Although MPH is often viewed as a safe drug with little addiction potential, emerging evidence indicates that MPH abuse is common and increasing2,3,4. We report here that a rat model of MPH abuse produces profound alterations in dopamine neurotransmission that may enhance vulnerability to addiction. First, we show that MPH self-administration results in escalation of MPH intake and enhances the neurochemical potency and reinforcing effects of MPH and dopamine releasers, but not DAT blockers. Second, increases in DAT levels are sufficient to augment the potency of releasers and MPH, but not blockers. Third, although MPH is traditionally classified as a DAT blocker, it shares some functional properties with releasers. Finally, models of therapeutic use and abuse of MPH have completely different neurochemical consequences.

With respect to the first finding, MPH self-administration resulted in escalation of intake over sessions and resulted in enhanced potency of releasers and MPH. The increased rate of intake over sessions was shown to be associated with an increased motivation to administer MPH, as measured by a PR schedule of reinforcement, which is consistent with previous work suggesting that escalation of cocaine intake is due to increased motivation to administer drugs26. Because of the fixed injection maximum per session we imposed to reduce variability in intake, animals cannot escalate in total intake. However, first-hour intake was increased, which is a hallmark of traditional escalation8,9,10,11,26. This first-hour escalation is consistent with work using long-access conditions showing that MPH intake does escalate at a number of doses11. Thus, like cocaine and AMPH, MPH intake transitions from low to high levels of intake over time.

In addition to enhanced MPH reinforcement, MPH self-administration also resulted in increased motivation to administer AMPH, a dopamine releaser, but not cocaine, a DAT blocker. The increased motivation to administer MPH and AMPH is likely driven by increased MPH and AMPH potency at the DAT and concomitant increases in extracellular dopamine. We found that increased DAT levels, observed following MPH self-administration, were sufficient to elicit the selective increase in MPH and releaser potencies. Indeed, genetic overexpression of the DAT in mice, in the absence of pharmacological intervention, recapitulated the neurochemical and behavioural alterations associated with MPH self-administration. In addition to providing a possible mechanism for the MPH self-administration-induced augmentation of the reinforcing efficacy of releasers, this demonstrates that fluctuations in DAT levels, regardless of how they occur, will change the potency of releaser compounds.

Next, we find that DAT level changes alter the potency of dopamine releasers and MPH, but not DAT blockers, despite the general consensus in the field that shifts in cell surface DAT expression lead to inverse shifts in cocaine, but not AMPH, potency. This theory originates from two main sources. The first is that historically, cocaine elevates dopamine in the NAc shell, where DAT levels are low, to a much greater extent than that in the dorsal striatum, where DAT levels are high27,28. Second, cell culture work shows that DAT overexpression in cells results in a decrease in cocaine potency with no change in the AMPH potency14.

With respect to the first, the increased potency of cocaine in the shell relative to the caudate has been demonstrated in vivo with microdialysis, and there are a number of factors other than DAT numbers that could influence these results. For example, there are many differences between afferent inputs to caudate and NAc shell, including levels of serotonin and norepinephrine innervations, which are also cocaine targets and influence presynaptic dopamine release. These differences could greatly influence the regional specificity of cocaine effects on dopamine29. In addition, previous work using voltammetry in freely moving animals has shown that regional variations in the potency of cocaine are likely attributable to differences in D2 autoreceptor sensitivity in regulating dopamine release, and not differences in DAT levels30.

With respect to discrepancies with cell culture work, one possibility is that recording endogenous dopamine fluctuations with voltammetry may produce different results than cell culture studies using exogenous [3H]-DA to measure uptake and uptake inhibition14. It is possible that [3H]-DA is sequestered into intracellular compartments differently than endogenous DA, and that releaser compounds interact differently with these compartments. Alternatively, it should be noted that many cell culture studies have determined the absolute effects of psychostimulants on dopamine levels, while not necessarily taking into account baseline rates of uptake in the overall effects of these drugs. This is particularly relevant as substantial work has suggested that behavioural outcomes are dependent on a change relative to baseline, not the absolute dopamine levels in isolation31,32,33. Voltammetry determines the effects of the drug while accounting for baseline uptake rates, and this approach correlates well with behavioural outcomes not only in this study but in previous work as well23.

The DAT-dependent changes in psychostimulant potencies are particularly relevant in clinical treatment settings because disorders such as post-traumatic stress disorder and ADHD are associated with increased DAT levels, ranging from 17 to 70% (refs 15, 16, 17, 18). These people could be more sensitive to the neurochemical, behavioural and neurotoxic effects of prescribed AMPH and MPH. In addition, these findings may partially explain the high frequency of drug abuse in untreated ADHD sufferers34.

Third, this work suggests that MPH is unique in the way in which it interacts with the DAT, and that the characterization of MPH solely as a prototypical DAT blocker may need revision. Indeed, although MPH is considered a DAT blocker, the increased behavioural and neurochemical potency of MPH following both MPH self-administration and DAT overexpression resembles the shift in AMPH’s effects, and not cocaine’s. Moreover, the acute effects of MPH following cocaine self-administration have previously been shown to be similar to releasers and not blockers. For example, Ferris et al.35 found that cocaine self-administration resulted in DAT tolerance to all blockers, but not releasers or MPH. In addition to the evidence provided by our laboratory, others have shown that, although MPH is not a substrate for the DAT36, it functions as a releaser at high concentrations37,38. In addition, MPH binds to the DAT in a manner that is distinct from prototypical blockers or releasers39,40, with significant overlap between the binding sites for both cocaine and AMPH. It is possible that although MPH is not transported into the cell, the interaction with the AMPH site results in conformational changes in the DAT, which promote reverse transport in the same way as AMPH. More work is needed to clarify the molecular mechanisms unique to MPH, but our findings suggest that MPH shares some characteristics of both blockers and releasers.

Our fourth major finding was that oral administration of low-dose MPH had different neurochemical consequences than self-administration of high-dose MPH. We found that oral administration of low, therapeutic doses had no discernible neurochemical consequences on the dopamine system. This is consistent with other work showing no changes in dopamine kinetics or stimulant potencies following low-dose therapeutic administration of MPH41,42. This differential effect of MPH at low and high doses may be due to the fact that high-dose MPH substantially elevates dopamine levels in the NAc, whereas low-dose MPH does not43,44. In addition, changes associated with MPH abuse models may be due to their higher doses and rapid onset, compared with therapeutic administration.

In summary, this study shows that MPH self-administration elevates DAT levels and leads to enhanced MPH and dopamine releaser potency. We showed that changes in DAT levels alone were sufficient to alter the potency of psychostimulant releasers and MPH, but not blockers. Thus, MPH is likely increasing DAT levels in individuals that are abusing the drug, which may lead to increased potency for dopamine uptake inhibition, neurotoxicity, and abuse potential for releasers and MPH. In addition, this study demonstrates that MPH potency is altered by transporter fluctuations in a manner that is not significantly different from releasers, although it is not a substrate for the DAT. Thus, we have discovered novel properties of MPH interactions with the DAT, which coincide with novel neuroadaptations following high-dose exposure.