Effects of isoflurane anesthesia and intravenous morphine self-administration on regional glucose metabolism ([18 F]FDG-PET) of male Sprague-Dawley rats

Metabolic brain imaging with glucosamine CEST MRI: in vivo characterization and first insights

The utility of chemical exchange saturation transfer (CEST) MRI for monitoring the uptake of glucosamine (GlcN), a safe dietary supplement, has been previously demonstrated in detecting breast cancer in both murine and human subjects. Here, we studied and characterized the detectability of GlcN uptake and metabolism in the brain. Following intravenous GlcN administration in mice, CEST brain signals calculated by magnetization transfer ratio asymmetry (MTRasym) analysis, were significantly elevated, mainly in the cortex, hippocampus, and thalamus. The in vivo contrast remained stable during 40 min of examination, which can be attributed to GlcN uptake and its metabolic products accumulation as confirmed using 13C NMR spectroscopic studies of brain extracts. A Lorentzian multi-pool fitting analysis revealed an increase in the hydroxyl, amide, and relayed nuclear Overhauser effect (rNOE) signal components after GlcN treatment. With its ability to cross the blood-brain barrier (BBB), the GlcN CEST technique has the potential to serve as a metabolic biomarker for the diagnosis and monitoring various brain disorders.

Neuroimaging of opioid exposure: a review of preclinical animal models to inform addiction research

Opioid use results in thousands of overdose deaths each year. To address this crisis, we need a better understanding of the neurobiological mechanisms that drive opioid abuse. The noninvasive imaging tools positron emission tomography (PET), functional magnetic resonance imaging (fMRI), and manganese-enhanced magnetic resonance imaging (MEMRI) can be used to identify how brain activity responds to acute opioid exposure and adapts to chronic drug treatment. These techniques can be performed in humans and animal models, and brain networks identified in animals closely map to the human brain. Animal models have the advantage of being able to systematically examine the independent effects of opioid exposure in a controlled environment accounting for the complex factors that drive opioid misuse in humans. This review synthesizes literature that utilized noninvasive neuroimaging tools (PET, fMRI, and MEMRI) measuring brain activity correlates in animals to understand the neurobiological consequences of exposure to abused opioids. A PubMed search in September 2023 identified 25 publications. These manuscripts were divided into 4 categories based on the route and duration of drug exposure (acute/chronic, active/passive administration). Within each category, the results were generally consistent across drug and imaging protocols. These papers cover a 20-year range and highlight the advancements in neuroimaging methodology during that time. These advances have enabled researchers to achieve greater resolution of brain regions altered by opioid exposure and to identify patterns of brain activation across regions (i.e., functional connectivity) and within subregions of structures. After describing the existing literature, we suggest areas where additional research is needed.

Neuroimaging revealed long-lasting glucose metabolism changes to morphine withdrawal in rats pretreated with the cannabinoid agonist CP-55,940 during periadolescence

This study evaluates the long-term effects of a six and 14-week morphine withdrawal in rats pretreated with a cannabinoid agonist (CP-55,940, CP) during periadolescence. Wistar rats (33 males; 32 females) were treated with CP or its vehicle (VH) from postnatal day (PND) 28-38. At PND100, rats performed morphine self-administration (MSA, 15d/12 h/session). Eight groups were defined according to pretreatment (CP), treatment (morphine), and sex. Three [¹⁸F]FDG-PET brain images were acquired: after MSA, and after six and 14 weeks of withdrawal. PET data were analyzed with SPM12. Endocannabinoid (EC) markers were evaluated in frozen brain tissue at endpoint. Females showed a higher mean number of self-injections than males. A main Sex effect on global brain metabolism was found. FDG uptake in males was discrete, whereas females showed greater brain metabolism changes mainly in areas of the limbic system after morphine treatment. Moreover, the morphine-induced metabolic pattern in females was exacerbated when CP was previously present. In addition, the CP-Saline male group showed reduced CB1R, MAGL expression, and NAPE/FAAH ratio compared to the control group, and morphine was able to reverse CB1R and MAGL expression almost to control levels. In conclusion, females showed greater and longer-lasting metabolic changes after morphine withdrawal than males, indicating a higher vulnerability and a different sensitivity to morphine in subjects pre-exposed to CP. In contrast, males primarily showed changes in EC markers. Together, our results suggest that CP pre-exposure contributes to the modulation of brain metabolism and EC systems in a sex-dependent manner.

Translation of Preclinical PET Imaging Findings: Challenges and Motion Correction to Overcome the Confounding Effect of Anesthetics

Preclinical brain positron emission tomography (PET) in animals is performed using anesthesia to avoid movement during the PET scan. In contrast, brain PET scans in humans are typically performed in the awake subject. Anesthesia is therefore one of the principal limitations in the translation of preclinical brain PET to the clinic. This review summarizes the available literature supporting the confounding effect of anesthesia on several PET tracers for neuroscience in preclinical small animal scans. In a second part, we present the state-of-the-art methodologies to circumvent this limitation to increase the translational significance of preclinical research, with an emphasis on motion correction methods. Several motion tracking systems compatible with preclinical scanners have been developed, each one with its advantages and limitations. These systems and the novel experimental setups they can bring to preclinical brain PET research are reviewed here. While technical advances have been made in this field, and practical implementations have been demonstrated, the technique should become more readily available to research centers to allow for a wider adoption of the motion correction technique for brain research.

Impact of anesthesia on static and dynamic functional connectivity in mice

A few studies have compared the static functional connectivity between awake and lightly anesthetized states in rodents by resting-state fMRI. However, impact of light anesthesia on static and dynamic fluctuations in functional connectivity has not been fully understood. Here, we developed a resting-state fMRI protocol to perform awake and anesthetized functional MRI in the same mice. Static functional connectivity showed a widespread decrease under light anesthesia, such as when under isoflurane or a mixture of isoflurane and medetomidine. Several interhemispheric and subcortical connections were key connections for anesthetized condition from awake state. Dynamic functional connectivity demonstrates the shift from frequent broad connections across the cortex, the hypothalamus, and the auditory-visual cortex to frequent local connections within the cortex only under light anesthesia compared with awake state. Fractional amplitude of low frequency fluctuation in the thalamic nuclei decreased under both anesthesia. These results indicate that typical anesthetics for functional MRI alters the spatiotemporal profile of the dynamic brain network in subcortical regions, including the thalamic nuclei and limbic system.

The Ameliorative Effects of Isorhynchophylline on Morphine Dependence Are Mediated Through the Microbiota-Gut-Brain Axis

Morphine abuse is a global public health problem. Increasing evidence has shown that gut microbiota dysbiosis plays an important role in several central nervous system diseases. However, whether there is an association between gut microbiota and morphine dependence remains unclear. In this study, the effects of isorhynchophylline on morphine dependence were evaluated based on the microbiota-gut-brain axis (MGBA). The results showed that isorhynchophylline could reverse the changes in alpha and beta diversity, composition, and richness of the intestinal flora occurring in morphine-dependent zebrafish, as well as the morphine-induced changes in the expression of MGBA-related genes in BV2 cells and the brain and intestine of zebrafish. Based on the results, we then used antibiotics to evaluate whether disrupting the gut microbiota would affect morphine addiction in zebrafish. The results showed that the antibiotic-induced intestinal floral imbalance changed the behavior of morphine-dependent zebrafish, the characteristics of the zebrafish intestinal flora, and the expression of MGBA-related genes in the zebrafish brain and intestine. Importantly, we also show that, following antibiotic administration, the ameliorative effects of isorhynchophylline on morphine addiction were lost. Together, our results indicate that the gut microbiota interacts with the brain, and dysbiosis of the intestinal flora may affect the efficacy of isorhynchophylline in the body. Our findings provide a novel framework for understanding the mechanisms of morphine addiction through the MGBA and may provide new therapeutic strategies for the use of Chinese medicines in the prevention of drug addiction.

Elucidating the complementarity of resting-state networks derived from dynamic [18F]FDG and hemodynamic fluctuations using simultaneous small-animal PET/MRI

Functional connectivity (FC) and resting-state network (RSN) analyses using functional magnetic resonance imaging (fMRI) have evolved into a growing field of research and have provided useful biomarkers for the assessment of brain function in neurological disorders. However, the underlying mechanisms of the blood oxygen level-dependant (BOLD) signal are not fully resolved due to its inherent complexity. In contrast, [¹⁸F]fluorodeoxyglucose positron emission tomography ([¹⁸F]FDG-PET) has been shown to provide a more direct measure of local synaptic activity and may have additional value for the readout and interpretation of brain connectivity. We performed an RSN analysis from simultaneously acquired PET/fMRI data on a single-subject level to directly compare fMRI and [¹⁸F]FDG-PET-derived networks during the resting state. Simultaneous [¹⁸F]FDG-PET/fMRI scans were performed in 30 rats. Pairwise correlation analysis, as well as independent component analysis (ICA), were used to compare the readouts of both methods. We identified three RSNs with a high degree of similarity between PET and fMRI-derived readouts: the default-mode-like network (DMN), the basal ganglia network and the cerebellar-midbrain network. Overall, [¹⁸F]FDG connectivity indicated increased integration between different, often distant, brain areas compared to the results indicated by the more segregated fMRI-derived FC. Additionally, several networks exclusive to either modality were observed using ICA. These networks included mainly bilateral cortical networks of a limited spatial extent for fMRI and more spatially widespread networks for [¹⁸F]FDG-PET, often involving several subcortical areas.

This is the first study using simultaneous PET/fMRI to report RSNs subject-wise from dynamic [¹⁸F]FDG tracer delivery and BOLD fluctuations with both independent component analysis (ICA) and pairwise correlation analysis in small animals. Our findings support previous studies, which show a close link between local synaptic glucose consumption and BOLD-fMRI-derived FC. However, several brain regions were exclusively attributed to either [¹⁸F]FDG or BOLD-derived networks underlining the complementarity of this hybrid imaging approach, which may contribute to the understanding of brain functional organization and could be of interest for future clinical applications.

Comparison of in vivo and in situ detection of hippocampal metabolites in mouse brain using 1 H-MRS

The study of cerebral metabolites relies heavily on detection methods and sample preparation. Animal experiments in vivo require anesthetic agents that can alter brain metabolism, whereas ex vivo experiments demand appropriate fixation methods to preserve the tissue from rapid postmortem degradation. In this study, the metabolic profiles of mouse hippocampi using proton magnetic resonance spectroscopy (¹ H-MRS) were compared in vivo and in situ with or without focused beam microwave irradiation (FBMI) fixation. Ten major brain metabolites, including lactate (Lac), N-acetylaspartate (NAA), total choline (tCho), myo-inositol (mIns), glutamine (Gln), glutamate (Glu), aminobutyric acid (GABA), glutathione (GSH), total creatine (tCr) and taurine (Tau), were analyzed using LCModel. After FBMI fixation, the concentrations of Lac, tCho and mIns were comparable with those obtained in vivo under isoflurane, whereas other metabolites were significantly lower. Except for a decrease in NAA and an increase in Tau, all the other metabolites remained stable over 41 hours in FBMI-fixed brains. Without FBMI, the concentrations of mIns (before 2 hours), tCho and GABA were close to those measured in vivo. However, higher Lac (P < .01) and lower NAA, Gln, Glu, GSH, tCr and Tau were observed (P < .01). NAA, Gln, Glu, GSH, tCr and Tau exhibited good temporal stability for at least 20 hours in the unfixed brain, whereas a linear increase of tCho, mIns and GABA was observed. Possible mechanisms of postmortem degradation are discussed. Our results indicate that a proper fixation method is required for in situ detection depending on the targeted metabolites of specific interests in the brain.

Optimizing clozapine for chemogenetic neuromodulation of somatosensory cortex

Clozapine (CLZ) has been proposed as an agonist for Designer Receptors Exclusively Activated by Designer Drugs (DREADDs), to replace Clozapine-N-oxide (CNO); however, there are no reliable guidelines for the use of CLZ for chemogenetic neuromodulation. We titrated the optimal dose of CLZ required to evoke changes in neural activity whilst avoiding off-target effects. We also performed [¹⁸F]Fluoro-deoxy-glucose micro positron emission tomography (FDG-microPET) scans to determine the global effect of CLZ-induced hM3D(Gq) DREADD activation in the rat brain. Our results show that low doses of CLZ (0.1 and 0.01 mg/kg) successfully induced neural responses without off-target effects. CLZ at 1 mg/kg evoked a stronger and longer-lasting neural response but produced off-target effects, observed as changes in locomotor behavior and FDG-microPET imaging. Unexpectedly, FDG-microPET imaging failed to demonstrate an increase in regional glucose metabolism in the stimulated cortex during CLZ chemogenetic neuromodulation. Therefore, caution should be used when interpreting FDG-PET images in the context of cortical chemogenetic activation.

Endogenous opiates and behavior: 2017

This paper is the fortieth consecutive installment of the annual anthological review of research concerning the endogenous opioid system, summarizing articles published during 2017 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides and receptors as well as effects of opioid/opiate agonists and antagonists. The review is subdivided into the following specific topics: molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors (1), the roles of these opioid peptides and receptors in pain and analgesia in animals (2) and humans (3), opioid-sensitive and opioid-insensitive effects of nonopioid analgesics (4), opioid peptide and receptor involvement in tolerance and dependence (5), stress and social status (6), learning and memory (7), eating and drinking (8), drug abuse and alcohol (9), sexual activity and hormones, pregnancy, development and endocrinology (10), mental illness and mood (11), seizures and neurologic disorders (12), electrical-related activity and neurophysiology (13), general activity and locomotion (14), gastrointestinal, renal and hepatic functions (15), cardiovascular responses (16), respiration and thermoregulation (17), and immunological responses (18).

Serotonin 5-HT1A Receptor Biased Agonists Induce Different Cerebral Metabolic Responses: A [18F]-Fluorodesoxyglucose Positron Emission Tomography Study in Conscious and Anesthetized Rats

Serotonin 5-HT_1A receptors constitute an attractive therapeutic target for various psychiatric or neurodegenerative disorders. These receptors are expressed in multiple brain regions on different neuronal populations and can be coupled with distinct G-protein subtypes; such functional diversity complicates the use of 5-HT_1A ligands in several pathologies where it would be desirable to stimulate the receptors in a precise region. Therefore, using "biased agonists" able to target specifically certain subpopulations of 5-HT_1A receptors would enable achievement of better therapeutic benefit. Several 5-HT_1A receptor biased agonists are currently in development, including NLX-101 (aka F15599) and NLX-112 (aka F13640, befiradol), with preclinical data suggesting that they preferentially target different populations of 5-HT_1A receptors. However, most previous studies used invasive and regionally limited approaches. In this context, [¹⁸F]-fluorodesoxyglucose (FDG)-positron emission tomography (PET) imaging constitutes an interesting technique as it enables noninvasive mapping of the regional brain activity changes following a pharmacological challenge in conscious animals. We report here the evaluation of cerebral glucose metabolism following intraperitoneal injection of different doses of NLX-112 or NLX-101 in conscious or isoflurane-anesthetized rats. The biased agonists produced different metabolic "fingerprints" with distinct regional preferences, consistent with previous studies. At equal doses, the effect of NLX-101 was less marked than NLX-112 in the piriform cortex, in the striatum (in terms of inhibition), and in the pontine nuclei and the cerebellum (in terms of activation); furthermore, only NLX-112 increased the glucose metabolism in the parietal cortex, whereas only NLX-101 induced a clear activation in the colliculi and the frontal cortex, which may be related to its distinctive procognitive profile. Both agonist effects were almost completely unapparent in anesthetized animals, underlining the importance of studying serotonergic neurotransmission in the conscious state. In this regard, [¹⁸F]FDG-PET imaging seems very complementary with other functional imaging techniques such as pharmacological MRI.

Anesthetics influence concussive head injury induced blood-brain barrier breakdown, brain edema formation, cerebral blood flow, serotonin levels, brain pathology and functional outcome

Several lines of evidences show that anesthetics influence neurotoxicity and neuroprotection. The possibility that different anesthetic agents potentially influence the pathophysiological and functional outcome following neurotrauma was examined in a rat model of concussive head injury (CHI). The CHI was produced by an impact of 0.224N on the right parietal bone by dropping a weight of 114.6g from a 20cm height under different anesthetic agents, e.g., inhaled ether anesthesia or intraperitoneally administered ketamine, pentobarbital, equithesin or urethane anesthesia. Five hour CHI resulted in profound volume swelling and brain edema formation in both hemispheres showing disruption of the blood-brain barrier (BBB) to Evans blue and radioiodine. A marked decrease in the cortical CBF and a profound increase in plasma or brain serotonin levels were seen at this time. Neuronal damages were present in several parts of the brain. These pathological changes were most marked in CHI under ether anesthesia followed by ketamine (35mg/kg, i.p.), pentobarbital (50mg/kg, i.p.), equithesin (3mL/kg, i.p.) and urethane (1g/kg, i.p.). The functional outcome on Rota Rod performances or grid walking tests was also most adversely affected after CHI under ether anesthesia followed by pentobarbital, equithesin and ketamine. Interestingly, the plasma and brain serotonin levels strongly correlated with the development of brain edema in head injured animals in relation to different anesthetic agents used. These observations suggest that anesthetic agents are detrimental to functional and pathological outcomes in CHI probably through influencing the circulating plasma and brain serotonin levels, not reported earlier. Whether anesthetics could also affect the efficacy of different neuroprotective agents in CNS injuries is a new subject that is currently being examined in our laboratory.

The effects of perinatal bisphenol A exposure on thyroid hormone homeostasis and glucose metabolism in the prefrontal cortex and hippocampus of rats

INTRODUCTION

Bisphenol A (BPA) is an endocrine disruptor widely used to manufacture consumer goods. Although the thyroid hormone (TH) disrupting potential of BPA has been thought to be responsible for the neuropsychiatric deficits in the animals that experienced perinatal BPA exposure, the TH availability change at the level of specific brain structures has not been subject to systematic investigation.

METHODS

In the present study the impacts of perinatal BPA exposure (0.1 mg/L in drinking water) spanning gestation and lactation on TH homeostasis in the prefrontal cortex (PFC) and hippocampus were assessed in male Sprague-Dawley rats at postnatal day 21 (PND21) and PND90. As TH regulates brain glucose metabolism at multiple levels，the effects of BPA treatment on glucose metabolism in the brain tissues were also assessed in adult rats.

RESULTS

The results showed heterogeneous changes in TH concentration induced by BPA between serum and brain tissues, additionally, in the BPA-treated pups, up-regulated expression of the TH transporter monocarboxylate 8 mRNA at PND21 and increased type 3 iodothyronine deiodinase mRNA expressions at PND21 and PND90 were observed. Meanwhile, decreased glucose metabolism was seen in the PFC and hippocampus, while deficits in locomotor activity, spatial memory and social behaviors occurred in BPA-treated groups.

CONCLUSION

These data support the concept that the developing brain possesses potent mechanisms to compensate for a small reduction in serum TH, such as serum hypothyrodism induced by BPA exposure, however, the long-term negative effect of BPA treatment on TH homeostasis and glucose metabolism may be attributable to neuropsychiatric deficits after mature.

Alteration of FDG uptake by performing novel object recognition task in a rat model of Traumatic Brain Injury

Traumatic Brain Injury (TBI) affects approximately 2.5 million people in the United States, of which 80% are considered to be mild (mTBI). Previous studies have shown that cerebral glucose uptake and metabolism are altered after brain trauma and functional metabolic deficits observed following mTBI are associated with changes in cognitive performance. Imaging of glucose uptake using [¹⁸F] Fluorodeoxyglucose (FDG) based Positron Emission Tomography (PET) with anesthesia during the uptake period demonstrated limited variability in results, but may have depressed uptake. Anesthesia has been found to interfere with blood glucose levels, and hence, FDG uptake. Conversely, forced cognitive testing during uptake may increase glucose demand in targeted regions, such as hippocampus, allowing for better differentiation of outcomes. Therefore, the objective of this study was to investigate the influence of a directed cognitive function task during the FDG uptake period on uptake measurements both in naïve rats and at 2 days after mild lateral fluid percussion (mLFP) TBI. Adult male Sprague Dawley rats underwent FDG uptake with either cognitive testing with the Novel Object Recognition (NOR) test or No Novel Object (NNO), followed by PET scans at baseline (prior to injury) and at 2days post mLFP. At baseline, FDG uptake in the right hippocampus was elevated in rats completing the NOR in comparison to the NNO (control group). Further, the NNO group rats demonstrated a greater fold change in the FDG uptake between baseline and post injury scans than the NOR group. Overall, these data suggest that cognitive activity during FDG uptake affects the regional uptake pattern in the brain, increasing uptake at baseline and suppressing the effects of injury.

Enhanced fear memories and brain glucose metabolism (18F-FDG-PET) following sub-anesthetic intravenous ketamine infusion in Sprague-Dawley rats

Ketamine is a multimodal dissociative anesthetic, which provides powerful analgesia for victims with traumatic injury. However, the impact of ketamine administration in the peri-trauma period on the development of post-traumatic stress disorder (PTSD) remains controversial. Moreover, there is a major gap between preclinical and clinical studies because they utilize different doses and routes of ketamine administration. Here, we investigated the effects of sub-anesthetic doses of intravenous (IV) ketamine infusion on fear memory and brain glucose metabolism (BGluM) in rats. Male Sprague-Dawley rats received an IV ketamine infusion (0, 2, 10, and 20 mg/kg, 2 h) or an intraperitoneal (IP) injection (0 and 10 mg/kg) following an auditory fear conditioning (3 pairings of tone and foot shock [0.6 mA, 1 s]) on day 0. Fear memory retrieval, fear extinction, and fear recall were tested on days 2, 3, and 4, respectively. The effects of IV ketamine infusion (0 and 10 mg/kg) on BGluM were measured using ¹⁸F-fluoro-deoxyglucose positron emission tomography (FDG-PET) and computed tomography (CT). The IV ketamine infusion dose-dependently enhanced fear memory retrieval, delayed fear extinction, and increased fear recall in rats. The IV ketamine (10 mg/kg) increased BGluM in the hippocampus, amygdala, and hypothalamus, while decreasing it in the cerebellum. On the contrary, a single ketamine injection (10 mg/kg, IP) after fear conditioning facilitated fear memory extinction in rats. The current findings suggest that ketamine may produce differential effects on fear memory depending on the route and duration of ketamine administration.

Cortico-Amygdala-Striatal Activation by Modafinil/Flecainide Combination

Background

Modafinil, a nonamphetaminic wake-promoting compound, is prescribed as first line therapy in narcolepsy, an invalidating disorder characterized by excessive daytime sleepiness and cataplexy. Although its mode of action remains incompletely known, recent studies indicated that modafinil modulates astroglial connexin-based gap junctional communication as administration of a low dose of flecainide, an astroglial connexin inhibitor, enhanced the wake-promoting and procognitive activity of modafinil in rodents and healthy volunteers. The aim of this study is to investigate changes in glucose cerebral metabolism in rodents, induced by the combination of modafinil+flecainide low dose (called THN102).

Methods

The impact of THN102 on brain glucose metabolism was noninvasively investigated using 18F-2-fluoro-2-deoxy-D-glucose Positron Emission Tomography imaging in Sprague-Dawley male rats. Animals were injected with vehicle, flecainide, modafinil, or THN102 and further injected with 18F-2-fluoro-2-deoxy-D-glucose followed by 60-minute Positron Emission Tomography acquisition. 18F-2-fluoro-2-deoxy-D-glucose Positron Emission Tomography images were coregistered to a rat brain template and normalized from the total brain Positron Emission Tomography signal. Voxel-to-voxel analysis was performed using SPM8 software. Comparison of brain glucose metabolism between groups was then performed.

Results

THN102 significantly increased regional brain glucose metabolism as it resulted in large clusters of 18F-2-fluoro-2-deoxy-D-glucose uptake localized in the cortex, striatum, and amygdala compared with control or drugs administered alone. These regions, highly involved in the regulation of sleep-wake cycle, emotions, and cognitive functions were hence quantitatively modulated by THN102.

Conclusion

Data presented here provide the first evidence of a regional brain activation induced by THN102, currently being tested in a phase II clinical trial in narcoleptic patients.

Locomotor activity does not predict individual differences in morphine self-administration in rats

Understanding factors contributing to individual differences in opioid addiction vulnerability is essential for developing more effective preventions and treatments. Sensation seeking has been implicated in addiction to several drugs of abuse, yet its relationship with individual differences in opioid addiction vulnerability has not been well established. The primary goal of this study was to evaluate the relationship between locomotor activity in a novel environment, a preclinical model of sensation-seeking, and individual differences in acquisition of i.v. morphine self-administration (SA) in rats. A secondary goal was to evaluate the relationship between activity and elasticity of demand (reinforcing efficacy) for morphine measured using a behavioral economic approach. Following an initial locomotor activity screen, animals were allowed to acquire morphine SA at a unit dose of 0.5 mg/kg/infusion in 4 hour/day sessions (Experiment 1) or 0.2 mg/kg/infusion in 2 hour/day sessions (Experiment 2) until infusion rates were stable. Unit price was subsequently manipulated via progressive reductions in unit dose (Experiment 1) or increases in response requirement per infusion (Experiment 2). Activity levels were not correlated with acquisition of morphine SA in either experiment. Morphine consumption was generally well described by an exponential demand function in both experiments (R² values > 0.95 for rats as a group), but activity did not correlate with behavioral economic measures. Locomotor activity in a novel environment did not predict individual differences in acquisition of morphine SA. These data complement findings from some human studies and suggest that the role of sensation seeking in individual differences in opioid addiction vulnerability may be limited.