Systemic Everolimus Does Not Block the Incubation of Methamphetamine-Craving: A Dose-Response Study

In rodent models, incubated craving for drugs and natural reinforcers is characterized by a time-dependent increase in cue-elicited operant responding, which posits that common biochemical mechanisms gate incubated craving across different types of reinforcers. Recently, our laboratory identified heightened mTOR activity in the prelimbic (PL) and infralimbic (IL) subregions of the medial prefrontal cortex (mPFC) as being associated with incubated craving for cocaine and sucrose, respectively. Importantly, Everolimus, an FDA-approved mTOR inhibitor, blocked the expression of incubated craving for both reinforcers. Hence, our current study investigated whether mTOR signalling also underpins cue-induced methamphetamine (METH)-seeking by evaluating the "therapeutic potential" of Everolimus to block incubated METH-craving. Male and female rats were trained under extended-access procedures (6-h/day × 10 days) to self-administer intravenous METH (0.1 mg/kg/0.1 mL infusion), paired with the presentation of a 20-s light + tone compound stimulus and then were subjected to a withdrawal period of 1 or 30 days (WD1 or WD30). Rats tested on WD30 were gavage-infused with either vehicle (VEH; 1% DMSO in water; vol: 1 mL/kg) or Everolimus (1 or 10 mg/kg), whereas WD1 rats were infused with VEH to index baseline craving. Thirty minutes later, rats underwent a 2-h test to measure cue-elicited responding. Akt/mTOR activity in the PL and IL was examined via immunoblotting procedures. Male and female rats exhibited comparable METH intake and craving, with no effect of estrous cycle observed for either measure in females. Unlike previous findings for incubated cocaine- and sucrose-craving, Everolimus did not reduce incubated METH-craving. Further, incubated METH-craving was not associated with changes in Akt/mTOR activation within mPFC subregions nor did Everolimus lower mTOR pathway activation below baseline levels in either METH-experienced or -naive rats. Together, these findings demonstrate that elevated mTOR signalling within mPFC is not involved in incubated METH-craving, highlighting the potential need for drug-specific targeted treatments for curbing incubated craving in protracted withdrawal.