Methylphenidate and the Salience of a Novel Object
Authors:Clinton Chapman, Alexander Posell
- Clinton Chapman, Assistant Professor of Psychology, Occidental College
- Nancy Dess, Professor of Psychology, Occidental College
Methylphenidate (MPD), commonly known as Ritalin, is the most commonly prescribed medication for Attention Deficit/Hyperactivity Disorder in the United States. A dopamine (DA) reuptake inhibitor, MPD’s effects include reduction of impulsive behaviors, increased concentration and, in some cases, amphetamine-like arousal. The Occidental high saccharin-preferring (HiS) and low saccharin-preferring (LoS) rats were bred along differential preference for the taste of saccharin solution; the two lines also display robust behavioral differences, including reactivity to stressors (LoS > HiS), impulsivity (HiS > LoS), and arousal levels (LoS > HiS). We explored the effects of MPD upon the two lines’ responses to novel object presentation in an “open-field” apparatus. It was hypothesized that MPD would show distinct arousal-increasing effects versus controls, but that novel object investigation would be reduced, with the latter effect being more pronounced in the HiS rats. Doses of 1.0, 5.0, and 10.0 mg/kg were administered.
LoS rats showed very little change in tendency to investigate the novel object regardless of dose, while, contrary to expectations, HiS rats showed a positive and nearly linear dose-response increase in object investigation. LoS had a higher baseline arousal state than HiS, and this remained the case regardless of dose, with both lines displaying maximum locomotion at 5.0 mg/kg.
It is very likely that excitation of distinct dopamine pathways contributed to the differential effects observed; mesocortical and mesolimbic pathways are involved with reward and attention, while the nigrostriatal pathway is primarily involved in motor response. HiS rats may have a higher sensitivity to DA in the former two pathways, while the lines may not differ significantly in the latter pathway.