John P. Kostrzewa

Pharmacological models of ADHD

SummaryFor 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.

Peculiarities of L-DOPA treatment of Parkinson’s disease

Summary.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 (D1–D5), an effect easily replicated by repeated DA agonist treatments (especially agonist of the D2 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 D3 receptors; and that this effect is associated with motor dyskinesias (and it is blocked by D3 antagonists). The recent evidence on mechanisms and effects of L-DOPA increases our understanding of this benefical anti-parkinsonian drug, and can lead to improvements in L-DOPA effects while providing avenues for reducing or eliminating L-DOPA’s deleterious effects.

Neuroprotective and neurotoxic roles of levodopa (L-DOPA) in neurodegenerative disorders relating to Parkinson's disease

Summary. Despite its being the most efficacious drug for symptom reversal in Parkinsons disease (PD), there is concern that chronic levodopa (L-DOPA) treatment may be detrimental. In this paper we review the potential for L-DOPA to 1) autoxidize from a catechol to a quinone, and 2) generate other reactive oxygen species (ROS). Overt toxicity and neuroprotective effects of L-DOPA, both in vivo and in vitro, are described in the context of whether L-DOPA may accelerate or delay progression of human Parkinsons disease.

Dopamine D2 agonist priming in intact and dopamine-lesioned rats

Receptor priming is a recently discovered phenomenon by which receptor agonists produce abrupt and long-lived supersensitization of receptors. Induction of dopamine (DA) D2 receptor supersensitivity by the agonist quinpirole was discovered approximately 15 years ago, and was found to occur consistently if rats were treated repeatedly at daily or weekly or monthly intervals with low or high doses of quinpirole. In this review we summarize and discuss some of the major studies that underlie DA D2 receptor supersen-sitivity, describe behavioral processes that are known to be altered by DA D2 receptor supersensitivity, and discuss the importance of DA innervation on expression of enhanced behaviors. DA D2 receptor supersen-sitivity represents one of the neural mechanisms implicated in psychiatric disorders. Also, DA D2 receptor supersensitivity and increased DA D3 receptor expression are associated with motor dyskinesias, as in L-DOPA-treated Parkinson’s disease patients. An understanding of receptor priming, a knowledge of the types of behavioral expression associated with DA D2 receptor supersensitivity, and an understanding of mechanisms associated with receptor supersen-sitization, can lead to improvements in the treatments of psychiatric and neurological disorders.

Ontogenetic quinpirole treatments fail to prime for D2 agonist-enhancement of locomotor activity in 6-hydroxydopamine-lesioned rats

Repeated treatments with a dopamine (DA) D2 receptor agonist result in the induction of DA D2 receptor supersensitivity, as evidenced by enhanced behavioral responses to subsequent D2 agonist treatments—a phenomenon known aspriming of receptors. Priming of D2 receptors has been well-studied in otherwise intact (non-lesioned) rats. In contrast to D2 priming, repeated treatments with a DA D1 agonist are unable to prime D1 receptors unless nigrostriatal DA fibers are largely destroyed in early postnatal ontogeny. In order to determine if D2 receptors could be primed in rats in which nigrostriatal DA fibers were largely destroyed in early postnatal ontogeny, rats were (a) lesioned at 3 days after birth with 6-hydroxydopamine (67 μg in each lateral ventricle; desipramine, 20 mg/kg IP, 1 h; 6-OHDA), (b) treated daily for the first 28 days after birth with the D2 agonist quinpirole HCl (3.0 mg/kg IP), and (c) observed in adulthood for both quinpirole-induced and SKF 38393- (D1 agonist-) induced locomotor activity and stereotyped activities. In 6-OHDA-lesioned rats in which endogenous striatal DA was reduced by 99%, quinpirole did not produce enhanced locomotor or stereotyped activities. However, SKF 38393 produced increased locomotor and stereotyped activities even after the first dose of SKF 38393. These findings demonstrate that D2 receptors are not primed by ontogenetic quinpirole treatments of neonatally 6-OHDA-lesioned rats, although D2 agonist treatments do at least partially prime D1 receptors in 6-OHDA-lesioned rats.

Dopamine receptor supersensitivity: Development, mechanisms, presentation, and clinical applicability

The process of receptor supersensitivity (RSS) has a long history and is an epiphenomenon of neuronal denervation. Dopamine (DA) RSS (DARSS) similarly occurs after DA-denervation, and this process is invoked in neuropsychiatric and neurodegenerative disorders. From studies largely over the past 25 years, much has been learned regarding DARSS. For example, overt D1 DARSS occurs after perinatal destruction of nigrostriatal DA fibers. However, following perinatal destruction of DA innervation, the mostprominent behavioral effects of a D1 agonist are observed after a series of D1 agonist treatments- a process known aspriming of D1DA receptors. Moreover, perinatal lesioning of DA fibers produces prominent serotonin (5-HT) RSS, and in fact 5-HT RSS appears to modulate D1 DA RSS. In rodents, receptor supersensitization by these means appears to be irreversible. In contrast to the observedD1 DARSS, D2 DARSS apparently does not occur after perinatal DA denervation. Also, while repeated D1 agonist treatment of intact rats has no observable effect, repeated D2 agonist treatments, during or after the ontogenetic phase, produces prominent life-long D2 RSS. The process may have an association with substance abuse. Therefore, production of D1 and D2 DARSS occurs by different means and under different circumstances, and in association with perhaps different neuronal phenotypes, and with greater incidence in either intact (D2) or DA-lesioned counterparts (D1). The physiological consequence of RSS are multiple.

Dopaminergic denervation enhances susceptibility to hydroxyl radicals in rat neostriatum

Summary. To determine if greater amounts of hydroxyl radical (•OH) are formed by dopamine (DA) denervation and treatment with L-dihydroxyphenylalanine (L-DOPA), the neostriatum was DA denervated (99% reduction in DA content) by 6-hydroxydopamine treatment (134 μg icv, desipramine pretreatment) of neonatal rats. At 10 weeks the peripherally restricted dopa decarboxylase inhibitor carbidopa (12.5 mg/kg i.p.) was administered 30 min before vehicle, L-DOPA (60 mg/kg i.p.), or the known generator of reactive oxygen species, 6-hydroxydopa (6-OHDOPA) (60 mg/kg i.p.); and this was followed 30 min later (and 15 min before termination) by the spin trap, salicylic acid (8 μmoles icv). By means of a high performance liquid chromatographic method with electrochemical detection, we found a 4-fold increase in the non-enzymatically formed spin trap product, 2,3-dihydroxybenzoic acid (2,3-DHBA), with neither L-DOPA nor 6-OHDOPA having an effect on 2,3-DHBA content of the neostriatum. Basal content of 2,5-DHBA, the enzymatically formed spin trap product, was 4-fold higher vs. 2,3-DHBA in the neostriatum of untreated rats, while L-DOPA and 6-OHDOPA each reduced formation of 2,5-DHBA. We conclude that DA innervation normally suppresses •OH formation, and that the antiparkinsonian drug L-DOPA has no effect (2,3-DHBA) or slightly reduces (2,5-DHBA) •OH formation in the neostriatum, probably by virtue of its bathing the system of newly formed •OH.

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 Parkinsons 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.

Dopamine receptor supersensitivity: an outcome and index of neurotoxicity.

The characteristics feature of neurotoxicity is a definable lesion which can account for observed deficits, corresponding to loss of nuclei or axonal fibers normally comprising a specific pathway or tract. However, with ontogenetic lesions, the operative definition fails. In rats lesioned as neonates with 6-hydroxydopamine (6-OHDA), near-total destruction of dopamine-(DA-) containing nerves is produced, and this itself is definable. However, the most prominent feature of rats so-lesioned is the DA receptor supersensitivity (DARSS) that develops and then persists throughtout the lifespan. DA D1 receptors show overt supersensitivity to agonists producing vacuous chewing movements (VCMs), while D1 receptors associated with locomotor activity have a latent supersensitivity that must be unmasked by repeated D1 or D2 agonist treatments — a ‘priming’ phenomenon. This D1 DARSS is not usually associated in either a change in D1 receptor number (Bmax) or affinity (Kd). In contrast to D1 DARSS, D2 receptors are not so predictably supersensitized by a lession of DA neurons. In reality, the permanently exaggerated response to an agonist by supersensitized receptors isper se a manifestation of neurotoxicity. Despite dramatic behavioral responses mediated by supersensitized receptors, DARSS has not been easy to correlate with enhanced production of second messengers or early response genes. Altered signaling (i.e., neuronal cross-talk) in defined pathways may represent the mechanism that produces so-called receptor supersensitization. Longlived agonist-induced behavioral abnormality, with or without anatomic evidence of a neuronal lesion, is one of the products of DA D1 receptor supersensitization — it self an index of neurotoxicity.

The catecholaminergic RCSN-3 cell line: A model to study dopamine metabolism

RCSN-3 cells are a cloned cell line derived from the substantia nigra of an adult rat. The cell line grows in monolayer and does not require differentiation to express catecholaminergic traits, such as (i) tyrosine hydroxylase; (ii) dopamine release; (iii) dopamine transport; (iv) norepinephrine transport; (v) monoamine oxidase (MAO)-A expression, but not MAO-B; (vi) formation of neuromelanin; (vii) vesicular monoamine transporter-2 (VMAT-2) expression. In addition, this cell line expresses serotonin transporters, divalent metal transporter, DMT1, dopamine receptor 1 mRNA under proliferating conditions, and dopamine receptor 5 mRNA after incubation with dopamine or dicoumarol. Expression of dopamine receptors D2, D3 and D4 mRNA were not detected in proliferating cells or when the cells were treated with dopamine, CuSO4, dicoumarol or dopamine-copper complex. Angiotensin II receptor mRNA was also found to be expressed, but it underwent down regulation in the presence of aminochrome. Total quinone reductase activity corresponded 94% to DT-diaphorase. The cells also express antioxidant enzymes such as superoxide dismutase, catalase and glutathione peroxidase. This cell line is a suitablein vitro model for studies of dopamine metabolism, since under proliferating conditions the cells express all the pertinent markers.