Background Positron Emission tomography (PET) imaging studies have shown that addiction to a number of substances of abuse is associated with a decrease in dopamine D2/3 receptor binding and decreased pre-synaptic dopamine release in the striatum. Some studies have also shown that these reductions are associated with the severity of addiction. For example, in cocaine dependence, low dopamine release is associated with the choice to self-administer cocaine. The goal of the present study was to investigate these parameters of striatal dopamine transmission in heroin dependence and their association with drug seeking behavior. Methods Heroin dependent and healthy control subjects were scanned with [11C]raclopride before and after stimulant administration (methylphenidate) to measure striatal D2/3 receptor binding and pre-synaptic dopamine release. Following the PET scans, the heroin dependent subjects performed heroin self-administration sessions. Results Both striatal D2/3 receptor binding and dopamine release were reduced in the heroin dependent subjects compared to healthy controls. However, neither PET measure of dopamine transmission predicted the choice to self-administer heroin. Conclusions These findings show that heroin addiction, like addiction to other drugs of abuse, is associated with low D2/3 receptor binding and low pre-synaptic dopamine. However, neither of these outcome measures were associated with the choice to self-administer heroin.

BACKGROUND Sublingual buprenorphine is an effective maintenance treatment for opioid dependence, yet intravenous buprenorphine misuse occurs. A buprenorphine/naloxone formulation was developed to mitigate this misuse risk. This randomized, double-blind, crossover study was conducted to assess the intravenous abuse potential of buprenorphine/naloxone compared with buprenorphine in buprenorphine-maintained injection drug users (IDUs). METHODS Intravenous heroin users (n=12) lived in the hospital for 8–9 weeks and were maintained on each of 3 different sublingual buprenorphine doses (2 mg, 8 mg, 24 mg). Under each maintenance dose, participants completed laboratory sessions during which the reinforcing and subjective effects of intravenous placebo, naloxone, heroin, and low and high doses of buprenorphine and buprenorphine/naloxone were examined. Every participant received each test dose under the 3 buprenorphine maintenance dose conditions. RESULTS Intravenous buprenorphine/naloxone was self-administered less frequently than buprenorphine or heroin (P < 0.0005). Participants were most likely to self-administer drug intravenously when maintained on the lowest sublingual buprenorphine dose. Subjective ratings of “drug liking” and “desire to take the drug again” were lower for buprenorphine/naloxone than for buprenorphine or heroin (P = 0.0001). Participants reported that they would pay significantly less money for buprenorphine/naloxone than for buprenorphine or heroin (P < 0.05). Seven adverse events were reported; most were mild and transient. CONCLUSIONS These data suggest that although the buprenorphine/naloxone combination has intravenous abuse potential, it is lower than for buprenorphine alone, particularly when participants received higher maintenance dosages and lower buprenorphine/naloxone challenge doses. Buprenorphine/naloxone may be a reasonable option for managing the risk for buprenorphine misuse during opioid dependence treatment.

Glial activation is hypothesized to contribute directly to opioid withdrawal. This study investigated the dose-dependent effects of a glial cell-modulator, ibudilast, on withdrawal symptoms in opioid-dependent volunteers after abrupt discontinuation of morphine administration. Methods Non-treatment seeking heroin-dependent volunteers (N = 31) completed the inpatient, double-blind, placebo-controlled, within-subject and between-group study. Volunteers were maintained on morphine (30 mg, QID) for 14 days and placebo (0 mg, QID) for the last 7 days of the 3-week study. Volunteers also received placebo (0 mg, PO, BID) capsules on Days 1–7. On days 8–21, volunteers were randomized to receive ibudilast (20 or 40 mg, PO, BID) or placebo capsules. Subjective and clinical ratings of withdrawal symptoms were completed daily using daily using the Subjective Opioid Withdrawal Scale (SOWS) and Clinical Opioid Withdrawal Scale (COWS). Medication side effects were also monitored. Results Relative to the first two weeks, all groups exhibited withdrawal during the third week as assessed by the SOWS and COWS (p ≤ 0.0001). Although overall SOWS scores did not differ between groups, exploratory analyses pooling the two ibudilast groups demonstrated that they had lower ratings of withdrawal symptoms on SOWS items (‘Anxious,’ ‘Perspiring,’ ‘Restless,’ ‘Stomach Cramps’) during detoxification relative to the placebo group. Ibudilast was well tolerated; no serious adverse events occurred during the study. Conclusion Pharmacological modulation of glial activity with ibudilast decreased some subjective ratings of opioid withdrawal symptoms. These exploratory findings are the first to demonstrate the potential clinical utility of glial modulators for treating opioid withdrawal in humans.

Despite the prevalent worldwide abuse of stimulants, such as amphetamines and cocaine, no medications are currently approved for treating this serious public health problem. Both preclinical and clinical studies suggest that the opioid antagonist naltrexone (NTX) is effective in reducing the abuse liability of amphetamine, raising the question of whether similar positive findings would be obtained for cocaine. The purpose of this study was to evaluate the ability of oral NTX to alter the cardiovascular and subjective effects of D-amphetamine (D-AMPH) and cocaine (COC). Non-treatment-seeking COC users (N=12) completed this 3-week inpatient, randomized, crossover study. Participants received 0, 12.5, or 50 mg oral NTX 60 min before active or placebo stimulant administration during 10 separate laboratory sessions. Oral AMPH (0, 10, and 20 mg; or all placebo) was administered in ascending order within a laboratory session using a 60-min interdose interval. Smoked COC (0, 12.5, 25, and 50 mg; or all placebo) was administered in ascending order within a laboratory session using a 14-min interdose interval. Active COC and AMPH produced dose-related increases in cardiovascular function that were of comparable magnitude. In contrast, COC, but not AMPH, produced dose-related increases in several subjective measures of positive drug effect (eg, high, liking, and willingness to pay for the drug). NTX did not alter the cardiovascular effects of AMPH or COC. NTX also did not alter positive subjective ratings after COC administration, but it did significantly reduce ratings of craving for COC and tobacco during COC sessions. These results show that (1) oral AMPH produces minimal abuse-related subjective responses in COC smokers, and (2) NTX reduces craving for COC and tobacco during COC sessions. Future studies should continue to evaluate NTX as a potential anti-craving medication for COC dependence.

The goal of this study was to evaluate the effects of intranasally administered opioids in rhesus monkeys using the tail-withdrawal assay, and to correlate these effects with measures of receptor occupancy using positron emission tomography (PET) imaging. Initial experiments characterized the antinociceptive effects of intranasal (IN) fentanyl and buprenorphine relative to intramuscular (IM) injection. Fentanyl (0.010-0.032 mg/kg) and buprenorphine (0.1-1.0 mg/kg) produced dose-dependent increases in tail-withdrawal latency that did not differ between routes of delivery. The second experiment compared the ability of IN and intravenous (IV) naloxone (NLX) to block the antinociceptive effects IV fentanyl, and to measure receptor occupancy at equipotent doses of NLX using PET imaging. IN and IV NLX (0.0032-0.032 mg/kg) produced dose-dependent decreases in fentanyl-induced antinociception. Again, there was no difference observed in overall potency between routes. PET imaging showed that IV and IN NLX produced similar decreases in receptor occupancy as measured by [11 C]carfentanil blocking, although there was a trend for IV NLX to produce marginally greater occupancy changes. This study validated the first procedures to evaluate the IN effects of opioids in rhesus monkeys.

Tolerance to the analgesic effects of opioids has been demonstrated in laboratory animals after repeated drug administration, yet this effect has been studied less frequently under controlled laboratory conditions in humans. This within-subject, double-blind, placebo-controlled study was designed to determine if tolerance developed to the analgesic, subjective, and physiological effects of the commonly prescribed opioid oxycodone when it was administered daily for 5 days. The effects of oxycodone’s (0, 5, and 20 mg/70 kg, p.o.) were compared, using a within-session cumulative dosing procedure, on the 1st and 5th days of the ‘daily’ dosing phase to assess for tolerance; active oxycodone was administered on the 2nd-4th days of the daily dosing phase. Changes in the effects of oxycodone were also compared when the medication was only administered on the 1st and 5th day of a 5-day ‘intermittent’ dosing phase; placebo medication was administered on the 2nd–4th days of the intermittent dosing phase. A 9-day ‘washout’ period occurred between phases when no medication was administered. Healthy volunteers (N=10) with no history of drug dependence or current drug use participated in this outpatient study. Analgesia was assessed using the Cold-Pressor Test (CPT), pain and drug effects were measured using a variety of questionnaires, and pupil diameter was monitored as an index of physiological effects. When administered daily, no differences were observed in oxycodone-induced analgesia between the 1st and 5th days, but tolerance did develop to some of the positive subjective effects of oxycodone. In contrast, oxycodone-induced analgesia and participant ratings of some positive subjective drug effects were greater on the 5th day compared to the 1st day of the intermittent dosing phase. No differences in the miotic effects of oxycodone between the 1st and 5th days of either dosing phase were detected. Though obtained under limited experimental conditions, these findings suggest that tolerance may not develop to the analgesic effects of therapeutic doses of oxycodone under short-term daily dosing conditions, even though some of its subjective effects may decrease. These data also suggest that intermittent administration may enhance the analgesic effects of oxycodone, while also increasing some of the drug’s positive subjective effects related to abuse liability.

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This double-blind, placebo-controlled study investigated effects of oral morphine (0, 45, 135 mg/70kg) and oral oxycodone (0, 15, 45 mg/70kg) in buprenorphine-maintained opioid addicts. Since a 3:1 morphine:oxycodone dose ratio had yielded equivalent subjective and physiological effects in non-dependent individuals, this ratio was used in the present study. Two self-administration laboratory procedures, i.e. a drug vs. money and a drug vs. drug procedure, were assessed. Study participants (N=12) lived in the hospital and were maintained on 4 mg/day sublingual buprenorphine. When participants chose between drug and money, money was preferred over all drug doses; only high-dose oxycodone was self-administered more than placebo. When participants chose between drug and drug, both drugs were chosen more than placebo, high doses of each drug were chosen over low doses, and high-dose oxycodone was preferred over high-dose morphine. The subjective, performance-impairing, and miotic effects of high-dose oxycodone were generally greater compared to high-dose morphine. The study demonstrated that a 3:1 dose ratio of morphine:oxycodone was not equipotent in buprenorphine-dependent subjects. Both self-administration procedures were effective for assessing the relative reinforcing effects of drugs; preference for one procedure should be driven by the specific research question of interest.