Hypodopaminergic and hypernoradrenergic activity in prefrontal cortex slices of an animal model for attention-deficit hyperactivity disorder--the spontaneously hypertensive rat.

Evidence supports dysfunction of dopaminergic and noradrenergic systems in patients with attention-deficit hyperactivity disorder (ADHD). Noradrenergic and dopaminergic systems exert distinct modulatory actions on the transfer of information through neural circuits that connect functionally distinct cortical areas with separate striatal regions and remain segregated in parallel striato-pallidal-thalamic and striato-substantia nigra pars reticulata-thalamic pathways. Prefrontal cortex performance is maximal at moderate stimulation of postsynaptic dopaminergic and noradrenergic receptors, and is reduced by either higher or lower levels of receptor stimulation. Spontaneously hypertensive rats (SHR) are generally considered to be a suitable genetic model for ADHD, since they display hyperactivity, impulsivity, poor stability of performance, impaired ability to withhold responses and poorly sustained attention, when compared with their normotensive Wistar-Kyoto (WKY) control rats. Evidence suggests that terminals of mesocortical, mesolimbic and nigrostriatal dopaminergic neurons of SHR release less dopamine in response to electrical stimulation and/or depolarization as a result of exposure to high extracellular K+ concentrations, than WKY. Vesicular storage of dopamine was suggested to be impaired in SHR, causing leakage of dopamine into the cytoplasm and increased d-amphetamine-induced transporter-mediated release. While electrically stimulated release of dopamine appears to be decreased in prefrontal cortex of SHR suggesting hypodopaminergic function, autoreceptor-mediated inhibition of norepinephrine release appears to be impaired in SHR, suggesting that noradrenergic function may be poorly regulated in the prefrontal cortex of the SHR. These findings are consistent with the hypothesis that the behavioral disturbances of ADHD are the result of an imbalance between noradrenergic and dopaminergic systems in the prefrontal cortex, with inhibitory dopaminergic activity being decreased and noradrenergic activity increased relative to controls.