Synthesis and Preclinical Evaluation of 6-[18F]Fluorine-α-methyl-l-tryptophan, a Novel PET Tracer for Measuring Tryptophan Uptake.

The positron emission tomography (PET) radioligand α-[11C]methyl-l-tryptophan ([11C]AMT) has been used to assess tryptophan metabolism in cancer, epilepsy, migraine, and autism. Despite its extensive application, the utility of this tracer is currently hampered by the short half-life of the radionuclide used for its labeling (11C, t1/2 = 20.4 min). We herein report the design, synthesis, radiolabeling, and initial in vivo evaluation of a fluorine-18 (18F, t1/2 = 109.7 min) labeled analogue that is fluorinated in the 6-position of the aromatic ring ([18F]6-F-AMTr). In a head-to-head comparison between [18F]6-F-AMTr and [11C]AMT in mice using PET, peak brain radioactivity, regional brain distribution, and kinetic profiles were similar between the two tracers. [18F]6-F-AMTr was however not a substrate for IDO1 or TPH as determined in in vitro enzymatic assays. The brain uptake of the tracer is thus more likely related to LAT1 transport over the blood-brain barrier than metabolism along the serotonin or kynurenine pathways.

Allele-selective transcriptional repression of mutant HTT for the treatment of Huntington's disease.

Huntington's disease (HD) is a dominantly inherited neurodegenerative disorder caused by a CAG trinucleotide expansion in the huntingtin gene (HTT), which codes for the pathologic mutant HTT (mHTT) protein. Since normal HTT is thought to be important for brain function, we engineered zinc finger protein transcription factors (ZFP-TFs) to target the pathogenic CAG repeat and selectively lower mHTT as a therapeutic strategy. Using patient-derived fibroblasts and neurons, we demonstrate that ZFP-TFs selectively repress >99% of HD-causing alleles over a wide dose range while preserving expression of >86% of normal alleles. Other CAG-containing genes are minimally affected, and virally delivered ZFP-TFs are active and well tolerated in HD neurons beyond 100 days in culture and for at least nine months in the mouse brain. Using three HD mouse models, we demonstrate improvements in a range of molecular, histopathological, electrophysiological and functional endpoints. Our findings support the continued development of an allele-selective ZFP-TF for the treatment of HD.

The MINDVIEW project: First results.

We present the first results of the MINDVIEW project. An innovative imaging system for the human brain examination, allowing simultaneous acquisition of PET/MRI images, has been designed and constructed. It consists of a high sensitivity and high resolution PET scanner integrated in a novel, head-dedicated, radio frequency coil for a 3T MRI scanner. Preliminary measurements from the PET scanner show sensitivity 3 times higher than state-of-the-art PET systems that will allow safe repeated studies on the same patient. The achieved spatial resolution, close to 1 mm, will enable differentiation of relevant brain structures for schizophrenia. A cost-effective and simple method of radiopharmaceutical production from 11C-carbon monoxide and a mini-clean room has been demonstrated. It has been shown that 11C-raclopride has higher binding potential in a new VAAT null mutant mouse model of schizophrenia compared to wild type control animals. A significant reduction in TSPO binding has been found in gray matter in a small sample of drug-naïve, first episode psychosis patients, suggesting a reduced number or an altered function of immune cells in brain at early stage schizophrenia.

Reduced metabolism in the hypothalamus of the anorectic anx/anx mouse.

The anorectic anx/anx mouse exhibits a mitochondrial complex I dysfunction that is related to aberrant expression of hypothalamic neuropeptides and transmitters regulating food intake. Hypothalamic activity, i.e. neuronal firing and transmitter release, is dependent on glucose utilization and energy metabolism. To better understand the role of hypothalamic activity in anorexia, we assessed carbohydrate and high-energy phosphate metabolism, in vivo and in vitro, in the anx/anx hypothalamus. In the fasted state, hypothalamic glucose uptake in the anx/anx mouse was reduced by ~50% of that seen in wild-type (wt) mice (P < 0.05). Under basal conditions, anx/anx hypothalamus ATP and glucose 6-P contents were similar to those in wt hypothalamus, whereas phosphocreatine was elevated (~2-fold; P < 0.001) and lactate was reduced (~35%; P < 0.001). The anx/anx hypothalamus had elevated total AMPK (~25%; P < 0.05) and GLUT4 (~60%; P < 0.01) protein contents, whereas GLUT1 and GLUT3 were similar to that of wt hypothalamus. Interestingly, the activation state of AMPK (ratio of phosphorylated AMPK/total AMPK) was significantly decreased in hypothalamus of the anx/anx mouse (~60% of that in wt; P < 0.05). Finally, during metabolic stress (ischemia), accumulation of lactate (measure of glycolysis) and IMP and AMP (breakdown products of ATP) were ~50% lower in anx/anx vs wt hypothalamus. These data demonstrate that carbohydrate and high-energy phosphate utilization in the anx/anx hypothalamus are diminished under basal and stress conditions. The decrease in hypothalamic metabolism may contribute to the anorectic behavior of the anx/anx mouse, i.e. its inability to regulate food intake in accordance with energy status.

Differential Effect of APOE Alleles on Brain Glucose Metabolism in Targeted Replacement Mice: An [18F]FDG-µPET Study.

BACKGROUND: The Apolipoprotein E (ApoE) alleles ɛ2, ɛ3, and ɛ4 are known to differentially modulate cerebral glucose metabolism and the risk for Alzheimer's disease (AD) via both amyloid-ß (Aß)-dependent and independent mechanisms. OBJECTIVE: We investigated the influence of ApoE on cerebral glucose metabolism in humanized APOE Targeted Replacement (TR) mice at ages that precede the comparison of Aß parenchymal deposits in APOE4-TR mice. METHODS: Fludeoxyglucose ([18F]FDG) positron emission tomography (PET) measures were performed longitudinally in homozygous APOE-TR mice (APOE2, APOE3, APOE4; n = 10 for each group) at 3, 5, 11, and 15 months. Results were quantified using standard uptake values and analyzed statistically using a linear mixed effects model. Levels of the Aß40 and Aß42 peptides were quantified ex vivo using enzyme-linked immunosorbent assay (ELISA) at 15 months in the same animals. RESULTS: APOE2 mice (versus APOE3) showed a significant increase in glucose metabolism starting at 6 months, peaking at 9 months. No evidence of hypometabolism was apparent in any region or time point for APOE4 mice, which instead displayed a hypermetabolism at 15 months. Whole brain soluble Aß40 and Aß42 levels were not significantly different between genotypes at 15 months. CONCLUSIONS: Introduction of human APOE alleles ɛ2 and ɛ4 is sufficient to produce alterations in brain glucose metabolism in comparison to the control allele ɛ3, without a concomitant alteration in Aß40 and Aß42 levels. These results suggest novel Aß-independent metabolic phenotypes conferred by ɛ2 and ɛ4 alleles and have important implications for preclinical studies using TR-mice.

Development of a Novel Carbon-11 Labeled PET Radioligand for Melanin- Concentrating Hormone Receptor 1.

BACKGROUND AND OBJECTIVE: Melanin-concentrating hormone (MCH) is an attractive target for antiobesity agents and many drug discovery programs have been dedicated to identify smallmolecule antagonists of melanin-concentrating hormone receptor 1 (MCHR1). The aim of this study was to develop a positron emission tomography (PET) tracer for MCHR1 for translation of preclinical pharmacology to clinic to enhance success rate of drug discovery programs. METHODS: We identified 4-(cyclopropylmethoxy)-N-[8-methyl-3-({[(1-methyl-1H-pyrrol-2-yl)methyl] amino}ethyl)quinolin-7-yl]benzamide (Compound II) from Takeda MCHR1 antagonist library by utilizing binding affinity, log D value, physicochemical parameters ideal for a central nerve system agent, and synthetic feasibility of corresponding carbon-11 labeled radioligands as selection parameters for tracer candidates. RESULTS: In the rat PET study, [11C] Compound II showed clear uptake in the caudate/putamen with the pretreatment of cyclosporine A and its uptake was higher than that in the cerebellum where expression of MCHR1 was reported to be low. CONCLUSION: In summary, [11C]Compound II is a promising lead compound for developing a suitable MCHR1 PET radioligand. [11C]Compound II, in combination with cyclosporine A, could be used as a research tool to visualize and quantify MCHR1 in rodents.

Longitudinal Small-Animal PET Imaging of the zQ175 Mouse Model of Huntington Disease Shows In Vivo Changes of Molecular Targets in the Striatum and Cerebral Cortex.

Since the discovery of the HTT gene in 1993, numerous animal models have been developed to study the progression of Huntington disease (HD) and to evaluate potential new therapeutics. In the present study, we used small-animal PET to characterize the expression of molecular targets in the recently reported HD animal model, the zQ175 mouse model. Methods: Male heterozygous zQ175 (Htttm1Mfc/190JChdi, CHDI-81003003) and wild-type (WT, C57BL/6J) animals were imaged with the dopamine D2 receptor radioligand 11C-raclopride, the PDE10A radioligand 18F-MNI-659, the dopamine D1 receptor radioligand 11C-NNC 112, and the 5-HT2A radioligand 11C-MDL 100907 at 6 and 9 mo of age. The outcome measure was the binding potential (BPND), using the cerebellum as the reference region. Selected regions of interest were the striatum for all radioligands and additionally the striatum, rostral cortex, caudal cortex, and hippocampus for 11C-NNC 112 and 11C-MDL 100907. Results: At 6 mo of age, the BPND in the striatum was lower in zQ175 than WT animals by 40% for 11C-raclopride, by 52% for 18F-MNI-659, by 28% for 11C-NNC, and by 11% for 11C-MDL 100907. In the rostral cortex, D1 receptor binding was 22% lower in zQ175 than WT animals. We found an overall reduction in D1 and 5-HT2A binding in the hippocampus of zQ175 compared with WT animals. The BPND of 11C-MDL 100907 in the caudal cortex was also lower in zQ175 WT animals. At 9 mo, there was a slight further reduction of D1, D2, and 5-HT2ABPND in the striatum, whereas PDE10A reached a plateau. Cortical markers were also slightly further decreased at 9 mo in zQ175 animals. Conclusion: Our study indicates a marked reduction of ligand binding to D1 and D2 and 5-HT2A receptors as well as loss of PDE10A enzyme in the striatum of zQ175 mice as compared with WT animals, in agreement with data obtained in clinical PET studies of patients with HD. The zQ175 mouse model recapitulates the expression pattern seen in humans with HD and may have value in further elucidating pathophysiologic events and therapeutic strategies.

Phosphodiesterase 10A Inhibition Improves Cortico-Basal Ganglia Function in Huntington's Disease Models.

Huntington's disease (HD) symptoms are driven to a large extent by dysfunction of the basal ganglia circuitry. HD patients exhibit reduced striatal phoshodiesterase 10 (PDE10) levels. Using HD mouse models that exhibit reduced PDE10, we demonstrate the benefit of pharmacologic PDE10 inhibition to acutely correct basal ganglia circuitry deficits. PDE10 inhibition restored corticostriatal input and boosted cortically driven indirect pathway activity. Cyclic nucleotide signaling is impaired in HD models, and PDE10 loss may represent a homeostatic adaptation to maintain signaling. Elevation of both cAMP and cGMP by PDE10 inhibition was required for rescue. Phosphoproteomic profiling of striatum in response to PDE10 inhibition highlighted plausible neural substrates responsible for the improvement. Early chronic PDE10 inhibition in Q175 mice showed improvements beyond those seen with acute administration after symptom onset, including partial reversal of striatal deregulated transcripts and the prevention of the emergence of HD neurophysiological deficits. VIDEO ABSTRACT.

Discovery and Preclinical Validation of [(11)C]AZ13153556, a Novel Probe for the Histamine Type 3 Receptor.

UNLABELLED: The histamine type 3 receptor (H3) is a G protein-coupled receptor implicated in several disorders of the central nervous system. Herein, we describe the radiolabeling and preclinical evaluation of a candidate radioligand for the H3 receptor, 4-(1S,2S)-2-(4-cyclobutylpiperazine-1-carbonyl)cyclopropyl]-N-methyl-benzamide (5), and its comparison with one of the frontrunner radioligands for H3 imaging, namely, GSK189254 (1). Compounds 1 and 5 were radiolabeled with tritium and carbon-11 for in vitro and in vivo imaging experiments. The in vitro binding of [(3)H]1 and [(3)H]5 was examined by (i) saturation binding to rat and nonhuman primate brain tissue homogenate and (ii) in vitro autoradiography on tissue sections from rat, guinea pig, and human brain. The in vivo binding of [(11)C]1 and [(11)C]5 was examined by PET imaging in mice and nonhuman primates. Bmax values obtained from Scatchard analysis of [(3)H]1 and [(3)H]5 binding were in good agreement. Autoradiography with [(3)H]5 on rat, guinea pig, and human brain slices showed specific binding in regions known to be enhanced in H3 receptors, a high degree of colocalization with [(3)H]1, and virtually negligible nonspecific binding in tissue. PET measurements in mice and nonhuman primates demonstrated that [(11)C]5 binds specifically and reversibly to H3 receptors in vivo with low nonspecific binding in brain tissue. Whereas [(11)C]1 showed similar binding characteristics in vivo, the binding kinetics appeared faster for [(11)C]5 than for [(11)C]1. CONCLUSIONS: [(11)C]5 has suitable properties for quantification of H3 receptors in nonhuman primate brain and has the potential to offer improved binding kinetics in man compared to [(11)C]1.

Acute neuroinflammation in a clinically relevant focal cortical ischemic stroke model in rat: longitudinal positron emission tomography and immunofluorescent tracking.

Adequate estimation of neuroinflammatory processes following ischemic stroke is essential for better understanding of disease mechanisms, and for the development of treatment strategies. With the TSPO (18 kDa translocator protein) positron emission tomography (PET) radioligand [(11)C]PBR28, we monitored longitudinally the inflammatory response post-transient cerebral ischemia in rats, using a recently developed rat stroke model that produces isolated focal cortical infarcts with clinical relevance in size and pathophysiology. Six Sprague-Dawley rats were subjected to 90 min transient endovascular occlusion of the M2 segment of the middle cerebral artery (M2CAO). Animals were imaged with a nanoScan(®) PET/MRI system at 1, 4, 7 and 14 days after M2CAO with a bolus injection of [(11)C]PBR28. In the infarct region, we found a significantly increased uptake of [(11)C]PBR28 on day 4, 7 and 14 compared to day 1 as well as compared to the contralateral cortex. No significant increase was detected in the contralateral cortex during the 14 days of imaging. The activation in the infarct region gradually decreased between day 4 and day 14. In an additional group of animals (n = 26), immunofluorescence studies were performed with antibodies for activated microglia/monocytes (Cd11b), phagocytes (Cd68), astrocytes (glial fibrillary acidic protein) and TSPO. The TSPO immunofluorescence signal indicated reactive microgliosis post injury, corresponding to PET findings. The present clinically relevant animal model and TSPO PET ligand appear to be well suited for studies on neuroinflammation after ischemic stroke.

Positron Emission Tomography studies with [11C]PBR28 in the Healthy Rodent Brain: Validating SUV as an Outcome Measure of Neuroinflammation.

UNLABELLED: Molecular imaging of the 18 kD Translocator protein (TSPO) with positron emission tomography (PET) is of great value for studying neuroinflammation in rodents longitudinally. Quantification of the TSPO in rodents is, however, quite challenging. There is no suitable reference region and the use of plasma-derived input is not an option for longitudinal studies. The aim of this study was therefore to evaluate the use of the standardized uptake value (SUV) as an outcome measure for TSPO imaging in rodent brain PET studies, using [11C]PBR28. In the first part of the study, healthy male Wistar rats (n = 4) were used to determine the correlation between the distribution volume (VT, calculated with Logan graphical analysis) and the SUV. In the second part, healthy male Wistar rats (n = 4) and healthy male C57BL/6J mice (n = 4), were used to determine the test-retest variability of the SUV, with a 7-day interval between measurements. Dynamic PET scans of 63 minutes were acquired with a nanoScan PET/MRI and nanoScan PET/CT. An MRI scan was made for anatomical reference with each measurement. The whole brain VT of [11C]PBR28 in rats was 42.9 ± 1.7. A statistically significant correlation (r2 = 0.96; p < 0.01) was found between the VT and the SUV. The test-retest variability in 8 brain region ranged from 8 to 20% in rats and from 7 to 23% in mice. The interclass correlation coefficient (ICC) was acceptable to excellent for rats, but poor to acceptable for mice. IN CONCLUSION: The SUV of [11C]PBR28 showed a high correlation with VT as well as good test-retest variability. For future longitudinal small animal PET studies the SUV can thus be used to describe [11C]PBR28 uptake in healthy brain tissue. Based on the present observations, further studies are needed to explore the applicability of this approach in small animal disease models, with special regard to neuroinflammatory models.

Astrocytosis precedes amyloid plaque deposition in Alzheimer APPswe transgenic mouse brain: a correlative positron emission tomography and in vitro imaging study.

PURPOSE: Pathological studies suggest that neuroinflammation is exacerbated by increased beta-amyloid (Aß) levels in the brain early in Alzheimer's disease (AD). The time course and relationships between astrocytosis and Aß deposition were examined using multitracer in vivo positron emission tomography (PET) imaging in an AD transgenic mouse model, followed by postmortem autoradiography and immunohistochemistry analysis. METHODS: PET imaging with the amyloid plaque tracer (11)C-AZD2184 and the astroglial tracer (11)C-deuterium-L-deprenyl ((11)C-DED) was carried out in APPswe mice aged 6, 8-15 and 18-24 months (4-6 animals/group) and in wild-type (wt) mice aged 8-15 and 18-24 months (3-6 animals/group). Tracer uptake was quantified by region of interest analysis using PMOD software and a 3-D digital mouse brain atlas. Postmortem brain tissues from the same APPswe and wt mice in all age groups were analysed for Aß deposition and astrocytosis by in vitro autoradiography using (3)H-AZD2184, (3)H-Pittsburgh compound B (PIB) and (3)H-L-deprenyl and immunostaining performed with antibodies for Aß42 and glial fibrillary acidic protein (GFAP) in sagittal brain sections. RESULTS: (11)C-AZD2184 PET retention in the cerebral cortices of APPswe mice was significantly higher at 18-24 months than in age-matched wt mice. Cortical and hippocampal (11)C-DED PET binding was significantly higher at 6 months than at 8-15 months or 18-24 months in APPswe mice, and it was also higher than at 8-15 months in wt mice. In vitro autoradiography (3)H-AZD2184 and (3)H-PIB binding confirmed the in vivo findings with (11)C-AZD2184 and demonstrated age-dependent increases in Aß deposition in APPswe cortex and hippocampus. There were no significant differences between APPswe and wt mice in (3)H-L-deprenyl autoradiography binding across age groups. Immunohistochemical quantification demonstrated more Aß42 deposits in the cortex and hippocampus and more GFAP(+) reactive astrocytes in the hippocampus at 18-24 months than at 6 months in APPswe mice. CONCLUSION: The findings provide further in vivo evidence that astrocytosis occurs early in AD, preceding Aß plaque deposition.

Molecular Imaging of PDE10A Knockout Mice with a Novel PET Radiotracer: [(11)C]T-773.

PURPOSE: [(11)C]T-773 is a new radioligand for positron emission tomography (PET) targeting the phosphodiesterase 10A enzyme (PDE10A). PDE10A is highly expressed in the striatum by medium spiny neurons, and it has been demonstrated to be involved in the regulation of striatal signaling through the reduction of medium spiny neuronal sensitivity towards glutamatergic excitation. PDE10A is associated with Parkinson's disease and different neuropsychiatric disorders such as Huntington's disease, obsessive-compulsive disorders (OCD) and schizophrenia. Studies have indicated that the inhibition of PDE10A may represent a novel therapeutic approach to the treatment of the aforementioned diseases characterized by the reduced activity of medium spiny neurons. An appropriate PET radioligand for PDE10A would help to facilitate drug development and drug evaluation. PROCEDURES: We have evaluated the [(11)C]T-773 ligand in PDE10A knockout mice (heterozygous [HET] and homozygous [HOM]) as well as in normal control animals (WILD) with PET. RESULTS: The regional percent standardized uptake values (%SUV; mean ± SD) in the striatum were 48.2 ± 1.0 (HOM), 63.6 ± 5.3 (HET) and 85.1 ± 6.3 (WILD). Between each animal group the striatal %SUV values were significantly different (p < 0.0001). The striatal BPND values (mean ± SD) were 0.0 ± 0.0 (HOM), 0.14 ± 0.07 (HET) and 0.56 ± 0.15 (WILD). The BPND values were significantly lower in homozygous and heterozygous animals compared to wild type (p < 0.0001). CONCLUSIONS: The novel PDE10A radioligand [(11)C]T-773 shows increased signals with higher levels of PDE10A and acceptable binding in the striatum in control animals compared to knockout mice.

ABC transporter-dependent brain uptake of the 5-HT1B receptor radioligand [ (11)C]AZ10419369: a comparative PET study in mouse, rat, and guinea pig.

BACKGROUND: We have explored the possibility that the serotonin 1B receptor radioligand [(11)C]AZ10419369 is a substrate for adenosine triphosphate (ATP)-binding cassette (ABC) transporters, such as P-glycoprotein (P-gp), Mrp4, and Bcrp, in rodents and whether there is a species difference regarding its blood-brain barrier (BBB) penetration. METHODS: In a series of preclinical positron emission tomography measurements, we have administered [(11)C]AZ10419369 to mice, rats, and guinea pigs under baseline conditions and, on separate experimental days, after administration of the ABC transporter inhibitor, cyclosporin A (CsA). RESULTS: During baseline conditions, the brain uptake was low in mice and rats, but not in guinea pigs. After CsA pretreatment, the peak whole brain uptake values of [(11)C]AZ10419369 increased by 207% in mice, 94% in rats, and 157% in guinea pigs. Binding potentials (BPND) could not be estimated during baseline conditions in mice and rats. After CsA pretreatment, the highest BPND values were obtained in the striatum and thalamus (BPND ≈ 0.4) in mice, while in rats, the highest binding areas were the striatum, thalamus, hypothalamus, and periaqueductal gray (BPND ≈ 0.5). In guinea pigs, we did not find any significant changes in BPND between baseline and CsA pretreatment, except in the striatum. CONCLUSIONS: The results indicate that BBB penetration of [(11)C]AZ10419369 was hindered by ABC transporter activity in mouse, rat, and guinea pig. This study highlights the importance of ABC transporters in the design of preclinical positron emission tomography (PET) studies.

Performance evaluation of the small-animal nanoScan PET/MRI system.

UNLABELLED: nanoScan is a high-resolution integrated system for consecutive PET and MR imaging of small laboratory animals. We evaluated the performance of the system, using the NEMA NU 4-2008 protocol for the PET component and the NEMA MS 1-2007, MS 2-2008, and MS 3-2007 standards for the MR imaging component. METHODS: The imaging system uses magnetically shielded position-sensitive photomultiplier tubes and a compact 1-T permanent-magnet MR imaging platform. Spatial resolution, sensitivity, counting rate capabilities, and image quality parameters were evaluated in accordance with the aforementioned NEMA standards. Further in vivo evaluation experiments complement the physical validation results. RESULTS: The spatial resolution of the PET system enabled the 0.8-mm rods of a Derenzo phantom to be resolved. With point source and 2-dimensional filtered backprojection reconstruction, the resolution varied from 1.50 to 2.01 mm in full width at half maximum in the radial direction and from 1.32 to 1.65 mm in the tangential direction within the radius of 25 mm. Peak absolute sensitivity was 8.41%. Scatter fraction was 17.3% and 34.0%, and maximum noise-equivalent counting rate was 406 and 119 kcps in the mouselike and ratlike phantom, respectively. The image quality test found a nonuniformity of 3.52% and a spillover ratio of 6.2% and 5.8% in water and air, respectively. In testing of the MR imaging component, artifact-free images with high signal-to-noise ratio were recorded. Geometric distortion was below 5%, and image uniformity was at least 94.5% and 96.6% for the 60- and 35-mm radiofrequency coils, respectively. CONCLUSION: The nanoScan integrated small-animal PET/MR imaging system has excellent spatial resolution and sensitivity. The performance characteristics of the PET and the MR imaging components are not compromised as a result of their integration onto a single platform. Because of its combination of features and performance parameters, the system provides crucial advantages for preclinical imaging studies over existing PET/CT systems, especially in neurologic and oncologic research.

Naltrexone attenuates amphetamine-induced locomotor sensitization in the rat.

Amphetamine, and other stimulants, readily induces behavioral sensitization, an effect hypothesized to reflect neurobiological changes that may underlie certain aspects of drug addiction. Apart from the effects on the dopamine system, previous studies have also shown that amphetamine interacts with other neurotransmitters, including the endogenous opioid system. The unselective opioid receptor antagonist naltrexone (NTX) modulates amphetamine-induced effects in both laboratory animals and humans. To further examine this interaction, the aim of the present study was to investigate the effect of NTX on the expression of locomotor sensitization and conditioned locomotor response in animals previously conditioned with amphetamine. Sensitization was induced by repeated administration of amphetamine (2 mg/kg) for 10 consecutive days. After a 10-day drug-free period, the rats were administered NTX (3 mg/kg) 30 minutes prior to the administration of a challenge dose of either amphetamine (0.5 mg/kg) (test for drug-induced sensitization) or saline (test for conditioned locomotor response). NTX had no effect on acute amphetamine-induced locomotor activity or on general locomotor activity in animals without a history of amphetamine conditioning. However, animals previously conditioned with amphetamine showed a sensitized locomotor response to the amphetamine challenge following the 10-day drug-free period. This sensitized response was significantly inhibited by NTX pre-treatment. In addition, NTX pre-treatment blocked the conditioned locomotor response when the amphetamine-conditioned animals were placed in the previously amphetamine-paired context. This study showed that NTX attenuates drug- and cue-induced locomotor behavior in amphetamine-conditioned animals, supporting recent clinical findings that indicated a potential role of NTX as a treatment for amphetamine dependence.

The effect of naltrexone on amphetamine-induced conditioned place preference and locomotor behaviour in the rat.

Whereas amphetamine and other psychostimulants primarily act on the dopamine system, there is also evidence that other neurotransmitter systems, such as the endogenous opioid system, modulate psychostimulant-induced effects. Several studies have investigated the role of opioid antagonists on cocaine-induced conditioned place preference (CPP), but there is limited information about the interaction with amphetamines. The aim of the present study was to investigate the effect of the opioid receptor antagonist, naltrexone (NTX) on the conditioning, expression and reinstatement of amphetamine-induced place preference. In addition, the effect of NTX on locomotor behaviour was measured during all sessions. During training, animals were conditioned with amphetamine (2 mg/kg) to induce place preference. In order to extinguish the conditioned behaviour, animals received saline for 12 days. Reinstatement of CPP was induced by a priming dose of amphetamine (0.5 mg/kg). The interaction of NTX and amphetamine was evaluated using three paradigms of CPP: with NTX (vehicle, 0.3, 1.0 and 3.0 mg/kg) administered either 30 minutes prior to amphetamine conditioning, or 30 minutes before the expression, or 30 minutes before the amphetamine priming to induce reinstatement. Naltrexone had no effect on the conditioning, the expression or the reinstatement induced by a priming dose of amphetamine. Further, NTX by itself did not induce place preference or place aversion. In contrast, NTX significantly attenuated the locomotor response to a priming dose of amphetamine without affecting general locomotor behaviour. The results suggest differences in opioid modulation of amphetamine-induced behaviours in the rat.

The opioid receptor antagonist naltrexone attenuates reinstatement of amphetamine drug-seeking in the rat.

Amphetamine produces its rewarding effects by enhancing dopamine transmission in the mesocorticolimbic pathway. Several studies have also suggested the involvement of the endogenous opioid system in mediating the neurochemical and behavioural effects of amphetamine. The aim of this study was to investigate the effect of the unselective opioid receptor antagonist naltrexone (NTX) on reinstatement of amphetamine self-administration in the rat. Animals were trained to self-administer amphetamine under a fixed ratio 1 (FR1) schedule (0.1mg/kg/infusion). After receiving a stable drug intake the amphetamine was replaced with saline and the animals went through an extinction period. After reaching the extinction criteria, animals were pre-treated with NTX (0, 0.3, 1.0 and 3.0mg/kg, s.c.) 30min before giving a priming dose of amphetamine (0.5mg/kg s.c). To study the effects of NTX on operant behaviour, animals were trained to lever press for food pellets under a FR1 schedule of reinforcement. Results from the present study shows that a single injection of amphetamine reinstated self-administration behaviour. NTX (0.3 and 1.0mg/kg) significantly attenuated the amphetamine-induced reinstatement but NTX had no effect at any dose studied on food taking behaviour. These results show that NTX attenuates reinstatement of amphetamine self-administration in rats without suppressing general behaviour, implicating a functional role for opioid receptors in modulating amphetamine seeking behaviour.