Toxicological analysis in rats subjected to heroin and morphine overdose

Substructure-Specific Antibodies Against Fentanyl Derivatives

Structural variants of the synthetic opioid fentanyl are a major threat to public health. Following an investigation showing that many derivatives are poorly detected by commercial lateral flow and related assays, we created hapten conjugate vaccines using an immunogenic virus-like particle carrier and eight synthetic fentanyl derivatives designed to mimic the structural features of several of the more dangerous analogues. Immunization of mice elicited strong antihapten humoral responses, allowing the screening of hundreds of hapten-specific hybridomas for binding strength and specificity. A panel of 13 monoclonal IgG antibodies were selected, each showing a different pattern of recognition of fentanyl structural variations, and all proving to be highly efficient at capturing parent fentanyl compounds in competition ELISA experiments. These results provide antibody reagents for assay development as well as a demonstration of the power of the immune system to create binding agents capable of both broad and specific recognition of small-molecule targets.

Development and preclinical testing of a naloxone prodrug depot for extended protection against opioid overdose

The opioid crisis, driven by synthetic opioids like fentanyl, demands innovative solutions. The opioid antidote naloxone has a short action ( ~ 1 hour), requiring repeated doses. To address this, we present a new and simple naloxone prodrug delivery system repurposing a hydrophilic derivative of acoramidis, a potent transthyretin ligand. When the fully soluble prodrug solution is administered subcutaneously, the prodrug forms a zwitterionic depot at physiological pH, enabling extended naloxone release. This non-polymeric depot-forming approach is rare and employs carboxylesterase 2 for selective bioactivation, ensuring controlled drug release. In male rats and cynomolgus monkeys, a single subcutaneous dose provides steady naloxone release over several days, reducing blood-brain barrier diffusion, withdrawal symptoms, and CNS toxicity. Preclinical studies demonstrated efficacy in rat overdose models and achieved monkey naloxone levels matching effective human therapeutic levels. Although monkey efficacy was not assessed, combined rat efficacy and monkey pharmacokinetics suggest strong potential for successful human translation.

Hypoxic effects of heroin and fentanyl and their basic physiological mechanisms

Respiratory depression that diminishes oxygen delivery to the brain is the most dangerous effect of opioid drugs. Although plethysmography is a valuable tool to examine drug-induced changes in respiration, the primary cause of brain abnormalities induced by opioids is the global decrease in brain oxygen levels. The primary goal of this review is to provide an overview and discussion on fluctuations in brain oxygen levels induced by opioids, with a focus on heroin and fentanyl. To evaluate fluctuations in brain oxygen levels, we used oxygen sensors coupled with high-speed amperometry in awake, freely moving rats. First, we provide an overview of brain oxygen responses induced by natural physiological stimuli and discuss the mechanisms regulating oxygen entry into brain tissue. Then, we present data on brain oxygen responses induced by heroin and fentanyl and review their underlying mechanisms. These data allowed us to compare the effects of these drugs on brain oxygen regarding their latency, potency, time-dependency, and potential lethality at high doses as well as their relationships with peripheral oxygen responses. We also discuss data on the effects of naloxone on brain oxygen responses induced by heroin and fentanyl in the paradigms of both the pretreatment and treatment, when naloxone is administered at different times after the primary opioid drug. Although most data discussed were obtained in rats, they may have clinical relevance for understanding the mechanisms underlying the physiological effects of opioids and developing rational treatment strategies to decrease acute lethality and long-term health complications of opioid misuse.

Effects of novel beta-lactam, MC-100093, and ceftriaxone on astrocytic glutamate transporters and neuroinflammatory factors in nucleus accumbens of C57BL/6 mice exposed to escalated doses of morphine

Chronic exposure to opioids can lead to downregulation of astrocytic glutamate transporter 1 (GLT-1), which regulates the majority of glutamate uptake. Studies from our lab revealed that beta-lactam antibiotic, ceftriaxone, attenuated hydrocodone-induced downregulation of GLT-1 as well as cystine/glutamate antiporter (xCT) expression in central reward brain regions. In this study, we investigated the effects of escalating doses of morphine and tested the efficacy of novel synthetic non-antibiotic drug, MC-100093, and ceftriaxone in attenuating the effects of morphine exposure in the expression of GLT-1, xCT, and neuroinflammatory factors (IL-6 and TGF-β) in the nucleus accumbens (NAc). This study also investigated the effects of morphine and beta-lactams in locomotor activity, spontaneous alternation percentage (SAP) and number of entries in Y maze since opioids have effects in locomotor sensitization. Mice were exposed to moderate dose of morphine (20 mg/kg, i.p.) on days 1, 3, 5, 7, and a higher dose of morphine (150 mg/kg, i.p.) on day 9, and these mice were then behaviorally tested and euthanized on Day 10. Western blot analysis showed that exposure to morphine downregulated GLT-1 and xCT expression in the NAc, and both MC-100093 and ceftriaxone attenuated these effects. In addition, morphine exposure increased IL-6 mRNA and TGF-β mRNA expression, and MC-100093 and ceftriaxone attenuated only the effect on IL-6 mRNA expression in the NAc. Furthermore, morphine exposure induced an increase in distance travelled, and MC-100093 and ceftriaxone attenuated this effect. In addition, morphine exposure decreased the SAP and increased the number of arm entries in Y maze, however, neither MC-100093 nor ceftriaxone showed any attenuating effect. Our findings demonstrated for the first time that MC-100093 and ceftriaxone attenuated morphine-induced downregulation of GLT-1 and xCT expression, and morphine-induced increase in neuroinflammatory factor, IL-6, as well as hyperactivity. These findings revealed the beneficial therapeutic effects of MC-100093 and ceftriaxone against the effects of exposure to escalated doses of morphine.

Chronic Voluntary Morphine Intake Is Associated with Changes in Brain Structures Involved in Drug Dependence in a Rat Model of Polydrug Use

Chronic opioid intake leads to several brain changes involved in the development of dependence, whereby an early hedonistic effect (liking) extends to the need to self-administer the drug (wanting), the latter being mostly a prefrontal-striatal function. The development of animal models for voluntary oral opioid intake represents an important tool for identifying the cellular and molecular alterations induced by chronic opioid use. Studies mainly in humans have shown that polydrug use and drug dependence are shared across various substances. We hypothesize that an animal bred for its alcohol preference would develop opioid dependence and further that this would be associated with the overt cortical abnormalities clinically described for opioid addicts. We show that Wistar-derived outbred UChB rats selected for their high alcohol preference additionally develop: (i) a preference for oral ingestion of morphine over water, resulting in morphine intake of 15 mg/kg/day; (ii) marked opioid dependence, as evidenced by the generation of strong withdrawal signs upon naloxone administration; (iii) prefrontal cortex alterations known to be associated with the loss of control over drug intake, namely, demyelination, axonal degeneration, and a reduction in glutamate transporter GLT-1 levels; and (iv) glial striatal neuroinflammation and brain oxidative stress, as previously reported for chronic alcohol and chronic nicotine use. These findings underline the relevance of polydrug animal models and their potential in the study of the wide spectrum of brain alterations induced by chronic morphine intake. This study should be valuable for future evaluations of therapeutic approaches for this devastating condition.

Effect of human mesenchymal stem cell secretome administration on morphine self-administration and relapse in two animal models of opioid dependence

The present study investigates the possible therapeutic effects of human mesenchymal stem cell-derived secretome on morphine dependence and relapse. This was studied in a new model of chronic voluntary morphine intake in Wistar rats which shows classic signs of morphine intoxication and a severe naloxone-induced withdrawal syndrome. A single intranasal-systemic administration of MSCs secretome fully inhibited (>95%; p < 0.001) voluntary morphine intake and reduced the post-deprivation relapse intake by 50% (p < 0.02). Since several studies suggest a significant genetic contribution to the chronic use of many addictive drugs, the effect of MSCs secretome on morphine self-administration was further studied in rats bred as high alcohol consumers (UChB rats). Sub-chronic intraperitoneal administration of morphine before access to increasing concentrations of morphine solutions and water were available to the animals, led UChB rats to prefer ingesting morphine solutions over water, attaining levels of oral morphine intake in the range of those in the Wistar model. Intranasally administered MSCs secretome to UChB rats dose-dependently inhibited morphine self-administration by 72% (p < 0.001); while a single intranasal dose of MSC-secretome administered during a morphine deprivation period imposed on chronic morphine consumer UChB rats inhibited re-access morphine relapse intake by 80 to 85% (p < 0.0001). Both in the Wistar and the UChB rat models, MSCs-secretome administration reversed the morphine-induced increases in brain oxidative stress and neuroinflammation, considered as key engines perpetuating drug relapse. Overall, present preclinical studies suggest that products secreted by human mesenchymal stem cells may be of value in the treatment of opioid addiction.

Heroin-Related Compounds and Metabolic Ratios in Postmortem Samples Using LC-MS-MS

Analysis of postmortem samples with the presence of morphine can sometimes be challenging to interpret. Tolerance complicates interpretation of intoxications and causes of death due to overlap in therapeutic and fatal concentrations. Determination of metabolites and metabolic ratios can potentially differentiate between abstinence, continuous administration, and perhaps time of administration. The purpose of this study was to (a) develop and validate a method for quantitation of morphine-3β-D-glucuronide, morphine-6β-D-glucuronide, normorphine, codeine-6β-D-glucuronide, norcodeine, codeine, 6-acetylmorphine, and ethylmorphine in urine using liquid chromatography–tandem mass spectrometry; (b) apply the method to opiate related deaths; (c) compare metabolic ratios in urine in different causes of death (CoD) and after different drug intakes and (d) compare heroin intoxications in rapid and delayed deaths. Validation parameters such as precision, bias, matrix effects, stability, process efficiency, and dilution integrity were assessed and deemed acceptable. Lower limits of quantitation ranged from 0.01–0.2 μg/mL for all analytes. Autopsy cases (n=135) with paired blood and urine samples were analyzed. Cases were divided into three groups based on CoD; opiate intoxication, intoxication with other drugs than opiates, and other CoD. The cases were classified by intake: codeine (n=42), heroin (n=36), morphine (n=49), and ethylmorphine (n=3). Five cases were classified as mixed intakes and excluded. Heroin intoxications (n=35) were divided into rapid (n=15) or delayed (n=20) deaths. Parent drug groups were compared using metabolic ratio morphine-3β-D-glucuronide/morphine and significant differences were observed between codeine vs morphine (p=0.005) and codeine vs heroin (p≤0.0001). Urine and blood concentrations, and metabolic ratios in rapid and delayed heroin intoxications were compared and determined a significant difference for morphine (p=0.001), codeine (p=0.009), 6-acetylmorphine (p=0.02) in urine, and morphine (p=0.02) in blood, but there was no significant difference (p=0.9) between metabolic ratios. Morphine-3β-D-glucuronide results suggested a period of abstinence prior to death in 25% of the heroin intoxications.

Development and validation of a UPLC-MS/MS method with a broad linear dynamic range for the quantification of morphine, morphine-3-glucuronide and morphine-6-glucuronide in mouse plasma and tissue homogenates

The aim of this study was to develop and to validate a UPLC-MS/MS method for the quantification of morphine, morphine-3-glucuronide, and morphine-6-glucuronide in mouse plasma and tissue homogenates to support preclinical pharmacokinetic studies. The sample preparation consisted of protein precipitation with cold (2-8 °C) methanol:acetonitrile (1:1, v/v), evaporation of the supernatant to dryness, and reconstitution of the dry-extracts in 4 mM ammonium formate pH 3.5. Separation was achieved on a Waters UPLC HSS T3 column (150 × 2.1 mm, 1.8 µm) maintained at 50 °C and using gradient elution with a total runtime of 6.7 min. Mobile phase A consisted of 4 mM ammonium formate pH 3.5 and mobile phase B of 0.1% formic acid in methanol:acetonitrile (1:1, v/v). Detection was carried out by tandem mass spectrometry with electrospray ionization in the positive ion mode. The method was validated within a linear range of 1-2,000 ng/mL, 10-20,000 ng/mL, and 0.5-200 ng/mL for morphine, morphine-3-glucuronide, and morphine-6-glucuronide, respectively. In human plasma, the intra- and inter-run precision of all analytes, including the lower limit of quantification levels, were ≤ 15.8%, and the accuracies were between 88.1 and 111.9%. It has been shown that calibration standards prepared in control human plasma can be used for the quantification of the analytes in mouse plasma and tissue homogenates. The applicability of the method was successfully demonstrated in a preclinical pharmacokinetic study in mice.

Systematic Structure-Based Virtual Screening Approach to Antibody Selection and Design of a Humanized Antibody against Multiple Addictive Opioids without Affecting Treatment Agents Naloxone and Naltrexone

Opioid drug use, especially heroin, is known as a growing national crisis in America. Heroin itself is a prodrug and is converted to the most active metabolite 6-monoacetylmorphine (6-MAM) responsible for the acute toxicity of heroin and then to a relatively less-active metabolite morphine responsible for the long-term toxicity of heroin. Monoclonal antibodies (mAbs) are recognized as a potentially promising therapeutic approach in the treatment of opioid use disorders (OUDs). Due to the intrinsic challenges of discovering an mAb against multiple ligands, here we describe a general, systematic structure-based virtual screening and design approach which has been used to identify a known anti-morphine antibody 9B1 and a humanized antibody h9B1 capable of binding to multiple addictive opioids (including 6-MAM, morphine, heroin, and hydrocodone) without significant binding with currently available OUD treatment agents naloxone, naltrexone, and buprenorphine. The humanized antibody may serve as a promising candidate for the treatment of OUDs. The experimental binding affinities reasonably correlate with the computationally predicted binding free energies. The experimental activity data strongly support the computational predictions, suggesting that the systematic structure-based virtual screening and humanization design protocol is reliable. The general, systematic structure-based virtual screening and design approach will be useful for many other antibody selection and design efforts in the future.

A vapor exposure method for delivering heroin alters nociception, body temperature and spontaneous activity in female and male rats

BACKGROUND

The ongoing crisis related to non-medical use of opioids makes it of continued importance to understand the risk factors for opioid addiction, the behavioral and neurobiological consequences of opioid exposure and to seek potential avenues for therapy. Pre-clinical rodent models have been critical to advancing understanding of opioid consequences for decades, but have been mostly limited to drug delivery by injection or by oral dosing. Inhalation, a significant route for many human users, has not been as well-established.

METHOD

We adapted an e-cigarette based exposure system, previously shown efficacious for delivery of other drugs to rats, to deliver heroin vapor. Effectsin vivo were assessed in male and female Sprague-Dawley rats using a warm-water assay for anti-nociception and an implanted radiotelemetry system for evaluating changes in body temperature and spontaneous activity rate.

RESULTS

Inhalation of vapor created by heroin 100 mg/mL in the propylene glycol (PG) vehicle significantly slowed tail-withdrawal from a 52 °C water bath, bi-phasically altered activity, and increased temperature in male and female rats. Inhalation of heroin 50 mg/mL for 15 min produced significant effects, as the lower bound on efficacy, whereas inhalation of heroin 100 mg/mL for 30 min produced robust effects across all endpoints and groups.

CONCLUSIONS

This work shows that e-cigarette devices deliver psychoactive doses of heroin to rats, using concentrations of ∼50-100 mg/mL and inhalation durations of 15-30 min. This technique may be useful to assess the health consequences of inhaled heroin and other opioid drugs.

Interactions of benzodiazepines with heroin: Respiratory depression, temperature effects, and behavior

Benzodiazepines are important therapeutic drugs, but they are often abused and co-abused with opioids. Clinical evidence suggests that benzodiazepines can inhibit respiration, and when combined with the respiratory-depressive effects of opioids, may increase likelihood of death. In this study we used oxygen sensors coupled with high-speed amperometry and multi-site thermorecording to examine how intravenous (iv) midazolam, a potent benzodiazepine, modulates the brain hypoxic and temperature effects of iv heroin in freely-moving rats. Oxygen levels and brain temperature were assessed with high temporal resolution in the nucleus accumbens (NAc), an important structure in the motivational-reinforcement circuit. When administered alone, midazolam (2 mg/kg) modestly decreased NAc temperature but had no evident effects on oxygen levels in this structure. In contrast, heroin (0.4 mg/kg) induced a strong decrease in NAc oxygen that was followed by a weaker, rebound-like oxygen increase. Midazolam pretreatment did not affect heroin-induced brain hypoxia but potentiated the initial hypothermia induced by heroin. However, co-administration of these drugs potentiated the heroin-induced oxygen decrease and enhanced heroin-induced brain hypothermia. Co-administration of heroin and midazolam also resulted in enhanced locomotor inhibition and loss of motor control. This effect caused some rats to collapse, resulting in nose and mouth occlusion, which caused a secondary hypoxic phase. These results could have important implications for human drug users, as the combined use of benzodiazepines with potent opioids not only results in sustained brain hypoxia but creates conditions of loss of motor control which could result in asphyxia and death. This article is part of the Special Issue entitled 'New Vistas in Opioid Pharmacology'.

Respiratory depression and brain hypoxia induced by opioid drugs: Morphine, oxycodone, heroin, and fentanyl

Opioid drugs are important tools to alleviate pain of different origins, but they have strong addictive potential and their abuse at higher doses often results in serious health complications. Respiratory depression that leads to brain hypoxia is perhaps the most dangerous symptom of acute intoxication with opioids, and it could result in lethality. The development of substrate-specific sensors coupled with amperometry made it possible to directly evaluate physiological and drug-induced fluctuations in brain oxygen levels in awake, freely-moving rats. The goal of this review paper is to consider changes in brain oxygen levels induced by several opioid drugs (heroin, fentanyl, oxycodone, morphine). While some of these drugs are widely used in clinical practice, they all are abused, often at doses exceeding the clinical range and often resulting in serious health complications. First, we consider some basic knowledge regarding brain oxygen, its physiological fluctuations, and mechanisms involved in regulating its entry into brain tissue. Then, we present and discuss data on brain oxygen changes induced by each opioid drug within a wide range of doses, from low, behaviorally relevant, to high, likely to be self-administered by drug users. These data allowed us to compare the effects of these drugs on brain oxygen in terms of their potency, time-course, and their potential danger when used at high doses via rapid-onset administration routes. While most data discussed in this work were obtained in rats, we believe that these data have clear human relevance in addressing the alarming rise in lethality associated with the opioid abuse.

Adolescent morphine exposure induces immediate and long-term increases in impulsive behavior

RATIONALE

Adolescence in humans represents a unique and critical developmental time point associated with increased risk-taking behavior. Converging clinical and epidemiological studies report a peak of drug use during adolescence, leading to the hypothesis that the developing adolescents brain is at risk to lose control over drug intake. Both adolescence and drug abuse are associated with significant cognitive and psychological changes such as lack of impulse control. A simple definition for impulsive behavior is the tendency to act prematurely without foresight. Increase in impulsivity is evident in acute morphine consumption, but to date, little is known with respect to subchronic morphine administration in impulsive behavior, particularly comparing time-dependent effects in adults, young adults, and adolescents.

METHODS

To evaluate this, adult, young adult, and adolescent rats were treated with a subchronic regimen of morphine or saline during 5 days (s.c.). Thereafter, we examined impulsive behavioral effects of morphine administration, 24 h and 25 days after administration in rats, while responding under a five-choice serial reaction time task (5-CSRTT).

RESULTS

Subchronic morphine administration increased premature responding 24 h after the last injection of morphine in adult, young adult, and adolescent rats without increasing motor activity but a significant change in motivation in adult and young adult rats only. After 25 days of abstinence, premature responses were significantly increased in comparison with baseline in adolescent rats but not in adults and young adults.

CONCLUSION

The main conclusion of this study is that morphine exposure in adolescents has a long-term profound effect on motor impulsive behavior later in adulthood. An implication of our findings might be that we should be especially careful about consuming and prescribing opioid drugs in adolescents.

Blocking drug activation as a therapeutic strategy to attenuate acute toxicity and physiological effects of heroin

Heroin is a growing national crisis in America. There is an increasing frequency of heroin overdoses. All of the currently used therapeutic approaches to treatment of heroin abuse and other opioid drugs of abuse focus on antagonizing a brain receptor (particularly µ-opiate receptors). However, it has been known that the therapeutic use of certain µ-opiate receptor antagonist may actually increase heroin overdose. Once overdosed, heroin addicts may continue to get overdosed again and again until fatal. Here we report our design and validation of a novel therapeutic strategy targeting heroin activation based on our analysis of the chemical transformation and functional change of heroin in the body. An effective blocker of heroin activation, such as ethopropazine tested in this study, may be used as a standalone therapy or in combination with a currently available, traditional medications targeting µ-opiate receptors (e.g. naltrexone or its extended-release formulation Vivitrol). The combination therapy would be ideal for heroin abuse treatment as the effects of two therapeutic agents targeting two independent mechanisms are cooperative.

Central and Peripheral Mechanisms Underlying Physiological and Drug-Induced Fluctuations in Brain Oxygen in Freely-Moving Rats

The goal of this work is to consider physiological fluctuations in brain oxygen levels and its changes induced by opioid drugs. This review article presents, as a comprehensive story, the most important findings obtained in our laboratory by using high-speed amperometry with oxygen sensors in awake, freely moving rats; most of these findings were separately published elsewhere. First, we show that oxygen levels in the nucleus accumbens (NAc) phasically increase following exposure to natural arousing stimuli. Since accumbal neurons are excited by arousing stimuli and NAc oxygen levels increase following glutamate (GLU) microinjections in the NAc, local neural activation with subsequent cerebral vasodilation appears to mediate the rapid oxygen increases induced by arousing stimuli. While it is established that intra-cerebral entry of oxygen depends on brain metabolism, physiological increases in NAc oxygen occurred more rapidly than increases in metabolic activity as assessed by intra-brain heat production. Therefore, due to neural activation and the subsequent rise in local cerebral blood flow (CBF), the brain receives more oxygen in advance of its metabolic requirement, thus preventing potential metabolic deficits. In contrast to arousing stimuli, three opioid drugs tested (heroin, fentanyl and oxycodone) decrease oxygen levels. As confirmed by our recordings in the subcutaneous space, a densely vascularized location with no metabolic activity of its own, these decreases result from respiratory depression with subsequent fall in blood oxygen levels. While respiratory depression was evident for all tested drugs, heroin was ~6-fold more potent than oxycodone, and fentanyl was 10-20-fold more potent than heroin. Changes in brain oxygen induced by respiratory depression appear to be independent of local vascular and blood flow responses, which are triggered, via neuro-vascular coupling, by the neuronal effects of opioid drugs.

Heroin Contaminated with Fentanyl Dramatically Enhances Brain Hypoxia and Induces Brain Hypothermia

While opioid abuse is an established medical and public health issue, the increased availability of highly potent synthetic opioids, such as fentanyl, has given rise to acute health complications, including a comatose state and death during drug overdose. Since respiratory depression that leads to acute hypoxia is the most dangerous complication of opioid drug use, we examined the effects of intravenous heroin and heroin contaminated with 10% fentanyl on oxygen levels in the nucleus accumbens (NAc) monitored using high-speed amperometry in freely moving rats. Additionally, we examined the effects of heroin, fentanyl, and their mixture on locomotion and temperatures in the NAc, temporal muscle, and skin. Both fentanyl and heroin at human-relevant doses (400 and 40 μg/kg, respectively) induced rapid, strong and transient decreases in NAc oxygen, indicative of brain hypoxia. When the heroin-fentanyl mixture was injected, the NAc hypoxic response was greatly potentiated in its duration, suggesting sustained hypoxia. In contrast to modest, monophasic brain temperature increases caused by heroin alone, the heroin-fentanyl mixture induced a biphasic temperature response, with a prominent postinjection decrease resulting from peripheral vasodilation. This hypothermic effect, albeit much smaller and more transient, was typical of fentanyl injected alone. Our findings indicate that accidental use of fentanyl instead of heroin, or even a relatively minor contamination of “street heroin” with fentanyl, poses great danger for acute health complications, including a comatose state and death.

Brain temperature effects of intravenous heroin: State dependency, environmental modulation, and the effects of dose

Here we examined how intravenous heroin at a dose that maintains self-administration (0.1 mg/kg) affects brain temperature homeostasis in freely moving rats under conditions that seek to mimic some aspects of human drug use. When administered under standard laboratory conditions (quiet rest at 22 °C ambient temperature), heroin induced moderate temperature increases (1.0-1.5 °C) in the nucleus accumbens (NAc), a critical structure of the brain motivation-reinforcement circuit. By simultaneously recording temperatures in the temporal muscle and skin, we demonstrate that the hyperthermic effects of heroin results primarily from inhibition of heat loss due to strong and prolonged skin vasoconstriction. Heroin-induced brain temperature increases were enhanced during behavioral activation (i.e., social interaction) and in a moderately warm environment (29 °C). By calculating the "net" effects of the drug in these two conditions, we found that this enhancement results from the summation of the hyperthermic effects of heroin with similar effects induced by either social interaction or a warmer environment. When the dose of heroin was increased (to 0.2, 0.4, 0.8, 1.6, 3.2, and 6.4 mg/kg), brain temperature showed a biphasic down-up response. The initial temperature decrease was dose-dependent and resulted from a transient inhibition of intra-brain heat production coupled with increased heat loss via skin surfaces-the effects typically induced by general anesthetics. These initial inhibitory effects induced by large-dose heroin injections could be related to profound CNS depression-the most serious health complications typical of heroin overdose in humans.

Reaction pathways and free energy profiles for cholinesterase-catalyzed hydrolysis of 6-monoacetylmorphine

As the most active metabolite of heroin, 6-monoacetylmorphine (6-MAM) can penetrate into the brain for the rapid onset of heroin effects. The primary enzymes responsible for the metabolism of 6-MAM to the less potent morphine in humans are acetylcholinesterase (AChE) and butyrylcholinesterase (BChE). The detailed reaction pathways for AChE- and BChE-catalyzed hydrolysis of 6-MAM to morphine have been explored, for the first time, in the present study by performing first-principles quantum mechanical/molecular mechanical free energy calculations. It has been demonstrated that the two enzymatic reaction processes follow similar catalytic reaction mechanisms, and the whole catalytic reaction pathway for each enzyme consists of four reaction steps. According to the calculated results, the second reaction step associated with the transition state TS2(a)/TS2(b) should be rate-determining for the AChE/BChE-catalyzed hydrolysis, and the free energy barrier calculated for the AChE-catalyzed hydrolysis (18.3 kcal mol(-1)) is 2.5 kcal mol(-1) lower than that for the BChE-catalyzed hydrolysis (20.8 kcal mol(-1)). The free energy barriers calculated for the AChE- and BChE-catalyzed reactions are in good agreement with the experimentally derived activation free energies (17.5 and 20.7 kcal mol(-1) for the AChE- and BChE-catalyzed reactions, respectively). Further structural analysis reveals that the aromatic residues Phe295 and Phe297 in the acyl pocket of AChE (corresponding to Leu286 and Val288 in BChE) contribute to the lower energy of TS2(a) relative to TS2(b). The obtained structural and mechanistic insights could be valuable for use in future rational design of a novel therapeutic treatment of heroin abuse.

Levels of heroin and its metabolites in blood and brain extracellular fluid after i.v. heroin administration to freely moving rats

BACKGROUND AND PURPOSE

Heroin, with low affinity for μ-opioid receptors, has been considered to act as a prodrug. In order to study the pharmacokinetics of heroin and its active metabolites after i.v. administration, we gave a bolus injection of heroin to rats and measured the concentration of heroin and its metabolites in blood and brain extracellular fluid (ECF).

EXPERIMENTAL APPROACH

After an i.v. bolus injection of heroin to freely moving Sprague-Dawley rats, the concentrations of heroin and metabolites in blood samples from the vena jugularis and in microdialysis samples from striatal brain ECF were measured by ultraperformance LC-MS/MS.

KEY RESULTS

Heroin levels decreased very fast, both in blood and brain ECF, and could not be detected after 18 and 10 min respectively. 6-Monoacetylmorphine (6-MAM) increased very rapidly, reaching its maximal concentrations after 2.0 and 4.3 min, respectively, and falling thereafter. Morphine increased very slowly, reaching its maximal levels, which were six times lower than the highest 6-MAM concentrations, after 12.6 and 21.3 min, with a very slow decline during the rest of the experiment and only surpassing 6-MAM levels at least 30 min after injection.

CONCLUSIONS AND IMPLICATIONS

After an i.v. heroin injection, 6-MAM was the predominant opioid present shortly after injection and during the first 30 min, not only in the blood but also in rat brain ECF. 6-MAM might therefore mediate most of the effects observed shortly after heroin intake, and this finding questions the general assumption that morphine is the main and most important metabolite of heroin.

Fundamental reaction pathway and free energy profile for butyrylcholinesterase-catalyzed hydrolysis of heroin

The pharmacological function of heroin requires an activation process that transforms heroin into 6-monoacetylmorphine (6-MAM), which is the most active form. The primary enzyme responsible for this activation process in human plasma is butyrylcholinesterase (BChE). The detailed reaction pathway of the activation process via BChE-catalyzed hydrolysis has been explored computationally, for the first time, in this study via molecular dynamics simulation and first-principles quantum mechanical/molecular mechanical free energy calculations. It has been demonstrated that the whole reaction process includes acylation and deacylation stages. The acylation consists of two reaction steps, i.e., the nucleophilic attack on the carbonyl carbon of the 3-acetyl group of heroin by the hydroxyl oxygen of the Ser198 side chain and the dissociation of 6-MAM. The deacylation also consists of two reaction steps, i.e., the nucleophilic attack on the carbonyl carbon of the acyl-enzyme intermediate by a water molecule and the dissociation of the acetic acid from Ser198. The calculated free energy profile reveals that the second transition state (TS2) should be rate-determining. The structural analysis reveals that the oxyanion hole of BChE plays an important role in the stabilization of rate-determining TS2. The free energy barrier (15.9 ± 0.2 or 16.1 ± 0.2 kcal/mol) calculated for the rate-determining step is in good agreement with the experimentally derived activation free energy (~16.2 kcal/mol), suggesting that the mechanistic insights obtained from this computational study are reliable. The obtained structural and mechanistic insights could be valuable for use in the future rational design of a novel therapeutic treatment of heroin abuse.

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.

The effect of morphine dependence on impulsive choice in rats

RATIONALE

In the human opiate-dependent population, the most consistently reported deficit in executive functioning is impulsivity. Previous research has shown that acute and chronic exposure to drugs of abuse can increase impulsive choice; however, the extent to which opiate dependence contributes to increased impulsivity has not been examined. We report here the effects of morphine dependence on rats' delay discounting (DD) of a sucrose reward.

METHODS

We assigned rats randomly to either a dependent group that received a nightly 30 mg/kg subcutaneous dose of morphine or a morphine-naive group that received a nightly saline injection. DD of a sucrose reward was examined in rats prior to initiation of the dosing regimen, 22.5 h after the daily maintenance dose, and after a 14-day abstinence period.

RESULTS

The groups did not differ at baseline, but rats showed accelerated DD while dependent on morphine. After withdrawal from morphine, DD in previously dependent rats was not significantly different from that of naive rats. Since dependent rats also showed reduced motivation to acquire the sucrose reinforcer, we performed a separate experiment to test whether such a decrease in motivation could cause an increase in impulsivity. We found that food-deprived rats switched to a free-feeding diet did not differ in DD from rats maintained at 85 % of free-feeding weight.

CONCLUSIONS

An increase in impulsivity can result from physical dependence on morphine and cannot be attributed to changes in motivation to acquire sucrose-reinforced responses.

The influence of heroin abuse on glutathione-dependent enzymes in human brain

Heroin is an illicit narcotic abused by millions of people worldwide. In our earlier studies we have shown that heroin intoxication changes the antioxidant status in human brain. In the present work we continued our studies by estimating the effect of heroin abuse on reduced glutathione (GSH) and enzymes related to this cofactor, such as glutathione S-transferase detoxifying electrophilics (GST) and organic peroxides (as Se-independent glutathione peroxidase-GSHPx), and Se-dependent glutathione peroxidase (Se-GSHPx) specific mainly for hydrogen peroxide. Studies were conducted on human brains obtained from autopsy of 9 heroin abusers and 8 controls. The level of GSH and the activity of glutathione-related enzymes were determined spectrophotometrically. The expression of GST pi on mRNA and protein level was studied by RT-PCR and Western blotting, respectively. The results indicated significant increase of GST and GSHPx activities, unchanged Se-GSHPx activity, and decreased level of GSH in frontal, temporal, parietal and occipital cortex, brain stem, hippocampus, and white matter of heroin abusers. GST pi expression was increased on both mRNA and protein levels, however the increase was lower in brain stem than in other regions. Heroin affects all regions of human brain, and especially brain stem. Its intoxication leads to an increase of organic rather then inorganic peroxides in various brain regions. Glutathione S-transferase plays an important role during heroin intoxication, however its protective effect is lower in brain stem than in brain cortex or hippocampus.

Neurotoxicity of heroin-cocaine combinations in rat cortical neurons

Cocaine and heroin are frequently co-abused by humans, in a combination known as speedball. Recently, chemical interactions between heroin (Her) or its metabolite morphine (Mor) and cocaine (Coc) were described, resulting in the formation of strong adducts. In this work, we evaluated whether combinations of Coc and Her affect the neurotoxicity of these drugs, using rat cortical neurons incubated with Coc, Her, Her followed by Coc (Her+Coc) and Her plus Coc (Her:Coc, 1:1). Neurons exposed to Her, Her+Coc and Her:Coc exhibited a decrease in cell viability, which was more pronounced in neurons exposed to Her and Her+Coc, in comparison with neurons exposed to the mixture (Her:Coc). Cells exposed to the mixture showed increased intracellular calcium and mitochondrial dysfunction, as determined by a decrease in intracellular ATP levels and in mitochondrial membrane potential, displaying both apoptotic and necrotic characteristics. Conversely, a major increase in cytochrome c release, caspase 3-dependent apoptosis, and decreased metabolic neuronal viability were observed upon sequential exposure to Her and Coc. The data show that drug combinations potentiate cortical neurotoxicity and that the mode of co-exposure changes cellular death pathways activated by the drugs, strongly suggesting that chemical interactions occurring in Her:Coc, such as adduct formation, shift cell death mechanisms towards necrosis. Since impairment of the prefrontal cortex is involved in the loss of impulse control observed in drug addicts, the data presented here may contribute to explain the increase in treatment failure observed in speedball abusers.

Drug context differently regulates cocaine versus heroin self-administration and cocaine- versus heroin-induced Fos mRNA expression in the rat

RATIONALE

We have previously reported that cocaine self-administration is facilitated in male rats not residing in the test chambers (Non Resident rats) relative to rats living in the test chambers at all times (Resident rats). Surprisingly, the opposite was found for heroin.

MATERIALS AND METHODS

We predicted that, when given access to both cocaine and heroin on alternate days, Non Resident rats would take more cocaine relative to heroin than Resident rats. Heroin (25.0 microg/kg) and cocaine (400 microg/kg), were made alternately available for 14 self-administration sessions, on a fixed ratio (FR) schedule that was progressively increased from FR1 to FR5. Next, some rats underwent a progressive-ratio procedure for heroin and cocaine. The other rats continued to alternate heroin and cocaine self-administration for 12 additional sessions, during which the FR schedule was progressively increased from FR10 to FR100. The second aim of the study was to investigate Fos mRNA expression in Resident and Non Resident rats treated with non-contingent intravenous infusion of "self-administration doses" of heroin (25.0 microg/kg) and cocaine (400 microg/kg).

RESULTS

We found that: (1) drug-taking context differentially modulates intravenous cocaine versus heroin self-administration; (2) very low doses of cocaine and heroin are sufficient to induce Fos mRNA expression in the posterior caudate; (3) drug-administration context differentially modulates cocaine- versus heroin-induced Fos mRNA expression.

CONCLUSIONS

Our study indicates that the context of drug taking can play a powerful role in modulating cocaine versus heroin intake in the laboratory rat.

Ambience and drug choice: cocaine- and heroin-taking as a function of environmental context in humans and rats

BACKGROUND

We have recently observed an unforeseen dissociation in the effect of environmental context on heroin versus cocaine self-administration in rats. Rats housed in the self-administration chambers (Residents) took more heroin than rats that were transferred to the self-administration chambers only for the test sessions (Nonresidents). The contrary was found for cocaine. The twofold aim of the present study was to investigate: 1) drug choice as a function of ambience in rats given access to both cocaine and heroin, and 2) ambience of choice for cocaine- versus heroin-taking in human addicts.

METHODS

Resident and Nonresident rats with double-lumen intrajugular catheters were trained to self-administer cocaine (400 microg/kg/infusion) and heroin (25 microg/kg/infusion) on alternate days and then given the opportunity to choose between the two drugs during seven daily sessions. In the human study, we asked heroin and cocaine abusers where they preferred to take these drugs.

RESULTS

Approximately 46.7% of Resident rats exhibited a preference for heroin over cocaine; 33.3% preferred cocaine, and 20% expressed no preference. In contrast, only 8.3% of Nonresident rats preferred heroin, whereas 66.7% preferred cocaine, and 25% expressed no preference. In the human study, 73% of co-abusers reported that they used heroin exclusively or mostly at home (22% used it outside the home), whereas only 25% reported using cocaine at home (67% took it outside their homes).

CONCLUSIONS

Environmental context plays an important role in drug choice in both humans and rats self-administering heroin and cocaine.

Post-mortem clinical pharmacology

Clinical pharmacology assumes that deductions can be made about the concentrations of drugs from a knowledge of the pharmacokinetic parameters in an individual; and that the effects are related to the measured concentration. Post-mortem changes render the assumptions of clinical pharmacology largely invalid, and make the interpretation of concentrations measured in post-mortem samples difficult or impossible. Qualitative tests can show the presence of substances that were not present in life, and can fail to detect substances that led to death. Quantitative analysis is subject to error in itself, and because post-mortem concentrations vary in largely unpredictable ways with the site and time of sampling, as a result of the phenomenon of post-mortem redistribution. Consequently, compilations of 'lethal concentrations' are misleading. There is a lack of adequate studies of the true relationship between fatal events and the concentrations that can be measured subsequently, but without such studies, clinical pharmacologists and others should be wary of interpreting post-mortem measurements.

A unique carrier for delivery of therapeutic compounds beyond the blood-brain barrier

Background

Therapeutic intervention in many neurological diseases is thwarted by the physical obstacle formed by the blood-brain barrier (BBB) that excludes most drugs from entering the brain from the blood. Thus, identifying efficacious modes of drug delivery to the brain remains a “holy grail” in molecular medicine and nanobiotechnology. Brain capillaries, that comprise the BBB, possess an endogenous receptor that ferries an iron-transport protein, termed p97 (melanotransferrin), across the BBB. Here, we explored the hypothesis that therapeutic drugs “piggybacked” as conjugates of p97 can be shuttled across the BBB for treatment of otherwise inoperable brain tumors.

Approach

Human p97 was covalently linked with the chemotherapeutic agents paclitaxel (PTAX) or adriamycin (ADR) and following intravenous injection, measured their penetration into brain tissue and other organs using radiolabeled and fluorescent derivatives of the drugs. In order to establish efficacy of the conjugates, we used nude mouse models to assess p97-drug conjugate activity towards glioma and mammary tumors growing subcutaneously compared to those growing intracranially.

Principal Findings

Bolus-injected p97-drug conjugates and unconjugated p97 traversed brain capillary endothelium within a few minutes and accumulated to 1–2% of the injected by 24 hours. Brain delivery with p97-drug conjugates was quantitatively 10 fold higher than with free drug controls. Furthermore, both free-ADR and p97-ADR conjugates equally inhibited the subcutaneous growth of gliomas growing outside the brain. Evocatively, only p97-ADR conjugates significantly prolonged the survival of animals bearing intracranial gliomas or mammary tumors when compared to similar cumulated doses of free-ADR.

Significance

This study provides the initial proof of concept for p97 as a carrier capable of shuttling therapeutic levels of drugs from the blood to the brain for the treatment of neurological disorders, including classes of resident and metastatic brain tumors. It may be prudent, therefore, to consider implementation of this novel delivery platform in various clinical settings for therapeutic intervention in acute and chronic neurological diseases.

(1)H MRS in the rat brain under pentobarbital anesthesia: accurate quantification of in vivo spectra in the presence of propylene glycol

Commercial solutions for pentobarbital anesthesia typically contain water H spectra. The purpose of the present study was to measure the concentration of metabolites in the rat brain in vivo under pentobarbital anesthesia using 1H MRS. Resonances of PG, but not ethanol, were observed in the rat brain. Chemical shifts and J-coupling constants for PG were measured at 37 degrees C and pH 7.1 and used for spectral simulation. Inclusion of the simulated PG spectrum in the basis set for LCModel analysis enabled accurate fitting of in vivo spectra. This work demonstrates that concentration of brain metabolites can be reliably measured using 1H spectroscopy under pentobarbital anesthesia. The chemical shifts and J-coupling values reported here can be used to simulate the spectrum of PG at any field strength, with various pulse sequences.

Endogenous opiates and behavior: 2006

This paper is the 29th consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning 30 years of research. It summarizes papers published during 2006 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurological disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); and immunological responses (Section 17).