Research reportThe spontaneously hypertensive rat/Izm (SHR/Izm) shows attention deficit/hyperactivity disorder-like behaviors but without impulsive behavior: Therapeutic implications of low-dose methylphenidate

- SHR/Izm is a SHR sub-strain derived from stroke-resistant SHR.
- SHR/Izm shows hyperactivity and inattention but not anxiety-related impulsivity.
- A low dose of methylphenidate ameliorates ADHD-like behaviors in SHR/Izm.
- SHR/Izm is a unique animal model of ADHD that lacks impulsivity.

The spontaneously hypertensive rat (SHR) has been used as a genetic animal model of attention deficit/hyperactivity disorder (ADHD). SHR/Izm is derived from stroke-resistant SHR as SHR/NIH and SHR/NCrl but from 22nd to 23rd generation descendants of the SHR/NIH ancestor and therefore may show different behavioral phenotypes compared to other SHR sub-strains. In this study, ADHD-like behaviors in SHR/Izm were evaluated compared to Wistar rats. SHR/Izm showed high locomotor activity in the habituation phase in a novel environment, although locomotor activity in the initial exploratory phase was low. In a behavioral test for attention, spontaneous alternation behavior in the Y-maze test was impaired in SHR/Izm. However, impulsive behavior in the elevated-plus maze test, which is designed to detect anxiety-related behavior but also reflects impulsivity for novelty seeking, was comparable to Wistar rats. Hyperactivity and inattention, detected as ADHD-like behaviors in SHR/Izm, were ameliorated with methylphenidate at a low dose (0.05 mg/kg, i.p.). Therefore, SHR/Izm represents a unique animal model of ADHD without anxiety-related impulsive behavior.