Supersensitized oral responses to a serotonin agonist in neonatal 6-OHDA-treated rats

Serotonergic control of the glutamatergic neurons of the subthalamic nucleus

The subthalamic nucleus (STN) houses a dense cluster of glutamatergic neurons that play a central role in the functional dynamics of the basal ganglia, a group of subcortical structures involved in the control of motor behaviors. Numerous anatomical, electrophysiological, neurochemical and behavioral studies have reported that serotonergic neurons from the midbrain raphe nuclei modulate the activity of STN neurons. Here, we describe this serotonergic innervation and the nature of the regulation exerted by serotonin (5-hydroxytryptamine, 5-HT) on STN neuron activity. This regulation can occur either directly within the STN or at distal sites, including other structures of the basal ganglia or cortex. The effect of 5-HT on STN neuronal activity involves several 5-HT receptor subtypes, including 5-HT_1A, 5-HT_1B, 5-HT_2C and 5-HT₄ receptors, which have garnered the highest attention on this topic. The multiple regulatory effects exerted by 5-HT are thought to be modified under pathological conditions, altering the activity of the STN, or due to the benefits and side effects of treatments used for Parkinson's disease, notably the dopamine precursor l-DOPA and high-frequency STN stimulation. Originally understood as a motor center, the STN is also associated with decision making and participates in mood regulation and cognitive performance, two domains of personality that are also regulated by 5-HT. The literature concerning the link between 5-HT and STN is already important, and the functional overlap is evident, but this link is still not entirely understood. The understanding of this link between 5-HT and STN should be increased due to the possible importance of this regulation in the control of fronto-STN loops and inherent motor and non-motor behaviors.

A Subset of Purposeless Oral Movements Triggered by Dopaminergic Agonists Is Modulated by 5-HT2C Receptors in Rats: Implication of the Subthalamic Nucleus

Dopaminergic medication for Parkinson’s disease is associated with troubling dystonia and dyskinesia and, in rodents, dopaminergic agonists likewise induce a variety of orofacial motor responses, certain of which are mimicked by serotonin2C (5-HT_2C) receptor agonists. However, the neural substrates underlying these communalities and their interrelationship remain unclear. In Sprague-Dawley rats, the dopaminergic agonist, apomorphine (0.03–0.3 mg/kg) and the preferential D2/3 receptor agonist quinpirole (0.2–0.5 mg/kg), induced purposeless oral movements (chewing, jaw tremor, tongue darting). The 5-HT_2C receptor antagonist 5-methyl-1-[[2-[(2-methyl-3-pyridyl)oxyl]-5-pyridyl]carbamoyl]-6-trifluoromethylindone (SB 243213) (1 mg/kg) reduced the oral responses elicited by specific doses of both agonists (0.1 mg/kg apomorphine; 0.5 mg/kg quinpirole). After having confirmed that the oral bouts induced by quinpirole 0.5 mg/kg were blocked by another 5-HT_2C antagonist (6-chloro-5-methyl-1-[6-(2-methylpiridin-3-yloxy)pyridine-3-yl carbamoyl] indoline (SB 242084), 1 mg/kg), we mapped the changes in neuronal activity in numerous sub-territories of the basal ganglia using c-Fos expression. We found a marked increase of c-Fos expression in the subthalamic nucleus (STN) in combining quinpirole (0.5 mg/kg) with either SB 243213 or SB 242084. In a parallel set of electrophysiological experiments, the same combination of SB 243213/quinpirole produced an irregular pattern of discharge and an increase in the firing rate of STN neurons. Finally, it was shown that upon the electrical stimulation of the anterior cingulate cortex, quinpirole (0.5 mg/kg) increased the response of substantia nigra pars reticulata neurons corresponding to activation of the “hyperdirect” (cortico-subthalamonigral) pathway. This effect of quinpirole was abolished by the two 5-HT_2C antagonists. Collectively, these results suggest that induction of orofacial motor responses by D2/3 receptor stimulation involves 5-HT_2C receptor-mediated activation of the STN by recruitment of the hyperdirect (cortico-subthalamonigral) pathway.

The role of glutamate receptors and their interactions with dopamine and other neurotransmitters in the development of tardive dyskinesia: preclinical and clinical results

Tardive dyskinesia is a serious, disabling, movement disorder associated with the ongoing use of antipsychotic medication. Current evidence regarding the pathophysiology of tardive dyskinesia is mainly based on preclinical animal models and is still not completely understood. The leading preclinical hypothesis of tardive dyskinesia development includes dopaminergic imbalance in the direct and indirect pathways of the basal ganglia, cholinergic deficiency, serotonin receptor disturbances, neurotoxicity, oxidative stress, and changes in synaptic plasticity. Although, the role of the glutamatergic system has been confirmed in preclinical tardive dyskinesia models it seems to have been neglected in recent reviews. This review focuses on the role and interactions of glutamate receptors with dopamine, acetylcholine, and serotonin in the neuropathology of tardive dyskinesia development. Moreover, preclinical and clinical results of the differentiated effectiveness of N-methyl-D-aspartate (NMDA) receptor antagonists are discussed with a special focus on antagonists that bind with the GluN2B subunit of NMDA receptors. This review also presents new combinations of drugs that are worth considering in the treatment of tardive dyskinesia.

Purposeless oral activity induced by meta-chlorophenylpiperazine (m-CPP): Undefined tic-like behaviors?

BACKGROUND

The pathophysiological hypothesis underlying tic disorders in Tourette syndrome (TS) is that basal ganglia are not capable of properly filtering cortical information, leading patients with difficulties in inhibiting unwanted behaviors or impulses. One of the main challenges for furthering such a hypothesis is to find appropriate animal models summarizing some aspects of the disease.

METHODS

It has been established for more than 25 years in rodents that the prototypical serotonin (5-HT) agonist meta-chlorophenylpiperazine (m-CPP) elicits purposeless oral movements including chewing behavior. These bouts of oral movements, originally thought to mimic human oral dyskinesia consequent to long-term administration of antipsychotic drugs or parkinsonian tremor, could correspond to an undefined form of tics. Here, we describe the nature of the purposeless oral movements triggered by m-CPP and other agonists which could be associated with obsessive compulsive disorders. We report the pharmacology of this response with a focus on the 5-HT_2C receptor subtype and the degree to which the dopaminergic and cholinergic systems are involved. The orofacial dyskinetic effects are related to the action of these compounds in associative/limbic territories of the basal ganglia, rather than sensorimotor ones, as expected from the human disease.

CONCLUSION

In spite of the low translational value of these oral movements, the neurobiological analysis of these oral movements could help to a better understanding of the pathophysiology of tics and compulsive disorders often cormorbid with TS.

Serotonergic modulation of the activity of mesencephalic dopaminergic systems: Therapeutic implications

Since their discovery in the mammalian brain, it has been apparent that serotonin (5-HT) and dopamine (DA) interactions play a key role in normal and abnormal behavior. Therefore, disclosure of this interaction could reveal important insights into the pathogenesis of various neuropsychiatric diseases including schizophrenia, depression and drug addiction or neurological conditions such as Parkinson's disease and Tourette's syndrome. Unfortunately, this interaction remains difficult to study for many reasons, including the rich and widespread innervations of 5-HT and DA in the brain, the plethora of 5-HT receptors and the release of co-transmitters by 5-HT and DA neurons. The purpose of this review is to present electrophysiological and biochemical data showing that endogenous 5-HT and pharmacological 5-HT ligands modify the mesencephalic DA systems' activity. 5-HT receptors may control DA neuron activity in a state-dependent and region-dependent manner. 5-HT controls the activity of DA neurons in a phasic and excitatory manner, except for the control exerted by 5-HT_2C receptors which appears to also be tonically and/or constitutively inhibitory. The functional interaction between the two monoamines will also be discussed in view of the mechanism of action of antidepressants, antipsychotics, anti-Parkinsonians and drugs of abuse.

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.

The enhanced oral response to the 5-HT2 agonist Ro 60-0175 in parkinsonian rats involves the entopeduncular nucleus: electrophysiological correlates

Lesions of nigrostriatal dopaminergic neurons as seen in Parkinson's disease (PD) increase orofacial responses to serotonergic (5-HT) agonists in rodents. Although this response to 5-HT agonists has been related to aberrant signalling in the basal ganglia, a group a subcortical structures involved in the control of motor behaviours, it deserves additional studies with respect to the specific loci involved. Using measurements of orofacial activity, as well as single-cell recordings in vivo, we have studied the role of the entopeduncular nucleus (EPN; equivalent to the internal globus pallidus of primates), an output structure of basal ganglia, in the hypersensitized responses to a 5-HT agonist in sham- or unilaterally dopamine-depleted rats. Intra-EPN injections of Ro 60-0175 (0.3 and 1 μg/100 nl) promoted robust oral movements in 6-OHDA rats without affecting oral activity in sham-depleted rats. Peripheral administration of Ro 60-0175 (3 mg/kg ip) decreased EPN neuronal firing rate in 6-OHDA rats compared to sham-depleted rats. Such an effect was also observed when the agonist (0.2 μg/20 nl) was locally applied onto EPN neurons. These data demonstrate the contribution of EPN to hypersensitized responses to 5-HT agonists in a rat model of PD.

Neonatal co-lesion by DSP-4 and 5,7-DHT produces adulthood behavioral sensitization to dopamine D(2) receptor agonists

To assess the possible modulatory effects of noradrenergic and serotoninergic neurons on dopaminergic neuronal activity, the noradrenergic and serotoninergic neurotoxins DSP-4 N-(2-chlorethyl)-N-ethyl-2-bromobenzylamine (50.0 mg/kg, sc) and 5,7-dihydroxytryptamine (5,7-DHT) (37.5 microg icv, half in each lateral ventricle), respectively, were administered toWistar rats on the first and third days of postnatal ontogeny, and dopamine (DA) agonist-induced behaviors were assessed in adulthood. At eight weeks, using an HPLC/ED technique, DSP-4 treatment was associated with a reduction in NE content of the corpus striatum (> 60%), hippocampus (95%), and frontal cortex (> 85%), while 5,7-DHT was associated with an 80-90% serotonin reduction in the same brain regions. DA content was unaltered in the striatum and the cortex. In the group lesioned with both DSP-4 and 5,7-DHT, quinpirole-induced (DA D(2) agonist) yawning, 7-hydroxy-DPAT-induced (DA D(3) agonist) yawning, and apomorphine-induced (non-selective DA agonist) stereotypies were enhanced. However, SKF 38393-induced (DA D(1) agonist) oral activity was reduced in the DSP-4 + 5,7-DHT group. These findings demonstrate that DA D(2)- and D(3)-agonist-induced behaviors are enhanced while DA D(1)-agonist-induced behaviors are suppressed in adult rats in which brain noradrenergic and serotoninergic innervation of the brain has largely been destroyed. This study indicates that noradrenergic and serotoninergic neurons have a great impact on the development of DA receptor reactivity (sensitivity).

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.

Amphetamine and mCPP effects on dopamine and serotonin striatal in vivo microdialysates in an animal model of hyperactivity

In the neonatally 6-hydroxydopamine (6-OHDA)-lesioned rat hyperlocomotor activity, first described in the 1970s, was subsequently found to be increased by an additional lesion with 5,7-dihydroxytryptamine (5,7-DHT) (i.c.v.) in adulthood. The latter animal model (i.e., 134 microg 6-OHDA at 3 d postbirth plus 71 microg 5,7-DHT at 10 weeks; desipramine pretreatments) was used in this study, in an attempt to attribute hyperlocomotor attenuation by D,L-amphetamine sulfate (AMPH) and m-chlorophenylpiperazine di HCl (mCPP), to specific changes in extraneuronal (i.e., in vivo microdialysate) levels of dopamine (DA) and/or serotonin (5-HT). Despite the 98-99% reduction in striatal tissue content of DA, the baseline striatal microdialysate level of DA was reduced by 50% or less at 14 weeks, versus the intact control group. When challenged with AMPH (0.5 mg/kg), the microdialysate level of DA went either unchanged or was slightly reduced over the next 180 min (i.e., 20 min sampling), while in the vehicle group and 5,7-DHT (alone) lesioned group, the microdialysate level was maximally elevated by approximately 225% and approximately 450%, respectively--and over a span of nearly 2 h. Acute challenge with mCPP (1 mg/kg salt form) had little effect on microdialysate levels of DA, DOPAC and 5-HT. Moreover, there was no consistent change in the microdialysate levels of DA, DOPAC, and 5-HT between intact, 5-HT-lesioned rats, and DA-lesioned rats which might reasonably account for an attenuation of hyperlocomotor activity. These findings indicate that there are other important neurochemical changes produced by AMPH- and mCPP-attenuated hyperlocomotor activity, or perhaps a different brain region or multiple brain regional effects are involved in AMPH and mCPP behavioral actions.

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.

Movement disorders: neurodevelopment and neurobehavioural expression

Braak and co-workers have recently shown that movement disorders such as Parkinson's disease develop progressively over years with early neuronal losses in brainstem regions caudal to the substantia nigra. The relevance of this finding to notions of comorbidity between movement disorders and psychiatric symptoms was recognised at the recent meeting concerning, "Implications of Comorbidity for the Etiology and Treatment of Neuropsychiatric Disorders" held in Oct. 2005 in Mazagon, Spain. The identification of stages in the early development of neurodegenerative disorders appeared to unify multiple, diverse findings. These included: novel therapeutic innovations for Parkinson's disease, Alzheimer's disease and depression in the aged; the neurochemical ontogeny of drug-induced oral dyskinesias; the types of chemical agents abused in neuropsychiatric states; postnatal iron overload effects upon the functional and interactive role of dopaminergic and noradrenergic pathways that contribute to the expression of movement disorders; and the spectrum of motor symptoms expressed in schizophrenia and attention deficit hyperactivity disorder and the eventual treatment of these disorders. A continued focus on a number of neuropsychiatric diseases as progressive disorders may lead to further advances in understanding their etiology and in developing better therapeutics.

DSP-4 prevents dopamine receptor priming by quinpirole

Repeated treatments of rats with the dopamine (DA) D2 receptor agonist quinpirole, consistently produce long-lived DA D2 receptor supersensitization, by the process that has been termed priming. Rats so-primed in ontogeny behaviorally demonstrate adulthood enhancement of low-dose quinpirole-induced yawning. Because 1) dopaminergic neurons originate in midbrain nuclei (substantia nigra and ventral tegmental area), and 2) noradrenergic neurons originate in pontine (locus coeruleus) and medullary areas, it might be presumed that these two monoaminergic systems are independent, not interdependent. However, in the present study we demonstrate that there was an attenuation of quinpirole-enhanced yawning at 8 weeks in rats that were 1) primed by repeated neonatal quinpirole HCl treatments (50 microg/kg per day SC) during the first ten days of postnatal ontogeny, and 2) lesioned at 3 days after birth with DSP-4 (N-2-chloroethyl-N-ethyl-2-bromobenzylamine hydrochloride, 50 mg/kg SC). Dose-effect curves indicated a 23-45% reduction in yawning by DSP-4 treatment of quinpirole-primed rats, acutely treated as adults with quinpirole (25, 50, or 100 microg/kg). Effectiveness of DSP-4 is reflected by the 95% and 99% reductions in norepinephrine contents of frontal cortex and hippocampus, respectively (HPLC/ED method). The findings are supportive of a modulatory role of noradrenergic fibers on dopamine receptor priming (supersensitization) in rat brain.

Enhanced 5-HT2C receptor signaling is associated with haloperidol-induced "early onset" vacuous chewing in rats: implications for antipsychotic drug therapy

RATIONALE

Haloperidol is a representative of typical antipsychotics that are still in clinical use and which can lead to abnormal motor activity following repeated administration. The mechanisms underlying antipsychotic-induced dyskinesias are not well understood but are widely held to be related to excessive loss of dopamine function. In several models of dopamine hypofunction, serotonin 5-HT2C receptors have been shown to mediate vacuous chewing movements (VCM), a form of abnormal motor activity. It is well established that repeated haloperidol administration also elicits VCM, but there is no information on how repeated haloperidol administration affects 5-HT2C receptor signaling.

OBJECTIVES

In the present study, we tested the hypothesis that repeated daily administration of haloperidol leads to enhanced serotonin 5-HT2C receptor signaling that is associated with increased 5-HT2C-mediated VCM.

METHODS

Rats were treated by subcutaneous injection once daily for 21 days with either vehicle, a low dose of haloperidol (0.1 mg kg(-1) day(-1)), or a high dose of haloperidol (1.0 mg kg(-1) day(-1)). Following 1-day withdrawal, rats were either used for behavioral scoring of VCM or sacrificed for biochemical assessment of 5-HT2 receptor-mediated phospholipase C activity and radioligand binding. VCM were scored following two successive "drug" challenges. The first challenge was an injection of vehicle (0.9% saline), and the second challenge was an injection of the 5-HT2C agonist meta-chlorophenylpiperazine (1.0 mg/kg). In this manner, a measure of "spontaneous" and "5-HT2C-elicited" orofacial activity could be made while minimizing animal use.

RESULTS

Following 21-day haloperidol treatment at either dose, there was an increase in expression of meta-chlorophenylpiperazine-induced VCM. In a separate experiment, meta-chlorophenylpiperazine-induced VCM were shown to be mediated through 5-HT2C receptors. Striatal 5-HT2C receptor-mediated phospholipase C (PLC) activity and high-affinity agonist-labeled 5-HT2C receptors were also increased following either dose of haloperidol as compared to vehicle treatment. GTP-stimulated PLC activity and striatal Gq proteins were unchanged by haloperidol suggesting that enhanced signaling could be accounted for by alterations at the level of the receptor and not at downstream mechanisms.

CONCLUSIONS

Repeated daily administration of haloperidol leads to an adaptive increase in 5-HT2C signaling which may contribute to abnormal motor function associated with antipsychotic use.

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.

Combined intrastriatal dopamine D1 and serotonin 5-HT2 receptor stimulation reveals a mechanism for hyperlocomotion in 6-hydroxydopamine-lesioned rats

Loss of dopaminergic innervation to the striatum increases the sensitivity of dopamine (DA) D1 and serotonin (5-HT) 5-HT2 receptor signaling. Previous work from our laboratory has shown that systemic co-administration of D1 and 5-HT2 receptor agonists leads to the synergistic overexpression of striatal preprotachykinin mRNA levels in the DA-depleted, but not intact animals. In the present study, we examined this mechanism as related to locomotor behavior. Adult male Sprague-Dawley rats were subject to bilateral i.c.v. 6-hydroxydopamine (6-OHDA; 200 microg in 10 microl/side) or vehicle (0.9% saline and 0.1% ascorbic acid). After 3 weeks, rats were tested for locomotor responses to bilateral intrastriatal infusions of vehicle (0.9% NaCl), the D1 agonist SKF82958 [(+/-)6-chloro-7,8-dihydroxy-3-allyl-1-phenyl-2,3,4,5-tetra-hydro-(1H)-3-benzazepine hydrobromide; 0.1, 1.0 or 10.0 microg/side], the 5-HT2 agonist DOI [(+/-)-1-(4-iodo-2,5-dimethoxyphenyl)-2-aminopropane; 0.1, 1.0 or 10.0 microg/side] or subthreshold doses of DOI and SKF82958 (0.1 microg+0.1 microg in 0.8 microl/side). Rats with DA loss demonstrated supersensitive locomotor responses to SKF82958, but not DOI. Combined administration of subthreshold SKF82958 and DOI doses (0.1 microg+0.1 microg) synergistically increased locomotor behavior only in 6-OHDA-lesioned rats. These effects were blocked by either the D1 antagonist SCH23390 3-methyl-1-phenyl-2,3,4,5-tetrahydro-7-chloro-8-hydroxy-(1H)-3-benzazepine or the 5-HT2 antagonist ritanserin (each 1.0 microg in 0.8 microl/side). The results of this study suggest that the behavioral synergy induced by local co-stimulation of D1 and 5-HT2 receptors within the 6-OHDA-lesioned striatum may lead to hyperkinesias that can occur with continued pharmacological treatment of Parkinson's disease.

Brain sites of movement disorder: genetic and environmental agents in neurodevelopmental perturbations

In assessing and assimilating the neurodevelopmental basis of the so-called movement disorders it is probably useful to establish certain concepts that will modulate both the variation and selection of affliction, mechanisms-processes and diversity of disease states. Both genetic, developmental and degenerative aberrations are to be encompassed within such an approach, as well as all deviations from the necessary components of behaviour that are generally understood to incorporate "normal" functioning. In the present treatise, both conditions of hyperactivity/hypoactivity, akinesia and bradykinesia together with a constellation of other symptoms and syndromes are considered in conjunction with the neuropharmacological and brain morphological alterations that may or may not accompany them, e.g. following neonatal denervation. As a case in point, the neuroanatomical and neurochemical points of interaction in Attention Deficit and Hyperactivity disorder (ADHD) are examined with reference to both the perinatal metallic and organic environment and genetic backgrounds. The role of apoptosis, as opposed to necrosis, in cell death during brain development necessitates careful considerations of the current explosion of evidence for brain nerve growth factors, neurotrophins and cytokines, and the processes regulating their appearance, release and fate. Some of these processes may possess putative inherited characteristics, like alpha-synuclein, others may to greater or lesser extents be endogenous or semi-endogenous (in food), like the tetrahydroisoquinolines, others exogenous until inhaled or injested through environmental accident, like heavy metals, e.g. mercury. Another central concept of neurodevelopment is cellular plasticity, thereby underlining the essential involvement of glutamate systems and N-methyl-D-aspartate receptor configurations. Finally, an essential assimilation of brain development in disease must delineate the relative merits of inherited as opposed to environmental risks not only for the commonly-regarded movement disorders, like Parkinson's disease, Huntington's disease and epilepsy, but also for afflictions bearing strong elements of psychosocial tragedy, like ADHD, autism and Savantism.

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.

Relationship between central serotonergic function and aggression in prepubertal boys: effect of age and attention-deficit/hyperactivity disorder

Data indicate that diminished central serotonergic (5-HT) function is related to aggression in adults, but discrepant findings in children suggest that age or the presence of attention-deficit/hyperactivity disorder (ADHD) may influence this relationship. This study examined whether age or ADHD affects the association between 5-HT and aggression in 7-11-year old clinically-referred boys. Forty-six boys were divided into non-aggressive ADHD, aggressive ADHD, and aggressive non-ADHD groups based on responses to interviews and ratings of behavior. Central 5-HT function was assessed by measuring the prolactin response to a 1-mg/kg oral dose of D,L-fenfluramine. There was no significant difference in the prolactin response across the three groups of boys. Furthermore, when examined dimensionally, prolactin response was largely unrelated to ratings of aggression, even after controlling for ADHD. Finally, age was not associated with prolactin response, and had no effect on the relationship between prolactin response and aggression. This study provides further evidence that there is no clear relationship between central 5-HT function and aggression in disruptive boys. Moreover, these data do not confirm the hypothesis that age or the presence of ADHD influence the relationship between 5-HT and childhood aggression.

Nigrostriatal lesions alter oral dyskinesia and c-Fos expression induced by the serotonin agonist 1-(m-chlorophenyl)piperazine in adult rats

The loss of dopaminergic innervation of the basal ganglia, a group of subcortical regions involved in motor control, is the hallmark of Parkinson's disease. The resulting molecular and cellular alterations mediate behavioral deficits and may modify neuronal responses to other neurotransmitters. In the present study, we sought to determine the effects of chronic dopamine (DA) depletion on responses mediated by stimulation of serotonergic 2C (5-HT(2C)) receptors, a serotonergic receptor subtype present in discrete regions of the basal ganglia. Specifically, the effects of unilateral lesions of nigrostriatal DA neurons on oral dyskinesia and Fos protein expression induced by the non-selective 5-HT(2C) agonist 1-(m-chlorophenyl)piperazine (m-CPP) were examined. Confirming previous findings, both peripheral and local injections of m-CPP into the subthalamic nucleus elicited oral dyskinesia. Nigrostriatal lesions markedly enhanced oral bouts induced by peripheral but not intrasubthalamic administration of m-CPP. In intact rats, Fos expression was increased by m-CPP (1 mg/kg, i.p.) in the striatum and the subthalamic nucleus. After nigrostriatal lesions, m-CPP-induced Fos expression remained unchanged in the subthalamic nucleus but was reduced in the medial quadrants of the striatum and was markedly enhanced in the entopeduncular nucleus. These data demonstrate regionally specific alterations in behavioral and cellular responses to a serotonergic agonist in an animal model of Parkinson's disease.

The effect of novel antipsychotics in rat oral dyskinesia

1. The effect of the D1 agonist SKF38393 and the 5HT2C agonist m-CPP on repetitive jaw movements (RJM) was studied in rats. Acute administration of SKF38393 and/or m-CPP induced RJM in a dose dependent manner. In rats treated with both drugs, RJM responses were about equal to the sum of those obtained with each drug alone. 2. The induction of RJM by SKF38393 was somewhat lower in rats pretreated with 5HT2C receptor antagonist, mianserin, whereas mianserin severely reduced RJM induced by m-CPP alone. 3. D1 antagonist SCH23390 inhibited SKF38393 induced RJM but had no effect on m-CPP induced chewing behavior. 4. The present study confirms earlier evidence that D1 agonists used at optimal doses for the induction of RJM do not involve the serotonergic system in a significant way. It does, however, implicate the system in the emergence of drug induced oral behavior in rats. 5. The effect of the atypical antipsychotics, clozapine, olanzapine and risperidone was studied on SKF38393 and m-CPP induced RJM. Pretreatment with the atypical antipsychotics clozapine and olanzapine inhibit SKF38393 and m-CPP induced RJM. Pretreatment with risperidone inhibits m-CPP induced oral behavior in rats while increases dose dependently SKF38393 induced RJM.

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.

Enhanced oral activity responses to intrastriatal SKF 38393 and m-CPP are attenuated by intrastriatal mianserin in neonatal 6-OHDA-lesioned rats

Enhanced oral activity is induced in neonatal 6-hydroxydopamine- (6-OHDA-) lesioned rats by systemic administration of the dopamine (DA) D1 receptor agonist SKF 38393 and serotonin (5-HT) 5-HT2A,2C agonist m-chlorophenylpiperazine (m-CPP). The DA D1 receptor antagonist SCH 23390 effectively attenuates the effect of SKF 38393 but not m-CPP. The 5-HT2 antagonist mianserin attenuates the effects of both m-CPP and SKF 38393, suggesting that DA agonist effects are mediated by 5-HT neurochemical systems. To test whether DA and 5-HT agonist effects and interactions might occur within the neostriatum, rats were implanted with permanent injection cannulae, with tips in the ventral striatum. One group of rats was lesioned at 3 days after birth with 6-OHDA HBr (100 micrograms salt form, in each lateral ventricle; desipramine HCl pretreatment, 20 mg/kg IP, base form, 1 h), while controls received the vehicle in place of 6-OHDA. Cannulae were implanted when rats weighed 200-250 g. During a 1-h observation session SKF 38393 (5 nmol per side) produced 74.3 +/- 19.2 oral movements in intact rats and 310.7 +/- 97.0 oral movements in 6-OHDA-lesioned rats. m-CPP (10 nmol per side) produced 72.6 +/- 15.1 and 274.5 +/- 65.0 oral movements in these respective groups. These responses were several-fold greater than the 25.3 +/- 7.3 and 41.8 +/- 9.5 oral movements in the same groups after saline (0.5 microliter per side) (P < 0.05). Mianserin (6 nmol per side) alone had no effect on oral activity but attenuated responses to both SKF 38393 and m-CPP in intact and 6-OHDA-lesioned rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Enhanced quinpirole response in rats lesioned neonatally with 5,7-dihydroxytryptamine

The ontogenic destruction of dopamine (DA) neurons in rat brain is associated with supersensitization of DA D1 receptors. This effect is attenuated when rats are cotreated in ontogeny with the serotonin (5-HT) neurotoxin, 5,7-dihydroxytryptamine (5,7-DHT). In an attempt to determine whether 5-HT fibers might have a similar modulatory role on the sensitivity of the DA D2 receptor complex, we pretreated rats with desipramine HCl (20 mg/kg, IP, base), 1 h before the DA neurotoxin, 6-hydroxydopamine (6-OHDA; 134 micrograms ICV, base) and/or 5,7-DHT (75 micrograms ICV) and/or vehicle. At about 3 months after birth dose-effect curves for quinpirole-induced oral activity were constructed for each group of rats. We found that quinpirole, an agonist for the DA D2 receptor complex, produced a dose-related increase in oral activity in all groups of rats. After a 200 micrograms/kg dose of quinpirole HCl, however, neonatal 5,7-DHT-lesioned rats had a peak oral response of 54.4 +/- 5.1 (mean and SEM) vs. 22.6 +/- 4.8 for control rats (p < 0.01). In neonatal 6-OHDA-lesioned rats this dose of quinpirole increased oral activity to 36.8 +/- 5.8 oral movements (p < 0.05 vs. control). In rats lesioned with both 5,7-DHT and 6-OHDA, the oral response was not different from control. The enhanced oral response to quinpirole in 5,7-DHT-lesioned rats was attenuated by spiperone, an antagonist for the DA D2 receptor complex.(ABSTRACT TRUNCATED AT 250 WORDS)

Dopamine receptor supersensitivity

Dopamine (DA) receptor supersensitivity refers to the phenomenon of an enhanced physiological, behavioral or biochemical response to a DA agonist. Literature related to ontogenetic aspects of this process was reviewed. Neonatal 6-hydroxydopamine (6-OHDA) destruction of rat brain DA neurons produces overt sensitization to D1 agonist-induced oral activity, overt sensitization of some D2 agonist-induced stereotyped behaviors and latent sensitization of D1 agonist-induced locomotor and some stereotyped behaviors. This last process is unmasked by repeated treatments with D1 (homologous "priming") or D2 (heterologous "priming") agonists. A serotonin (5-HT) neurotoxin (5,7-dihydroxytryptamine) and 5-HT2C receptor antagonist (mianserin) attenuate some enhanced behavioral effects of D1 agonists, indicating that 5-HT neurochemical systems influence D1 receptor sensitization. Unlike the relative absence of change in brain D1 receptor number, DA D2 receptor proliferation accompanies D2 sensitization in neonatal 6-OHDA-lesioned rats. Robust D2 receptor supersensitization can also be induced in intact rats by repeated treatments in ontogeny with the D2 agonist quinpirole. In these rats quinpirole treatments produce vertical jumping at 3-5 wk after birth and subsequent enhanced quinpirole-induced antinociception and yawning. The latter is thought to represent D3 receptor sensitization. Except for enhanced D1 agonist-induced expression of c-fos, there are no changes in the receptor or receptor-mediated processes which account for receptor sensitization. Adaptive mechanisms by multiple "in series" neurons with different neurotransmitters may account for the phenomenon known as receptor supersensitivity.

Hypersensitivity to serotonin and its agonists in serotonin-hyperinnervated neostriatum after neonatal dopamine denervation

Neonatal destruction of the nigrostriatal dopamine projection by intraventricular 6-hydroxydopamine leads to a serotonin (5-hydroxytryptamine, 5-HT) hyperinnervation of the adult neostriatum accompanied by increased radioligand binding to 5-HT1B, 5-HT1nonAB and 5-HT2 receptors. The consequences of such 5-HT receptor changes on neuronal responsiveness to 5-HT and corresponding receptor agonists were assessed with a quantitative iontophoretic approach. For comparative purposes, similar data were also obtained from rats 6-hydroxydopamine lesioned as adults, showing severe neostriatal dopamine denervation but no 5-HT hyperinnervation. In controls, 5-HT and its receptor agonists, m-chlorophenylpiperazine (mCPP; 5-HT1B/2C agonist) and dimethoxy-iodophenyl-aminopropane (DOI; 5-HT2A/2C agonist), depressed the firing rate of a majority of the unit tested. Three months after neonatal 6-hydroxydopamine lesion (5-HT-hyperinnervated tissue), inhibitory responses to all three agents were significantly increased and comparable results were obtained for 5-HT and DOI in the rostral versus caudal neostriatum. After 6-hydroxydopamine lesion in adults, neither responsiveness to 5-HT, mCPP or DOI nor the density of 5-HT1B or 5-HT2A binding were significantly different from control. Thus, the up-regulation of 5-HT1B, 5-HT2A and possibly 5-HT2C receptors accompanying the 5-HT hyperinnervation after neonatal but not after adult dopamine denervation was associated with increased responsiveness (IT50) of neostriatal neurons to iontophoresed 5-HT and its receptor agonists. Under these conditions, neostriatal 5-HT transmission might be enhanced in spite of a basal release seemingly comparable to normal (Jackson and Abercrombie, 1992, J. Neurochem. 58, 890).(ABSTRACT TRUNCATED AT 250 WORDS)

Dopamine neoinnervation in the substantia nigra and hyperinnervation in the interpeduncular nucleus of adult rat following neonatal cerebroventricular administration of 6-hydroxydopamine

An aberrant network of dopamine axons was found to pervade the rat substantia nigra following neonatal destruction of its dopamine nerve cell bodies and dendrites by cerebroventricular administration of 6-hydroxydopamine. Light-microscopic immunocytochemistry with a primary monoclonal antibody directed against dopamine-glutaraldehyde-protein was used to investigate the time-course of development and the critical period of induction of this ectopic dopamine innervation (neoinnervation). In rats 6-hydroxydopamine-lesioned at postnatal day 3 (P3) and examined at P7, P10, P15, P30 or later, some dopamine fibers were already present in the substantia nigra at P7; their number increased sharply until P15 and only slightly thereafter, assuming a topographic distribution reminiscent of the missing dopamine nerve cell bodies and dendrites. A similar growth of dopamine fibers took place in the substantia nigra after lesions made at P6, P9 and P12, but was less pronounced after lesion at P15 and absent after lesion at P21 or later. Excessive innervation by dopamine axons (hyperinnervation) was concomitantly observed in the nearby interpeduncular nucleus. The sprouting of dopamine axons in both regions was therefore rapid and coincided in time and space with the developmental redistribution of mesencephalic dopamine neurons in normal rat. It is conceivable that these aberrant dopamine innervations play a role in the peculiar behavior and responsiveness to dopaminergic agents manifested by neonatally 6-hydroxydopamine-lesioned rats. It will be of particular interest to investigate the functional consequences of the dopamine neoinnervation in the substantia nigra, where an eventual axonal release might thus be replacing the normal somatodendritic release of this amine.

Enhanced oral activity response to A77636 in neonatal 6-hydroxydopamine-lesioned rats

To study the role of dopamine D1 receptors in enhanced oral activity effects of SKF 38393 ((+-)-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine-7,8-diol) in neonatal 6-hydroxydopamine-lesioned rats, SKF 38393 was compared to the full agonist, A77636 ((1R,3S)-3-(1'-adamantyl)-1-aminomethyl-3,4-dihydro-5,6-dihydroxy- 1H-2-benzopyran). At 3 days after birth rats were treated with 6-hydroxydopamine HBr (200 micrograms salt form, i.c.v.; desipramine (20 mg/kg i.p.), 1 h) or vehicle. At 6-8 months a 0.01 mg/kg dose of A77636 HCl increased oral activity in 6-hydroxydopamine vs. control rats (P < 0.01). A77636 and SKF 38393 produced identical maximal responses of 35-36 oral movements at 0.1 and 1.0 mg/kg, respectively. SCH 23390 (R-(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benz azepine ) HCl (0.3 mg/kg i.p.) attenuated the response to A77636. Neither A77636 HCl (0.01-1.0 mg/kg i.p.) nor SKF 38393 HCl (0.03-3.0 mg/kg i.p.) induced oral activity in intact rats. The findings demonstrate that A77636 is more potent than SKF 38393, and that supersensitized dopamine D1 receptors are involved in the induction of oral behavior in neonatal 6-hydroxydopamine-lesioned rats.

Proposed animal model of attention deficit hyperactivity disorder

Dopamine (DA) neurons are implicated in the hyperlocomotion of neonatal 6-hydroxydopamine (6-OHDA)-lesioned rats, an animal model of attention deficit hyperactivity disorder (ADHD). Because serotonin (5-HT) neurons mediate some DA agonist effects, we investigated the possible role of 5-HT neurons on locomotor activity. Rats were treated at 3 days after birth with vehicle or 6-OHDA (134 micrograms ICV; desipramine pretreatment, 20 mg/kg IP, 1 h), and at 10 weeks with vehicle or 5,7-dihydroxytryptamine (5,7-DHT; 75 micrograms ICV; pretreatment with desipramine and pargyline, 75 mg/kg IP, 30 min), to destroy DA and/or 5-HT fibers. Intense spontaneous hyperlocomotor activity was produced in rats lesioned with both 6-OHDA and 5,7-DHT. Locomotor time in this group was 550 +/- 17 s in a 600 s session, vs. 127 +/- 13 s in the 6-OHDA group and < 75 s in 5,7-DHT and intact control groups (p < 0.001). Oral activity dose-effect curves established that 5,7-DHT attenuated DA D1 receptor supersensitivity and further sensitized 5-HT2c receptors. Acute treatment with dextroamphetamine (0.25 mg/kg SC) reduced locomotor time in 6-OHDA + 5,7-DHT-lesioned rats to 76 +/- 37 s (p < 0.001). Striatal DA was reduced by 99% and 5-HT was reduced by 30% (vs. 6-OHDA group). Because combined 6-OHDA (to neonates) and 5,7-DHT (to adults) lesions produce intense hyperlocomotion that is attenuated by amphetamine, we propose this as a new animal model of ADHD. The findings suggest that hyperactivity in ADHD may be due to injury or impairment of both DA and 5-HT neurons.

Dose-related effects of a neonatal 6-OHDA lesion on SKF 38393- and m-chlorophenylpiperazine-induced oral activity responses of rats

Neonatal 6-hydroxydopamine (6-OHDA) treatment of rats is associated with concurrent supersensitization of dopamine (DA) D1 and serotonin 5-HT1C receptors, for agonist-induced oral activity. The present study was conducted to determine if graded reduction of striatal DA content and/or graded elevation of striatal 5-HT content by 6-OHDA would alter sensitivity of either receptor type, and thereby influence oral activity responses to DA and 5-HT agonists. At 3 days after birth, groups of rats were pretreated with desipramine (20 mg/kg i.p.), 1 h before administration of a range of doses of 6-OHDA HBr (15, 30, 60, 100, 150 and 200 micrograms, i.c.v., salt form; half in each lateral ventricle) or the vehicle, saline (0.85%)-ascorbic acid (0.1%). Between 2 and 4 months, a series of challenge doses of SKF 38393 HCl (0.30 to 3.0 mg/kg i.p.) and m-chlorophenylpiperazine 2HCl (0.30 to 6.0 mg/kg i.p.; m-CPP 2HCl) were administered to each group of rats and oral activity was observed. Oral activity was determined for 1 min every 10 min during a 60-min period, starting 10 min after injection of agonist or vehicle. SKF 38393 dose-response curves demonstrated enhanced oral activity responses in rats lesioned neonatally with 150 or 200 micrograms of 6-OHDA. m-CPP dose-response curves demonstrated enhanced oral activity responses in these 2 groups of rats, as well as those lesioned neonatally with 100 micrograms of 6-OHDA. Striatal DA content was reduced by > 97% in these 3 groups of rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Ontogenetic SKF 38393 treatments sensitize dopamine D1 receptors in neonatal 6-OHDA-lesioned rats

Neonatal 6-hydroxydopamine (6-OHDA) treatment of rats is associated with supersensitization of the dopamine (DA) D1 agonist induction of stereotyped and locomotor behaviors. The present study was conducted to determine whether ontogenetic treatments of these rats with the DA D1 receptor agonist, SKF 38393, would produce a maximal DA D1 receptor supersensitivity, as measured by locomotor behavior in adulthood. Rat pups were treated daily with SKF 38393-HCl (3.0 mg/kg per day, i.p.) or saline vehicle for 28 consecutive days from birth. These animals were additionally treated at 3 days after birth with 6-OHDA-HBr (100 micrograms, in each lateral ventricle, salt form) or its vehicle. Between 6 and 9 weeks locomotor activity or stereotyped behaviors were observed after weekly challenge doses of SKF 38393-HCl (3.0 mg/kg, i.p.). In the neonatal 6-OHDA group, successive SKF 38393 treatments produced progressively greater locomotor activity. In the group of rats treated during postnatal ontogeny with both 6-OHDA and SKF 38393 daily treatments, the first adult challenge dose of SKF 38393 produced an enhanced locomotor response, greater than that seen in other groups (P < 0.01). Subsequent SKF 38393 treatments of this group produced increasingly greater locomotor responses. SKF 38393-induced stereotyped behavioral effects were greater in the 6-OHDA-lesioned groups, whether or not SKF 38393 was administered ontogenetically. Profound reductions (> 99%) of DA and its metabolites were found in the striatum of neonatal 6-OHDA treated rats, regardless of whether SKF 38393 was co-administered ontogenetically.(ABSTRACT TRUNCATED AT 250 WORDS)

Age-dependence of a 6-hydroxydopamine lesion on SKF 38393- and m-chlorophenylpiperazine-induced oral activity responses of rats

Neonatal 6-hydroxydopamine (6-OHDA) treatment is associated with destruction of dopamine (DA) fibers and subsequent sprouting of serotonin (5-HT) fibers in the striatum of rats. Enhanced oral activity responses to SKF 38393 and m-chlorophenylpiperazine (m-CPP), respective agonists for the DA D1 receptor complex and 5-HT2C receptor complex, ensue. To study the ontogenetic nature of this effect, rats were treated at birth, 3 days, 7 days, 10 days or 14 days with 6-OHDA-HBr (200 micrograms i.c.v.; salt form), following desipramine-HCl pretreatment (20 mg/kg i.p., 1 h; base form). Another group of rats was treated at 35 days and again at 42 days with 6-OHDA-HBr (300 micrograms i.c.v.), following desipramine-HCl (20 mg/kg i.p., 1 h) and pargyline-HCl (50 mg/kg i.p., 30 min). In rats treated from birth to 10 days, 6-OHDA reduced striatal DA content at 5 months by > or = 94%. Striatal 5-HT content was elevated by 28% to 51%, but only in rats treated with 6-OHDA at 7 days from birth or earlier. An enhanced oral activity response to SKF 38393-HCl (0.03 to 1.0 mg/kg i.p.) was absent in rats treated 7 days or later, and the change in SKF 38393 effect was correlated with a change in striatal DA content. An enhanced response to m-CPP.2HCl (0.3 to 6.0 mg/kg i.p.) was absent after treatment at 14 or 35 days, when striatal DA content was reduced only 44% to 63% and 5-HT content was not changed.(ABSTRACT TRUNCATED AT 250 WORDS)

MIF-1 fails to modify agonist-induced oral activity in neonatal 6-OHDA-treated rats

L-Prolyl-L-leucyl-glycinamide (MIF-1) is known to attenuate apomorphine-induced stereotypes in adult rats that are lesioned as neonates with 6-hydroxydopamine (6-OHDA). To test whether MIF-1 would affect dopamine (DA) agonist-induced and serotonin (5-HT) agonist-induced oral activity, both intact and neonatal 6-OHDA-treated rats were studied. Rats at 3 days from birth were injected with desipramine (20 mg/kg, IP), 1 h before 6-OHDA HBr (100 micrograms, salt form, in each lateral ventricle) or its vehicle, saline-ascorbic acid (0.1%). At approximately 6 months rats were treated with MIF-1 (0.1, 1.0, or 10.0 mg/kg, IP), 10 min before SKF 38393 HCl (1.0 mg/kg, IP) or m-chlorophenylpiperazine 2HCl (m-CPP 2HCl; 0.5 mg/kg, IP), DA D1 and 5-HT1C,2 receptor agonists, respectively. Although both agonists increased oral activity in control and neonatal 6-OHDA-treated rats, MIF-1 did not modify the response. In rats that received either of the three doses of MIF-1 for 21 consecutive days, there was still no observed effect of MIF-1 on the oral response of control and 6-OHDA-lesioned rats to SKF 38393 and m-CPP. These findings indicate that MIF-1 does not modify the oral activity response of supersensitized D1 and 5-HT1C receptors in adult rats that are lesioned neonatally with 6-OHDA.

Enhanced pilocarpine-induced oral activity responses in neonatal 6-OHDA treated rats

Neonatal destruction of rat nigrostriatal dopaminergic fibers results in an enhanced oral activity response to both dopamine (DA) D1 and serotonin (5-HT) agonists. Because cholinergic systems represent another one of the neural circuits involved in oral behavior, it was of interest to determine whether muscarinic receptors might also be sensitized in the lesioned rats. At 3 days after birth, rats were pretreated with desipramine HCl (20 mg/kg, IP) 1 h before 6-hydroxydopamine (6-OHDA) HBr (100 micrograms in each lateral ventricle) or saline-ascorbic acid (0.1%) vehicle. Between 2 and 4 months, behavioral supersensitivity to a D1 agonist (SK&F 38393) and 5-HT agonist (m-chlorophenylpiperazine; m-CPP) was established before rats were challenged with the muscarinic receptor agonist, pilocarpine HCl (0.125 to 10.0 mg/kg, IP). The pilocarpine dose-effect curve was shifted to the left, with a maximal effect of 63.7 +/- 8.6 oral movements being produced by a 1.0 mg/kg pilocarpine HCl dose in the 6-OHDA lesioned rats, versus 15.0 +/- 2.4 oral movements in the control group (p < 0.001). The enhanced response to pilocarpine was attenuated by the muscarinic receptor antagonist, scopolamine HCl (0.1 mg/kg IP). These findings indicate that neonatal 6-OHDA treatment produces supersensitization of muscarinic receptors in rats.

Serotonin 5-HT1 and 5-HT2 receptors in adult rat brain after neonatal destruction of nigrostriatal dopamine neurons: a quantitative autoradiographic study

Neonatal destruction of nigrostriatal dopamine neurons by cerebroventricular injection of 6-hydroxydopamine (6-OHDA) results in a serotonin (5-HT) hyperinnervation of the rostral neostriatum in adult rat. Quantitative ligand-binding autoradiography was used to compare the density of various 5-HT receptor subtypes in the adult brain of control and neonatally 6-OHDA-lesioned rats. 5-HT1A, 5-HT1B, 5HT1nonAB and 5-HT2 sites were labeled with [3H]8-OH-DPAT, [125I]cyanopindolol, [3H]5-HT and [125I]DOI, respectively, and measured in the rostral and caudal halves of neostriatum and selected forebrain or midbrain regions. 5-HT1A binding, measured after 6 months, was unchanged in all regions examined including the dorsal raphe nucleus. Three months after the lesion, 5-HT1B binding was increased throughout the neostriatum (30%), but also in the substantia nigra (50%) and globus pallidus (30%), suggesting an up-regulation and an increased axonal transport of these receptors in neostriatal projection neurons. 5-HT1nonAB binding was also increased throughout the neostriatum (40%) and in the substantia nigra (50%), but unchanged in the globus pallidus, as if this up-regulation preferentially involved striatonigral as opposed to striatopallidal neurons. 5-HT2 binding showed an even greater increase (60%), which was restricted to the rostral half of neostriatum and also seemed imputable to an up-regulation as heteroreceptors. Even though the exact cause(s) of these receptor increases could not be determined, their anatomical distribution suggested that they were somehow related to the initial dopamine denervation in the case of the 5-HT1B and 5-HT1nonAB receptors, and more tightly linked to the 5-HT hyperinnervation in the case of the 5-HT2 receptors. Such receptor changes could participate in adaptive mechanisms implicating other transmitters and behavioral disturbances observed in this particular experimental model. Interestingly, they could also account for an enhancement of neostriatal 5-HT function even in a condition where extracellular levels of 5-HT apparently remain normal because of increased uptake.

The cimetidine-induced increase in prolactin secretion in schizophrenia: effect of clozapine

There is considerable interest in the role of serotonin (5-HT) in the pathophysiology of schizophrenia and in the mechanism of action of clozapine, an atypical antipsychotic agent and a potent dopamine (DA), 5-HT2/5-HT1C and histamine (H) antagonist. Cimetidine, an H2 antagonist, produces robust, transient increase in plasma prolactin (PRL) levels in man following intravenous administration. This effect has been attributed, in part, to indirect central serotonergic mechanisms involving 5-HT2 receptors in the hypothalamus, but the evidence is inconclusive. This study investigated the effects of cimetidine on plasma PRL levels in unmedicated schizophrenic patients versus normal controls and the effect of chronic treatment with clozapine on the cimetidine-induced PRL response. The PRL response to cimetidine was significantly blunted in male but not female schizophrenic patients. The PRL response in male schizophrenic patients was inversely related to psychopathology. Chronic treatment with clozapine completely suppressed the plasma PRL response following cimetidine. These data are consistent with the hypothesis of an abnormality of serotonergic activity, including downregulation of 5-HT2 receptors, in male but not female schizophrenic patients. The role of antagonism of 5-HT2 receptors in the action of clozapine is discussed.