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The protocol for this trial and the CONSORT checklist are available as supporting information; see Checklist S1 and Protocol S1. We recruited for the aerobic exercise training program 14 cannabis-dependent adults who met criteria for primary cannabis dependence according to the Substance Abuse module of the Structured Clinical Interview for DSM-IV (SCID-IV)[10], but who were not interested in reducing or quitting cannabis use or seeking treatment. A positive urine drug test for cannabis on the study day was an absolute criterion for participation. Participants were recruited using flyers and by word of mouth, signed an informed consent document approved by the Vanderbilt University Institutional Review Board, and were compensated for transportation and the study visits. The study was conducted between March and August 2010. This manuscript does not include results from the fMRI part of the protocol performed separately (see Protocol S1). Exclusion criteria included presence of another Axis 1 DSM-IV diagnosis in the past 6 months, having a chronic medical or neurological illness, and having taken any psychotropic or vasoactive medications within 6 weeks of screen day. Also excluded were persons who smoked more than 10 cigarettes per day in the last year, and who had current dependence, as determined by the SCID, on any psychoactive substance other than nicotine and/or cannabinoids. Persons participating in an organized form of exercise or exercised more than 2 h per week [11] in the last month and persons having orthopedic or other problems precluding them from performing the exercise protocol were also excluded. Of those recruited, two (1 male, 1 female) did not complete the study; therefore, these results are reported on a final sample of 12 participants (age 24.8±2.9 years, 8 females).
Written informed consent was obtained from all participants after they were given a complete description of the study. The Institutional Review Board of Vanderbilt University approved the protocol and consent procedure; see Consent Form S1 and Approval Letter S1.
Sedentary or minimally active (<60 min/week of routine exercise) cannabis-dependent adults participated in this 2-week treadmill exercise program. After screening and a 7-day run-in period, participants came for 10 scheduled and supervised exercise intervention sessions (see Figure 1). On days with no exercise session scheduled, participants were encouraged to follow their daily routine and perform exercise in their normal living environment. Participants were able to make up for missed sessions with no more than 1 session per day. During the follow-up period (2 weeks), participants were asked to continue their daily routine without encouraging or discouraging them to continue exercise. Self-reported drug use was assessed for 1-week before, during, and 2-weeks after the exercise intervention.
Figure data removed from full text. Figure identifier and caption: 10.1371/journal.pone.0017465.g001 Experimental design.The study included Run-In (1 week), Exercise intervention (10 daily 30-minute treadmill sessions in ∼2 weeks), and Follow-Up (2 weeks) periods. Cannabis, other drug, caffeine and tobacco use were assessed from self-reports during all study periods. Cannabis craving was assessed pre- and-post each exercise session after viewing cannabis-related visual cues.
At every session, before and after performing the exercise, each participant was presented with a set of visual cannabis cues on a computer monitor in conjunction with assessment of subjective cannabis craving determined using the Marijuana Craving Questionnaire (MCQ-SF) [12]. The cannabis -related cues included pictures of cannabis in different forms, people using cannabis, and paraphernalia. Three sets of cues were randomly assigned and were viewed during a 2-minute session performed in a separate semi-dark room. Exercise was performed on a treadmill (Vision Fitness, Lake Mills, WI, US) at a target intensity of 60% of heart rate reserve that corresponds to approximately 60% of maximal aerobic capacity [13] for 30 minutes. Heart rate reserve, calculated as a difference between maximal and resting heart rate, was used as an indicator of intensity for exercise prescriptions [13] and monitored using an automatic monitor (DINAMAP® PRO, GE, St. Paul, MN, US). The intensity was adjusted to individual aerobic capacity and followed current guidelines that recommend limited thresholds of exertion level and time of exercise in sedentary populations [13]. All exercise sessions were conducted by the study exercise physiologists under medical oversight.
Cannabis-related cues were selected from a variety of sources and altered using a visual graphics program to ensure clarity, brightness, color balance, and size. The stimulus set includes pictures of cannabis in different forms and its use (people smoking joints) and related paraphernalia (e.g. bongs, pipes). Craving was assessed after presentation of visual stimuli using the Marijuana Craving Questionnaire (MCQ-SF) [12]. The MCQ-SF is a Likert-based, 12-item self-assessment instrument for situational cannabis craving measurement with four factors (compulsivity, emotionality, expectancy, and purposefulness). The MCQ-SF validity to monitor the course of change in craving over time has been reported [12]. Each item is rated on a scale from 1 (strongly disagree) to 7 (strongly agree). Each of the factors is comprised of 3 items. Scores for each of the factors are derived by averaging the component item responses [14]. Internal consistency of the scores as measured by the Cronbach's alpha statistic were .86 (compulsivity), .93 (emotionality), .75 (expectancy), and .89 (purposefulness) scale in our study.
Lifetime drug use was assessed using a questionnaire that follows principles of Timeline Follow-back Method [15] and includes prompts for all major classes of drugs and includes assessments of age at onset, frequency of drug use, desired effects of drug or drug combination use [16], [17], [18], [19].
Participants received a calendar with instructions on how to use it to record drug use during the run-in, exercise intervention, and 2-week follow-up periods. Information was collected daily during the run-in period and every 5–7 days during the follow-up period. During the exercise intervention, the information was collected daily from the participants. The record included form (i.e. joints, blunts, bongs, chillums, bowls) and quantity of cannabis used, as well as use of any other drugs and alcohol. A trained interviewer reviewed the calendar with the participant using the Timeline Follow-back Method [15] to verify the information about drug use. Reported records were independently reviewed for reliability and correctness and entered into the statistical database.
Lifetime tobacco and caffeine use history was assessed using a validated questionnaire and current use was assessed daily by self-report [16], [17], [18], [19].
Descriptive statistics were used to summarize the participant characteristics, as well as the variables of interest in this study. Because drug use history data were extremely skewed, median, minimum and maximum values are presented to summarize those distributions; otherwise mean ± SD are reported. Analysis of differences in overall cannabis use between each of the run-in, exercise, and follow-up periods were conducted using Wilcoxon Signed Ranks tests. Friedman tests were used to analyze changes in use within each of the periods. Wilcoxon Signed Ranks Tests were used to assess the statistical significance of changes in reported craving (MCQ Factor scores) from pre-to-post exercise. An alpha level of .05 was used for determination of statistical significance.
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