Karen K. Szumlinski

Homer Proteins Regulate Sensitivity to Cocaine

Drug addiction involves complex interactions between pharmacology and learning in genetically susceptible individuals. Members of the Homer gene family are regulated by acute and chronic cocaine administration. Here, we report that deletion of Homer1 or Homer2 in mice caused the same increase in sensitivity to cocaine-induced locomotion, conditioned reward, and augmented extracellular glutamate in nucleus accumbens as that elicited by withdrawal from repeated cocaine administration. Moreover, adeno-associated virus-mediated restoration of Homer2 in the accumbens of Homer2 KO mice reversed the cocaine-sensitized phenotype. Further analysis of Homer2 KO mice revealed extensive additional behavioral and neurochemical similarities to cocaine-sensitized animals, including accelerated acquisition of cocaine self-administration and altered regulation of glutamate by metabotropic glutamate receptors and cystine/glutamate exchange. These data show that Homer deletion mimics the behavioral and neurochemical phenotype produced by repeated cocaine administration and implicate Homer in regulating addiction to cocaine.

Behavioral and neurochemical phenotyping of Homer1 mutant mice: possible relevance to schizophrenia

Homer proteins are involved in the functional assembly of postsynaptic density proteins at glutamatergic synapses and are implicated in learning, memory and drug addiction. Here, we report that Homer1‐knockout (Homer1‐KO) mice exhibit behavioral and neurochemical abnormalities that are consistent with the animal models of schizophrenia. Relative to wild‐type mice, Homer1‐KO mice exhibited deficits in radial arm maze performance, impaired prepulse inhibition, enhanced ‘behavioral despair’, increased anxiety in a novel objects test, enhanced reactivity to novel environments, decreased instrumental responding for sucrose and enhanced MK‐801‐ and methamphetamine‐stimulated motor behavior. No‐net‐flux in vivo microdialysis revealed a decrease in extracellular glutamate content in the nucleus accumbens and an increase in the prefrontal cortex. Moreover, in Homer1‐KO mice, cocaine did not stimulate a rise in frontal cortex extracellular glutamate levels, suggesting hypofrontality. These behavioral and neurochemical data derived from Homer1 mutant mice are consistent with the recent association of schizophrenia with a single‐nucleotide polymorphism in the Homer1 gene and suggest that the regulation of extracellular levels of glutamate within limbo‐corticostriatal structures by Homer1 gene products may be involved in the pathogenesis of this neuropsychiatric disorder.

Chronic ethanol consumption by C57BL/6 mice promotes tolerance to its interoceptive cues and increases extracellular dopamine, an effect blocked by naltrexone.

BACKGROUNDnC57BL/6 (B6) mice voluntarily consume ethanol. Although preingestive factors might be accountable, the fact that B6 mice voluntarily consume sufficient ethanol to set the conditions for an ethanol-deprivation effect suggest that post-ingestive pharmacological induced changes also occur. In this study, we determined the amounts of ethanol voluntarily consumed by B6 mice and associated blood ethanol levels (BEL), the effects of this consumption on extracellular dopamine (DA) and how this was altered by naltrexone, as well as on its interoceptive discriminative cues.nnnMETHODSnIn experiment 1, the amounts of 12% ethanol consumed at 2, 4, and 6 hr into the active phase of the circadian cycle and associated BEL were determined. In experiment 2, dialysate samples were collected for 1 hr to establish basal DA levels. Mice were then injected with saline or naltrexone (6 mg/kg) and given access to water and 12% ethanol or to water only, and samples were collected at 20-min intervals for the next 2 hr. In experiment 3, mice were trained to discriminate ethanols interoceptive cues via operant techniques, and half were given 3 weeks access to ethanol and water, the other half water only. Ethanol-consuming and water control mice were again tested for their ability to discriminate the drugs interoceptive cues.nnnRESULTSnMice ingested nearly 6 g/kg of ethanol and attained BEL near 100 mg/100 mL by 6 hr into the active phase. Ethanol intake at 2-hr into the dark phase was approximately 2.5 g/kg, and increased DA to approximately 100% above basal levels. Naltrexone reduced ethanol consumption and blocked the DA increase. Ethanol consumption for 3 weeks attenuated its discriminative cues.nnnCONCLUSIONSnB6 mice voluntarily consume sufficient ethanol (1) to produce intoxicating BEL; (2) to increase DA levels in nucleus accumbens, an effect blocked by naltrexone; and (3) to attenuate its discriminative cues.

Repeated cocaine administration alters the electrophysiological properties of prefrontal cortical neurons

Recently it has become clear that some of the symptoms of addiction such as relapse to drug-taking behavior arise, in part, from a dysfunction in cognitive and emotional processing. This realization has promoted investigations into the physiology and pathophysiology of forebrain circuits that are both innervated by dopamine and play an important role in cognitive processing, including the prefrontal cortex. In order to study long-term neuroadaptations occurring in the prefrontal cortex of the rat as a consequence of psychostimulant administration, cocaine was repeatedly administered in either a contingent or a non-contingent manner. At least 2 weeks following the last cocaine injection, in vivo intracellular recordings were made from neurons located in the deep layers of the prefrontal cortex. Repeated cocaine administration abolished the presence of membrane bistability normally present in neurons located in the limbic prefrontal cortex. These results indicate that repeated exposure to cocaine produces enduring changes in the basal activity of neurons in the prefrontal cortex that may contribute to previously identify cognitive and emotional dysfunctions in cocaine addicts.

Unconditioned and conditioned factors contribute to the 'reinstatement' of cocaine place conditioning following extinction in C57BL/6 mice.

Relapse to drug use following prolonged periods of abstinence results, in part, from the ability of contextual cues paired previously with self-administered drug to elicit drug craving and -seeking behavior. Given the popularity of the mouse for the genetic analysis of drug-induced behaviors, a place conditioning model of drug-seeking behavior was used to examine the ability of cocaine (COC) to reinstate extinguished conditioned reward in mice. In a series of experiments, COC place conditioning was produced in male C57BL/6 (B6) mice by four pairings of COC (15 or 25 mg/kg, IP) with the non-preferred compartment of a two-compartment place conditioning apparatus. Following a post-conditioning test (Post-Test), place conditioning was extinguished by repeated testing. The mice were then challenged with one of five COC doses (0, 5, 10, 15 or 25 mg/kg, IP) and allowed free access to both environments. Following extinction, COC injections reinstated place conditioning to 100% or greater, relative to the Post-Test. In a control experiment, mice received either COC or SAL paired with non-preferred compartment and were then challenged with either COC (15 mg/kg, IP) or SAL on the Post-Test. COC-conditioned, but not SAL-conditioned, mice exhibited place conditioning when tested in a COC-free state. Interestingly, COC injection on the Post-Test elicited an increase in approach behavior in both SAL- and COC-conditioned mice and this increase was equivalent to that produced by COC conditioning alone. No direct relationships were observed between the magnitude of place conditioning and either COC-induced or -conditioned locomotor hyperactivity in the non-preferred compartment. Thus, at least two independent processes appear to underlie the ability of a COC injection to elicit approach behavior towards the non-preferred compartment of a biased place conditioning apparatus in mice-reactivation of the conditioned incentive motivational properties of COC-paired cues and elicitation of unconditioned behavioral disinhibition. One or both of these processes sensitizes with the passage of time, increasing the propensity of B6 mice to approach non-preferred environments upon COC re-administration.

Rapamycin attenuates the expression of cocaine-induced place preference and behavioral sensitization

The mammalian target of rapamycin (mTOR) is a serine‐threonine kinase that controls global protein synthesis, in part, by modulating translation initiation, a rate‐limiting step for many mRNAs. Previous studies implicate mTOR in regulating stimulant‐induced sensitization and antidepressive‐like behavior in rodents, as well as drug craving in abstinent heroin addicts. To determine if signaling downstream of mTOR is affected by repeated cocaine administration in reward‐associated brain regions, and if inhibition of mTOR alters cocaine‐induced behavioral plasticity, C57BL/6J mice received four intraperitoneal (i.p.) injections of 15u2003mg/kg cocaine and levels of phosphorylated P70S6 kinase and ribosomal S6 protein—two translational regulators directly downstream of mTOR—were analyzed by immunoblotting across several brain regions. Cocaine place preference and locomotor sensitization were elicited by four pairings of cocaine with a distinct environment and the effects of mTOR inhibition were assessed by pre‐treating the mice with 10u2003mg/kg rapamycin, 1 hour prior to: (1) each saline/cocaine conditioning session; (2) a post‐conditioning test; or (3) a test for locomotor sensitization conducted at 3 weeks withdrawal. While systemic pre‐treatment with 10u2003mg/kg rapamycin during conditioning failed to alter the development of a cocaine place preference or locomotor sensitization, pre‐treatment prior to the post‐conditioning test attenuated the expression of the place preference. Additionally, rapamycin pre‐treatment prior to a cocaine challenge 3 weeks post‐conditioning blocked the expression of the sensitized locomotor response. These findings suggest a role for mTOR activity, and perhaps translational control, in the expression of cocaine‐induced place preference and locomotor sensitization.

Evidence for a relationship between Group 1 mGluR hypofunction and increased cocaine and ethanol sensitivity in Homer2 null mutant mice.

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Genetic variation in heroin‐induced changes in behaviour: effects of B6 strain dose on conditioned reward and locomotor sensitization in 129‐B6 hybrid mice

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Homer2 Gene Deletion in Mice Produces a Phenotype Similar to Chronic Cocaine Treated Rats

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Pretreatment with serotonin 5-HT3 receptor antagonists produces no observable blockade of long-term motor sensitization to cocaine in rats

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