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Open Access Publications from the University of California

Scholarly Works (7 results)

  • Thesis
  • Peer Reviewed
UC San Diego Electronic Theses and Dissertations (2015)

Addiction results in persistent neural and behavioral modifications characterized by compulsive drug use, compulsive drug seeking, and repeated relapse. The neural substrates associated with drug-use and addiction are believed to overlap with those involved in memory. Many protein kinases, transcription factors, and receptors are implicated in both processes; however, drug related and traditional associative memory may produce different neural modifications. One such addiction related memory is sensitization, a persistent increase in the response to a drug following repeated administration. The purpose of my dissertation is to examine whether certain post-translational mechanisms are similarly involved in associative forms of memory and sensitization. In my first experiment, I investigated the role of atypical protein kinase C (PKC) in the development and expression of locomotor sensitization. I used zeta inhibitory peptide (ZIP) to block atypical PKC activity. We found that the induction, but not maintenance, of locomotor sensitization was impaired when ZIP was administered both continuously and with a single infusion. In the second and third series of experiments, I examined the effects of disrupting the ubiquitin-proteasome system (UPS), a pathway that mediates protein degradation, on learning, memory, and addiction using two lines of novel knockin mice. Both mice had a mutation at a single amino acid residue, serine 120, a site on Rpt6, part of the proteasome’s regulatory component. This site is important as it is phosphorylated by CaMKII in an activity-dependent manner. In one line of mice the serine was switched to an aspartic acid to create a phospho-mimetic mutation (S120D), whereas in the second line of mice serine was switched to an alanine to create a phospho-dead (S120A) mutation. We found this mechanism to be a critical regulator of the UPS during addiction-related memory, as sensitization was completely blocked in phospho-mimetic mice. In contrast, both lines of knockin mice displayed intact associative memory, as assessed using Pavlovian fear conditioning and context discrimination . Therefore, while atypical PKCs and the UPS may be involved in both traditional, associative memory and addiction-related memory, the functional role of these mechanisms seems to differ between the two processes.

    Cover page: The involvement of atypical PKC and the ubiquitin-proteasome system in memory and addiction
    • Article
    • Peer Reviewed
    UC San Diego Previously Published Works (2014)
    Methylphenidate (MPH), introduced more than 60 years ago, accounts for two-thirds of current prescriptions for attention deficit hyperactivity disorder (ADHD). Although many studies have modeled MPHs effect on executive function, almost none have directly modeled its effect on long-term memory (LTM), even though improvement in LTM is a critical target of therapeutic intervention in ADHD. We examined the effects of a wide range of doses of MPH (0.01-10 mg/kg, i.p.) on Pavlovian fear learning, a leading model of memory. MPHs effects were then compared to those of atomoxetine (0.1-10 mg/kg, i.p.), bupropion (0.5-20 mg/kg, i.p.), and citalopram (0.01-10 mg/kg, i.p.). At low, clinically relevant doses, MPH enhanced fear memory; at high doses it impaired memory. MPHs memory-enhancing effects were not confounded by its effects on locomotion or anxiety. Further, MPH-induced memory enhancement seemed to require both dopamine and norepinephrine transporter inhibition. Finally, the addictive potential of MPH (1 mg/kg and 10 mg/kg) was compared to those of two other psychostimulants, amphetamine (0.005 mg/kg and 1.5 mg/kg) and cocaine (0.15 mg/kg and 15 mg/kg), using a conditioned place preference and behavioral sensitization paradigm. We found that memory-enhancing effects of psychostimulants observed at low doses are readily dissociable from their reinforcing and locomotor activating effects at high doses. Together, our data suggest that fear conditioning will be an especially fruitful platform for modeling the effects of psychostimulants on LTM in drug development.
      Cover page: Animal model of methylphenidates long-term memory-enhancing effects.
      • Article
      • Peer Reviewed
      UC San Diego Previously Published Works (2014)
      Psychostimulants containing methylphenidate (MPH) are increasingly being used both on and off-label to enhance learning and memory. Still, almost no studies have investigated MPH's ability to specifically improve spatial or long-term memory. Here we examined the effect of training with 1 or 10mg/kg MPH on hidden platform learning in the Morris water maze. 10mg/kg MPH improved memory acquisition and retention, while 1mg/kg MPH improved memory retention. Taken together with prior evidence that low, clinically relevant, doses of MPH (0.01-1mg/kg MPH) enhance fear memory we conclude that MPH broadly enhances memory.
        Cover page: Methylphenidate enhances acquisition and retention of spatial memory
        • Article
        • Peer Reviewed
        UC San Diego Previously Published Works (2014)
        Methylphenidate (MPH), introduced more than 60 years ago, accounts for two-thirds of current prescriptions for attention deficit hyperactivity disorder (ADHD). Although many studies have modeled MPH's effect on executive function, almost none have directly modeled its effect on long-term memory (LTM), even though improvement in LTM is a critical target of therapeutic intervention in ADHD. We examined the effects of a wide range of doses of MPH (0.01-10 mg/kg, i.p.) on Pavlovian fear learning, a leading model of memory. MPH's effects were then compared to those of atomoxetine (0.1-10 mg/kg, i.p.), bupropion (0.5-20 mg/kg, i.p.), and citalopram (0.01-10 mg/kg, i.p.). At low, clinically relevant doses, MPH enhanced fear memory; at high doses it impaired memory. MPH's memory-enhancing effects were not confounded by its effects on locomotion or anxiety. Further, MPH-induced memory enhancement seemed to require both dopamine and norepinephrine transporter inhibition. Finally, the addictive potential of MPH (1 mg/kg and 10 mg/kg) was compared to those of two other psychostimulants, amphetamine (0.005 mg/kg and 1.5 mg/kg) and cocaine (0.15 mg/kg and 15 mg/kg), using a conditioned place preference and behavioral sensitization paradigm. We found that memory-enhancing effects of psychostimulants observed at low doses are readily dissociable from their reinforcing and locomotor activating effects at high doses. Together, our data suggest that fear conditioning will be an especially fruitful platform for modeling the effects of psychostimulants on LTM in drug development.
          Cover page: Animal model of methylphenidate's long-term memory-enhancing effects
          • Article
          • Peer Reviewed
          UC San Diego Previously Published Works (2018)
          Repeated exposure to cocaine produces structural and functional modifications at synapses from neurons in several brain regions including the nucleus accumbens. These changes are thought to underlie cocaine-induced sensitization. The ubiquitin proteasome system plays a crucial role in the remodeling of synapses and has recently been implicated in addiction-related behavior. The ATPase Rpt6 subunit of the 26S proteasome is phosphorylated by Ca2+/calmodulin-dependent protein kinases II alpha at ser120 which is thought to regulate proteasome activity and distribution in neurons. Here, we demonstrate that Rpt6 phosphorylation is involved in cocaine-induced locomotor sensitization. Cocaine concomitantly increases proteasome activity and Rpt6 S120 phosphorylation in cultured neurons and in various brain regions of wild type mice including the nucleus accumbens and prefrontal cortex. In contrast, cocaine does not increase proteasome activity in Rpt6 phospho-mimetic (ser120Asp) mice. Strikingly, we found a complete absence of cocaine-induced locomotor sensitization in the Rpt6 ser120Asp mice. Together, these findings suggest a critical role for Rpt6 phosphorylation and proteasome function in the regulation cocaine-induced behavioral plasticity.
            Cover page: Proteasome phosphorylation regulates cocaine-induced sensitization
            • Article
            • Peer Reviewed
            UC San Diego Previously Published Works (2014)
            The atypical PKC isoforms, PKMζ and PKCλ have been proposed as integral substrates of long-term memory (LTM). Inhibition of these isoforms has recently been demonstrated to be sufficient for impairing the expression and maintenance of long-term potentiation. Additionally, the pseudosubstrate inhibitor, zeta inhibitory peptide (ZIP), which effectively blocks PKMζ and PKCλ, has previously been shown to disrupt associative memory; very little is known about its effects on pathological nonassociative forms of memory related to addiction. The neural and molecular substrates of memory and addiction have recently been argued to overlap. Here, we used ZIP to disrupt PKMζ and PKCλ activity to examine their role in cocaine sensitization, a nonassociative, addiction-related memory argued to underlie the transition from casual to pathological drug use. We examined the effects of both continuous and acute administration of ZIP. Even a single application of ZIP blocked the development of sensitization; sustained inhibition using osmotic pumps produced an almost complete blockade of sensitization. Further, a single application of ZIP was shown to reduce membrane-bound AMPAR expression. These results demonstrate a novel, critical role for the atypical PKC isoforms in nonassociative memory and cocaine addiction.
              Cover page: Inhibition of PKC disrupts addiction-related memory
              • Article
              • Peer Reviewed
              UC San Diego Previously Published Works (2021)
              Dynamic control of protein degradation via the ubiquitin proteasome system (UPS) is thought to play a crucial role in neuronal function and synaptic plasticity. The proteasome subunit Rpt6, an AAA ATPase subunit of the 19S regulatory particle (RP), has emerged as an important site for regulation of 26S proteasome function in neurons. Phosphorylation of Rpt6 on serine 120 (S120) can stimulate the catalytic rate of substrate degradation by the 26S proteasome and this site is targeted by the plasticity-related kinase Ca2+/calmodulin-dependent kinase II (CaMKII), making it an attractive candidate for regulation of proteasome function in neurons. Several in vitro studies have shown that altered Rpt6 S120 phosphorylation can affect the structure and function of synapses. To evaluate the importance of Rpt6 S120 phosphorylation in vivo, we created two mouse models which feature mutations at S120 that block or mimic phosphorylation at this site. We find that peptidase and ATPase activities are upregulated in the phospho-mimetic mutant and downregulated in the phospho-dead mutant [S120 mutated to aspartic acid (S120D) or alanine (S120A), respectively]. Surprisingly, these mutations had no effect on basal synaptic transmission, long-term potentiation (LTP), and dendritic spine dynamics and density in the hippocampus. Furthermore, these mutants displayed no deficits in cued and contextual fear memory. Thus, in a mouse model that blocks or mimics phosphorylation at this site, either compensatory mechanisms negate these effects, or small variations in proteasome activity are not enough to induce significant changes in synaptic structure, plasticity, or behavior.
                Cover page: Altered phosphorylation of the proteasome subunit Rpt6 has minimal impact on synaptic plasticity and learning
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