Methylphenidate (MPH) is the most commonly prescribed drug to treat Attention Deficit Hyperactivity Disorder (ADHD).
ADHD is now considered a life-long disorder; therefore, patients take MPH from adolescence into adulthood, highlighting the need for research studying chronic MPH use. MPH increases dopamine and norepinephrine within the synaptic cleft; therefore, chronic use of MPH may lead to changes within important dopaminergic pathways.
One pathway, the mesolimbic pathway, includes the hippocampus, an area where adult neurogenesis occurs. We investigated the effects of chronic low and high doses of MPH on neurogenesis and examined levels of a few key proteins linked to cell proliferation in the hippocampus.
Low dose MPH appears to increase cell proliferation and cell survival in the hippocampus, and these effects are accompanied by increases in vascular endothelial growth factor (VEGF), the receptor for brain-derived neurotrophic factor (TrkB), and beta-catenin.
While high dose MPH may initially increase neuronal proliferation, newly-generated neurons are unable to survive long-term, and decreases in VEGF, TrkB, and beta-catenin are observed with chronic high dose MPH.
Another major dopaminergic pathway is the nigrostriatal pathway, which is involved in motor control and degenerates with Parkinson’s disease.
Chronic use of MPH appears to sensitize dopaminergic neurons within this pathway to the Parkinsonian toxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), but the cause of this sensitization is unknown.
The autooxidation of excess dopamine forms dopamine-quinones that lead to free radical production, but the antioxidant, glutathione, can protect neurons.
However, we showed that chronic MPH increases dopamine-quinone formation and causes a subsequent glutathione depletion within the striatum.
Therefore, oxidative stress may sensitize dopamine neurons to MPTP.
We also assessed the vulnerability of dopaminergic neurons in the nigrostriatal pathway to MPTP after chronic MPH in females.
Interestingly, proestrus (high estrogen) females were more sensitive to MPTP than anestrus (low estrogen) females.
Similar to males, chronic MPH caused a depletion in glutathione that was further decreased following MPTP exposure.
However, chronic MPH did not significantly alter dopaminergic neuronal numbers or quinone formation in females. These studies highlight some of the potential effects of chronic MPH use.