Sex differences in operant discrimination behaviour in an animal model of attention-deficit hyperactivity disorder

The present study was aimed at determining whether the behaviour of the spontaneously hypertensive rat (SHR), an animal model of attention-deficit hyperactivity disorder (ADHD), showed sex differences parallel to those seen in ADHD children. The experimental protocol contained an operant discrimination task, a two-component multiple (mult) 2-min fixed interval (FI) 5-min extinction (EXT) schedule of water reinforcement, a reliable behavioural paradigm for testing activity levels, discrimination behaviour and impulsiveness. The results indicated that both male and female SHRs show some of the most important behavioural traits of ADHD. Both were hyperactive and showed discrimination problems in terms of a behavioural extinction deficit towards the end of the EXT component. Still their behaviour differed markedly, which was probably due to quite different underlying mechanisms. The behavioural characteristics of the female SHRs may be compatible with an attention-deficit interpretation, whereas the behavioural characteristics of the male SHRs may be due to a shorter than normal delay-of-reinforcement gradient. The present study strengthens the position of SHR as an animal model of ADHD for future studies that may elucidate details in the underlying neurobiological deficits and for testing various treatment strategies.