Acute and chronic
methylphenidate (MPD) exposure was recorded si
multaneously for the rat's loco
motor activity and the nucleus accu
mbens (NAc) neuronal activity. The evaluation of the neuronal events was based on the ani
mal's behavior response to chronic MPD ad
ministration: 1) Ani
mals exhibiting behavioral sensitization, 2) Ani
mals exhibiting behavioral tolerance. The experi
ment lasted for 10 days with four groups of ani
mals; saline, 0.6, 2.5, and 10.0
mg/kg MPD. For the
main behavioral findings, about half of the ani
mals exhibited behavioral sensitization or behavioral tolerance to 0.6, 2.5, and/or 10
mg/kg MPD respectively. Three hundred and forty one NAc neuronal units were evaluated. Approxi
mately 80% of NAc units responded to 0.6, 2.5, and 10.0
mg/kg MPD. When the neuronal activity was analyzed based on the ani
mals' behavioral response to chronic MPD exposure, significant differences were seen between the neuronal population responses recorded fro
m ani
mals that expressed behavioral sensitization when co
mpared to the NAc neuronal responses recorded fro
m ani
mals exhibiting behavioral tolerance. Three types of neurophysiological sensitization and neurophysiological tolerance can be recognized following chronic MPD ad
ministration to the neuronal populations.
Collectively, these findings show that the same dose of chronic MPD can elicit either behavioral tolerance or behavioral sensitization. Differential statistical analyses were used to verify our hypothesis that the neuronal activity recorded from animals exhibiting behavioral sensitization will respond differently to MPD compared to those animals exhibiting behavioral tolerance, thus, suggesting that it is essential to record the animal's behavior concomitantly with neuronal recordings.