Exposure to methylphenidate in adolescence and adulthood modulates cross-sensitization to amphetamine in adulthood in three genetically variant female rat strains.

Attention Deficit Hyperactivity Disorder (ADHD) is a developmental, behavioral disorder that is characterized by patterns of impulsivity and limited attention. Stimulants, such as methylphenidate (MPD) and amphetamine (Amph), are utilized as first-line agents in the treatment of ADHD. While Amph is known to elicit dependence, MPD is not. Interdependence between MPD and Amph is a growing concern. Therefore, it is important to explore this interaction in animal models to gain insight on its mechanisms. In experimental studies, drugs that elicit behavioral sensitization and cross-sensitization in animals are considered to have the potential to elicit dependence. This study evaluated whether chronic repetitive MPD exposure in adolescence and/or adulthood in various genetic strains of female rats elicits behavioral sensitization as well as cross-sensitization with Amph. We used three strains: SHR (ADHD model), WKY, and SD rats. The three rat strains were exposed to chronic MPD in their adolescence and adulthood or exposed to chronic MPD only in adulthood. When the female rats were treated with MPD only in adulthood, the SHR strain exhibited cross-sensitization to Amph but the WKY and SD strains did not. Whereas, when the female rats were pretreated with MPD in adolescence and adulthood, the SHR and WKY strains exhibited cross-sensitization response to Amph, but the SD strain did not. This study showed that the genetic strain and age of exposure to MPD plays a crucial role in cross-sensitization to Amph in female rodents. Furthermore, we showed that genetics associated with ADHD pre-dispose animals to dependence between drugs even if the exposure starts in adulthood compared to control strains that did not show dependence with adult only MPD exposures. Genetic variability, age of initial drug exposure, and sex of the subject are key variables that should be accounted in studies that explore effects of psychostimulants.