Pharmacological models of ADHD.

For more than 50 years, heavy metal exposure during pre- or post-natal ontogeny has been known to produce long-lived hyperactivity in rodents. Global brain injury produced by neonatal hypoxia also produced hyperactivity, as did (mainly) hippocampal injury produced by ontogenetic exposure to X-rays, and (mainly) cerebellar injury produced by the ontogenetic treatments with the antimitotic agent methylazoxymethanol or with polychlorinated biphenyls (PCBs). More recently, ontogenetic exposure to nicotine has been implicated in childhood hyperactivity. Because attention deficits most often accompany the hyperactivity, all of the above treatments have been used as models of attention deficit hyperactivity disorder (ADHD). However, the causation of childhood hyperactivity remains unknown. Neonatal 6-OHDA-induced dopaminergic denervation of rodent forebrain also produces hyperactivity - and this model, or variations of it, remain the most widely-used animal model of ADHD. In all models, amphetamine (AMPH) and methylphenidate (MPH), standard treatments of childhood ADHD, typically attenuate the hyperactivity and/or attention deficit. On the basis of genetic models and the noted animal models, monoaminergic phenotypes appear to most-closely attend the behavioral dysfunctions, notably dopaminergic, noradrenergic and serotoninergic systems in forebrain (basal ganglia, nucleus accumbens, prefrontal cortex). This paper describes the various pharmacological models of ADHD and attempts to ascribe a neuronal phenotype with specific brain regions that may be associated with ADHD.

Proposed animal model of severe Parkinson's disease: neonatal 6-hydroxydopamine lesion of dopaminergic innervation of striatum.

Rats lesioned shortly after birth with 6-hydroxydopamine are posed as a near-ideal model of severe Parkinson's disease, because of the non-lethality of the procedure, near-total destruction of nigrostriatal dopaminergic fibers, near-total dopamine (DA)-denervation of striatum, reproducibility of effect, and relative absence of overt behavioral effects--there is no aphasia, no adipsia, and no change in motor activity. In vivo microdialysis findings reinforce the utility of the animal model, clearly demonstrating L-DOPA beneficial actions without an increase in hydroxyl radical production.

Peculiarities of L: -DOPA treatment of Parkinson's disease.

L-Dihydroxyphenylalanine (L: -DOPA), the anti-parkinsonian drug affording the greatest symptomatic relief of parkinsonian symptoms, is still misunderstood in terms of its neurotoxic potential and the mechanism by which generated dopamine (DA) is able to exert an effect despite the absence of DA innervation of target sites in basal ganglia. This review summaries important aspects and new developments on these themes. On the basis of L: -DOPA therapy in animal models of Parkinson's disease, it appears that L: -DOPA is actually neuroprotective, not neurotoxic, as indicated by L: -DOPA's reducing striatal tissue content of the reactive oxygen species, hydroxyl radical (HO(*)), and by leaving unaltered the extraneuronal in vivo microdialysate level of HO(*). In addition, the potential beneficial anti-parkinsonian effect of L: -DOPA is actually increased because of the fact that the basal ganglia are largely DA-denervated. That is, from in vivo microdialysis studies it can be clearly demonstrated that extraneuronal in vivo microdialysate DA levels are actually higher in the DA-denervated vs. the intact striatum of rats - owing to the absence of DA transporter (i.e., uptake sites) on the absent DA nerve terminal fibers in parkinsonian brain. In essence, there are fewer pumps removing DA from the extraneuronal pool. Finally, the undesired motor dyskinesias that commonly accompany long-term L: -DOPA therapy, can be viewed as an outcome of L: -DOPA's sensitizing DA receptors (D(1)-D(5)), an effect easily replicated by repeated DA agonist treatments (especially agonist of the D(2) class) in animals, even if the brain is not DA-denervated. The newest findings demonstrate that L: -DOPA induces BDNF release from corticostriatal fibers, which in-turn enhances the expression of D(3) receptors; and that this effect is associated with motor dyskinesias (and it is blocked by D(3) antagonists). The recent evidence on mechanisms and effects of L: -DOPA increases our understanding of this beneficial anti-parkinsonian drug, and can lead to improvements in L: -DOPA effects while providing avenues for reducing or eliminating L: -DOPA's deleterious effects.