Pharmacokinetics and pharmacodynamics of cannabis-based medicine in a patient population included in a randomized, placebo-controlled, clinical trial.

Information on the pharmacokinetics (PK) and pharmacodynamics (PD) of orally administered cannabis-based medicine (CBM) in capsule formulation in patient populations is sparse. In this exploratory study, we aimed to evaluate the PK and PD in a probable steady state of CBM in neuropathic pain and spasticity in a population of patients with multiple sclerosis (MS). Of 134 patients participating in a randomized, double-blinded, placebo-controlled, trial, 23 patients with MS (17 female) mean age 52 years (range 21-67) were enrolled in this substudy. They received oral capsules containing Δ9 -tetrahydrocannabinol (THC, n = 4), cannabidiol (CBD, n = 6), a combination (THC&CBD, n = 4), or placebo (n = 9). Maximum doses were 22.5 mg (THC) and 45 mg (CBD) a day divided into three administrations. PD parameters were evaluated for pain and spasticity. Blood samples were analyzed using an ultra-high-performance liquid chromatography-tandem mass spectrometer after protein precipitation and phospholipid removal. PK parameters were estimated using computerized modeling. The variation in daily dose and PK between individuals was considerable in a steady state, yet comparable with previous reports from healthy controls. Based on a simulation of the best model, the estimated PK parameters (mean) for THC (5 mg) were Cmax 1.21 ng/mL, Tmax 2.68 h, and half-life 2.75 h, and for CBD (10 mg) were Cmax 2.67 ng/mL, Tmax 0.10 h, and half-life 4.95 h, respectively. No effect was found on the PD parameters, but the placebo response was considerable. More immediate adverse events were registered in the active treatment groups compared with the placebo group.

Distribution of morphine and methadone to the brain in a developmental chicken embryo model.

The use and/or misuse of opioids by pregnant women would expose the fetuses to these drugs during critical stages of development with serious effects for the newborn, like the neonatal abstinence syndrome (NAS). We have revisited an established chicken model for NAS to describe the distribution of morphine and methadone to the brain and explore its validity as a valuable alternative to rodent models. For this purpose, chicken eggs were injected with a single dose of 10 mg/kg or 20 mg/kg morphine or 20 mg/kg methadone onto the chorioallantoic membrane (CAM) on embryonal day 13. Whole brains and lungs were harvested and the concentrations of morphine, methadone and their subsequent metabolites (morphine-3-glucuronide and EDDP, respectively) determined in the brain and lungs at different time points using LC-MS/MS. Morphine and methadone, as well as their metabolites, were detected both in the brain and lungs, with significantly higher concentrations in the lungs. Pharmacokinetic modelling showed that the distribution of morphine to the brain followed a first-order absorption with transit compartments and linear elimination, with concentrations linearly dependent on dose. Moreover, methadone, but not morphine, reduced µ receptor (the main morphine receptor) binding, which can be of relevance for opioid tolerance. The present study is the first to report the brain distribution of morphine, which can be described by standard pharmacokinetic processes, and methadone in the developing chicken embryo. The present findings supplement the already established model and support the use of this chicken model to study NAS.

Heroin metabolism in human blood and its impact for the design of an immunotherapeutic approach against heroin effects.

Immunotherapeutic interventions that block drug effects by binding drug molecules to specific antibodies in the bloodstream have shown promising effects in animal studies. For heroin, which effects are mainly mediated by the metabolites 6-acetylmorphine (6-AM; also known as 6-monoacetylmorphine or 6-MAM) and morphine, the optimal antibody specificity has been discussed. In rodents, 6-AM specific antibodies have been recommended based on the rapid metabolism of heroin to 6-AM in the bloodstream. Since the metabolic rate of heroin in blood is unsettled in humans, we examined heroin metabolism with state-of-the-art analytical methodology (UHPLC-MS/MS) in freshly drawn human whole blood incubated with a wide range of heroin concentrations (1-500 µM). The half-life of heroin was highly concentration dependent, ranging from 1.2-1.7 min for concentrations at or above 25 µM, and gradually increasing to approximately 20 min for 1 µM heroin. At concentrations that can be attained in the bloodstream shortly after an i.v. injection, approximately 70% was transformed into 6-AM within 3 min, similar to previous observations in vivo. Our results indicate that blood enzymes play a more important role for the rapid metabolism of heroin in humans than previously assumed. This points to 6-AM as an important target for an efficient immunotherapeutic approach to block heroin effects in humans.

Increased heroin intake and relapse vulnerability in intermittent relative to continuous self-administration: Sex differences in rats.

BACKGROUND AND PURPOSE: Studies using intermittent-access drug self-administration show increased motivation to take and seek cocaine and fentanyl, relative to continuous access. In this study, we examined the effects of intermittent- and continuous-access self-administration on heroin intake, patterns of self-administration and cue-induced heroin-seeking, after forced or voluntary abstinence, in male and female rats. We also modelled brain levels of heroin and its active metabolites. EXPERIMENTAL APPROACH: Rats were trained to self-administer a palatable solution and then heroin (0.075 mg·kg-1 per inf) either continuously (6 h·day-1 ; 10 days) or intermittently (6 h·day-1 ; 5-min access every 30-min; 10 days). Brain levels of heroin and its metabolites were modelled using a pharmacokinetic software. Next, heroin-seeking was assessed after 1 or 21 abstinence days. Between tests, rats underwent either forced or voluntary abstinence. The oestrous cycle was measured using a vaginal smear test. KEY RESULTS: Intermittent access exacerbated heroin self-administration and was characterized by a burst-like intake, yielding higher brain peaks of heroin and 6-monoacetylmorphine concentrations. Moreover, intermittent access increased cue-induced heroin-seeking during early, but not late abstinence. Heroin-seeking was higher in females after intermittent, but not continuous access, and this effect was independent of the oestrous cycle. CONCLUSIONS AND IMPLICATIONS: Intermittent heroin access in rats resembles critical features of heroin use disorder: a self-administration pattern characterized by repeated large doses of heroin and higher relapse vulnerability during early abstinence. This has significant implications for refining animal models of substance use disorder and for better understanding of the neuroadaptations responsible for this disorder. LINKED ARTICLES: This article is part of a themed issue on Advances in Opioid Pharmacology at the Time of the Opioid Epidemic. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v180.7/issuetoc.

Peripherally administered persistent organic pollutants distribute to the brain of developing chicken embryo in concentrations relevant for human exposure.

Persistent organic pollutants (POPs) can reach the fetal brain and contribute to developmental neurotoxicity. To explore the distribution of POPs to the fetal brain, we exposed chicken embryos to a POP mixture, containing 29 different compounds with concentrations based on blood levels measured in the Scandinavian human population. The mixture was injected into the allantois at embryonic day 13 (E13), aiming at a theoretical concentration of 10 times human blood levels. POPs concentrations in the brain were measured at 0.5, 1, 2, 4, 6, 24, 48, and 72 h after administration. Twenty-seven of the individual compounds were detected during at least one of the time-points analyzed. Generally, the concentrations of most of the measured compounds were within the order of magnitude of those reported in human brain samples. Differences in the speed of distribution to the brain were observed between the per- and polyfluoroalkyl substances (PFASs), which have protein binding potential, and the lipophilic polychlorinated biphenyls (PCBs), organochlorine pesticides (OCPs) and brominated flame retardants (BFRs). Based on pharmacokinetic modeling, PFASs were best described by a one compartment model. PFASs displayed relatively slow elimination (Kel) and persisted at high levels in the brain. Lipophilic OCPs and PCBs could be fitted to a 2-compartment model. These showed high levels in the brain relative to the dose administrated as calculated by area under the curve (AUC)/Dose. Altogether, our study showed that chicken is a suitable model to explore the distribution of POPs into the developing brain at concentrations which are relevant for humans.

Does the preparation for intravenous administration affect the composition of heroin injections? A controlled laboratory study.

AIMS: To study whether the preparation procedure, and its acidic and heating conditions, used by heroin users to prepare heroin for intravenous administration affects the final composition of the fluid to be injected. METHODS: Samples from different seizures of illegal heroin provided by the Norwegian police were prepared by adding water and ascorbic acid before heating under controlled conditions in the laboratory. Further, three seizures were prepared with different amounts of ascorbic or citric acid relative to their diacetylmorphine content. Pure diacetylmorphine base or salt was also submitted to the procedure applying two different heating intensities. The seizures and the final product after preparation were analysed for diacetylmorphine, 6-acetylmorphine and morphine using liquid chromatography with tandem mass spectrometry (LC-MS-MS). RESULTS: After preparation, a decrease of 19.8% (25th and 75th percentiles = -29.2 and -15.3) in the initial diacetylmorphine content was observed. Both the 6-acetylmorphine and morphine content increased but, due to their low content in the initial product, diacetylmorphine still represented 83.9% (25th and 75th percentiles = 77.3 and 88.0) of the sum of these three opioids in the final solution. The loss of water during preparation caused an increase in the concentration of diacetylmorphine, 6-acetylmorphine and morphine, depending on the heating intensity applied. The content of these opioids was affected by the quantity and type of acid added in relation to the heroin purity and the level of diacetylmorphine dissolved being proportional to the amount of ascorbic acid, but not citric acid, in the sample with high heroin purity. CONCLUSIONS: Preparation of heroin for intravenous injection appears to change the amount or concentration of diacetylmorphine and its active metabolites, 6-acetylmorphine and morphine in the final product, depending on heroin purity, amount and type of acid used or heating conditions. These circumstances can contribute to unintentional variations in the potency of the final injected solution, and therefore affect the outcome after injection.

Association of a CAMK2A genetic variant with logical memory performance and hippocampal volume in the elderly.

Calcium/Calmodulin-dependent kinase alpha (αCaMKII) has been shown to play an essential role in synaptic plasticity and in learning and memory in animal models. However, there is little evidence for an involvement in specific memories in humans. Here we tested the potential involvement of the αCaMKII coding gene CAMK2A in verbal logical memory in two Caucasian populations from Germany, in a sample of 209 elderly people with cognitive impairments and a sample of 142 healthy adults. The association of single nucleotide polymorphisms (SNPs) located within the genomic region of CAMK2A with verbal logical memory learning and retrieval from the Wechsler Memory Scale was measured and hippocampal volume was assessed by structural MRI. In the elderly people, we found the minor allele of CAMK2A intronic SNP rs919741 to predict a higher hippocampal volume and better logical memory retrieval. This association was not found in healthy adults. The present study may provide evidence for an association of a genetic variant of the CAMK2A gene specifically with retrieval of logical memory in elderly humans. This effect is possibly mediated by a higher hippocampal volume.

CaMKII is activated in opioid induced conditioned place preference, but αCaMKII Thr286 autophosphorylation is not necessary for its establishment.

Activation of calcium/calmodulin-dependent protein kinase II (CaMKII), particularly its α isoform, is known to be important for neuronal processes central for learning and memory and has also been implicated in the maladaptive learning involved in drug addiction.Thr286 autophosphorylation of αCaMKII has been shown to be indispensable for establishment of cocaine-induced CPP (Easton et al., 2014). To study the contribution of CaMKII in opioid induced conditioned learning, we examined how establishment of conditioned place preference (CPP) induced by 10 or 30 µmol/kg morphine or its active metabolite morphine-6-glucuronide (M6G) affects the levels and Thr286 autophosphorylation of the α- and ß-isoforms of CaMKII, as well as ß-actin levels, in dorsal and ventral striatum and in hippocampus of mice. An acute and a sub-chronic treatment were used as controls. Whereas an acute single administration of morphine or M6G caused increases in CaMKII levels and phosphorylation at Thr286 and ß-actin in striatal areas, CPP induced by these opioids was accompanied primarily by an increase in the protein levels of both CaMKII isoforms and ß-actin in dorsal striatum and hippocampus. Decreases in CaMKII Thr286 phosphorylation were observed in dorsal striatum after the sub-chronic pharmacological treatment. Despite the changes observed in αCaMKII activity in wild type mice, morphine-induced CPP was not affected in αCaMKIIT286A autophosphorylation-deficient mice. These results indicate that opioid-induced CPP is accompanied by activation of α- and ßCaMKII in striatum and hippocampus, but, in opposition to what has been observed with cocaine, αCaMKII autophosphorylation is not essential for establishment of opioid-induced CPP.

The role of 6-acetylmorphine in heroin-induced reward and locomotor sensitization in mice.

We have previously demonstrated that heroin's first metabolite, 6-acetylmorphine (6-AM), is an important mediator of heroin's acute effects. However, the significance of 6-AM to the rewarding properties of heroin still remains unknown. The present study therefore aimed to examine the contribution of 6-AM to heroin-induced reward and locomotor sensitization. Mice were tested for conditioned place preference (CPP) induced by equimolar doses of heroin or 6-AM (1.25-5 µmol/kg). Psychomotor activity was recorded during the CPP conditioning sessions for assessment of drug-induced locomotor sensitization. The contribution of 6-AM to heroin reward and locomotor sensitization was further examined by pretreating mice with a 6-AM specific antibody (anti-6-AM mAb) 24 hours prior to the CPP procedure. Both heroin and 6-AM induced CPP in mice, but heroin generated twice as high CPP scores compared with 6-AM. Locomotor sensitization was expressed after repeated exposure to 2.5 and 5 µmol/kg heroin or 6-AM, but not after 1.25 µmol/kg, and we found no correlation between the expression of CPP and the magnitude of locomotor sensitization for either opioid. Pretreatment with anti-6-AM mAb suppressed both heroin-induced and 6-AM-induced CPP and locomotor sensitization. These findings provide evidence that 6-AM is essential for the rewarding and sensitizing properties of heroin; however, heroin caused stronger reward compared with 6-AM. This may be explained by the higher lipophilicity of heroin, providing more efficient drug transfer to the brain, ensuring rapid increase in the brain 6-AM concentration.

The active heroin metabolite 6-acetylmorphine has robust reinforcing effects as assessed by self-administration in the rat.

Previous studies have suggested that at least some of the behavioral effects of heroin might be mediated by its active metabolite 6-acetylmorphine (6-AM). The aim of the present study was to investigate the reinforcing effects of 6-AM and its role in mediating those of heroin. We used an intravenous self-administration procedure in male Sprague-Dawley rats including four phases: acquisition, extinction, reinstatement of drug-seeking, and re-acquisition. Independent groups of rats readily learned to self-administer equimolar doses (0.135 µmol/kg) of either 6-AM (44.3 µg/kg) or heroin (50 µg/kg). Under a fixed ratio 1 (FR1) schedule of reinforcement, the rate of responding was the same for 6-AM and heroin, but it was significantly higher for 6-AM than for heroin under a FR2 schedule. A non-contingent infusion ('priming') of 0.068 µmol/kg of either 6-AM or heroin reinstated non-reinforced drug-seeking (relapse). The rats readily re-acquired self-administration behaviour when given access to one of two doses (0.068 and 0.135 µmol/kg) of 6-AM or heroin. Pretreatment with a specific monoclonal antibody (mAb) against 6-AM blocked the priming effect of 6-AM, and modified the rate of lever-pressing on re-acquisition of 6-AM self-administration in a manner compatible with a shift to the right of the dose-effect curve. The mAb did not affect heroin responding. The present results show that 6-AM possesses reinforcing effects similar to those of heroin. The lack of effect of 6-AM mAb on heroin priming and heroin self-administration calls for further studies to clarify the role of heroin and its metabolites in heroin reward. This article is part of the Special Issue entitled 'Opioid Neuropharmacology: Advances in treating pain and opioid addiction'.

Restoration of Cognitive Performance in Mice Carrying a Deficient Allele of 8-Oxoguanine DNA Glycosylase by X-ray Irradiation.

Environmental stressors inducing oxidative stress such as ionizing radiation may influence cognitive function and neuronal plasticity. Recent studies have shown that transgenic mice deficient of DNA glycosylases display unexpected cognitive deficiencies related to changes in gene expression in the hippocampus. The main objectives of the present study were to determine learning and memory performance in C57BL/6NTac 8-oxoguanine DNA glycosylase 1 (Ogg1)+/- (heterozygote) and Ogg1+/+ (wild type, WT) mice, to study whether a single acute X-ray challenge (0.5 Gy, dose rate 0.457 Gy/min) influenced the cognitive performance in the Barnes maze, and if such differences were related to changes in gene expression levels in the hippocampus. We found that the Ogg1+/- mice exhibited poorer early-phase learning performance compared to the WT mice. Surprisingly, X-ray exposure of the Ogg1+/- animals improved their early-phase learning performance. No persistent effects on memory in the late-phase (6 weeks after irradiation) were observed. Our results further suggest that expression of 3 (Adrb1, Il1b, Prdx6) out of in total 35 genes investigated in the Ogg1+/- hippocampus is correlated to spatial learning in the Barnes maze.

Comparison of (+)- and (-)-Naloxone on the Acute Psychomotor-Stimulating Effects of Heroin, 6-Acetylmorphine, and Morphine in Mice.

Toll-like receptor 4 (TLR4) signaling is implied in opioid reinforcement, reward, and withdrawal. Here, we explored whether TLR4 signaling is involved in the acute psychomotor-stimulating effects of heroin, 6-acetylmorphine (6-AM), and morphine as well as whether there are differences between the three opioids regarding TLR4 signaling. To address this, we examined how pretreatment with (+)-naloxone, a TLR4 active but opioid receptor (OR) inactive antagonist, affected the acute increase in locomotor activity induced by heroin, 6-AM, or morphine in mice. We also assessed the effect of pretreatment with (-)-naloxone, a TLR4 and OR active antagonist, as well as the pharmacokinetic profiles of (+) and (-)-naloxone in the blood and brain. We found that (-)-naloxone reduced acute opioid-induced locomotor activity in a dose-dependent manner. By contrast, (+)-naloxone, administered in doses assumed to antagonize TLR4 but not ORs, did not affect acute locomotor activity induced by heroin, 6-AM, or morphine. Both naloxone isomers exhibited similar concentration versus time profiles in the blood and brain, but the brain concentrations of (-)-naloxone reached higher levels than those of (+)-naloxone. However, the discrepancies in their pharmacokinetic properties did not explain the marked difference between the two isomers' ability to affect opioid-induced locomotor activity. Our results underpin the importance of OR activation and do not indicate an apparent role of TLR4 signaling in acute opioid-induced psychomotor stimulation in mice. Furthermore, there were no marked differences between heroin, 6-AM, and morphine regarding involvement of OR or TLR4 signaling.

Drugged driving arrests in Norway before and after the implementation of per se law.

Norway introduced legislative limits for driving under the influence of drugs (DUID) February 1st, 2012, to harmonize with the legislation on driving under the influence of alcohol. Per se limits corresponding to blood alcohol concentrations (BACs) of 0.02% were established for 20 drugs and concentration limits for graded sanctions corresponding to BACs of 0.05% and 0.12% were established for 13 of these drugs as well. The new system is not applied to individuals with valid prescriptions for medicinal drugs. The aim of this study was to investigate if the implementation of legislative limits for drugs affected the number of blood samples taken from suspected drugged drivers, drug findings and the number of expert witness statement requests. The number of blood samples taken in suspected DUID cases increased by 20% after introduction of legislative limits (3320 cases in 2010 and 3970 in 2013). The number of samples with at least one drug above the per se limit corresponding to BAC of 0.02% increased by 17% (from 2646 in 2010 to 3090 in 2013), whereas the number of expert witness statements was reduced by the half (from 63.4% in 2010 and 28.7% in 2013).

A monoclonal antibody specific for 6-monoacetylmorphine reduces acute heroin effects in mice.

Immunotherapy against drugs of abuse is being studied as an alternative treatment option in addiction medicine and is based on antibodies sequestering the drug in the bloodstream and blocking its entry into the brain. Producing an efficient vaccine against heroin has been considered particularly challenging because of the rapid metabolism of heroin to multiple psychoactive molecules. We have previously reported that heroin's first metabolite, 6-monoacetylmorphine (6-MAM), is the predominant mediator for heroin's acute behavioral effects and that heroin is metabolized to 6-MAM primarily prior to brain entry. On this basis, we hypothesized that antibody sequestration of 6-MAM is sufficient to impair heroin-induced effects and therefore examined the effects of a monoclonal antibody (mAb) specific for 6-MAM. In vitro experiments in human and rat blood revealed that the antibody was able to bind 6-MAM and block the metabolism to morphine almost completely, whereas the conversion of heroin to 6-MAM remained unaffected. Mice pretreated with the mAb toward 6-MAM displayed a reduction in heroin-induced locomotor activity that corresponded closely to the reduction in brain 6-MAM levels. Intraperitoneal and intravenous administration of the anti-6-MAM mAb gave equivalent protection against heroin effects, and the mAb was estimated to have a functional half-life of 8 to 9 days in mice. Our study implies that an antibody against 6-MAM is effective in counteracting heroin effects.

3-Methoxynaltrexone is not a selective antagonist for the acute psychomotor stimulating effects of heroin and 6-monoacetylmorphine in mice.

The opioid receptor antagonist 3-methoxynaltrexone (3-MeONtx) has previously been shown in rodents to selectively reverse the analgesic actions of heroin and its metabolites 6-monoacetylmorphine (6-MAM), and morphine-6-glucuronide (M6G), but not that of morphine. Based on these and other results, a heroin/6-MAM/M6G µ-opioid receptor binding site or subreceptor mediating their analgesic activity has been proposed. It is however unknown whether this also accounts for the acute psychomotor stimulating properties of these opioids. The aim of the present study was therefore to explore if the acute psychomotor stimulating effects of heroin, 6-MAM, and morphine are mediated by distinct µ-opioid receptor binding sites or subreceptors. To address this aim, we examined how pretreatment with 3-MeONtx or naltrexone (NTX) affected the acute increase in locomotor activity induced by heroin, 6-MAM, or morphine in mice. The pharmacokinetic profiles of 3-MeONtx and NTX were also assessed in mouse brain. We found that 3-MeONtx similarly antagonized the acute increase in locomotor activity induced by equipotent doses of heroin, 6-MAM, or morphine. This antagonistic effect was comparable to the one observed following administration of NTX, and both antagonists gave similar pharmacokinetic profiles in mouse brain. Our findings do not support that different µ-opioid receptor subtypes or a distinct binding site at the µ-opioid receptor is involved in morphine-induced versus heroin/6-MAM-induced psychomotor activation. This might suggest that the opioid-induced psychomotor stimulation is mediated by different µ-opioid subreceptors than those responsible for their analgesic effects.

Has the intake of THC by cannabis users changed over the last decade? Evidence of increased exposure by analysis of blood THC concentrations in impaired drivers.

The main psychoactive substance, Δ9-tetrahydrocannabinol (THC) can be present in highly variable amounts in different cannabis preparations. An increase in THC content in cannabis products has been suggested, and reported from several countries. However, it has not yet been investigated if products with high potency lead to increased human exposure, and thus to higher risk of adverse effects. In this study, we examined the mean concentrations of THC in whole blood samples from drivers apprehended in Norway in the period between 2000 and 2010 suspected of driving under the influence of drugs. Cases with only THC (n=1747) have been compared to cases with only ethanol (n=38796) or amphetamines (n=2493). The increase in mean THC concentration measured from 2000 to 2010 was from 4.0 ± 0.3 to 6.6 ± 0.4 ng/ml (58%), compared to 3% for ethanol and 16% for the amphetamines. This increase in THC concentrations was to some extent paralleled by an increase in the percentage of drivers which were judged as lightly impaired by a physician. Monitoring concentrations of drugs of abuse in blood from apprehended drivers indicated an increasing exposure to THC in Norway. If similar trends are observed globally, it should be further explored if this type of information could be used to elucidate the drug consumption patterns in a population and accordingly the consequences with regard to adverse effects of cannabis from a public health perspective.

Pharmacokinetic modeling of subcutaneous heroin and its metabolites in blood and brain of mice.

High blood-brain permeability and effective delivery of morphine to the brain have been considered as explanations for the high potency of heroin. Results from Andersen et al. indicate that 6-monoacetylmorphine (6-MAM), and not morphine, is the active metabolite responsible for the acute effects observed for heroin. Here, we use pharmacokinetic modeling on data from the aforementioned study to calculate parameters of the distribution of heroin, 6-MAM and morphine in blood and brain tissue after subcutaneous heroin administration in mice. The estimated pharmacokinetic parameters imply that the very low heroin and the high 6-MAM levels observed both in blood and brain in the original experiment are likely to be caused by a very high metabolic rate of heroin in blood. The estimated metabolic rate of heroin in brain was much lower and cannot account for the low heroin and high 6-MAM levels in the brain, which would primarily reflect the concentrations of these compounds in blood. The very different metabolic rates for heroin in blood and brain calculated by the model were confirmed by in vitro experiments. These results show that heroin's fast metabolism in blood renders high concentrations of 6-MAM which, due to its relatively good blood-brain permeability, results in high levels of this metabolite in the brain. Thus, it is the high blood metabolism rate of heroin and the blood-brain permeability to 6-MAM, and not to heroin, which could account for the highly efficient delivery of active metabolites to the brain after heroin administration.

Increased locomotor activity induced by heroin in mice: pharmacokinetic demonstration of heroin acting as a prodrug for the mediator 6-monoacetylmorphine in vivo.

We investigated the relative importance of heroin and its metabolites in eliciting a behavioral response in mice by studying the relationship between concentrations of heroin, 6-monoacetylmorphine (6MAM), and morphine in brain tissue and the effects on locomotor activity. Low doses (subcutaneous) of heroin (< or =5 micromol/kg) or 6MAM (< or =15 micromol/kg) made the mice run significantly more than mice given equimolar doses of morphine. There were no differences in the response between heroin and 6MAM, although we observed a shift to the left of the dose-response curve for the maximal response of heroin. The behavioral responses were abolished by pretreatment with 1 mg/kg naltrexone. Heroin was detected in brain tissue after injection, but the levels were low and its presence too short-lived to be responsible for the behavioral response observed. The concentration of 6MAM in brain tissue increased shortly after administration of both heroin and 6MAM and the concentration changes during the first hour roughly reflected the changes in locomotor activity. Both the maximal and the total concentration of 6MAM were higher after administration of heroin than after administration of 6MAM itself. The morphine concentration increased slowly after injection and could not explain the immediate behavioral response. In summary, the locomotor activity response after injection of heroin was mediated by 6MAM, which increased shortly after administration. Heroin acted as an effective prodrug. The concentration of morphine was too low to stimulate the immediate response observed but might have an effect on the later part of the heroin-induced behavioral response curve.