Neurobehavioral adaptations to methylphenidate: the issue of early adolescent exposure

Exposure to psychostimulants, including both abused and therapeutic drugs, can occur first during human adolescence. Animal modeling is useful not only to reproduce adolescent peculiarities but also to study neurobehavioral adaptations to psychostimulant consumption. Human adolescence (generally considered as the period between 9/12 and 18 years old) has been compared with the age window between postnatal days (pnd) 28/35 and 50 in rats and mice. These adolescent rodents display basal hyperlocomotion and higher rates of exploration together with a marked propensity for sensation-seeking and risk-taking behaviors. Moreover, peculiar responses to psychostimulants, including enhanced locomotor sensitization, no drug-induced stereotypy and reduced place conditioning have been described in adolescent rodents. During this age window, forebrain dopamine systems undergo profuse remodeling, thus providing a neuro-biological substrate to explain behavioral peculiarities observed during adolescence, as well as the reported vulnerabilities to several drugs. Further, methylphenidate (MPH, better known as Ritalin®), a psychostimulant extensively prescribed to children and adolescents diagnosed with attention-deficit/hyperactivity disorder (ADHD), raises concerns for its long-term safety. Using magnetic resonance techniques, MPH-induced acute effects appear to be different in adolescent rats compared to adult animals. Moreover, adolescent exposure to MPH seems to provoke persistent neurobehavioral consequences: long-term modulation of self-control abilities, decreased sensitivity to natural and drug reward, enhanced stress-induced emotionality, together with an enhanced cortical control over sub-cortical dopamine systems and an enduring up-regulation of Htr7 gene expression within the nucleus accumbens (NAcc). In summary, additional studies in animal models are necessary to better understand the long-term consequences of adolescent MPH, and to further investigate the safety of the prescription and administration of such pharmacological treatment at early life stages.