective in blocking cocaine's effects they would have to be given at doses that achieve almost complete DAT occupancy.

Symptoms of withdrawal after oral delta9-tetrahydrocannabinol (THC) administration have been reported, yet little is known about the development of dependence on smoked marijuana in humans. In a 21-day residential study, marijuana smokers (n = 12) worked on five psychomotor tasks during the day (0915-1700 hours), and in the evening engaged in recreational activities (1700-2330 hours); subjective-effects measures were completed 10 times/day. Food and beverages were available ad libitum from 0830 to 2330 hours. Marijuana cigarettes (0.0, 1.8, 3.1% THC) were smoked at 1000, 1400, 1800, and 2200 hours. Placebo marijuana was administered on days 1-4 . One of the active marijuana doses was administered on days 5-8, followed by 4 days of placebo marijuana (days 9-12). The other concentration of active marijuana cigarettes was administered on days 13-16, followed by 4 days of placebo marijuana (days 17-20); the order in which the high and low THC-concentration marijuana cigarettes were administered was counter-balanced between groups. Both active doses of marijuana increased ratings of "High," and "Good Drug Effect," and increased food intake, while decreasing verbal interaction compared to the placebo baseline (days 1-4). Abstinence from active marijuana increased ratings such as "Anxious," "Irritable," and "Stomach pain," and significantly decreased food intake compared to baseline. This empirical demonstration of withdrawal from smoked marijuana may suggest that daily marijuana use may be maintained, at least in part, by the alleviation of abstinence symptoms.

No abstract

Symptoms of dependence and withdrawal after the frequent administration of high doses (210 mg/day) of oral delta9-tetrahydrocannabinol (THC) have been reported, yet little is known about dependence on lower oral THC doses, more relevant to levels attained by smoking marijuana. In a 20-day residential study, male (n = 6) and female (n = 6) marijuana smokers worked on five psychomotor tasks during the day (0915-1700 hours), and in the evening engaged in private or social recreational activities (1700-2330 hours); subjective-effects measures were completed 10 times/day, and a sleep questionnaire was completed each morning. Food and beverages were available ad libitum from 0830 to 2330 hours. Capsules were administered at 1000, 1400, 1800, and 2200 hours. Placebo THC was administered on days 1-3, 8-11, and 16-19. Active THC was administered on days 4-7 (20 mg qid) and on days 12-15 (30 mg qid). Both active doses of THC increased ratings of "High," "Good Drug Effect," and "Willingness to Take Dose Again" compared to baseline (days 1-3). THC also increased food intake by 35-45%, and decreased verbal interaction among participants compared to placebo baseline. Tolerance developed to the subjective effects of THC but not to its effects on food intake or social behavior. Abstinence from THC increased ratings of "Anxious," "Depressed," and "Irritable," decreased the reported quantity and quality of sleep, and decreased food intake by 20-30% compared to baseline. These behavioral changes indicate that dependence develops following exposure to lower daily doses of THC than have been previously studied, suggesting that the alleviation of abstinence symptoms may contribute to the maintenance of daily marijuana use.

Cocaine dependence is associated with impairment of dopamine function, and this impairment appears to play a critical role in relapse.