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  • Addiction, Drugs
  • Information from Lay-Language Summaries is Embargoed Until the Conclusion of the Scientific Presentation

    546—Cocaine: Neural Mechanisms of Addiction IV

    Tuesday, November 12, 2013, 8:00 am - 12:00 noon

    546.20: Beta-arrestin 1 dependent regulation of cocaine self-administration in mice

    Location: Halls B-H

    1Univ. of Texas At Austin, Austin, TX; 2California State Univ. San Bernardino, San Bernardino, CA; 3Univ. of California Los Angeles, Los Angeles, CA

    Abstract Body: AMPARs (2-amino-3-(3-hydroxy-5-methyl-isoxazol-4-yl)propanoic acid receptors) rely on the 3-dimensional structure of the postsynaptic density to be trafficked into and removed from the synapse as necessary. This is mediated by the rapid rearrangement of actin filaments, which is orchestrated by a series of proteins, including the actin severing protein, cofilin, Lim domain kinase (LIMK), the upstream kinase, and Slingshot, a phosphatase. These proteins are in turn associated with one of the two non-visual arrestins, thereby providing spatiotemporal regulation of actin turnover. These protein interactions predict that mice lacking one of the arrestin isoforms, β-arrestin 1, may demonstrate altered AMPAR function. As changes in AMPAR subunit composition and function are a well-recognized hallmark of single and repeated exposure to psychostimulants such as cocaine, we examined whether mice lacking B-arrestin1 show an altered profile of cocaine self-administration and of AMPAR function. We first defined the profile of intravenous self-administration of cocaine in β-arrestin 1 knockout (βarr1-/-) and wildtype (WT, βarr1+/+) mice. Mice were trained to self-administer cocaine (1 mg/kg/infusion) on schedules of reinforcement that progressively advanced: fixed ratio (FR) 1, FR2, and FR5. Sessions were conducted on sequential days and each lasted 2 hours. Relative to their WT littermates, βarr1-/- mice earned fewer cocaine infusions during the early stages of acquisition (at FR1; p=0.016), but not thereafter (at FR2 or FR5). After stable responding at FR5 was achieved, 5 days of extinction followed, in which the mice were placed in the operant boxes but no drug delivered when the active lever was pressed. During early extinction learning WT mice showed a faster decline in the number of lever presses as compared to βarr1-/- mice (p=0.015). Thereafter, both genotypes showed equivalent cue-induced reinstatement behaviors. At the end of either the acquisition or reinstatement phases, mice were removed from the study to assess the AMPA/NMDA (A/N) ratio from evoked excitatory post-synaptic currents (eEPSCs) in medium spiny neurons in the nucleus accumbens shell. We found that the A/N ratio from naive WT mice was lower than that of naïve βarr1-/- mice (p=0.047). Acquisition and reinstatement of cocaine self-administration was associated with a progressive increase in the A/N ratio in WT mice. In contrast, the A/N ratio from βarr1-/- was not altered at any stage. These data suggest that β-arrestin 1 plays an important role in the acquisition and extinction of cocaine self-administration, and in the adaptations of AMPAR function that may underlie these behaviors.

    Lay Language Summary: Our research indicates that beta-arrestin 1, a ‘scaffolding’ protein, controls how signaling proteins are moved along the actin ‘backbone’ of brain cells, or neurons, and in and out of specialized regions called synapses. This affects communication between neurons, and regulates learning specific changes in cocaine-related behaviors. These findings highlight the role of beta-arrestin 1 in facilitating cocaine abuse and outline a novel pathway. This may facilitate the development of new treatments emphasizing specific components of drug addiction.
    Arrestins are a class of proteins that serve as ‘scaffolding’ units, which connect proteins to actin ‘tracks', and allow receptor movement to or away from a synapse. We proposed that beta-arrestin 1 is able to link AMPA receptors to the actin scaffold and regulate their function. This hypothesis was tested in genetically manipulated mice that lacked the beta-arrestin 1 protein.
    Using a behavioral test that is similar to how humans self-administer drugs, we examined how these mice self-administer cocaine. We inserted catheters into the jugular veins of beta-arrestin 1-lacking or, beta-arrestin 1-containing, normal, mice. Thereafter, both groups of mice were trained to press a lever, initially once, then twice, and finally five times, to receive a single infusion of cocaine through the catheter. This initial phase tested how well the mice learned to obtain cocaine. Then, by increasing the lever-pressing requirement, we measured their desire, or motivation, for this drug. Both groups of mice were then moved onto a period of ‘forced rehabilitation’, during which no cocaine was delivered no matter how many times they pressed the lever. After a week of ‘rehabilitation’, they were finally allowed to press the lever to obtain cocaine, so modeling ‘relapse’ seen in humans.
    The arrestin-lacking mice were initially slower in learning to press the lever for cocaine. However, they did eventually learn to administer similar amounts of cocaine as the normal mice showing no change in their desire to obtain cocaine. Then, during the rehabilitation phase, the arrestin-lacking mice continued to press the lever for a longer time than the normal mice. These experiments showed that beta-arrestin 1 is important for two components of self-administering cocaine; the initial phase of learning to press a lever for cocaine and the later phase of learning that lever-pressing will not deliver cocaine.
    We then used micro-electrodes to measure AMPA receptor function in the neurons of these mice after each of these behaviors. This test is considered a ‘gold standard’ for measuring learning in neurons. Similar to previous studies, we found that cocaine self-administration increased AMPA receptor function in normal mice. But the arrestin-lacking mice did not show this increase in AMPA receptor function after cocaine self-administration. Measuring the activation of GluR1, one of four molecules that make up AMPA receptors, also showed that arrestin-lacking mice did not show an increase in AMPA receptor function, further confirming our findings. Next, we aim to visualize this receptor movement, and identify additional proteins involved in the restructuring of synapses after cocaine use.
    Continuous signaling and rewiring of connections between neurons forms the basis of learning and memory. Drugs of abuse target these systems to strengthen a person’s memories and feelings associated with drug use. This in turn leads to a stronger desire or craving for the drug. AMPA receptors are signaling proteins that play an important role in the rewiring of these connections, or synapses. Indeed, even a single exposure to cocaine can lead to changes in AMPA receptor function in brain regions associated with drug use. This change in function is a result of the rapid movement of AMPA receptors in and out of synapses on the surface of neurons.