Neonatal 6-hydroxydopamine lesioning of rats and dopaminergic neurotoxicity: proposed animal model of Parkinson's disease.

The neurotoxin 6-hydroxydopamine (6-OHDA), following pretreatment with the norepinephrine transport inhibitor desipramine, selectively destroys dopaminergic neurons. When given to rats, neonatal 6-OHDA (n6-OHDA) crosses the blood-brain barrier to destroy 90-99% of dopaminergic nerves in pars compacta substantia nigra (SNpc). The n6-OHDA-lesioned rat is posed as a reasonable animal model for PD: (a) the magnitude of dopaminergic neuronal destruction is expansive, (b) mapping of dopaminergic denervation has been defined, (c) effects on dopamine (DA) receptor alterations have been elucidated (d) as well as changes in receptor sensitivity status, (e) reactive sprouting of serotoninergic innervation (i.e. hyperinnervation) has been mapped, and (f) interplay between serotoninergic and dopaminergic systems is characterized. (g) A broad range of locomotor and stereotyped behaviors has been assessed and (h) large numbers of neurochemical assessments have been attained. (i) n6-OHDA-lesioned rats survive 6-OHDA lesioning and (j) the rat is behaviorally indistinguishable from controls. Dopaminergic destruction in early ontogeny rather in adulthood is a 'treatment liability' of this model, yet other animal models have liability issues of a serious nature-the initial one being use of a neurotoxin to produce the animal model of PD. The n6-OHDA-lesioned rat is proposed as a PD model for its value in associating the SNpc dopaminergic lesion with behavioral outcomes, also for replicability of dopaminergic destruction, and the accompanying neuronal adaptations and interplay between neuronal phenotypes in brain-which provide a means to better define and understand the range of deficits and neuronal adaptations that are likely to occur in human PD.

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 < 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.

Dopamine D2 Receptor Supersensitivity as a Spectrum of Neurotoxicity and Status in Psychiatric Disorders.

Abnormality of dopamine D2 receptor (D2R) function, often observed as D2R supersensitivity (D2RSS), is a commonality of schizophrenia and related psychiatric disorders in humans. Moreover, virtually all psychotherapeutic agents for schizophrenia target D2R in brain. Permanent D2RSS as a feature of a new animal model of schizophrenia was first reported in 1991, and then behaviorally and biochemically characterized over the next 15-20 years. In this model of schizophrenia characterized by production of D2RSS in ontogeny, there are demonstrated alterations of signaling processes, as well as functional links between the biologic template of the animal model and ability of pharmacotherapeutics to modulate or reverse biologic and behavioral modalities toward normality. Another such animal model, featuring knockout of trace amine-associated receptor 1 (TAAR1), demonstrates D2RSS with an increase in the proportion of D2R in the high-affinity state. Currently, TAAR1 agonists are being explored as a therapeutic option for schizophrenia. There is likewise an overlay of D2RSS with substance use disorder. The aspect of adenosine A2A-D2 heteroreceptor complexes in substance use disorder is highlighted, and the association of adenosine A2A receptor antagonists in discriminative and rewarding effects of psychostimulants is outlined. In summary, these new animal models of schizophrenia have face, construct, and predictive validity, and distinct advantages over earlier models. While the review summarizes elements of D2RSS in schizophrenia per se, and its interplay with substance use disorder, a major focus is on presumed new molecular targets attending D2RSS in schizophrenia and related clinical entities.

BMAA and Neurodegenerative Illness.

The cyanobacterial toxin ß-N-methylamino-L-alanine (BMAA) now appears to be a cause of Guamanian amyotrophic lateral sclerosis/parkinsonism dementia complex (ALS/PDC). Its production by cyanobacteria throughout the world combined with multiple mechanisms of BMAA neurotoxicity, particularly to vulnerable subpopulations of motor neurons, has significantly increased interest in investigating exposure to this non-protein amino acid as a possible risk factor for other forms of neurodegenerative illness. We here provide a brief overview of BMAA studies and provide an introduction to this collection of scientific manuscripts in this special issue on BMAA.

Neuroteratology and Animal Modeling of Brain Disorders.

Over the past 60 years, a large number of selective neurotoxins were discovered and developed, making it possible to animal-model a broad range of human neuropsychiatric and neurodevelopmental disorders. In this paper, we highlight those neurotoxins that are most commonly used as neuroteratologic agents, to either produce lifelong destruction of neurons of a particular phenotype, or a group of neurons linked by a specific class of transporter proteins (i.e., dopamine transporter) or body of receptors for a specific neurotransmitter (i.e., NMDA class of glutamate receptors). Actions of a range of neurotoxins are described: 6-hydroxydopamine (6-OHDA), 6-hydroxydopa, DSP-4, MPTP, methamphetamine, IgG-saporin, domoate, NMDA receptor antagonists, and valproate. Their neuroteratologic features are outlined, as well as those of nerve growth factor, epidermal growth factor, and that of stress. The value of each of these neurotoxins in animal modeling of human neurologic, neurodegenerative, and neuropsychiatric disorders is discussed in terms of the respective value as well as limitations of the derived animal model. Neuroteratologic agents have proven to be of immense importance for understanding how associated neural systems in human neural disorders may be better targeted by new therapeutic agents.

Exercise and Nutritional Benefits in PD: Rodent Models and Clinical Settings.

Physical exercise offers a highly effective health-endowering activity as has been evidence using rodent models of Parkinson's disease (PD). It is a particularly useful intervention in individuals employed in sedentary occupations or afflicted by a neurodegenerative disorder, such as PD. The several links between exercise and quality-of-life, disorder progression and staging, risk factors and symptoms-biomarkers in PD all endower a promise for improved prognosis. Nutrition provides a strong determinant for disorder vulnerability and prognosis with fish oils and vegetables with a mediterranean diet offering both protection and resistance. Three factors determining the effects of exercise on disorder severity of patients may be presented: (i) Exercise effects upon motor impairment, gait, posture and balance, (ii) Exercise reduction of oxidative stress, stimulation of mitochondrial biogenesis and up-regulation of autophagy, and (iii) Exercise stimulation of dopamine (DA) neurochemistry and trophic factors. Running-wheel performance, as measured by distance run by individual mice from different treatment groups, was related to DA-integrity, indexed by striatal DA levels. Finally, both nutrition and exercise may facilitate positive epigenetic outcomes, such as lowering the dosage of L-Dopa required for a therapeutic effect.

Perinatal Treatments with the Dopamine D2-Receptor Agonist Quinpirole Produces Permanent D2-Receptor Supersensitization: a Model of Schizophrenia.

Repeated daily treatments of perinatal rats with the dopamine D2-receptor (D2-R) agonist quinpirole for a week or more produces the phenomenon of 'priming'-gradual but long-term sensitization of D2-R. In fact a daily dose of quinpirole as low as 50 µg/kg/day is adequate for sensitizing D2-R. Primed rats as neonates and in adolescence, when acutely treated with quinpirole display enhanced eating/gnawing/nursing on dams, also horizontal locomotor activity. Between 3 and 5 weeks of age, acute quinpirole treatment of primed rats produces profound vertical jumping with paw treading-a behavior that is not observed in control rats. At later ages acute quinpirole treatment is associated with enhanced yawning, a D2-R-associated behavior. This long-term D2-R supersensitivity is believed to be life-long, despite the relatively brief period of D2-R priming near the time of birth. D2-R supersensitivity is not associated with an increase in the number or affinity of D2-R, as assessed in the striatum of rats; nor is it induced with the D3-R agonist 7-OH-DPAT. However, quinpirole-induced D2-R supersensitivity is associated with cognitive deficits, also a deficit in pre-pulse inhibition and in neurotrophic factors, and low levels of the transcript regulator of G-protein signaling (RGS) RGS9 in brain; and acute reversal of these alterations by the antipsychotic agent olanzapine. In sum, rats ontogenetically D2-R supersensitized have face validity, construct validity and predictive ability for schizophrenia.

Lifelong Rodent Model of Tardive Dyskinesia-Persistence After Antipsychotic Drug Withdrawal.

Tardive dyskinesia (TD), first appearing in humans after introduction of the phenothiazine class of antipsychotics in the 1950s, is now recognized as an abnormality resulting predominately by long-term block of dopamine (DA) D2 receptors (R). TD is thus reproduced in primates and rodents by chronic administration of D2-R antagonists. Through a series of studies predominately since the 1980s, it has been shown in rodent modeling of TD that when haloperidol or other D2-R antagonist is added to drinking water, rats develop spontaneous oral dyskinesias, vacuous chewing movements (VCMs), after ~3 months, and this TD is associated with an increase in the number of striatal D2-R. This TD persists for the duration of haloperidol administration and another ~2 months after haloperidol withdrawal. By neonatally lesioning dopaminergic nerves in brain in neonatal rats with 6-hydroxydopamine (6-OHDA), it has been found that TD develops sooner, at ~2 months, and also is accompanied by a much higher number of VCMs in these haloperidol-treated lesioned rats, and the TD persists lifelong after haloperidol withdrawal, but is not associated with an increased D2-R number in the haloperidol-withdrawn phase. TD apparently is related in part to supersensitization of both D1-R and serotoninergic 5-HT2-R, which is also a typical outcome of neonatal 6-OHDA (n6-OHDA) lesioning. Testing during the haloperidol-withdrawn phase in n6-OHDA rats displaying TD reveals that receptor agonists and antagonists of a host of neuronal phenotypic classes have virtually no effect on spontaneous VCM number, except for 5-HT2-R antagonists which acutely abate the incidence of VCMs in part. Extrapolating to human TD, it appears that (1) 5-HT2-R supersensitization is the crucial alteration accounting for persistence of TD, (2) dopaminergic-perhaps age-related partial denervation-is a risk factor for the development of TD, and (3) 5-HT2-R antagonists have the therapeutic potential to alleviate TD, particularly if/when an antipsychotic D2-R blocker is withdrawn.

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 indistinguishable 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 serotoninergic 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.

Perinatal Lesioning and Lifelong Effects of the Noradrenergic Neurotoxin 6-Hydroxydopa.

6-hydroxydopa (6-OHDOPA) was synthesized with the expectation that it would be able to cross the blood-brain barrier to be enzymatically decarboxylated to 6-hydroxydopamine (6-OHDA), the newly discovered neurotoxin for noradrenergic and dopaminergic neurons. In part, 6-OHDOPA fulfilled these criteria. When administered experimentally to rodents, 6-OHDOPA destroyed peripheral sympathetic noradrenergic nerves and did exert neurotoxicity to noradrenergic nerves in brain-in large part, from its conversion to 6-OHDA. However, the efficacy of 6-OHDOPA was less than that of 6-OHDA; also, 6-OHDOPA was relatively selective for noradrenergic neurons; near-lethal doses of 6-OHDOPA were required to damage dopaminergic nerves; and ultimately, 6-OHDOPA was found to be an agonist at AMPA receptors, thus accounting for more non-specificity. Nevertheless, 6-OHDOPA was found to be a particularly valuable tool in uncovering processes and mechanisms associated with noradrenergic nerve regeneration and sprouting, particularly when administered to perinatal rodents. Also, 6-OHDOPA was a good tool for selective mapping of noradrenergic nerve tracts in brain, since dopaminergic tracts were unaffected and did not interfere with the histofluorescent methodology used for this purpose in the early 1970s. As an experimental research tool, 6-OHDOPA was valuable in a short time-window, but its utility is largely limited because of newer research technologies that provide better means today for nerve tract mapping, and for experimental approaches engaged toward study of processes and mechanisms attending nerve regeneration. AMPA actions of 6-OHDOPA have not been extensively studied, so this avenue may enliven use of 6-OHDOPA in the future.

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.

Physical Exercise Alleviates Health Defects, Symptoms, and Biomarkers in Schizophrenia Spectrum Disorder.

Schizophrenia spectrum disorders are characterized by symptom profiles consisting of positive and negative symptoms, cognitive impairment, and a plethora of genetic, epigenetic, and phenotypic biomarkers. Assorted animal models of these disorders and clinical neurodevelopmental indicators have implicated neurodegeneration as an element in the underlying pathophysiology. Physical exercise or activity regimes--whether aerobic, resistance, or endurance--ameliorate regional brain and functional deficits not only in affected individuals but also in animal models of the disorder. Cognitive deficits, often linked to regional deficits, were alleviated by exercise, as were quality-of-life, independent of disorder staging and risk level. Apoptotic processes intricate to the etiopathogenesis of schizophrenia were likewise attenuated by physical exercise. There is also evidence of manifest benefits endowed by physical exercise in preserving telomere length and integrity. Not least, exercise improves overall health and quality-of-life. The notion of scaffolding as the outcome of physical exercise implies the "buttressing" of regional network circuits, neurocognitive domains, anti-inflammatory defenses, maintenance of telomeric integrity, and neuro-reparative and regenerative processes.

Inhibition of connexin43 improves functional recovery after ischemic brain injury in neonatal rats.

Connexin43 (Cx43) is one of the most abundant gap junction proteins in the central nervous system. Abnormal opening of Cx43 hemichannels after ischemic insults causes apoptotic cell death. In this study, we found persistently increased expression of Cx43 8 h to 7 d after hypoxia/ischemia (HI) injury in neonatal rats. Pre-treatment with Gap26 and Gap27, two Cx43 mimetic peptides, significantly reduced cerebral infarct volume. Gap26 treatment at 24 h after ischemia improved functional recovery on muscle strength, motor coordination, and spatial memory abilities. Further, Gap26 inhibited Cx43 expression and reduced active astrogliosis. Gap26 interacted and co-localized with Cx43 together in brain tissues and cultured astrocytes. After oxygen glucose deprivation, Gap26 treatment reduced the total Cx43 level in cultured astrocytes; but Cx43 level in the plasma membrane was increased. Degradation of Cx43 in the cytoplasm was mainly via the ubiquitin proteasome pathway. Concurrently, phosphorylated Akt, which phosphorylates Cx43 on Serine(373) and facilitates the forward transport of Cx43 to the plasma membrane, was increased by Gap26 treatment. Microdialysis showed that increased membranous Cx43 causes glutamate release by opening Cx43 hemichannels. Extracellular glutamate concentration was significantly decreased by Gap26 treatment in vivo. Finally, we found that cleaved caspase-3, an apoptosis marker, was attenuated after HI injury by Gap26 treatment. Effects of Gap27 were analogous to those of Gap26. In summary, our findings demonstrate that modulation of Cx43 expression and astroglial function is a potential therapeutic strategy for ischemic brain injury.

Neurotoxin mechanisms and processes relevant to Parkinson's disease: an update.

The molecular mechanism responsible for degenerative process in the nigrostriatal dopaminergic system in Parkinson's disease (PD) remains unknown. One major advance in this field has been the discovery of several genes associated to familial PD, including alpha synuclein, parkin, LRRK2, etc., thereby providing important insight toward basic research approaches. There is an consensus in neurodegenerative research that mitochon dria dysfunction, protein degradation dysfunction, aggregation of alpha synuclein to neurotoxic oligomers, oxidative and endoplasmic reticulum stress, and neuroinflammation are involved in degeneration of the neuromelanin-containing dopaminergic neurons that are lost in the disease. An update of the mechanisms relating to neurotoxins that are used to produce preclinical models of Parkinson´s disease is presented. 6-Hydroxydopamine, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine, and rotenone have been the most wisely used neurotoxins to delve into mechanisms involved in the loss of dopaminergic neurons containing neuromelanin. Neurotoxins generated from dopamine oxidation during neuromelanin formation are likewise reviewed, as this pathway replicates neurotoxin-induced cellular oxidative stress, inactivation of key proteins related to mitochondria and protein degradation dysfunction, and formation of neurotoxic aggregates of alpha synuclein. This survey of neurotoxin modeling-highlighting newer technologies and implicating a variety of processes and pathways related to mechanisms attending PD-is focused on research studies from 2012 to 2014.

Perinatal manganese exposure and hydroxyl radical formation in rat brain.

The present study was designed to investigate the role of pre- and postnatal manganese (Mn) exposure on hydroxyl radical (HO(•)) formation in the brains of dopamine (DA) partially denervated rats (Parkinsonian rats). Wistar rats were given tap water containing 10,000 ppm manganese chloride during the duration of pregnancy and until the time of weaning. Control rat dams consumed tap water without added Mn. Three days after birth, rats of both groups were treated with 6-hydroxydopamine at one of three doses (15, 30, or 67 µg, intraventricular on each side), or saline vehicle. We found that Mn content in the brain, kidney, liver, and bone was significantly elevated in dams exposed to Mn during pregnancy. In neonates, the major organs that accumulated Mn were the femoral bone and liver. However, Mn was not elevated in tissues in adulthood. To determine the possible effect on generation of the reactive species, HO(•) in Mn-induced neurotoxicity, we analyzed the contents of 2.3- and 2.5-dihydroxybenzoic acid (spin trap products of salicylate; HO(•) being an index of in vivo HO(•) generation), as well as antioxidant enzyme activities of superoxide dismutase (SOD) isoenzymes and glutathione S-transferase (GST). 6-OHDA-depletion of DA produced enhanced HO(•) formation in the brain tissue of newborn and adulthood rats that had been exposed to Mn, and the latter effect did not depend on the extent of DA denervation. Additionally, the extraneuronal, microdialysate, content of HO(•) in neostriatum was likewise elevated in 6-OHDA-lesioned rats. Interestingly, there was no difference in extraneuronal HO(•) formation in the neostriatum of Mn-exposed versus control rats. In summary, findings in this study indicate that Mn crosses the placenta but in contrast to other heavy metals, Mn is not deposited long term in tissues. Also, damage to the dopaminergic system acts as a "trigger mechanism," initiating a cascade of adverse events leading to a protracted increase in HO(•) generation, and the effects of Mn and 6-OHDA are compounded. Moreover, HO(•) generation parallels the suppression of SOD isoenzymes and GST in the brains of rats lesioned with 6-OHDA and/or intoxicated with Mn-the most prominent impairments being in frontal cortex, striatum, and brain stem. In conclusion, ontogenetic Mn exposure, resulting in reactive oxygen species, HO(•) formation, represents a risk factor for dopaminergic neurotoxicity and development of neurodegenerative disorders.

A comprehensive view of the epigenetic landscape part I: DNA methylation, passive and active DNA demethylation pathways and histone variants.

In multicellular organisms, all the cells are genetically identical but turn genes on or off at the right time to promote differentiation into specific cell types. The regulation of higher-order chromatin structure is essential for genome-wide reprogramming and for tissue-specific patterns of gene expression. The complexity of the genome is regulated by epigenetic mechanisms, which act at the level of DNA, histones, and nucleosomes. Epigenetic machinery is involved in many biological processes, including genomic imprinting, X-chromosome inactivation, heterochromatin formation, and transcriptional regulation, as well as DNA damage repair. In this review, we summarize the recent understanding of DNA methylation, cytosine derivatives, active and passive demethylation pathways as well as histone variants. DNA methylation is one of the well-characterized epigenetic signaling tools. Cytosine methylation of promoter regions usually represses transcription but methylation in the gene body may have a positive correlation with gene expression. The attachment of a methyl group to cytosine residue in the DNA sequence is catalyzed by enzymes of the DNA methyltransferase family. Recent studies have shown that the Ten-Eleven translocation family enzymes are involved in stepwise oxidation of 5-methylcytosine, creating new cytosine derivatives including 5-hydroxymethylcytosine, 5-formylcytosine, and 5-carboxylcytosine. Additionally, histone variants into nucleosomes create another strategy to regulate the structure and function of chromatin. The replacement of canonical histones with specialized histone variants regulates accessibility of DNA, and thus may affect multiple biological processes, such as replication, transcription, DNA repair, and play a role in various disorders such as cancer.

Elevated gene expression of glutamate receptors in noradrenergic neurons from the locus coeruleus in major depression.

Glutamate receptors are promising drug targets for the treatment of urgent suicide ideation and chronic major depressive disorder (MDD) that may lead to suicide completion. Antagonists of glutamatergic NMDA receptors reduce depressive symptoms faster than traditional antidepressants, with beneficial effects occurring within hours. Glutamate is the prominent excitatory input to the noradrenergic locus coeruleus (LC). The LC is activated by stress in part through this glutamatergic input. Evidence has accrued demonstrating that the LC may be overactive in MDD, while treatment with traditional antidepressants reduces LC activity. Pathological alterations of both glutamatergic and noradrenergic systems have been observed in depressive disorders, raising the prospect that disrupted glutamate-norepinephrine interactions may be a central component to depression and suicide pathobiology. This study examined the gene expression levels of glutamate receptors in post-mortem noradrenergic LC neurons from subjects with MDD (most died by suicide) and matched psychiatrically normal controls. Gene expression levels of glutamate receptors or receptor subunits were measured in LC neurons collected by laser capture microdissection. MDD subjects exhibited significantly higher expression levels of the NMDA receptor subunit genes, GRIN2B and GRIN2C, and the metabotropic receptor genes, GRM4 and GRM5, in LC neurons. Gene expression levels of these receptors in pyramidal neurons from prefrontal cortex (BA10) did not reveal abnormalities in MDD. These findings implicate disrupted glutamatergic-noradrenergic interactions at the level of the stress-sensitive LC in MDD and suicide, and provide a theoretical mechanism by which glutamate antagonists may exert rapid antidepressant effects.

Ontogenetic manganese exposure with perinatal 6-OHDA lesioning alters behavioral responses of rats to dopamine D1 and D2 agonist treatments.

The effect of neonatal manganese (Mn) exposure in a 6-hydroxydopamine (6-OHDA) rat model of Parkinson's disease was investigated. Pregnant Wistar rats were given drinking water with 10,000 ppm of Manganese (MnCl2.4H2O) from the time of conception until weaning on the 21st day after delivery. Control rats consumed tap water. Three days after the birth, other groups of neonatal rat pups were pretreated with desipramine (20 mg/kg ip 1h) prior to bilateral ICV administration of 6-OHDA or its vehicle, saline-ascorbic (0.1%) (control). Two months after the birth, striatal dopamine and homovanilic acid efflux measured by an in vivo microdialysis method were reduced in rats lesioned with 6-OHDA. Co-exposure to perinatal Mn did not modify neurotransmission alterations. However, there were prominent abnormalities in behavioral testing in rats perinatally exposed to Mn and treated neonatally with 6-OHDA. These findings demonstrate that although Mn did not further damage neurotransmitter activity in the neostriatum, ontogenetic exposure to Mn enhances the behavioral toxicity to 6-OHDA.

Antinociceptive effects of H3 (R-methylhistamine) and GABA(B) (baclofen)-receptor ligands in an orofacial model of pain in rats.

The present study explored the antinociceptive effects of H3 (R-methylhistamine) and GABA(B) (baclofen) receptor ligands in an orofacial model of pain in rats. Orofacial pain was induced by subcutaneous injection of formalin (50 µl, 5 %) in the upper lip region, and the number of jumps and time spent face rubbing was recorded for 40 min. Formalin produced a marked biphasic pain response; first phase, 0-10 min (jumps), and second phase, 15-40 min, (rubbing). Baclofen (50 µg) injected into the rat wiskerpad 5 min before formalin administration suppressed both phases of pain whereas R-alpha-methylhistamine (12.5 µg) abolished the first phase only. Brains were taken immediately after behavioral testing was completed. HPLC/ED analysis showed that 5-hydroxytryptamine (5-HT) turnover was increased in hippocampus, thalamus, and brain stem of all formalin groups, excepting the baclofen group in which the balance of 5-HT metabolism was restored to control values. These findings demonstrate that GABA(B) receptors represent peripheral targets for analgesia. Consequently, locally administered baclofen may be a useful approach in treating inflammatory trigeminal pain.