ADHD-like hyperactivity, with no attention deficit, in adult rats after repeated hypoxia during the equivalent of extreme prematurity

The most common behavioural disorder seen in children and adolescents born extremely prematurely is attention deficit hyperactivity disorder (ADHD). The hyperactive/impulsive sub-type of ADHD or the inattentive sub-type or the hyperactive/impulsive/inattentive sub-type can be evident. These sub-types of ADHD can persist into adulthood. The aim of this study was to investigate the relevance of a new immature rat model of repeated hypoxic exposure to these behavioural characteristics of extreme prematurity. More specifically, this study aimed to measure ADHD-like hyperactivity in response to delayed reward, and inattention, in repeated hypoxic versus repeated normoxic rats. Sprague-Dawley rats were exposed to either repeated hypoxia or repeated normoxia during postnatal days (PN) 1–3. The rat brain during PN1–3 is generally considered to be developmentally equivalent to the human brain during extreme prematurity. The rats were then behaviourally tested at 16 months-of-age on a multiple component fixed interval–extinction test. This test detects ADHD-like hyperactivity in response to delayed reward, as well as inattention. It was found that the repeated hypoxic rats exhibited ADHD-like hyperactivity in response to delayed reward, but no attention deficit, when compared to repeated normoxic rats. These findings provide a new animal model to investigate the biological mechanisms and treatment of ADHD-like hyperactivity due to repeated hypoxia during the equivalent of extreme prematurity.