Rapamycin reduces motivated responding for cocaine and alters GluA1 expression in the ventral but not dorsal striatum

Abstract

The mechanistic target of rapamycin complex 1 (mTORC1) regulates synaptic protein synthesis and therefore synaptic function and plasticity. A role for mTORC1 has recently been demonstrated for addiction-related behaviors. For example, central or intra-accumbal injections of the mTORC1 inhibitor rapamycin attenuates several indices of cocaine-seeking including progressive ratio (PR) responding and reinstatement. These behavioral effects are associated with decreased mTORC1 activity and synaptic protein translation in the nucleus accumbens (NAC) and point to a possible therapeutic role for rapamycin in the treatment of addiction. Currently, it is unclear whether similar behavioral and biochemical effects can be achieved by administering rapamycin systemically, which represents a more clinically-appropriate route of administration. Here, we assessed the effects of repeated, systemic administration of rapamycin (10 mg/kg, i.p.) on PR responding for cocaine. We also assessed whether systemic rapamycin was associated with changes in measures of mTORC1 activity and GluA1 expression in the ventral and dorsal striatum. We report that systemic rapamycin treatment reduced PR breakpoints to levels comparable to intra-NAC rapamycin. Systemic rapamycin treatment also reduced phosphorylated p70S6K and GluA1 AMPARs within the NAC but not dorsal striatum. Thus, systemic administration of rapamycin is as effective at reducing drug seeking behavior and measures of mTORC1 activity compared to direct accumbal application and may therefore represent a possible therapeutic option in the treatment of addiction. Possible caveats of this treatment approach are discussed.

Introduction

The mechanistic target of rapamycin (mTOR) is a serine-threonine kinase expressed ubiquitously in all eukaryotic cells, including neurons. mTOR forms two active complexes by binding to scaffolding proteins Raptor and Rictor, as well as several other components, to form either the mTOR complex 1 (mTORC1) or mTOR complex 2 (mTORC2). Studies in acute brain slices utilizing the allosteric inhibitor of mTORC1 activity rapamycin indicate that this system controls activity-dependent translation of proteins required for synaptic plasticity. This process involves phosphorylation of several intracellular targets, including p70 S6 kinase (p70S6K) and eukaryotic initiation factor 4E-binding proteins (4EBPs); (Hay and Sonenberg, 2004, Raught et al., 2001). Through this signaling cascade, mTORC1 regulates the translation of a number of plasticity-related proteins, including Ca²⁺/Calmodulin-dependent kinase II alpha (CAMKIIα), AMPA and NMDA receptor subunits, including GluA1 (Dayas et al., 2012, Hou and Klann, 2004, Mameli et al., 2007). Given the well-characterized changes in these proteins in response to drug exposure, it has been suggested that dysregulated mTORC1 signaling may play a key role in modulating drug seeking and addiction.

Acute and chronic exposure to drugs of abuse is associated with increased mTORC1 activity in reward-relevant brain regions, particularly the nucleus accumbens (NAC) (James et al., 2014, Neasta et al., 2010, Wu et al., 2011). Further, accumbal levels of S6K and 4EBP are enhanced following exposure to drug-associated cues and contexts, suggesting that the ventral striatum is a key site at which changes in mTORC1 activity may modulate drug-seeking behavior. To this end, several studies have examined the effect of intra-accumbal infusions of the mTORC1 inhibitor rapamycin on drug seeking behavior. For example, we recently showed that repeated (5d) intra-accumbal infusions of rapamycin during self-administration training had no effect on cocaine responding on an FR1 schedule, but significantly attenuated breakpoints on a progressive ratio schedule (James et al., 2014). This treatment regime also significantly reduced responding on a subsequent discriminative cue-induced reinstatement test up to 6 weeks later, as well as levels of GluA1 AMPAR levels in the NAC (James et al., 2014). These findings are generally consistent with studies showing that acute intra-accumbal rapamycin infusions are effective at reducing psychostimulant reinstatement (Wang et al., 2010) and sensitization (Narita et al., 2005) behavior.

Whilst these findings point to a potential therapeutic role for rapamycin in the treatment of addiction, there is a need for a more complete understanding of the effects of rapamycin when delivered systemically – a more clinically relevant route of administration. Although a small number of studies have utilized systemic rapamycin treatment regimes, they have been largely restricted to either alcohol seeking (Barak et al., 2013) or paradigms that involve non-operant administration of psychostimulants, such as conditioned place preference (Wu et al., 2011, Bailey et al., 2012). As such, the effect of systemic rapamycin administration on psychostimulant self-administration behavior remains unclear.

Accordingly, we assessed the effects of rapamycin on progressive ratio responding for cocaine, a sensitive test of motivated responding for drug reinforcement. We chose a repeated (3d) systemic dosing regime to explore the effectiveness of subchronic rapamycin treatment on this behavior. To assess whether systemic rapamycin treatment is effective at producing physiologically-relevant reductions in mTORC1 signaling and AMPAR subunit levels within the NAC, we assessed mTORC1 activity, as well as GluA1 levels following treatment. To evaluate the specificity of these biochemical changes, we also assessed rapamycin-induced changes in mTORC1 activity in the dorsal striatum, another region known to be important for drug-seeking behaviors (Balleine et al., 2009, Corbit et al., 2014a, Corbit et al., 2014b, Everitt and Robbins, 2013).