The spontaneous hypertensive rat - model for the human Attention
Deficit / Hyperactivity Disorder
Department of Neurophysiology, University of Oslo, Norway.
Childhood Hyperkinesis, also known as Attention Deficit/Hyperactivity
Disorder (AD/HD) affects at least 3 per cent of all children in all
countries. The causes of this disorder as well as the mechanism(s) of
treatment are poorly understood. There is no brain damage in AD/HD, but a
genetic factor gives rise to neurochemical imbalances that cause the
behavioural problems: deficits in sustained attention (inability to maintain
an attentional set), overactivity (inappropriate motor activity),
impulsiveness, and an increased variability in all behaviour. Impulsiveness
is increasingly seen as a key characteristic of the disorder. The
behavioural problems may arise from alterations in the effectiveness of
delayed reinforcers (rewards). Children with AD/HD prefer an immediate
reward and cannot wait for a delayed one. The symptoms are reduced by
psychomotor stimulating drugs like d-amphetamine and methylphenidate.
The Spontaneously Hypertensive Rat (SHR) is a putative animal model that may
be used in order to study in detail the behaviour, drug effects and
neurobiology of ADD. SHR has (in simple forms) all the major symptoms of
AD/HD. Behavioural analyses of the model showed increased efficacy of
reinforcers, but not higher maximal response rates in the hyperactives.
Neurobiological studies of the animal model show that reduced catecholamine
neurotransmission may be the neuronal basis of the symptoms. Such a
shortage may explain why methylphenidate is the drug of choice: this drug
enhances catecholamine neurotransmission.
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