p-Chloroamphetamine-Enhanced Neostriatal Dopamine Exocytosis in Rats Neonatally Co-lesioned with 6-OHDA and 5,7-DHT: Relevance to Parkinson’s Disease

Serotoninergic nerves are known to modulate sensitization of dopamine receptors (DA-R) in a rodent model of Parkinson’s disease (PD). However, serotoninergic nerves are not known to have a prominent role on DA exocytosis in intact rats. The current study was undertaken to explore the possible influence of serotoninergic nerves on DA exocytosis in Parkinsonian rats. Rat pups were treated at 3 days after birth with the neurotoxin 6-hydroxydopamine (6-OHDA; 134 μg icv, half into each lateral ventricle; desipramine, 1 h pretreatment), in order to produce marked long-lasting destruction of neostriatal dopaminergic innervation, as evidenced by the 90–95% depletion of DA (p \u3c 0.001) [HPLC/ED] into adulthood. Controls received vehicle/desipramine in place of 6-OHDA. Other groups received the serotoninergic neurotoxin 5,7-dihydroxytryptamine (5,7-DHT; 25 μg base, icv, half in each lateral ventricle; desipramine, 1 h; 75 mg/kg pargyline HCl, 30 min) at 3 days post-birth; or both 6-OHDA+5,7-DHT treatments. In adulthood, an in vivo microdialysis study was undertaken to ascertain that p-chloroamphetamine (PCA, 1 mM in the microdialysate)-evoked DA release in the neostriatum was reduced approximately 50% in the 6-OHDA group, while PCA-evoked DA release in the 6-OHDA+5,7-DHT group was substantially increased, to a level equivalent to that of the vehicle control. The baseline neostriatal microdialysate level of 3,4-dihydroxyphenylacetic acid (DOPAC) was also higher in the 6-OHDA+5,7-DHT group vs 6-OHDA group; also, during the 2nd hour of PCA infusion. PCA-enhanced DA exocytosis occurred in the absence of changes in hydroxyl radical (HO·) in the microdialysate (i.e., assay of 2,3- and 2,5-dihydroxybenzoic acid, 2,3-DHBA; 2,5-DHBA). The overall findings demonstrate that an adulthood serotoninergic nerve lesion enhanced PCA-evoked DA exocytosis in a rodent model of severe PD, while susceptibility to oxidative stress was unchanged. The implication is that serotoninergic nerves may normally suppress the release of DA and/or act as an uptake site and storage sink for accumulated DA in parkinsonian-like neostriatum. Potentially, serotoninergic agonists or antagonists, targeting subtype-selective serotonin receptors, may be viable therapeutic adjuncts in PD

Botulinum Neurotoxin: Progress in Negating Its Neurotoxicity; And in Extending Its Therapeutic Utility via Molecular Engineering. Minireview

While the poisonous effects of botulinum neurotoxin (BoNT) have been recognized since antiquity, the overall actions and mechanisms of effects of BoNT have been elucidated primarily over the past several decades. The general utility of BoNT is described in the paper, but the focus is mainly on the approaches towards negating the toxic effects of BoNT, and on the projection of an engineered BoNT molecule serving as a Trojan Horse to deliver a therapeutic load for treatment of a host of medical disorders. The BoNT molecule is configured with a binding domain, a zinc-dependent protease with specificity primarily for vesicular proteins, and a translocation domain for delivery of the metalloprotease into the cytoplasm. The anti-toxin approaches for BoNT include the use of vaccines, antibodies, block of BoNT binding or translocation, inhibition of metalloprotease activity, impeded translocation of the protease/catalytic domain, and inhibition of the downstream Src signaling pathway. Projections of BoNT as a therapeutic include its targeting to non-cholinergic nerves, also targeting to non-neuronal cells for treatment of hypersecretory disorders (e.g., cystic fibrosis), and treatment of hormonal disorders (e.g., acromegaly). Still in the exploratory phase, there is the expectation of major advances in BoNT neuroprotective strategies and burgeoning utility of engineered BoNTs as therapeutics

Stereotypic Progressions in Psychotic Behavior

Dopamine receptor supersensitivity (DARSS) often is invoked as a mechanism possibly underlying disordered thought processes and agitation states in psychiatric disorders. This review is focused on identified means for producing DARSS and associating the role of other monoaminergic systems in modulating DARSS. Dopamine (DA) receptors, experimentally, are prone to become supersensitive and to thus elicit abnormal behaviors when coupled with DA or a receptor agonist. In intact (control) rats repeated DA D1 agonist treatments fail to sensitize D1 receptors, while repeated D 2 agonist treatments sensitize D2 receptors. D2 RSS is attenuated by a lesion with DSP-4 (N-(2-chlorethyl)-N-ethyl-2- bromobenzylamine) in early postnatal ontogeny, indicating that noradrenergic nerves have a permissive effect on D2 DARSS. However, if DSP-4 is co-administered with 5,7-dihydroxytryptamine to destroy serotonin (5-HT) nerves, then D2 RSS is restored. In rats treated early in postnatal ontogeny with the neurotoxin 6-hydroxydopamine to largely destroy DA innervation of striatum, both repeated D1 and D2 agonists sensitize D1 receptors. 5-HT nerves appear to have a permissive effect on D1 DARSS, as a 5-HT lesion reduces the otherwise enhanced effect of a D1 agonist. The series of findings demonstrate that DARSS is able to be produced by repeated agonist treatments, albeit under different circumstances. The involvement of other neuronal phenotypes as modulators of DARSS provides the potential for targeting a variety of sites in the aim to prevent or attenuate DARSS. This therapeutic potential broadens the realm of approaches toward treating psychiatric disorders

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 \u27priming\u27 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 is per 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

Perinatal 6-Hydroxydopamine to Produce a Lifelong Model of Severe Parkinson’s Disease

The classic rodent model of Parkinson’s disease (PD) is produced by unilateral lesioning of pars compacta substantia nigra (SNpc) in adult rats, producing unilateral motor deficits which can be assessed by dopamine (DA) D2 receptor (D2-R) agonist induction of measurable unilateral rotations. Bilateral SNpc lesions in adult rats produce life-threatening aphagia, adipsia, and severe motor disability resembling paralysis-a PD model that is so compromised that it is seldom used. Described in this paper is a PD rodent model in which there is bilateral 99% loss of striatal dopaminergic innervation, produced by bilateral intracerebroventricular or intracisternal 6-hydroxydopamine (6-OHDA) administration to perinatal rats. This procedure produces no lethality and does not shorten the life span, while rat pups continue to suckle through the pre-weaning period; and eat without impairment post-weaning. There is no obvious motor deficit during or after weaning, except with special testing, so that parkinsonian rats are indistin-guishable from control and thus allow for behavioral assessments to be conducted in a blinded manner. L-DOPA (L-3,4-dihydroxyphenylalanine) treatment increases DA content in striatal tissue, also evokes a rise in extraneuronal (i.e.,in vivo microdialysate) DA, and is able to evoke dyskinesias. D2-R agonists produce effects similar to those of L-DOPA. In addition, effects of both D1-and D2-R agonist effects on overt or latent receptor supersensitization are amenable to study. Elevated basal levels of reactive oxygen species (ROS), namely hydroxyl radical, occurring in dopaminergic denervated striatum are suppressed by L-DOPA treatment. Striatal serotoninergic hyperinnervation ensuing after perinatal dopaminergic denervation does not appear to interfere with assessments of the dopaminergic system by L-DOPA or D1-or D2-R agonist challenge. Partial lesioning of serotonin fibers with a selective neurotoxin either at birth or in adulthood is able to eliminate sero-toninergic hyperinnervation and restore the normal level of serotoninergic innervation. Of all the animal models of PD, that produced by perinatal 6-OHDA lesioning provides the most pronounced destruction of nigrostriatal neurons, thus representing a model of severe PD, as the neurochemical outcome resembles the status of severe PD in humans but without obvious motor deficits

Perinatal 6-Hydroxydopamine Modeling of ADHD

The neonatally 6-hydroxydopamine (n6-OHDA)-lesioned rat has been the standard for 40 years, as an animal model of attention-deficit hyperactivity disorder (ADHD). Rats so lesioned during postnatal ontogeny are characterized by ∽99% destruction of dopaminergic nerves in pars compacta substantia nigra, with comparable destruction of the nigrostriatal tract and lifelong ∽99 % dopaminergic denervation of striatum, with lesser destructive effect on the ventral tegmental nucleus and associated lesser dopaminergic denervation of nucleus accumbens and prefrontal cortex. As a consequence of striatal dopaminergic denervation, reactive serotoninergic hyperinnervation of striatum ensues. The striatal extraneuronal milieu of DA and serotonin is markedly altered. Also, a variety of sensitization changes occur for dopaminergic D1 and D2 receptors, and for serotoninergic receptors. Behaviorally, these rats in adulthood display spontaneous hyperlocomotor activity, attentional deficits, and cognitive impairment-all of which are acutely attenuated by the psychostimulants amphetamine (AMPH) and methylphenidate (MPH) (i.e.,opposite to the acute effects of AMPH and MPH in intact control rats). The acute behavioral effects of AMPH and MPH in intact and lesioned rats are analogous to their respective acute effects in non-ADHD and in ADHD humans. The neurochemical template of brain, and behavioral series of changes in n6-OHDA-lesioned rats, is described in the review. Despite the fact that nigrostriatal damage is not an underlying pathophysiological process of human ADHD (i.e.,lacking construct validity), the described animal model has face validity (behavioral profile) and predictive validity (mirror of ADHD/MPH effects, as well as putative and new ADHD treatment effects). Also described in this review is a modification of the n6-OHDA rat, produced by adulthood partial lesioning of the serotoninergic fiber overgrowth. This ADHD model has even more accentuated hyperlocomotor and attentional deficits, counteracted by AMPH-thus providing a more robust means of animal modeling of ADHD. The n6-OHDA rat as a model of ADHD continues to be important in the search for new ADHD treatments

Dopamine D\u3csub\u3e2\u3c/sub\u3e 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\u27s 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 D\u3csub\u3e2\u3c/sub\u3e 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 as priming 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

Modeling Tardive Dyskinesia: Predictive 5-HT\u3csub\u3e2c\u3c/sub\u3e Receptor Antagonist Treatment

Tardive dyskinesia (TD), a movement disorder produced by long-term treatment with a classical antipsychotic drug, is generally considered to be a disorder of dopamine (DA) systems, since classical antipsychotics are potent DA D2 receptor blockers. Also, acute DA D1 agonist treatment of rats is known to produce vacuous chewing movements (VCMs), a behavioral feature resembling the oral dyskinesia that is so prominent in most instances of TD. In this paper we outline a series of studies in a new animal model of TD in which DA D1 receptor supersensitivity was produced by neonatal 6-hydroxydopamine (6-OHDA)-induced destruction of nigrostriatal DA fibers. In rats so-lesioned 5-HT receptor supersensitivity is additionally produced, and in fact 5-HT receptor antagonists attenuate enhanced DA D16-lesioned rats treated with haloperidol for one year, there is a 2-fold increase in numbers of VCMs (versus intact rats treated with haloperidol); and this high frequency of VCMs persists for more than 6 months after discontinuing haloperidol treatment. During this stage, 5-HT2 receptor antagonists, but not DA D1 receptor antagonists, attenuate the incidence of VCMs. This series of findings implicates the 5-HT neuronal phenotype in TD, and promotes 5-HT2 receptor antagonists, more specifically 5-HT2C receptor antagonists, as a rational treatment approach for TD in humans