It depends. (Keep in mind that lower levels of dopamine D2 receptors has been associated with addiction.):
After two months of treatment, and again after eight months, the scientists performed positron emission tomography (PET) scans to measure the levels of dopamine D2 receptors, a type of brain receptor important for experiencing reward and pleasure that has been linked to pleasure and drug abuse. After the eight-month treatment, animals were also tested for their propensity to self-administer cocaine.
Rats given the 2mg/kg dose of methylphenidate were significantly less likely to press a lever to self-administer cocaine, and received fewer self-initiated infusions of the drug following eight months of treatment than the lower-dose group or the control rats.
The changes observed in brain chemistry were specific to the age and duration of methylphenidate treatment: Specifically, after two months of treatment, brain scans revealed that both groups of treated rats had lower levels of dopamine D2 receptors in their brains than did control animals.
In contrast, after eight months of treatment, the brain scans revealed elevated levels of dopamine D2 receptors in treated rats compared with controls, with the higher-dose treatment group showing the highest level of D2 receptors. In the control group, D2 receptor levels declined with age. Research at Brookhaven and elsewhere has suggested that low levels of dopamine D2 receptors may increase the likelihood of drug abuse, while elevated levels of dopamine D2 receptors may attenuate the propensity to abuse drugs.
Read the rest here.