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A Method for Evaluating the Reinforcing Properties of Ethanol in Rats without Water Deprivation, Saccharin Fading or Extended Access Training

Operant oral self-administration methods are commonly used to study the reinforcing properties of ethanol in animals. However, the standard methods ...

linköping university

Assessing the relative value of cocaine and how it changes with chronic drug use represents a long-standing goal in addiction research. Surprisingly, recent experiments in rats--by far the most frequently used animal model in this field--suggest that the value of cocaine is lower than previously thought.

Cocaine is low on the value ladder of rats: possible evidence for resilience to addiction.

When facing a choice between cocaine and a potent, albeit inessential, non-drug alternative (i.e. water sweetened with saccharin), most cocaine self-administering rats abstain from cocaine in favor of the non-drug pursuit, regardless of the dose available and even after extended drug use. Only a minority continues to take the drug despite the opportunity of making a different choice and increasing stakes. This pattern of individual variation could suggest that the majority of rats are resilient to addiction, taking cocaine by default of other options. Only a minority would be vulnerable to addiction. This study tested the hypothesis that rats choose to refrain from cocaine self-administration because cocaine would be conflictual, having both rewarding and anxiogenic properties. Contrary to this hypothesis, however, we report here that diazepam-a broad-spectrum benzodiazepine anxiolytic-did not decrease, but instead, further increased cocaine abstinence. Interestingly, although diazepam decreased locomotion, rats adapted to this effect by spending more time near the lever associated with the preferred reward, a behavior that minimized the need for locomotion at the moment of choice. When responding for cocaine or saccharin was analyzed separately, we found that diazepam decreased responding for cocaine without affecting responding for saccharin. Finally, the abstinence-promoting effects of diazepam were also induced in cocaine-preferring rats treated chronically with diazepam. Overall, this study demonstrates that abstinence from cocaine cannot be explained away by the anxiogenic effects of cocaine, thereby reinforcing the notion of resilience to addiction. It also supports the use of benzodiazepines in the treatment of cocaine addiction.

Diazepam promotes choice of abstinence in cocaine self-administering rats.

Cell fate and proliferation are tightly linked to the regulation of the mitochondrial energy metabolism. Hence, mitochondrial biogenesis regulation, a complex process that requires a tight coordination in the expression of the nuclear and mitochondrial genomes, has a major impact on cell fate and is of high importance. Here, we studied the molecular mechanisms involved in the regulation of mitochondrial biogenesis through a nutrient-sensing pathway, the Ras-cAMP pathway. Activation of this pathway induces a decrease in the cellular phosphate potential that alleviates the redox pressure on the mitochondrial respiratory chain. One of the cellular consequences of this modulation of cellular phosphate potential is an increase in the cellular glutathione redox state. The redox state of the glutathione disulfide-glutathione couple is a well known important indicator of the cellular redox environment, which is itself tightly linked to mitochondrial activity, mitochondria being the main cellular producer of reactive oxygen species. The master regulator of mitochondrial biogenesis in yeast (i.e. the transcriptional co-activator Hap4p) is positively regulated by the cellular glutathione redox state. Using a strain that is unable to modulate its glutathione redox state (Δglr1), we pinpoint a positive feedback loop between this redox state and the control of mitochondrial biogenesis. This is the first time that control of mitochondrial biogenesis through glutathione redox state has been shown.

cAMP-induced mitochondrial compartment biogenesis: role of glutathione redox state.

We describe a protocol for screening compulsive drug users among cocaine self-administering rats, the most frequently used animal model in addiction research. Rats are first trained on several alternating days to self-administer either cocaine (i.v.) or saccharin-sweetened water (by mouth)--a potent, albeit nonessential, nondrug reward. Then rats are allowed to choose between the two rewards over several days until the preference stabilizes. Most rats choose to stop using cocaine and pursue the alternative reward. Only a minority of Wistar strain rats (generally 15%) persist in taking the drug, regardless of the severity of past cocaine use and even when made hungry and offered the possibility to relieve their physiological need. Persistence of cocaine use in the face of a high-stakes choice is a core defining feature of compulsion. This choice-based screening method for compulsive drug users is easy to implement, has several important applications, and compares well with other methods in the field.

A choice-based screening method for compulsive drug users in rats.

Identification of genes influencing complex traits is hampered by genetic heterogeneity, the modest effect size of many alleles, and the likely involvement of rare and uncommon alleles. Etiologic complexity can be simplified in model organisms. By genomic sequencing, linkage analysis, and functional validation, we identified that genetic variation of Grm2, which encodes metabotropic glutamate receptor 2 (mGluR2), alters alcohol preference in animal models. Selectively bred alcohol-preferring (P) rats are homozygous for a Grm2 stop codon (Grm2 *407) that leads to largely uncompensated loss of mGluR2. mGluR2 receptor expression was absent, synaptic glutamate transmission was impaired, and expression of genes involved in synaptic function was altered. Grm2 *407 was linked to increased alcohol consumption and preference in F2 rats generated by intercrossing inbred P and nonpreferring rats. Pharmacologic blockade of mGluR2 escalated alcohol self-administration in Wistar rats, the parental strain of P and nonpreferring rats. The causal role of mGluR2 in altered alcohol preference was further supported by elevated alcohol consumption in Grm2 (-/-) mice. Together, these data point to mGluR2 as an origin of alcohol preference and a potential therapeutic target.

Loss of metabotropic glutamate receptor 2 escalates alcohol consumption.

Group II metabotropic glutamate receptors (mGluR2 and mGluR3) may control relapse of alcohol seeking, but previously available Group II agonists were unable to discriminate between mGluR2 and mGluR3. Here we use AZD8529, a novel positive allosteric mGluR2 modulator, to determine the role of this receptor for alcohol-related behaviors in rats. We assessed the effects of AZD8529 (20 and 40 mg/kg s.c.) on male Wistar rats trained to self-administer 20% alcohol and determined the effects of AZD8529 on self-administration, as well as stress-induced and cue-induced reinstatement of alcohol seeking. The on-target nature of findings was evaluated in Indiana P-rats, a line recently shown to carry a mutation that disrupts the gene encoding mGluR2. The behavioral specificity of AZD8529 was assessed using self-administration of 0.2% saccharin and locomotor activity tests. AZD8529 marginally decreased alcohol self-administration at doses that neither affected 0.2% saccharin self-administration nor locomotor activity. More importantly, cue- but not stress-induced alcohol seeking was blocked by the mGluR2 positive allosteric modulator. This effect of AZD8529 was completely absent in P rats lacking functional mGluR2s, demonstrating the receptor specificity of this effect. Our findings provide evidence for a causal role of mGluR2 in cue-induced relapse to alcohol seeking. They contribute support for the notion that positive allosteric modulators of mGluR2 block relapse-like behavior across different drug categories.

Eric Augier

Center for Social and Affective Neuroscience,
IKE,
Center for Social and Affective Neuroscience, IKE

linköping university

A Method for Evaluating the Reinforcing Properties of Ethanol in Rats without Water Deprivation, Saccharin Fading or Extended Access Training

Eric Augier

.css-o250i3{font-size:18px;font-family:Open Sans,sans-serif;line-height:21.6px;}Center for Social and Affective Neuroscience,IKE,Center for Social and Affective Neuroscience, IKE

linköping university

A Method for Evaluating the Reinforcing Properties of Ethanol in Rats without Water Deprivation, Saccharin Fading or Extended Access Training

Center for Social and Affective Neuroscience,
IKE,
Center for Social and Affective Neuroscience, IKE