(1S, 3S)-3-amino-4-difluoromethylenyl-1-cyclopentanoic acid (CPP-115), a potent γ-aminobutyric acid aminotransferase inactivator for the treatment of cocaine addiction.

Vigabatrin, a GABA aminotransferase (GABA-AT) inactivator, is used to treat infantile spasms and refractory complex partial seizures and is in clinical trials to treat addiction. We evaluated a novel GABA-AT inactivator (1S, 3S)-3-amino-4-difluoromethylenyl-1-cyclopentanoic acid (CPP-115, compound 1) and observed that it does not exhibit other GABAergic or off-target activities and is rapidly and completely orally absorbed and eliminated. By use of in vivo microdialysis techniques in freely moving rats and microPET imaging techniques, 1 produced similar inhibition of cocaine-induced increases in extracellular dopamine and in synaptic dopamine in the nucleus accumbens at (1)/(300) to (1)/(600) the dose of vigabatrin. It also blocks expression of cocaine-induced conditioned place preference at a dose (1)/(300) that of vigabatrin. Electroretinographic (ERG) responses in rats treated with 1, at doses 20-40 times higher than those needed to treat addiction in rats, exhibited reductions in ERG responses, which were less than the reductions observed in rats treated with vigabatrin at the same dose needed to treat addiction in rats. In conclusion, 1 can be administered at significantly lower doses than vigabatrin, which suggests a potential new treatment for addiction with a significantly reduced risk of visual field defects.

Concentration-dependent conditioned place preference to inhaled toluene vapors in rats.

OBJECTIVES: Toluene is present in many commercial products and is subject to abuse by inhalation. The goal of this study was to extend previous reports indicating that rats will exhibit a positive conditioned place preference to inhaled toluene vapors and to determine the dose-response relationship for inhaled toluene in terms of exposure concentration and number of exposures. For the conditioned place preference experiments rats were exposed to toluene vapors at concentrations of 800, 2000, 3000 or 5000 ppm in one compartment of a three-compartment box. RESULTS: Following six conditioning sessions with toluene, a significant place preference was obtained at 2000 and 3000 ppm, but not at 800 or 5000 ppm. Extending the number of toluene pairings at the 2000 and 3000 ppm concentration to 12 significantly enhanced the place preference compared to that at six pairings. CONCLUSIONS: These experiments extend our previous finding that rats will show a conditioned place preference to inhaled toluene, and indicate that a reinforcing "dose" of toluene depends on both the concentration and number of pairings.

PET imaging in clinical drug abuse research.

Over the last two decades, SPECT (single photon emission computed tomography) and especially PET (positron emission tomography) have proven increasingly effective imaging modalities in the study of human psychopharmacology. Abusing populations can be studied at multiple times after abstinence begins, to give information about neurochemical and physiological adaptations of the brain during recovery from addiction. Individual human subjects can be studied using multiple positron labeled radiotracers, so as to probe more than one facet of brain function. PET and SPECT have been used to help our understanding of many aspects of the pharmacokinetics and pharmacodynamics of abused drugs, and have made valuable contributions in terms of drug mechanisms, drug interactions (e.g. cocaine and alcohol) and drug toxicities. They have also been employed to study the acute effects of drugs on populations of active drug abusers and of normal controls, and to evaluate the neurochemical consequences of candidate therapies for drug abuse. A particularly productive strategy has been the use of PET in conjunction with neuropsychological testing of subjects, to allow correlation of imaging data with uniquely human aspects of the effects of drugs, such as euphoria and craving.

Synthesis and evaluation of inhaled [11C]butane and intravenously injected [11C]acetone as potential radiotracers for studying inhalant abuse.

The phenomenon of inhalant abuse is a growing problem in the US and many countries around the world. Yet, relatively little is known about the pharmacokinetic properties of inhalants that underlie their abuse potential. While the synthesis of 11C-labeled toluene, acetone and butane has been proposed in the literature, none of these compounds has been developed as radiotracers for PET studies. In the present report we extend our previous studies with [11C]toluene to include [11C]acetone and [11C]butane with the goal of comparing the pharmacokinetic profiles of these three volatile abused substances. Both [11C]toluene and [11C]acetone were administered intravenously and [11C]butane was administered via inhalation to anesthesized baboons. Rapid and efficient uptake of radiolabeled toluene and acetone into the brain was followed by fast clearance in the case of toluene and slower kinetics in the case of acetone. [11C]Butane was detected in the blood and brain following inhalation, but the levels of radioactivity in both tissues dropped to half of the maximal values over the period of less than a minute. To our knowledge, this is the first reported study of the in vivo brain pharmacokinetics of labeled acetone and butane in nonhuman primates. These data provide insight into the pharmacokinetic features possibly associated with the abuse liability of toluene, acetone and butane.

Toluene inhalation produces a conditioned place preference in rats.

Toluene is a widely abused solvent with demonstrated addictive potential in humans. Here we explore if conditioned place preference can be used to study the abuse-related effects of inhaled toluene in rats. Animals were confined to a distinctive compartment of a three-compartment chamber while exposed to toluene vapor and later tested for preference for that compartment compared to appropriate control subjects. In this study, a flame ionization detector was used for on-line monitoring of toluene vapor concentrations inside the conditioning apparatus coupled with computerized recording of the time spent by the animals on the test day in each of the chambers. Sprague-Dawley rats were exposed to 810, 1895 or 4950 ppm of toluene vapors in either the black or white compartment during 30-min pairing sessions given every other day alternating with air exposure for the total of six pairings for each treatment. Rats that received air in both sides (control group) did not show any preference for either side with approximately equal time spent in each compartment on the test day (241 +/- 33 and 234 +/- 34 s, for white and black box, respectively). However, the 1895- and 4950-ppm test groups, but not the 810-ppm group, demonstrated a significant preference for the side paired with toluene exposure. When a subsequent test session was performed during toluene exposures, no conditioned place preference was observed. Thus, toluene produced a clear conditioned place preference that appears to be most evident when animals are not intoxicated. This procedure should be useful for further studies of the abuse-related effects of abused inhalants.

Selegiline potentiates cocaine-induced increases in rodent nucleus accumbens dopamine.

Selegiline has been proposed as a treatment for cocaine addiction and studies in humans suggest that it attenuates cocaine's reinforcing effects. Here we assessed the effects of selegiline treatment on cocaine-induced increases in nucleus accumbens (NAc) dopamine (DA) in freely moving rodents. Chronic treatment with selegiline (L-deprenyl, 0.25/mg/kg, 24 days) potentiated cocaine-induced increases in NAc DA from 350-600%. However, this enhanced response was abolished when animals were treated chronically with both cocaine and selegiline. Inasmuch as increases in NAc DA are associated with the reinforcing effects of cocaine, these results obtained in rodents suggest that MAO-A and -B inhibition may not be a suitable strategy to antagonize cocaine's reinforcing effects during cocaine detoxification. On the other hand, chronic selegiline treatment may improve DA deficits, which are thought to contribute to relapse through a decreased response to natural rewards.

Acute handling stress modulates methylphenidate-induced catecholamine overflow in the medial prefrontal cortex.

Although stress is an extensively investigated phenomenon, the effects of specific stressors on the pharmacologic activity of routinely administered drugs are less well characterized. We designed the present study to investigate the effect of handling stress on catecholaminergic responsivity following an acute methylphenidate (MP, Ritalin) challenge in the medial prefrontal cortex (mPFC). Norepinephrine (NE) and dopamine (DA) levels were simultaneously measured in 15-min samples of PFC dialysate using HPLC coupled with electrochemical detection. Sprague-Dawley rats were handled for 15 min, which produced an increase from basal extracellular DA and NE levels. Handling stress attenuates the DA response when administered 2 h prior to IP MP, whereas handling stress enhances the DA response when administered simultaneously with IG MP. These findings suggest that persistent alterations in mesocorticolimbic DA-ergic activity are induced by a short exposure to restraint stress as evidenced by the altered response to MP challenge.

Effect of vehicle on brain uptake of [11C]toluene.

With the goal of investigating the pharmacokinetics of the abused solvent, toluene we have adapted the rapid coupling of methyl iodide with tributylphenylstannane mediated by palladium(0) complex to the synthesis of no-carrier-added [11C]toluene starting with 11CH(3)I. Two methods for purification and formulation of the tracer were developed. The first one yielded [11C]toluene dissolved in dimethylacetamide/saline solution, for the second one we adapted supercritical fluid technology where the tracer was purified using and conventional C(18) HPLC column and pure supercritical CO(2) fluid as a mobile phase operating at 2000 psi. Formulation of the tracer in cyclodextrin resulted in a significantly higher integrated uptake and distribution volume values. Additionally, we observed higher uptake and slower clearance of 11C-toluene in white matter, consistent with higher lipid content and neurotoxicological evidence indicating restricted and diffuse white matter changes in toluene abusers. This trend was observed when either DMA or cyclodextrin was used as a vehicle. It appears then, that the choice of a vehicle affected only the degree of bioavailability, but not the regional brain pharmacokinetics. Finally, we demonstrated the effect of a decreased percent difference between DV values for the studies performed on the same day, that is, test/retest variability was lower for all brain regions in beta-cyclodextrin experiments. Present results clearly demonstrate that the choice of a vehicle has a significant effect on tracer uptake and should be considered as a potential factor contributing to the pharmacokinetic measurements.

Supercritical CO2 fluid radiochromatography system used to purify [11C]toluene for PET.

Abuse of inhalants in today's society has become such a widespread problem among today's adolescents that in many parts of the world their use exceeds that of many other illicit drugs or alcohol. Even so, little is known how such inhalants affect brain function to an extent that can lead to an abuse liability. While methodologies exist for radiolabeling certain inhalants of interest with short-lived positron emitting radioisotopes that would allow their investigation in human subjects using positron emission tomography (PET), the purification methodologies necessary to separate these volatile substances from the organic starting materials have not been developed. We've adapted supercritical fluid technology to this specific PET application by building a preparative-scale supercritical CO2 fluid radiochromatograph, and applied it to the purification of [11C]toluene. We've demonstrated that [11C]toluene can be separated from the starting materials using a conventional C18 HPLC column and pure supercritical CO2 fluid as the mobile phase operating at 2000 psi and 40 degrees C. We've also shown that the purified radiotracer can be quantitatively captured on Tenax GR, a solid support material, as it exits the supercritical fluid stream, thus allowing for later desorption into a 1.5% cyclodextrin solution that is suitable for human injection, or into a breathing tube for direct inhalation.

Systemic administration of 1R,4S-4-amino-cyclopent-2-ene-carboxylic acid, a reversible inhibitor of GABA transaminase, blocks expression of conditioned place preference to cocaine and nicotine in rats.

We examined the effect of 1R,4S-4-amino-cyclopent-2-ene-carboxylic acid (ACC), a reversible inhibitor of GABA transaminase, on the expression of conditioned place preference response to cocaine and nicotine in rats. Cocaine (20 mg/kg i.p.) and nicotine (0.4 mg/kg s.c.), but not vehicle or 300 mg/kg i.p. of ACC, produced a significant conditioned place preference response. Pretreatment of animals with 300 and 75 mg/kg i.p. of ACC significantly attenuated the expression of the cocaine- and nicotine-induced conditioned place preference responses, respectively. These results are the first to suggest that reversible inhibition of GABA transaminase may be useful in blocking cue-induced relapse to nicotine and cocaine.

Toluene inhalation produces regionally specific changes in extracellular dopamine.

The aim of the present study was to investigate the effect of toluene inhalation on dopaminergic transmission in two distinct brain areas presumably involved in mediating the reward processes important for toluene abuse. Extracellular dopamine (DA) levels were measured in prefrontal cortex (PFC) and nucleus accumbens (NACC) of freely moving rats using in vivo microdialysis. Inhalation of a behaviorally relevant concentration of toluene (3000 ppm) produced a significant increase in the PFC but not in the NACC. However, the odorant isoamyl acetate, increased PFC DA levels by only 37%, significantly less than the 96% increase observed following toluene exposure. When toluene inhalation was combined with cocaine administration (20 mg/kg i.p.), the response to the combined challenge was not different from the response to toluene alone in the PFC. However, the combination of these two drugs produced a supradditive response of 802% in the NACC, compared with the 450% increase observed following cocaine alone. Recent reports indicate that toluene influences the function of several ionotropic receptors in a subunit specific manner. As further evidence of specific effects, our results indicate regionally specific changes in dopaminergic transmission following toluene exposure.