Long-term effects of chronic treatment for ADHD on cognition and neural development in an adolescent rat model

Attention Deficit/Hyperactivity Disorder (ADHD) has become the most commonly diagnosed childhood disorder. However, ADHD diagnosis involves subjective interpretation of diagnostic criterion which could lead to misdiagnosis. ADHD symptoms are thought to arise due to irregularities in the function of the neurotransmitters dopamine and noradrenaline. In order to normalise these irregularities, powerful psychostimulant medications such as Ritalin® (methylphenidate, MPH) are front line treatments for this disorder. MPH is generally administered over long periods of time where the onset and duration of chronic treatment corresponds with significant periods in brain development and maturation. This thesis was conducted to 1) investigate the long-term effects of chronic MPH treatment during adolescence on cognitive and neural development of misdiagnosed individuals; and 2) investigate the neural mechanisms underlying MPH's impact on impulsive behaviours. -- Appropriate animal models are necessary to conduct these investigations. The Spontaneously Hypertensive rat (SHR) has been extensively studied and is a commonly used animal model for ADHD. However, the methodology of previous research was recently criticized by Alsop (2007). The aim of the first experimental chapter of this thesis (Chapter 2) was to re-assess the validity of the SHR as an animal model of ADHD, in light of Alsop's criticisms. The results showed that the SHR had increased locomotor activity and were more sensitive to increases in delay in an impulsivity task when compared to the control strain, Wistar-Kyoto rats (WKY). These findings provide further support for the SHR as a valid animal model of ADHD. -- Previous pre-clinical research has employed drug administration methods which limit the generalisation of their findings to the human condition. When children are treated with MPH they receive low oral doses, whilst the majority of pre-clinical research administers large doses of MPH via intraperitoneal injection. The aim of the second experimental study (Chapter 3) was to develop an oral method of drug administration that was less invasive than dosing by gavage and that required minimal training. For this, the rats learned to consume a MPH suspension through a drinking spout. Following this novel oral drug administration, there was a dose-dependent increase in locomotor activity that was similar in effect to MPH administration by gavage. -- The following experimental study (Chapter 4) incorporated this novel oral administration method to assess the long-term effects on cognitive development of chronic MPH treatment during adolescence. While the animal model of ADHD is the SHR, the focus of this study was on the WKY as the non-ADHD or misdiagnosed strain. The rats were orally treated with either MPH or distilled water over 4 weeks throughout adolescence (PND 27 - 52). MPH was administered twice daily to model clinical dosing schedules in children. Locomotor activity was measured at the beginning of each week of treatment and cognitive-behavioural tests were completed in adulthood after cessation of treatment. The findings of this study suggest there are enduring behavioural changes in adulthood when rats inappropriately received chronic MPH treatment throughout adolescence. However, when chronic MPH treatment was appropriately given to the ADHD rats, there were no long-term effects observed in adulthood. -- The effects of chronic MPH treatment on neural development were assessed in the following study (Chapter 5). Following 4 weeks of MPH treatment, immunostaining for Tyrosine Hydroxylase (TH) positive neurons in the prefrontal cortex (PFC) was performed at 3 stages. Group 1 and Group 2 were euthanized 1 week and 12 weeks, respectively, after cessation of treatment for neural tissue analysis. Group 3 (from Chapter 4) were euthanized upon completion of cognitive-behavioural testing for neural tissue analysis (12 weeks post treatment). The results suggest that pre-exposure to MPH in non-ADHD rats may interfere with the maturation of the PFC and may subsequently alter future neural adaptations to behavioural experiences. -- The final experimental chapter of this thesis was conducted to elaborate on the neurochemistry which may underlie the persistent behavioural changes in the adult WKY. Specifically, the final study investigated the role of dopamine and noradrenaline on impulsivity mediated by regions within the PFC (Chapter 6). Alterations in impulsivity were assessed following local infusions of dopamine antagonists or noradrenaline agonists into regions of the PFC. The results indicated that blockade of different dopamine receptors increased impulsivity depending on their location within the PFC, while noradrenaline receptor activation of the PFC was found to have no impact on impulsivity. -- In conclusion, using a method of drug administration that closely models clinical treatment in children, the findings of this thesis suggests inappropriate chronic childhood treatment with MPH has long-term effects on cognition and may interfere with brain maturation. Furthermore, the potential cause of these deficits may be alterations in DA functioning. The findings of this thesis highlight the need for more stringent diagnostic criteria for ADHD.