Behavioural Analysis and Neurobiolology of Attention-Deficit/Hyperactivity Disorder (AD/HD) Terje Sagvolden, Department of Physiology, University of Oslo, Norway Operant analysis of behaviour has been shown to be a superior tool in the analysis of the inattentiveness, overactivity and impulsiveness presently regarded as the main clinical symptoms of AD/HD. We have been conducting a series of comparative studies using the Spontaneously Hypertensive Rat (SHR) as the animal model of AD/HD (for references and abstracts see: http://www.uio.no/~terjesa/publ.html). At the neurobiological level, AD/HD symptoms may to a large extent be caused by hypofunctioning dopamine systems: A hypofunctioning meso-cortical dopamine system may produce frontal cortical hypofunctioning resulting in "cognitive" impulsiveness. A hypofunctioning meso-limbic dopamine system may produce a shorter delay-of-reinforcement gradient, on a behavioural level giving rise to deficient sustained attention, hyperactivity and motor impulsiveness. A hypofunctioning nigro-striatal dopamine system will cause poor motor control (for figures see http://www.uclm.es/inabis2000/symposia/files/123/figure1.htm). The various dopaminergic systems might not be equally dysfunctional in all individuals with AD/HD. Further, these systems are not operating in isolation, but are closely linked to other neurotransmitter and neuromodulator systems. An imbalance in one system will inevitably, to some degree, also affect the functioning of the other systems. Dopamine hypofunctioning might in some cases be genetically determined while in other cases be induced by environmental factors like drugs of abuse or pollutants. This can explain why the prevalence of AD/HD differs between the various areas of the world: the prevalence of genetically-based AD/HD might well be the same everywhere, but not all areas are equally polluted by organic environmental pollutants. Key words:

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