Quantification of the serotonin hyperinnervation in adult rat neostriatum after neonatal 6-hydroxydopamine lesion of nigral dopamine neurons

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.

Animal Models of Self-Injurious Behavior: An Update

Although self-injurious behavior is a common comorbid behavior problem among individuals with neurodevelopmental disorders, little is known about its etiology and underlying neurobiology. Interestingly, it shows up in various forms across patient groups with distinct genetic errors and diagnostic categories. This suggests that there may be shared neuropathology that confers vulnerability in these disparate groups. Convergent evidence from clinical pharmacotherapy, brain imaging studies, postmortem neurochemical analyses, and animal models indicates that dopaminergic insufficiency is a key contributing factor. This chapter provides an overview of studies in which animal models have been used to investigate the biochemical basis of self-injury and highlights the convergence in findings between these models and expression of self-injury in humans.

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.

New therapeutic opportunities for 5-HT2C receptor ligands in neuropsychiatric disorders

The 5-HT2C receptor (R) displays a widespread distribution in the CNS and is involved in the action of 5-HT in all brain areas. Knowledge of its functional role in the CNS pathophysiology has been impaired for many years due to the lack of drugs capable of discriminating among 5-HT2R subtypes, and to a lesser extent to the 5-HT1B, 5-HT5, 5-HT6 and 5-HT7Rs. The situation has changed since the mid-90s due to the increased availability of new and selective synthesized compounds, the creation of 5-HT2C knock out mice, and the progress made in molecular biology. Many pharmacological classes of drugs including antipsychotics, antidepressants and anxiolytics display affinities toward 5-HT2CRs and new 5-HT2C ligands have been developed for various neuropsychiatric disorders. The 5-HT2CR is presumed to mediate tonic/constitutive and phasic controls on the activity of different central neurobiological networks. Preclinical data illustrate this complexity to a point that pharmaceutical companies developed either agonists or antagonists for the same disease. In order to better comprehend this complexity, this review will briefly describe the molecular pharmacology of 5-HT2CRs, as well as their cellular impacts in general, before addressing its central distribution in the mammalian brain. Thereafter, we review the preclinical efficacy of 5-HT2C ligands in numerous behavioral tests modeling human diseases, highlighting the multiple and competing actions of the 5-HT2CRs in neurobiological networks and monoaminergic systems. Notably, we will focus this evidence in the context of the physiopathology of psychiatric and neurological disorders including Parkinson's disease, levodopa-induced dyskinesia, and epilepsy.

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.

Serotonin2C Receptors and the Motor Control of Oral Activity

Data from many experiments has shown that serotonin2C (5-HT2C) receptor plays a role in the control of orofacial activity in rodents. Purposeless oral movements can be elicited either by agonists or inverse agonists implying a tight control exerted by the receptor upon oral activity. The effects of agonists has been related to an action of these drugs in the subthalamic nucleus and the striatum, the two input structures for cortical efferents to the basal ganglia, a group of subcortical structures involved in the control of motor behaviors. The oral effects of agonists are dramatically enhanced in case of chronic blockade of central dopaminergic transmission induced by neuroleptics or massive destruction of dopamine neurons. The mechanisms involved in the hypersensitized oral responses to 5-HT2C agonists are not clear and deserve additional studies. Indeed, while the oral behavior triggered by 5-HT2C drugs would barely correspond to the dyskinesia observed in humans, the clinical data have consistently postulated that 5-HT2C receptors could be involved in these aberrant motor manifestations.

Multiple controls exerted by 5-HT2C receptors upon basal ganglia function: from physiology to pathophysiology

Serotonin2C (5-HT2C) receptors are expressed in the basal ganglia, a group of subcortical structures involved in the control of motor behaviour, mood and cognition. These receptors are mediating the effects of 5-HT throughout different brain areas via projections originating from midbrain raphe nuclei. A growing interest has been focusing on the function of 5-HT2C receptors in the basal ganglia because they may be involved in various diseases of basal ganglia function notably those associated with chronic impairment of dopaminergic transmission. 5-HT2C receptors act on numerous types of neurons in the basal ganglia, including dopaminergic, GABAergic, glutamatergic or cholinergic cells. Perhaps inherent to their peculiar molecular properties, the modality of controls exerted by 5-HT2C receptors over these cell populations can be phasic, tonic (dependent on the 5-HT tone) or constitutive (a spontaneous activity without the presence of the ligand). These controls are functionally organized in the basal ganglia: they are mainly localized in the input structures and preferentially distributed in the limbic/associative territories of the basal ganglia. The nature of these controls is modified in neuropsychiatric conditions such as Parkinson's disease, tardive dyskinesia or addiction. Most of the available data indicate that the function of 5-HT2C receptor is enhanced in cases of chronic alterations of dopamine neurotransmission. The review illustrates that 5-HT2C receptors play a role in maintaining continuous controls over the basal ganglia via multiple diverse actions. We will discuss their interest for treatments aimed at ameliorating current pharmacotherapies in schizophrenia, Parkinson's disease or drugs abuse.

Plasticity of tyrosine hydroxylase and serotonergic systems in the regenerating spinal cord of adult zebrafish

Monoaminergic innervation of the spinal cord has important modulatory functions for locomotion. Here we performed a quantitative study to determine the plastic changes of tyrosine hydroxylase-positive (TH1(+); mainly dopaminergic), and serotonergic (5-HT(+)) terminals and cells during successful spinal cord regeneration in adult zebrafish. TH1(+) innervation in the spinal cord is derived from the brain. After spinal cord transection, TH1(+) immunoreactivity is completely lost from the caudal spinal cord. Terminal varicosities increase in density rostral to the lesion site compared with unlesioned controls and are re-established in the caudal spinal cord at 6 weeks post lesion. Interestingly, axons mostly fail to re-innervate more caudal levels of the spinal cord even after prolonged survival times. However, densities of terminal varicosities correlate with recovery of swimming behavior, which is completely lost again after re-lesion of the spinal cord. Similar observations were made for terminals derived from descending 5-HT(+) axons from the brain. In addition, spinal 5-HT(+) neurons were newly generated after a lesion and transiently increased in number up to fivefold, which depended in part on hedgehog signaling. Overall, TH1(+) and 5-HT(+) innervation is massively altered in the successfully regenerated spinal cord of adult zebrafish. Despite these changes in TH and 5-HT systems, a remarkable recovery of swimming capability is achieved, suggesting significant plasticity of the adult spinal network during regeneration.

Animal models of self-injurious behaviour: an overview

Self-injurious behaviour is highly prevalent in neurodevelopmental disorders. Interestingly, it is not restricted to any individual diagnostic group. Rather, it is exhibited in various forms across patient groups with distinct genetic defects and classifications of disorders. This suggests that there may be shared neuropathology that confers vulnerability. Convergent evidence from clinical pharmacotherapy, brain imaging studies, postmortem neurochemical analyses, and animal models indicates that dopaminergic insufficiency is a key culprit. This chapter provides an overview of studies in which animal models have been used to investigate the biochemical basis of self-injury, and highlights the convergence in findings between these models and expression of self-injury in humans.

Distribution and ultrastructural features of the serotonin innervation in rat and squirrel monkey subthalamic nucleus

The main purpose of this light and electron microscopic immunocytochemical study was to characterize and compare the serotonin (5-HT) innervation of the subthalamic nucleus (STN) in rats and squirrel monkeys (Saimiri sciureus) following labeling with an antibody against the 5-HT transporter (SERT). Unbiased counts of SERT+ axon varicosities revealed an average density of 5-HT innervation higher in monkeys (1.52 x 10(6) varicosities/mm3) than rats (1.17 x 10(6)), particularly in the anterior half of the nucleus (1.70 x 10(6)). As measured by electron microscopy, SERT+ axon varicosity profiles in the STN of both species were smaller than unlabeled profiles. The number of SERT+ profiles displaying a synaptic junction indicated that, in both rat and monkey STN, approximately half of 5-HT axon varicosities were asynaptic. In monkeys, all synaptic junctions made by SERT+ varicosities were asymmetrical, as opposed to only 77% in rats. Despite the higher density of 5-HT innervation in the anterior half of monkey STN, the ultrastructural features of its SERT+ varicosities, including synaptic incidence, did not significantly differ from those in its posterior half. These findings suggest that, throughout the rat and monkey STN, 5-HT afferents may exert their influence via both synaptic delivery and diffusion of 5-HT, and that an ambient level of 5-HT maintained in STN by these two modes of transmission might also modulate neuronal activity and influence motor behavior. A better understanding of the factors governing the complex interplay between these signaling processes would greatly improve our knowledge of the physiopathology of the STN.

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 most-prominent behavioral effects of a D1 agonist are observed after a series of D1 agonist treatments--a process known as priming of D1 DA 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 observed D1 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.

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.

Modeling tardive dyskinesia: Predictive 5-HT2C 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 D(2) receptor blockers. Also, acute DA D(1) 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 D(1) 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 D(1) induction of VCMs. Moreover, in 6-OHDA-lesioned rats treated with haloperidol for one year, there a 2-fold increase in numbers of VCMs (vs 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-HT(2) receptor antagonists, but not DA D(1) receptor antagonists, attenuate the incidence of VCMs. This series of findings implicates the 5-HT neuronal phenotype in TD, and promotes 5-HT(2) receptor antagonists, more specifically 5-HT(2C) receptor antagonists, as a rational treatment approach for TD in humans.

Diminished serotonergic innervation of adult medial prefrontal cortex after 6-OHDA lesions in the newborn rat

The development of the serotonergic (5HT) and dopaminergic (DA) systems may contribute to the onset of psychotic disorders during late adolescence and early adulthood. Previous studies in our laboratory have suggested that these systems may compete for functional territory on neurons during development, as lesions of the serotonergic system at postnatal day 5 (P5) result in an increase in the density of dopaminergic fibers in rat medial prefrontal cortex (mPFC). In the present study, the dopaminergic system of P5 rats was lesioned with intracisternal injections of 6-hydroxydopamine (6-OHDA). Quantification of serotonin-immunoreactivity (5HT-IR) in mPFC at adulthood (P70) revealed a significant decrease in fiber density within layers II and III of the Cg3 subdivision of mPFC in lesioned rats compared to sham controls. We propose that the decrease in serotonergic fibers in mPFC in response to a neonatal depletion of dopamine may be due to the loss of a trophic effect of this system on 5HT neurons and/or fibers during development. Taken together with previous findings, our data suggest that there may be an "inverse trophic" relationship between the cortical DA and 5HT systems whereby dopamine facilitates the ingrowth of 5HT fibers, while serotonin suppresses the ingrowth of DA fibers. We present a model based on neurotrophic interactions at the cortical and brainstem levels that could potentially explain these unexpected results.

The neonate-6-hydroxydopamine-lesioned rat: a model for clinical neuroscience and neurobiological principles

In 1973, a technique of administering 6-hydroxydopamine (2,4,5-trihydroxyphenylethylamine) intracisternally to neonate rats was introduced to selectively reduce brain dopamine (neonate-lesioned rat). This neonate treatment proved unique when compared to rats lesioned as adults with 6-hydroxydopamine--prompting the discovery of differing functional characteristics resulting from the age at which brain dopamine is reduced. A realization was that neonate-lesioned rats modeled the loss of central dopamine and the increased susceptibility for self-injury in Lesch-Nyhan disease, which allowed identification of drugs useful in treating self-injury in mentally retarded patients. The neonate-lesioned rat has also been proposed to model the hyperactivity observed in attention-deficit hyperactivity disorder. Because the neonate-lesioned rat exhibits enhanced sensitization to repeated NMDA receptor antagonist administration and has functional changes characteristic of schizophrenia, the neonate lesioning is believed to emulate the hypothesized NMDA hypofunction in this psychiatric disorder. Besides modeling features of neurological and psychiatric disorders, important neurobiological concepts emerged from pharmacological studies in the neonate-lesioned rats. One was the discovery of coupling of D1/D2-dopamine receptor function. Another was the progressive increase in responsiveness to repeated D1-dopamine agonist administration referred to as "priming" of D1-dopamine receptor function. Additionally, a unique profile of signaling protein expression related to neonate reduction of dopamine has been identified. Thus, from modeling characteristics of disease to defining adaptive mechanisms related to neonatal loss of dopamine, the neonate-lesioned rat has had a persisting influence on neuroscience. Despite an extraordinary legacy from studies of the neurobiology of this treatment, a host of unknowns remain that will inspire future investigations.

The dopamine D4 receptor is essential for hyperactivity and impaired behavioral inhibition in a mouse model of attention deficit/hyperactivity disorder

The dopamine D4 receptor (D4R) is a candidate gene for attention deficit/hyperactivity disorder (ADHD) based on genetic studies reporting that particular polymorphisms are present at a higher frequency in affected children. However, the direct participation of the D4R in the onset or progression of ADHD has not been tested. Here, we generated a mouse model with high face value to screen candidate genes for the clinical disorder by neonatal disruption of central dopaminergic pathways with 6-hydroxydopamine (6-OHDA). The lesioned mice exhibited hyperactivity that waned after puberty, paradoxical hypolocomotor responses to amphetamine and methylphenidate, poor behavioral inhibition in approach/avoidance conflict tests and deficits in continuously performed motor coordination tasks. To determine whether the D4R plays a role in these behavioral phenotypes, we performed 6-OHDA lesions in neonatal mice lacking D4Rs (Drd4(-/-)). Although striatal dopamine contents and tyrosine hydroxylase-positive midbrain neurons were reduced to the same extent in both genotypes, Drd4(-/-) mice lesioned with 6-OHDA did not develop hyperactivity. Similarly, the D4R antagonist PNU-101387G prevented hyperactivity in wild-type 6-OHDA-lesioned mice. Furthermore, wild-type mice lesioned with 6-OHDA showed an absence of behavioral inhibition when tested in the open field or the elevated plus maze, while their Drd4(-/-) siblings exhibited normal avoidance for the unprotected areas of these mazes. Together, our results from a combination of genetic and pharmacological approaches demonstrate that D4R signaling is essential for the expression of juvenile hyperactivity and impaired behavioral inhibition, relevant features present in this ADHD-like mouse model.

Serotonin 5-HT2A Receptors Underlie Increased Motor Behaviors Induced in Dopamine-Depleted Rats by Intrastriatal 5-HT2A/2C Agonism

Gene expression studies have suggested that dopamine (DA) depletion increases the sensitivity of striatal direct pathway neurons to the effects of serotonin (5-HT) via the 5-HT(2) receptor. The present study examined the possible influence(s) of 5-HT(2A) or 5-HT(2C) receptor-mediated signaling locally within the striatum on motor behavior triggered by 5-HT(2) receptor agonism in the neonatal DA-depleted rat. Male Sprague-Dawley rats were treated with 6-hydroxydopamine (6-OHDA; 60 microg in 5 microl per lateral ventricle) on postnatal day 3 to achieve near-total DA depletion bilaterally. Sixty days later, sham-operated (saline-injected) or 6-OHDA-treated rats were challenged with the 5-HT(2A/2C) agonist DOI [(+/-)-1-(4-iodo-2,5-dimethoxyphenyl)-2-aminopropane] or saline either by systemic treatment or bilateral intrastriatal infusion. Motor behavior was quantified for 60 min after agonist injection using computerized activity monitors. Systemic DOI treatment (0.2 or 2.0 mg/kg i.p.) was more effective in inducing motor activity in the DA-depleted group compared with intact controls. Intrastriatal DOI infusion (1.0 or 10.0 microg/side) also produced a significant rise in motor activity in the DA-depleted group during the 30- to 60-min period of behavioral analysis but did not influence behavior in intact animals. The effects of intrastriatal DOI infusion were blocked by intrastriatal coinfusion of the 5-HT(2) antagonist ketanserin (1.0 microg) and the 5-HT(2A)-preferring antagonist M100907 [(R)(+)-alpha-(2,3-dimethoxyphenyl)-1-[2-(4-fluorophenyl)ethyl]-4-piperidinemethanol; 1.0 microg] but not the 5-HT(2C)-preferring antagonist RS102221 [8-[5-(2,4-dimethoxy-5-(4-trifluoromethylsulfo-amido)phenyl-5-oxopentyl]-1,3,8-triazaspiro[4.5]decane-2,4-dione; 1.0 microg]. Such results support the hypothesis that 5-HT(2A) receptor-mediated signaling events are strengthened within the striatum under conditions of DA depletion to provide a more potent regulation of motor activity.

Dopamine depletion abolishes apomorphine- and amphetamine-induced increases in extracellular serotonin levels in the striatum of conscious rats: a microdialysis study

We investigated how serotonergic neurotransmission was affected by 6-hydroxydopamine (6-OHDA) lesioning of the adult rat brain dopamine (DA) system. In this animal model for Parkinson's disease (PD), the effect of destroying ascending DA pathways on extracellular levels of serotonin (5-HT) and 5-HT innervation in rat striatum were examined. Profound unilateral lesions of the nigrostriatal DA pathways were made by infusing 6-OHDA unilaterally into either the right medial forebrain bundle or the right substantia nigra. At 5 weeks after lesioning extracellular levels of DA and 5-HT were determined with microdialysis and high-pressure liquid chromatography under basal conditions and after systemic injections of apomorphine or amphetamine. DA nerve-terminal destruction and 5-HT innervation were determined with quantitative autoradiography. 6-OHDA lesioning reduced extracellular levels of DA below detection limits and led to statistically significant increases in extracellular 5-HT. Apomorphine, and amphetamine, respectively increased extracellular 5-HT to 8.2- and 2.2-fold above baseline levels in intact animals; these effects were absent in 6-OHDA-lesioned animals. Basal levels of [(3)H]paroxetine binding to 5-HT transporters in caudate-putamen increased by 41% in 6-OHDA-lesioned animals. These results suggest that 6-OHDA lesioning led to hyperinnervation of 5-HT nerve terminals and increases in basal extracellular 5-HT levels, but also to an unexplained loss of apomorphine and amphetamine-induced release of 5-HT. Addressing whether this impairment has significance in the onset of PD might lead to development of new strategies to manage parkinsonian symptoms.

Dopamine receptor supersensitivity: an outcome and index of neurotoxicity

The characteristic 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 throughout the lifespan. DA D(1) receptors show overt supersensitivity to agonists producing vacuous chewing movements (VCMs), while D(1) receptors associated with locomotor activity have a latent supersensitivity that must be unmasked by repeated D(1) or D(2) agonist treatments - a 'priming' phenomenon. This D(1) DARSS is not usually associated in either a change in D(1) receptor number (B(max)) or affinity (K(d)). In contrast to D(1) DARSS, D(2) receptors are not so predictably supersensitized by a lesion 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. Long-lived agonist-induced behavioral abnormality, with or without anatomic evidence of a neuronal lesion, is one of the products of DA D(1) receptor supersensitization -- itself an index of neurotoxicity.

Serotonin transporter binding increases in caudate-putamen and nucleus accumbens after neonatal 6-hydroxydopamine lesions in rats: implications for motor hyperactivity

We examined serotonin (5-HT) transporters in rat forebrain using quantitative autoradiography at three distinct developmental stages after neonatal 6-hydroxydopamine lesions. The lesions substantially increased 5-HT transporter binding in both caudate-putamen and nucleus accumbens, but not cerebral cortex. The effects reached maximal levels as early as postnatal day (PD) 24, and were sustained until early adulthood. Behavioral analyses indicated that neonatal lesions resulted in motor hyperactivity on PD 24, but not on PD 36 or 59. These findings suggest that excess 5-HT transporters reflect serotonin hyperinnervation reported to occur in lesioned rats, and may modulate motor hyperactivity.

Distribution of serotonin, its metabolites and 5-HT transporters in the neostriatum of Lurcher and weaver mutant mice

Serotonin (5-HT) uptake sites, or transporters, were measured in the neostriatum (caudate putamen) of wild type (+/+) mice and heterozygous (wv/+) and homozygous (wv/wv) weaver, as well as in heterozygous Lurcher (Lc/+) mutants. These topological surveys were carried out by quantitative ligand binding autoradiography using the uptake site antagonist [3H]-citalopram as a probe of innervation densities in four quadrants of the rostral neostriatum and in two halves of the caudal neostriatum. In addition, tissue concentrations of 5-HT, 5-hydroxyindole-3-acetic acid and 5-hydroxytryptophol were measured by high-performance liquid chromatography with electrochemical detection in these neostriatal divisions. In +/+ mice and in Lc/+ mutants there was a dorso-ventral gradient of increasing 5-HT levels, and they exhibited a similar heterogeneity of [3H]-citalopram labeling. In contrast, the gradients of 5-HT concentrations and [3H]-citalopram binding disappeared in the weaver mutants, suggesting a rearrangement of the 5-HT innervation. This reorganization of the 5-HT system in the neostriatum was more obvious in the wv/wv and is compatible with the hypothesis that the postnatal dopaminergic deficiencies that characterize weaver mutants lead to a sprouting of fibers and thus constitute a genetic model of dopaminergic denervation that leads to a 5-HT hyperinnervation.

Central serotonin system in Dystonia musculorum mutant mice: biochemical, autoradiographic and immunocytochemical data

The autosomal recessive mutation dystonia musculorum (dt(J)/dt(J)) causes degenerative alterations of peripheral and central sensory pathways that lead to ataxia. To investigate possible changes in the central serotonin system of these mice, HPLC measurements of 5-hydroxytryptophan, 5-hydroxy-tryptamine (serotonin; 5-HT), and 5-HT metabolites were obtained from 22 brain regions and the spinal cord of wild type and dt(J)/dt(J) mutant mice. Also, 5-HT transporters were quantified by [(3)H]citalopram autoradiography in 72 brain regions, subregions, and nuclei, and the 5-HT innervation visualized by immunocytochemistry throughout the brain and spinal cord. In all brain regions measured for indoleamine content, there were no significant differences between the two genotypes. In the spinal cord, an increased tissue concentration of 5-HT (+34%), 5-hydroxyindole-3-acetic acid (+33%), 5-hydroxytryptophol (+21%), and 5-hydroxytryptophan (+45%) in dt(J)/dt(J) actually corresponded to the same total amount of each of these indoleamines in the entire spinal cord, when taking into account its reduced size in the mutants. Quantification of the binding to 5-HT transporters showed increases in the medial geniculate nucleus (+14%), medial (+24%) and lateral (+18%) hypothalamus, interpeduncular (+13%), vestibular (+22%), and deep cerebellar nuclei (+37%) of dt(J)/dt mice, and decreases in the ventral tegmental area (-13%), median and linear raphe nuclei (-20%), as well as in the solitary complex (-35%). There were no apparent differences in the distribution of 5-HT-immunostained fibers in these and other regions of brain and in the spinal cord of dt(J)/dt(J) compared to wild type mice. The bulk of these results indicates a relative sparing of the central 5-HT system in the dt(J)/dt(J) mice, even though alterations in 5-HT transporters could justify attempts at improving the sensorimotor dysfunction by administration of serotoninergic agents in these mice.

Temporal changes of dopaminergic and glutamatergic receptors in 6-hydroxydopamine-treated rat brain

Quantitative receptor autoradiography was used to examine the sequential patterns of changes in dopaminergic and glutamatergic receptors in the brain of rats lesioned with 6-hydroxydopamine. The animals were unilaterally lesioned in the medial forebrain bundle and the brains were analyzed at 1, 2, 4 and 8 weeks of postlesion. Degeneration of the nigrostriatal pathway caused a significant increase in dopamine D(2) receptors in the ipsilateral striatum from 1 to 8 weeks of postlesion. In the ipsilateral substantia nigra (SN), a significant decrease in dopamine D(2) receptors was also observed from 1 to 8 weeks of postlesion. On the other hand, dopamine D(1) receptors were increased in the ipsilateral ventromedial striatum from 2 to 4 weeks of postlesion. In the ipsilateral SN, a transient increase in dopamine D(1) receptors was observed only 1 week after lesioning. However, other regions in both ipsilateral and contralateral sides showed no significant change in dopamine D(1) and D(2) receptors during postlesion except for a transient change in a few regions. N-Methyl-D-aspartate (NMDA) receptors showed no significant changes in all brain regions studied during the postlesion. In contrast, a transient increase in excitatory amino acid transport sites was observed only in the frontal cortex and ventromedial striatum of the ipsilateral side at 2 weeks of postlesion. However, glycine receptors showed a significant change in any brain areas of both ipsilateral and contralateral sides after lesioning. The change in the brain areas of contralateral side was more pronounced than that of ipsilateral side for glycine receptors. In addition, dopamine uptake sites showed a severe damage in the ipsilateral striatum from 1 to 8 weeks after lesioning. In the contralateral side, in contrast, no significant change in dopamine uptake sites was found in the striatum during the postlesion. These results indicate that unilateral injection of 6-hydroxydopamine in the medial forebrain bundle can cause a significant increase in dopamine D(1) and D(2) receptors in the striatum. The increase in dopamine D(2) receptors was more pronounced than that in dopamine D(1) receptors in the striatum after 6-hydroxydopamine treatment. In contrast, dopamine uptake sites showed a severe damage in the striatum during the postlesion. Furthermore, our results support the existence of dopamine D(2) receptors on the neurons of SN, but not dopamine D(1) receptors. For glutamatergic receptor system, the present study suggests that the changes in glycine receptors may be more susceptible to degeneration of nigrostriatal pathway than NMDA receptors and excitatory amino acid transport sites. Thus, our findings are of interest in relation of degeneration of the nigrostriatal pathway that occurs in Parkinson's disease

Alterations in serotonin receptors in the neostriatum of weaver mutant mice

Mice that carry the autosomal recessive gene weaver show a distinctive loss of nigrostriatal dopamine innervation, with the greatest deficits in the dorsal caudate-putamen and almost complete sparing in the nucleus accumbens and ventral caudate. In addition to loss of dopamine in this model, it has recently been shown that markers of serotonin (5-hydroxytryptamine, 5-HT) innervation including 5-HT content, synaptosomal uptake of [3H]5-HT and [3H]citalopram binding were elevated in the dorsal neostriatum of the weaver mutant mouse. Using quantitative autoradiography of specific ligands for dopamine and 5-HT uptake sites as well as serotonin 5-HT1 and 5-HT2A receptors, we found an increased density of 5-HT uptake sites and 5-HT1 receptors restricted to the dorsal portion of the neostriatum of the weaver mouse. In contrast, 5-HT2A receptors were increased in both the dorsal and ventral portions of the rostral neostriatum as well as the nucleus accumbens. The behavioural and functional relevance of these receptor changes is unclear, although, adaptations in 5-HT may play a role in certain aspects of spontaneous behaviour in the weaver mutant mouse.

Serotonin 2A receptor mRNA levels in the neonatal dopamine-depleted rat striatum remain upregulated following suppression of serotonin hyperinnervation

Sixty days after bilateral dopamine (DA) depletion (>98%) with 6-hydroxydopamine (6-OHDA) in neonatal rats, serotonin (5-HT) content doubled and 5-HT(2A) receptor mRNA expression rose 54% within the rostral striatum. To determine if striatal 5-HT(2A) receptor mRNA upregulation is dependent on increased 5-HT levels following DA depletion, neonatal rats received dual injections of 6-OHDA and 5,7-dihydroxytryptamine (5,7-DHT) which suppressed 5-HT content by approximately 90%. In these 6-OHDA/5,7-DHT-treated rats, striatal 5-HT(2A) receptor mRNA expression was still elevated (87% above vehicle controls). Comparative analysis of 5-HT(2C) receptor mRNA expression yielded no significant changes in any experimental group. These results demonstrate that upregulated 5-HT(2A) receptor biosynthesis in the DA-depleted rat is not dependent on subsequent 5-HT hyperinnervation.

In vivo electrochemical measurements of serotonin clearance in rat striatum: effects of neonatal 6-hydroxydopamine-induced serotonin hyperinnervation and serotonin uptake inhibitors

Diffusion and clearance of extracellular serotonin (5-HT) was examined using in vivo chronoamperometry with "delayed-pulse" recordings after pressure ejections of 1 to 60 picomoles 5-HT into rat striatum at a fixed distance from a Nafion-coated carbon fiber electrode. Signals obtained were identified based on the signal characteristics to consist of 5-HT. Clearance times of 5-HT decreased, while amplitudes and rise times increased with serotonergic hyperinnervation induced by neonatal 6-hydroxydopamine (6-OHDA) lesions of dopamine (DA) neurons. Local applications of the 5-HT uptake inhibitors zimelidine or fluoxetine, in conjunction with 5-HT ejections, produced increased clearance times in both normal and 6-OHDA-treated animals. Thus, direct in vivo evidence was obtained for the importance of high affinity nerve terminal uptake as a key mechanism for clearance of 5-HT from the extracellular space. Inhibitors of 5-HT uptake appear to prolong the extracellular presence of 5-HT by increasing its clearance time.

Morphological and biochemical adaptations to unilateral dopamine denervation of the neostriatum in newborn rats

Basal ganglia of adult rats were examined for morphological and biochemical changes resulting from neonatal unilateral dopamine denervation of the striatum with increasing doses of 6-hydroxydopamine (4, 12 and 20 microg). Rotational behaviour induced by apomorphine (0.1 mg/kg) was observed in all rats injected with the high dose (20 microg) and totally absent in those injected with the low dose (4 microg). As assessed with tyrosine hydroxylase immunocytochemistry, the extent of dopamine denervation within the injected striatum was clearly related to the dose injected. In the mesencephalon, losses of tyrosine hydroxylase-immunoreactive cell bodies were proportional to the dose injected and the extent of neostriatal dopamine denervation. This retrograde cell loss predominated in the ventromedial and lateral parts of the substantia nigra pars compacta, with relative sparing of the ventral tegmental area. After the injection of the intermediate (12 microg) and the high (20 microg) doses, a network of thin tyrosine hydroxylase-immunoreactive fibres was visualized in the ventral part of the pars reticulata ipsilateral to the injected striatum, suggesting a neoinnervation of this structure by dopamine axons. After the high dose, the density of serotonin-immunoreactive fibres was enhanced in the anterior half of the lesioned striatum. Associated changes in dopamine and serotonin content and turnover were also documented on both sides, in the striatum and in two output structures of the basal ganglia, the globus pallidus and the substantia nigra. Dopamine content was decreased only on the injected side. After the low dose, equal reductions (-60%) were observed in the anterior striatum and the substantia nigra, whereas a more marked decrease was measured in the anterior striatum (-93%) than in the substantia nigra (-60% to -74%) after the intermediate and high doses. In the globus pallidus, dopamine tissue content was decreased (-51%) only after the high dose. Dopamine turnover was unchanged after the low dose in all structures examined and was increased in the striatum, on the lesioned side only, after the intermediate and high doses. Serotonin content was increased only on the injected side in the anterior striatum (+50% after the low and +92% after the high dose). Serotonin turnover was unchanged on the injected side but increased by +118% and by +81% in the contralateral anterior striatum after the low and high doses, respectively. It was also increased in both substantia nigra after the high dose. In conclusion, morphological changes similar to those described after a bilateral neonatal lesion were observed on the injected side in the model of the unilateral neonatal nigrostriatal dopamine denervation. Biochemical changes were, however, not restricted to the lesioned side. Notably, changes in serotonin turnover developed on the contralateral side. These morphological and biochemical adaptative changes need to be taken into account in considering the mechanisms implicated in the rotional behaviour measured in these animals.

Evaluation of three transporter ligands as quantitative markers of serotonin innervation density in rat brain

Direct counting of axon terminals (varicosities) labeled by uptake/storage of a tritiated monoamine provides a means to test radioligands of the corresponding membrane transporter as quantitative markers of regional monoamine innervation density in brain tissue. In autoradiographs from alternate rat brain slices, counts of [3H]5-HT-labeled axon terminals were matched with densitometric measurements of the specific binding of tritiated cyanoimipramine (CYI), citalopram (CITAL), and 6-nitroquipazine (6-NTQ), under conditions of hypo-, normo-, or hyper-5-HT innervation of the neostriatum. A total of 267 pairs of data were subjected to a multilevel analysis (iterative generalized least square procedure). With all three ligands, there was a linear relationship between the density of 5-HT innervation and the density of specific binding and no change in the slope of the regression lines as a function of 5-HT innervation density. Thus, none of these ligands gave any sign of down- or up-regulation of the 5-HT transporter consequent to 5-HT hypo- or hyper-innervation. The regression lines for CYI and CITAL were not significantly different from one another and crossed the ordinate near zero, whereas the regression line for 6-NTQ was less steep and had a higher intercept with the ordinate. In addition, the dispersion of values around the regression line (residuals) was lower with CYI and CITAL than 6-NTQ. It was concluded that both CYI and CITAL may serve as quantitative markers of 5-HT innervation density, at least in vitro, whereas 6-NTQ demonstrates a certain lack of specificity and sensitivity. Further work will be needed to assess the potential of CYI and CITAL for positron emission tomographic studies of living brain. Such empirical testing should also be applicable for screening radioligands of the dopamine or the noradrenaline transporters.

Changes in steady-state levels of tryptophan hydroxylase protein in adult rat brain after neonatal 6-hydroxydopamine lesion

A recently developed technique of immunoautoradiography on nitrocellulose transfers of serial frozen sections was used to determine tryptophan hydroxylase concentration in selected areas of the adult rat brain following neonatal 6-hydroxydopamine destruction of nigrostriatal dopamine neurons. Particular attention was paid to the neostriatum, known to be serotonin-hyperinnervated under these conditions, and to the nucleus raphe dorsalis, containing the cell bodies of origin for these nerve terminals. The hippocampus was also investigated as a territory of structurally intact serotonin innervation arising primarily from the nucleus raphe medianus. Tryptophan hydroxylase protein was measured at successive transverse levels across the entire caudorostral extent of all these regions. Similar measurements of tyrosine hydroxylase protein across the substantia nigra and the neostriatum verified the disappearance of the nigrostriatal dopamine neurons. The average tryptophan hydroxylase tissue concentration in the dorsal third of the serotonin-hyperinnervated neostriatum was up by 36% above control, i.e. significantly less than the number of its serotonin axon terminals or varicosities. This was therefore indicative of a lowering of the tryptophan hydroxylase protein content per serotonin ending. Interestingly, a tight correlation between the respective level-by-level concentrations of tryptophan hydroxylase and tyrosine hydroxylase protein in the control neostriatum allowed the prediction the tryptophan hydroxylase concentration after dopamine denervation with a serotonin hyperinnervation. Tryptophan hydroxylase concentration was also significantly reduced in both the nucleus raphe dorsalis and nucleus raphe medianus, notably at those raphe dorsalis levels known to give rise to the serotonin hyperinnervation of neostriatum. It is hypothesized that the lower steady-state level of tryptophan hydroxylase inside the terminals and cell bodies of hyperinnervating serotonin neurons was the result of a feedback inhibition of the synthesis of the enzyme by its end-product, presumably because of the increased amount of serotonin in these terminals.