Glutamate and dopamine in the VTA participate differently in the acute and chronic effect of methylphenidate

Psychostimulants such as methylphenidate (MPD) have long been the treatment of choice in behavioral disorders such as attention deficit/hyperactivity disorder (ADHD) and narcolepsy in both children and adults. However, its abuse by healthy children and adults for academic enhancement or recreation is on the rise. This raises concern for brain chemistry alteration leading to dependence during a period of neuroplasticity and brain development. Psychostimulants such as MPD are indirect dopamine antagonists and are known to act on the dopaminergic system of the brain to produce their effects. The ventral tegmental area (VTA) is one of the primary sources of dopamine in the CNS and is a part of the reward circuits affected by MPD. In order to elucidate the role of the VTA in MPD exposure, five groups of rats were used: VTA intact control, sham VTA surgery, nonspecific electrolytic VTA lesion, glutamatergic specific VTA chemical lesion, and dopaminergic specific VTA chemical lesion. Baseline locomotor activity was established, then the surgeries were performed followed by several days of recovery and establishment of post-surgical baseline. Following the recovery period, the rats were challenged with 6 days of MPD exposure, followed by 3 washout days, then a re-challenge of MPD to assess chronic MPD exposure on animals behavior. Locomotive activity was recorded for 120 min after each injection by a computerized animal activity monitor system. The results indicate that glutamatergic synapses in the ventral tegmental area are critical for acute and chronic MPD response, while dopaminergic synapses contribute to tonic inhibition of the ventral tegmental area on rat locomotor excitation.