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

Motor cortico-nigral and cortico-entopeduncular information transmission and its modulation by buspirone in control and after dopaminergic denervation

Cortical information is transferred to the substantia nigra pars reticulata (SNr) and the entopeduncular nucleus (EP), the output structures of the basal ganglia (BG), through three different pathways: the hyperdirect trans-subthalamic and the direct and indirect trans-striatal pathways. The nigrostriatal dopamine (DA) and the activation of 5-HT1A receptors, distributed all along the BG, may modulate cortical information transmission. We aimed to investigate the effect of buspirone (5-HT1A receptor partial agonist) and WAY-100635 (5-HT1A receptor antagonist) on cortico-nigral and cortico-entopeduncular transmission in normal and DA loss conditions. Herein, simultaneous electrical stimulation of the motor cortex and single-unit extracellular recordings of SNr or EP neurons were conducted in urethane-anesthetized sham and 6-hydroxydopamine (6-OHDA)-lesioned rats before and after drug administrations. Motor cortex stimulation evoked monophasic, biphasic, or triphasic responses, combination of an early excitation, an inhibition, and a late excitation in both the SNr and EP, while an altered pattern of evoked response was observed in the SNr after 6-OHDA lesion. Systemic buspirone potentiated the direct cortico-SNr and cortico-EP transmission in sham animals since increased duration of the inhibitory response was observed. In DA denervated animals, buspirone administration enhanced early excitation amplitude in the cortico-SNr transmission. In both cases, the observed effects were mediated via a 5-HT1A-dependent mechanism as WAY-100635 administration blocked buspirone's effect. These findings suggest that in control condition, buspirone potentiates direct pathway transmission and DA loss modulates responses related to the hyperdirect pathway. Overall, the results may contribute to understanding the role of 5-HT1A receptors and DA in motor cortico-BG circuitry functionality.

Tetrabenazine Mitigates Aberrant Release and Clearance of Dopamine in the Nigrostriatal System, and Alleviates L-DOPA-Induced Dyskinesia in a Mouse Model of Parkinson's Disease

BACKGROUND

L-DOPA-induced dyskinesia (LID), occurring with aberrant processing of exogenous L-DOPA in the dopamine-denervated striatum, is a main complication of levodopa treatment in Parkinson's disease.

OBJECTIVE

To characterize the effects of the vesicular antagonist tetrabenazine (TBZ) on L-DOPA-induced behavior, neurochemical signals, and underlying protein expressions in an animal model of Parkinson's disease.

METHODS

20-week-old MitoPark mice were co-treated or separately administered TBZ and L-DOPA for 14 days. Abnormal involuntary movements (AIMs) and locomotor activity were analyzed. To explore dopamine (DA) transmission, fast scan cyclic voltammetry was used to assess presynaptic DA dynamics in striatal slices following treatments. PET imaging with 4-[18F]-PE2I, ADAM and immunoblotting assays were used to detect receptor protein changes in the DA-denervated striatum. Finally, nigrostriatal tissues were collected for HPLC measures of DA, serotonin and their metabolites.

RESULTS

A single injection of TBZ given in the interval between the two L-DOPA/Carbidopa treatments significantly attenuated L-DOPA-induced AIMs expression and locomotor hyperactivity. TBZ was shown to reduce tonic and phasic release of DA following L-DOPA treatment in DA-denervated striatal tissue. In the DA-depleted striatum, TBZ decreased the expression of L-DOPA-enhanced D1 receptors and the serotonin reuptake transporter. Neurochemical analysis indicated that TBZ attenuated L-DOPA-induced surges of DA levels by promoting DA turnover in the nigrostriatal system.

CONCLUSIONS

Our findings demonstrate that TBZ diminishes abnormal striatal DA transmission, which involves the ability of TBZ to modulate the presymptomatic dynamics of DA, and then mitigate aberrant release of exogenous L-DOPA from nerve terminals. The results support the potential of repositioning TBZ to counteract LID development.

The effect of 5-HT1A receptor agonists on the entopeduncular nucleus is modified in 6-hydroxydopamine-lesioned rats

BACKGROUND AND PURPOSE

l-DOPA prolonged treatment leads to disabling motor complications as dyskinesia that could be decreased by drugs acting on 5-HT1A receptors. Since the internal segment of the globus pallidus, homologous to the entopeduncular nucleus in rodents, seems to be involved in the etiopathology of l-DOPA-induced dyskinesia, we investigated whether the entopeduncular nucleus is modulated by the 5-HT1A receptor partial and full agonists, buspirone, and 8-hydroxy-2-(di-n-propylamino)-tetralin (8-OH-DPAT) in control and 6-hydroxydopamine (6-OHDA)-lesioned rats with or without long-term l-DOPA treatment.

EXPERIMENTAL APPROACH

Extracellular single-unit electrocorticogram and local field potential recordings under anaesthesia, immunostaining assays and optogenetic manipulation coupled to electrophysiological recordings were performed.

KEY RESULTS

Systemic buspirone reduced the entopeduncular nucleus firing rate in the sham animals and burst activity in the 6-OHDA-lesioned rats (with or without l-DOPA treatment), while local administration reduced entopeduncular nucleus activity in all the groups, regardless of DA integrity. Systemic 8-OH-DPAT also induced inhibitory effects only in the sham animals. Effects triggered by buspirone and 8-OH-DPAT were reversed by the 5-HT1A receptor antagonist, WAY-100635. Neither buspirone nor 8-OH-DPAT modified the low-frequency oscillatory activity in the entopeduncular nucleus or its synchronization with the motor cortex. Buspirone did not alter the response induced by subthalamic nucleus opto-stimulation in the entopeduncular nucleus.

CONCLUSION AND IMPLICATIONS

Systemic 5-HT1A receptor activation elicits different effects on the electrophysiological properties of the entopeduncular nucleus depending on the integrity of the nigrostriatal pathway and it does not alter the relationship between subthalamic nucleus and entopeduncular nucleus neuron activity.

Dopamine, a key factor of mitochondrial damage and neuronal toxicity on rotenone exposure and also parkinsonic motor dysfunction-Impact of asiaticoside with a probable vesicular involvement

Persuasive evidence propose that the toxicity of dopamine in parkinsonism and the loss of dopaminergic neurons are the earliest events during the pathogenesis of Parkinson's disease (PD). In our earlier study, Asiaticoside (AS), a triterpenoid saponin isolated from Centella asiatica was shown to exert a neuroprotective effect against hemiparkinsonism, purportedly due to phosphoinositides (PI)-assisted cytodynamics and synaptic function. Here, we evaluate AS in the modulation of dopamine (DA), mitochondrial integrity and neurite variations in vitro and motor dysfunctions in vivo. PC12 cells challenged with rotenone-(ROT) (0.1 μM/mL) were exposed to AS and l-DOPA (10 mM and 20 μM/mL respectively). The protein expressions of Bax and Bcl-2 that regulate cell death were assessed following neurite length assays. Rats were distributed into 6 groups (6 rats/group): Sham, Vehicle controls, ROT-infused (6 μg/μl/kg), AS- treated (50 mg/kg/day), Drug control, and ROT + L-DOPA-treated (6 mg/kg/day) groups. At the end of the experimental period, the rats were sacrificed after performing motor behavioral analysis, and the striatum was dissected out. The contents of synaptic vesicular and cytosolic DA were analyzed. Further, the levels of striatal PI were also measured. ROT had caused significant reduction in the neurite outgrowth in the exposed PC12 cells while the tested concentrations of AS and l-DOPA can exert their protective effect on the stunted neurite growth. The levels of Bax, Bcl-2, and cytochrome c which were significantly disturbed by ROT, could also be affected by AS thereby suggesting its effect on neurons. AS treatment caused an improved motor performance, vesicular and cytosolic DA, and striatal PI. These pre-clinical findings force us to speculate that AS could be a potential drug candidate in combating ROT-induced variations that are possibly precipitated by varied vesicular trafficking of DA.

Psychosis in Parkinson's Disease: Epidemiology, Pathophysiology, and Management

Psychotic symptoms are common in Parkinson's disease (PD) and are associated with poorer quality of life and increased caregiver burden. PD psychosis is correlated with several factors, such as more advanced disease, cognitive impairment, depression, and sleep disorders. The underlying causes of psychosis in PD thus involve a complex interplay between exogenous (e.g., drugs, intercurrent illnesses) and endogenous (e.g., PD disease pathology) factors. Current theories of the pathophysiology of PD psychosis have come from several neuropathological and neuroimaging studies that implicate pathways involving visual processing and executive function, including temporo-limbic structures and neocortical gray matter with altered neurotransmitter functioning (e.g., dopamine, serotonin, and acetylcholine). Treatment of PD psychosis requires a step-wise process, including initial careful investigation of treatable triggering conditions and a comprehensive evaluation with adjustment of PD medications and/or initiation of specific antipsychotic therapies. Clozapine remains the only recommended drug for the treatment of PD psychosis; however, because of regular blood monitoring, quetiapine is usually first-line therapy, although less efficacious. Emerging studies have focused on agents involving other neurotransmitters, including the serotonin 5-HT2A receptor inverse agonist pimavanserin, cholinesterase inhibitors, and antidepressants and anxiolytics.

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.

Striatal Plasticity in L-DOPA- and Graft-Induced Dyskinesia; The Common Link?

One of the major symptoms of the neurodegenerative condition Parkinson's disease (PD) is a slowness or loss of voluntary movement, yet frustratingly therapeutic strategies designed to restore movement can result in the development of excessive abnormal movements known as dyskinesia. These dyskinesias commonly develop as a result of pharmacotherapy in the form of L-DOPA administration, but have also been identified following deep brain stimulation (DBS) and intrastriatal cell transplantation. In the case of L-DOPA these movements can be treatment limiting, and whilst they are not long lasting or troubling following DBS, recognition of their development had a near devastating effect on the field of cell transplantation for PD.Understanding the relationship between these therapeutic approaches and the development of dyskinesia may improve our ability to restore function without disabling side effects. Interestingly, despite the fact that dopaminergic cell transplantation repairs many of the changes induced by the disease process and through L-DOPA treatment, there appears to be a relationship between the two. In rodent models of the disease, the severity of dyskinesia induced by L-DOPA prior to the transplantation procedure correlated with post-transplantation, graft-induced dyskinesia. A review of clinical data also suggested that the worse preoperational dyskinesia causes worsened graft-induced dyskinesia (GID). Understanding how these aberrant behaviors come about has been of keen interest to open up these therapeutic options more widely and one major underlying theory is the effects of these approaches on the plasticity of synapses within the basal ganglia. This review uniquely brings together developments in understanding the role of striatal synaptic plasticity in both L-DOPA and GID to guide and stimulate further investigations on the important striatal plasticity.

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.

Functional Interplay between Dopaminergic and Serotonergic Neuronal Systems during Development and Adulthood

The complex integration of neurotransmitter signals in the nervous system contributes to the shaping of behavioral and emotional constitutions throughout development. Imbalance among these signals may result in pathological behaviors and psychiatric illnesses. Therefore, a better understanding of the interplay between neurotransmitter systems holds potential to facilitate therapeutic development. Of particular clinical interest are the dopaminergic and serotonergic systems, as both modulate a broad array of behaviors and emotions and have been implicated in a wide range of affective disorders. Here we review evidence speaking to an interaction between the dopaminergic and serotonergic neuronal systems across development. We highlight data stemming from developmental, functional, and clinical studies, reflecting the importance of this transmonoaminergic interplay.

Interaction between the 5-HT system and the basal ganglia: functional implication and therapeutic perspective in Parkinson's disease

The neurotransmitter serotonin (5-HT) has a multifaceted function in the modulation of information processing through the activation of multiple receptor families, including G-protein-coupled receptor subtypes (5-HT1, 5-HT2, 5-HT4-7) and ligand-gated ion channels (5-HT3). The largest population of serotonergic neurons is located in the midbrain, specifically in the raphe nuclei. Although the medial and dorsal raphe nucleus (DRN) share common projecting areas, in the basal ganglia (BG) nuclei serotonergic innervations come mainly from the DRN. The BG are a highly organized network of subcortical nuclei composed of the striatum (caudate and putamen), subthalamic nucleus (STN), internal and external globus pallidus (or entopeduncular nucleus in rodents, GPi/EP and GPe) and substantia nigra (pars compacta, SNc, and pars reticulata, SNr). The BG are part of the cortico-BG-thalamic circuits, which play a role in many functions like motor control, emotion, and cognition and are critically involved in diseases such as Parkinson's disease (PD). This review provides an overview of serotonergic modulation of the BG at the functional level and a discussion of how this interaction may be relevant to treating PD and the motor complications induced by chronic treatment with L-DOPA.

Anticataleptic effects of 5-HT(1B) receptors in the globus pallidus

The globus pallidus occupies an important position in the indirect pathway of the basal ganglia. Being a monoamine neurotransmitter, 5-HT is involved in mediating many physiological functions and pathophysiological processes in several movement disorders. Morphological studies have revealed that the globus pallidus receives serotonergic innervation arising from the raphe nuclei, mainly the dorsal raphe nucleus. A high level of 5-HT and 5-HT(1B) receptors were detected in the globus pallidus. In the present study, bilateral microinjection of 5-HT or 5-HT(1B) receptor agonist, CP-93129, into the globus pallidus significantly alleviated the symptoms of rigidity caused by haloperidol. To further elucidate 5-HT(1B) receptor-induced anticatalepsy, in vivo extracellular recordings were performed to examine the effects of 5-HT(1B) receptor activation on the firing activity of the globus pallidus neurons under the presence of haloperidol. Micro-pressure ejection of 5-HT or CP-93129 increased the spontaneous firing rate of the pallidal neurons. Furthermore, by using immunohistochemistry, positive staining of 5-HT(1B) receptor was observed in the globus pallidus neurons. Taken together, the present findings provide evidence that activation of 5-HT(1B) receptor may exert anticataleptic effects by increasing the activity of pallidal neurons.

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.

Rodent models of ADHD

The neonatal 6-OHDA-lesioned rat, coloboma mouse, DAT-KO mouse, and spontaneously hypertensive rat (SHR) models all bear a phenotypic resemblance to ADHD in that they express hyperactivity, inattention, and/or impulsivity. The models also illustrate the heterogeneity of ADHD: the initial cause (chemical depletion or genetic abnormality) of the ADHD-like behaviors is different for each model. Neurochemical and behavioral studies of the models indicate aberrations in monoaminergic neurotransmission. Hyperdopaminergic neurotransmission is implicated in the abnormal behavior of all models. Norepinephrine has a dual enhancing/inhibitory role in ADHD symptoms, and serotonin acts to inhibit abnormal dopamine and norepinephrine signaling. It is unlikely that symptoms arise from a single neurotransmitter dysfunction. Rather, studies of animal models of ADHD suggest that symptoms develop through the complex interactions of monoaminergic neurotransmitter systems.

The serotonergic system in Parkinson's disease

Although the cardinal manifestations of Parkinson's disease (PD) are attributed to a decline in dopamine levels in the striatum, a breadth of non-motor features and treatment-related complications in which the serotonergic system plays a pivotal role are increasingly recognised. Serotonin (5-HT)-mediated neurotransmission is altered in PD and the roles of the different 5-HT receptor subtypes in disease manifestations have been investigated. The aims of this article are to summarise and discuss all published preclinical and clinical studies that have investigated the serotonergic system in PD and related animal models, in order to recapitulate the state of the current knowledge and to identify areas that need further research and understanding.

Presynaptic control of serotonin on striatal dopamine function

RATIONALE

The influences of the serotonergic system on dopamine (DA) neuron activity have received considerable attention during the last three decades due to the real opportunity to improve disorders related to central DA neuron dysfunctions such as Parkinson's disease, schizophrenia, or drug abuse with serotonergic drugs. Numerous biochemical and behavioral data indicate that serotonin (5-HT) affects dopaminergic terminal function in the striatum.

OBJECTIVE

The authors propose a thorough examination of data showing controversial effects induced by striatal 5-HT on dopaminergic activity.

RESULTS

Inhibitory and excitatory effects of exogenous 5-HT have been reported on DA release and synthesis, involving various striatal 5-HT receptors. 5-HT also promotes an efflux of DA through reversal of the direction of DA transport. By analogy with the mechanism of action described for amphetamine, the consequences of 5-HT entering DA terminals might explain both the excitatory and inhibitory effects of 5-HT on presynaptic DA terminal activity, but the physiological relevance of this mechanism is far from clear. The recent data suggest that the endogenous 5-HT system affects striatal DA release in a state-dependent manner associated with the conditional involvement of various 5-HT receptors such as 5-HT(2A), 5-HT(2C), 5-HT(3), and 5-HT(4) receptors.

CONCLUSION

Methodological and pharmacological issues have prevented a comprehensive overview of the influence of 5-HT on striatal DA activity. The distribution of striatal 5-HT receptors and their restricted influence on DA neuron activity suggest that the endogenous 5-HT system exerts multiple and subtle influences on DA-mediated behaviors.

Role of serotonin in central dopamine dysfunction

The interaction between serotonin (5-HT) and dopamine (DA)-containing neurons in the brain is a research topic that has raised the interest of many scientists working in the field of neuroscience since the first demonstration of the presence of monoamine-containing neurons in the mid 1960. The bulk of neuroanatomical data available clearly indicate that DA-containing neurons in the brain receive a prominent innervation from serotonin (5-hydroxytryptamine, 5-HT) originating in the raphe nuclei of the brainstem. Compelling electrophysiological and neurochemical data show that 5-HT can exert complex effects on the activity of midbrain DA neurons mediated by its various receptor subtypes. The main control seems to be inhibitory, this effect being more marked in the mesocorticolimbic DA system as compared to the DA nigrostriatal system. In spite of a direct effect of 5-HT by its receptors located on DA cells, 5-HT can modulate their activity indirectly, modifying gamma-aminobutyric (GABA)-ergic and glutamatergic input to the ventral tegmental area (VTA) and substantia nigra pars compacta (SNc). Although 5-HT/DA interaction in the brain has been extensively studied, much work remains to be done to clarify this issue. The recent development of subtype-selective ligands for 5-HT receptors will not only allow a detailed understanding of this interaction but also will lead to the development of new treatment strategies, appropriate for those neuropsychiatric disorders in which an alteration of the 5-HT/DA balance is supposed.

The use of an antagonist 5-HT2a/c for depression and motor function in Parkinson' disease

OBJECTIVE

To test the ability of a 5HT2a/c (trazodone) antagonist, to improve depression and motor function in Parkinson' disease (PD).

METHOD

Twenty PD patients with and without depression were randomly assigned to receive trazodone (group 1) or not (group 2). They were evaluated through UPDRS and Hamilton Depression Rating Scale (HAM-D).

RESULTS

For the UPDRS the mean score of group 2 was 33.1 +/- 19.7 and 37.1 +/- 18.0 at the end. For the group 1, the corresponding scores were 31.4 +/- 11.3 and 25.9 +/- 13.7. The variations in the Mann-Whitney test were 0.734 at the initial moment and 0.208 at the final moment. The variation in the comparison of the initial moment with the final moment was 0.005 providing statistical significance. For the HAM-D, the mean score went up 4 points in group 2, contrary to a 5.5 points decrease in group 1.

CONCLUSION

Data analysis shows that this agent significantly improves depression, but the motor function improved only in the depressed patients. Because of the known anti-dopaminergic property of the 5-HT2c receptors, a possible approach for depression in PD could be the use of 5-HT2c antagonists, similarly to the use of atypical neuroleptics in case of psychotic symptoms.

Effects of 5-HT in globus pallidus on haloperidol-induced catalepsy in rats

Histological, behavioral and electrophysiological studies have suggested that 5-HT may regulate motor function by affecting globus pallidus neurons activity. In this study, the effects of 5-HT in globus pallidus on haloperidol-induced catalepsy and its possible receptor mechanisms were examined in rats using bar tests. Bilateral microinjection of 5-HT (10microM) into globus pallidus significantly attenuated haloperidol-induced catalepsy. This anticataleptic effect was completely counteracted by selective 5-HT(1B) receptors antagonist SB-224289 (10microM), while partly reversed by selective 5-HT(4) receptors antagonist GR-113808 (1microM). In addition, the selective 5-HT(7) receptors antagonist SB-269970 (1microM) partly reversed the anticataleptic effect of 5-HT only at the incipient period after the intrapallidal injection. In conclusion, 5-HT in globus pallidus could attenuate haloperidol-induced catalepsy via multiple receptor mechanisms.

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.

Serotonin 2A receptor antagonist treatment reduces dopamine D1 receptor-mediated rotational behavior but not L-DOPA-induced abnormal involuntary movements in the unilateral dopamine-depleted rat

Previous experiments have demonstrated that serotonin (5-HT) 2A receptor antagonists suppress hyperkinetic behaviors associated with dopamine (DA) D1 receptor supersensitivity in rats with 6-hydroxydopamine (6-OHDA) lesions. Since l-DOPA induced dyskinesia (LID) may be mediated by over-sensitive D1-mediated signaling, the present study examined the effects of the selective 5-HT2A antagonist M100907 on LID behaviors in DA-depleted rats. Adult male Sprague-Dawley rats with unilateral 6-OHDA lesions received daily l-DOPA treatments to produce dyskinetic behaviors as measured by abnormal involuntary movements (AIMs) testing. In these animals, M100907 (0.01, 0.1 or 1.0mg/kg, ip) given 30 min before l-DOPA did not alter the appearance or intensity of AIMs behaviors. Because l-DOPA induced AIMs in rats are dependent upon D1 and D2 receptor activation, a second study was performed to determine if M100907 could suppress D1- or D2-mediated rotational behaviors. Contralateral rotations induced by the D1 agonist SKF82958 were significantly reduced by pre-treatment with M100907. However, M100907 was ineffective in reducing rotations induced by the D2 agonist quinpirole. The finding that M100907 suppresses rotations induced by D1, but not D2, agonists may provide a partial explanation for the lack of effect of a selective 5-HT2A antagonist on l-DOPA-induced AIMs behaviors.

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.

A serotonin 5-HT1A receptor agonist prevents behavioral sensitization to L-DOPA in a rodent model of Parkinson's disease

Marked fluctuation of dopamine concentration in the striatum following long-term L-DOPA administration contributes to the development of L-DOPA-induced motor complications including L-DOPA-induced dyskinesias and wearing-off in patients with Parkinson's disease. We have shown that pretreatment with 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), a 5-HT1A (5-hydroxytryptamine) receptor agonist, alleviates fluctuation of dopamine levels in the dopamine-denervated striatum of 6-hydroxydopamine-lesioned (hemiparkinsonian) rats after L-DOPA treatment. To determine whether co-administration of 8-OH-DPAT with L-DOPA prevents L-DOPA-induced motor complications, we examined rotation behavior and levels of messenger RNAs coding for dynorphin and glutamic acid decarboxylase in the striatum of 6-hydroxydopamine-lesioned rats treated with L-DOPA alone or L-DOPA + 8-OH-DPAT, twice daily, for 2 weeks. Co-administration of 8-OH-DPAT inhibited an increase of rotation behavior to L-DOPA and L-DOPA-induced increases in levels of messenger RNAs coding for dynorphin and glutamic acid decarboxylase in the dopamine-denervated striatum, both of which are established indices of L-DOPA-induced motor complications. These results suggest that pharmaceutical products that stimulate 5-HT1A receptors could prove useful in prevention of the development of L-DOPA-induced motor complications in patients with Parkinson's disease.

Transmitter receptors and functional anatomy of the cerebral cortex

The currently available architectonic maps of the human cerebral cortex do not match the high degree of cortical segregation as shown by functional imaging. Such functional imaging studies have demonstrated a considerable number of functionally specialized areas not displayed in the architectonic maps. We therefore analysed the regional and laminar distribution of various transmitter receptors in the human cerebral cortex, because these signalling molecules play a crucial role in cortical functions. They may provide a novel and functionally more relevant insight into the regional organization of the cortex, which cannot be achieved by architectonic observations in cell body- or myelin-stained sections. Serial cryostat sections through whole human hemispheres were used for quantitative receptor autoradiography. The regional and laminar densities of numerous receptors of classic transmitter systems were analysed. Alternating sections were stained for comparisons based on cyto- or myeloarchitectonic criteria. Our results demonstrate that the regional distribution of transmitter receptors reflects well-established cyto- and myeloarchitectonically defined borders of cortical areas, but in addition enables the identification of more cortical areas than previously demonstrated. Moreover, the laminar distribution patterns of a given receptor type in different cortical areas as well as those of different receptor types in the same cortical area reveal novel and functionally relevant data concerning the intracortical organization in the human cerebral cortex.

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.

Effect of acute and chronic olanzapine treatment on phencyclidine-induced behavioral sensitization in rats with neonatal dopamine loss

In agreement with previous work, adult rats given selective lesions to dopamine (DA)-containing neurons as neonates exhibited a greater behavioral sensitization to repeated phencyclidine (PCP) treatment in comparison to sham-lesioned controls. Acute administration of olanzapine (1-5 mg/kg ip) or clozapine (15 mg/kg ip) decreased sensitized PCP-induced activity in both lesioned and control animals. Acute haloperidol (0.5 mg/kg ip) had no impact on PCP responsiveness in lesioned animals, but significantly antagonized PCP effects in sham-lesioned controls. Ketanserin, a selective 5-HT(2A)/5-HT(2C)-receptor antagonist, significantly reduced PCP activation in both lesioned and control rats, suggesting that the efficacy of atypical antipsychotics against PCP-induced sensitized responses may be mediated by one of the 5-HT(2)-receptor subtypes. A 6-week chronic regimen of orally administered olanzapine, clozapine, or haloperidol failed to block the sensitization induced by repeated PCP exposure. However, a 10-month oral olanzapine treatment significantly blunted the behavioral sensitization to repeated PCP exposure in lesioned animals, even after withdrawal from chronic olanzapine for more than 3 weeks. A 10-month oral haloperidol treatment had no effect on the sensitization induced by repeated PCP dosing. The persistent effect of chronic olanzapine administration on PCP sensitization may be relevant to the chronic therapeutic efficacy of atypical antipsychotics treating schizophrenia-a clinical syndrome linked to enhanced sensitivity to N-methyl-d-aspartate (NMDA)-receptor antagonists.

Altered emotional behaviors and the expression of 5-HT1A and M1 muscarinic receptors in ?-opioid receptor knockout mice

Anxiety and depression alterations have been reported in micro-opioid receptor knockout mice after exon 2 disruption. However, emotional behaviors, such as novelty and emergence responses have not been reported in micro-opioid receptor knockout mice due to the disruptions of exon 2 and 3. Here, we report that mu-opioid receptor knockout mice, with deletion of exon 2 and 3, display significant emotional behavior changes; they showed less anxiety in the elevated plus maze and emergence tests, reduced response to novel stimuli in the novelty test, and less depressive-like behavior in the forced-swim test. Analysis of the compensatory mechanism in mu-opioid receptor knockout mice revealed that the M1 mRNA levels were reduced in the cortex, caudate putamen, nucleus accumbens, and hippocampus, and that M1 receptor levels were reduced in the nucleus accumbens, CA1, and the dentate gyrus of the hippocampus, versus the wild-type. However, 5-HT1A receptor levels were significantly elevated in the cerebral cortex and in the hypothalamus of mu-opioid receptor knockout mice versus the wild-type. These aberrant emotional behavioral phenotypes are possibly related to M1 and 5-HT1A receptor alterations in the micro-opioid receptor knockout mice. Overall, our study suggests that micro-opioid receptor may play a role in the modification of emotional responses to novelty, anxiety, and depression.

3,4-methylenedioxymethamphetamine (ecstasy) inhibits dyskinesia expression and normalizes motor activity in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-treated primates

Ecstasy [3,4-methylenedioxymethamphetamine (MDMA)] was shown to prolong the action of L-3,4-dihydroxyphenylalanine (L-DOPA) while suppressing dyskinesia in a single patient with Parkinson's disease (PD). The clinical basis of this effect of MDMA is unknown but may relate to its actions on either dopaminergic or serotoninergic systems in brain. In normal, drug-naive common marmosets, MDMA administration suppressed motor activity and exploratory behavior. In 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated, L-DOPA-primed common marmosets, MDMA transiently relieved motor disability but over a period of 60 min worsened motor symptoms. When given in conjunction with L-DOPA, however, MDMA markedly decreased dyskinesia by reducing chorea and to a lesser extent dystonia and decreased locomotor activity to the level observed in normal animals. MDMA similarly alleviated dyskinesia induced by the selective dopamine D2/3 agonist pramipexole. The actions of MDMA appeared to be mediated through 5-HT mechanisms because its effects were fully blocked by the selective serotonin reuptake inhibitor fluvoxamine. Furthermore, the effect of MDMA on L-DOPA-induced motor activity and dyskinesia was partially inhibited by 5-HT1a/b antagonists. The ability of MDMA to inhibit dyskinesia results from its broad spectrum of action on 5-HT systems. Serotoninergic receptors appear to play an important modulatory role in l-DOPA-induced dyskinesia, and this study may provide a framework for the use of serotoninergic agents in the treatment of L-DOPA-induced dyskinesia.

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.

Role of serotonin(2C) receptors in the control of brain dopaminergic function

There is substantial evidence that the functional status of mesocorticolimbic dopaminergic (DA) system originating in the ventral tegmental area (VTA) is under a phasic and tonic inhibitory control by the serotonergic system, which acts by stimulating serotonin(2C) (5-HT(2C)) receptor subtypes. This assertion is based upon a number of electrophysiological and biochemical data showing that 5-HT(2C) receptor agonists decrease, while 5-HT(2C) receptor antagonists enhance mesocorticolimbic DA function. On the other hand, it does not seem that 5-HT(2C) receptors play a relevant role in the control of nigrostriatal DA system originating in the substantia nigra pars compacta (SNc). The authors of this article review the most relevant data regarding the role of 5-HT(2C) receptors in the control of brain DA function and underline the importance of this subject in the search of new therapies for neuropsychiatric disorders such as depression, schizophrenia, drug addiction, and Parkinson's disease.

Regulation of striatal neuropeptide mRNAs: effects of the 5-HT(2) antagonist SR46349B in adult rats with a neonatal 6-hydroxydopamine lesion

The intrastriatal injection of 6-hydroxydopamine (6-OHDA) in newborn rats produces a marked striatal dopamine (DA) depletion, accompanied by a serotonin (5-HT) hyperinnervation and an up-regulation of 5-HT receptors. The aim of the present study was to investigate whether the increase in 5-HT(2) receptors could compensate for some of the DA lesion-induced effects, such as the increase in striatal preproenkephalin (PPE) and the decrease in preprotachykinin A (PPT-A) mRNA levels. Three months after the DA lesion, the effect of the selective 5-HT(2) antagonist SR46349B was investigated by a subacute treatment (10 mg/kg, IP, twice per day for 3.5 days). In sham-operated rats, the blockade of 5-HT(2) receptors decreased PPE mRNA levels in the striatum and, by contrast, had no effect on PPT-A mRNA levels. In rats with a unilateral neonatal DA lesion, SR46349B had no more effect on PPE mRNA levels in the intact striatum and was unable to modify the lesion induced-increase in PPE mRNA. The decrease in PPT-A mRNA levels induced by the neonatal DA lesion was not changed after SR46349B treatment in the posterior part of the lesioned striatum. Our results suggest that SR46349B indirectly decreases PPE mRNA levels in striatopallidal neurons in intact animals through a desinhibition of DA neuron activity. This is further evidenced by the lack of PPE mRNA changes in the DA lesioned striatum despite the up-regulation of 5-HT(2) receptor transmission induced in this model. Finally, the absence of any effect of 5-HT(2) antagonist on the expression of PPT-A mRNA in intact animals is discussed. The precise role of 5-HT(2) receptor on PPT-A mRNA biosynthesis after a neonatal lesion should be clarified by further experiments using 5-HT(2) agonists.

Neonatal dopamine depletion reveals a synergistic mechanism of mRNA regulation that is mediated by dopamine(D1) and serotonin(2) receptors and is targeted to tachykinin neurons of the dorsomedial striatum

It has been hypothesized that dopamine(D1) and serotonin(2) receptors become sensitized to agonist-mediated regulation of gene expression following loss of dopaminergic innervation to the striatum. We have previously demonstrated that the combined administration of dopamine(D1) and serotonin(2) receptor agonists to dopamine-depleted adult rats induced preprotachykinin mRNA expression within the periventricular rostral striatum to levels which were significantly different than what could be elicited by either agonist alone. In the present study, we have determined that this phenomenon is revealed only after dopamine depletion. In addition, it is targeted primarily to tachykinin producing neurons of the dorsomedial striatum and is dependent on both dopamine(D1) and serotonin(2) receptor activation. Preprotachykinin mRNA levels in the intact striatum were unaltered 4 h following an i.p. injection of either SKF-38393 (1 mg/kg, dopamine(D1) partial agonist) or (+/-)-1-(4-Iodo-2,5-dimethoxyphenyl)-2-aminopropane (DOI 1 mg/kg, serotonin(2) agonist). However, the combined application of both agonists increased (+44%) preprotachykinin message levels, but these changes were restricted to the dorsomedial striatum. In adult animals depleted of dopamine as neonates, striatal preprotachykinin mRNA expression was reduced by approximately 50%. From this lowered level of basal expression, DOI or SKF-38393 raised preprotachykinin mRNA levels within the dorsomedial, but not the dorsolateral striatum. Furthermore, co-stimulation of dopamine(D1) and serotonin(2) receptors produced a nearly four-fold induction of preprotachykinin message levels in the dorsomedial striatum that was significantly greater than either agonist alone. Application of both agonists also elevated preprotachykinin mRNA expression within the dorsolateral striatum, but to a lesser extent. All increases in preprotachykinin mRNA resulting from co-application of SKF-38393 and DOI were prevented by pretreatment with either SCH-23390 (1 mg/kg, dopamine(D1) antagonist) or ritanserin (1 mg/kg, serotonin(2) antagonist). Alternately, preproenkephalin mRNA expression was unaffected by dopamine(D1) receptor stimulation, but was slightly elevated by DOI or both agonists together (42-58%) in intact animals. However, neither agonist treatment in this experiment significantly altered preproenkephalin mRNA expression in the dopamine-depleted striatum which was elevated in response to dopamine lesion alone. Dopamine depletion appears to promote a synergistic interaction between dopamine(D1) and serotonin(2) receptors that leads to enhanced expression of striatal preprotachykinin mRNA levels. The localization of this phenomenon to tachykinin neurons of the direct striatonigral pathway specifically within the dorsomedial regions of the rostral striatum may be relevant to the problem of dyskinetic behaviors which arise during the pharmacological treatment of movement disorders.

Serotonin 2A receptor regulation of striatal neuropeptide gene expression is selective for tachykinin, but not enkephalin neurons following dopamine depletion

Serotonin (5-HT) 2A receptor-mediated regulation of striatal preprotachykinin (PPT) and preproenkephalin (PPE) mRNAs was studied in adult rodents that had been subjected to near-total dopamine (DA) depletion as neonates. Two months following bilateral 6-hydroxydopamine (6-OHDA) lesion, PPT mRNA levels decreased 59-73% across dorsal subregions of the rostral and caudal striatum while PPE transcripts increased 61-94%. Four hours after a single injection of the serotonin 2A/2C receptor agonist, (+/-)-1-(2,5-Dimethoxy-4-iodophenyl)-2-aminopropane (DOI; 1 mg/kg), PPT mRNA expression was significantly increased in DA-depleted rats across all dorsal subregions of the rostral and caudal striatum as compared to 6-OHDA-treated animals alone. In the intact rat, DOI did not influence PPT mRNA levels in the rostral striatum, but did raise expression in the caudal striatum where 5-HT2A receptors are prominent. DOI did not regulate PPE mRNA levels in any striatal sub-region of the intact or DA-depleted rat. Prior administration of the 5-HT2A/2C receptor antagonist, ritanserin (1 mg/kg) or the 5-HT2A receptor antagonist, ketanserin (1 mg/kg) completely blocked the DOI-induced increases in striatal PPT mRNA in both lesioned and intact animals. The ability of ketanserin to produce identical results as ritanserin suggests that 5-HT2A receptor-mediated regulation is selectively strengthened within tachykinin neurons of the rostral striatum which are suppressed by DA depletion. The selectivity suggests that 5-HT2A receptor upregulation following DA depletion is capable of regulating tachykinin biosynthesis without influencing enkephalin expression in striatal output neurons.

Altered serotonin and dopamine metabolism in the CNS of serotonin 5-HT(1A) or 5-HT(1B) receptor knockout mice

Measurements of serotonin (5-HT), dopamine (DA), and noradrenaline, and of 5-HT and DA metabolites, were obtained by HPLC from 16 brain regions and the spinal cord of 5-HT(1A) or 5-HT(1B) knockout and wild-type mice of the 129/Sv strain. In 5-HT(1A) knockouts, 5-HT concentrations were unchanged throughout, but levels of 5-HT metabolites were higher than those of the wild type in dorsal/medial raphe nuclei, olfactory bulb, substantia nigra, and locus coeruleus. This was taken as an indication of increased 5-HT turnover, reflecting an augmented basal activity of midbrain raphe neurons and consequent increase in their somatodendritic and axon terminal release of 5-HT. It provided a likely explanation for the increased anxious-like behavior observed in 5-HT(1A) knockout mice. Concomitant increases in DA content and/or DA turnover were interpreted as the result of a disinhibition of DA, whereas increases in noradrenaline concentration in some territories of projection of the locus coeruleus could reflect a diminished activity of its neurons. In 5-HT(1B) knockouts, 5-HT concentrations were lower than those of the wild type in nucleus accumbens, locus coeruleus, spinal cord, and probably also several other territories of 5-HT innervation. A decrease in DA, associated with increased DA turnover, was measured in nucleus accumbens. These changes in 5-HT and DA metabolism were consistent with the increased aggressiveness and the supersensitivity to cocaine reported in 5-HT(1B) knockout mice. Thus, markedly different alterations in CNS monoamine metabolism may contribute to the opposite behavioral phenotypes of these two knockouts.

Serotonin 2A and 2C receptor biosynthesis in the rodent striatum during postnatal development: mRNA expression and functional linkage to neuropeptide gene regulation

The present study was designed to determine if there are region-specific differences in serotonin (5-HT) neurotransmission and 5-HT receptor expression that may limit the stimulatory effects of the 5-HT releaser p-chloroamphetamine (pCA) on striatal neuropeptide gene expression to the posterior striatum (P-STR) during postnatal maturation. Sprague-Dawley rat brains from postnatal days (PND) 1-35 were processed for 5-HT(2A) and 5-HT(2C) receptor mRNA expression by in situ hybridization and monoamine analysis by HPLC. Within the P-STR, 5-HT(2A) receptor mRNA expression reached young adult (PND 35) levels by PND 3, while levels in the A-STR were significantly less (range: 1.43 +/- 0.219-6. 36 +/- 0.478) than P-STR (5.36 +/- 0.854-12.11 +/- 1.08) at each respective age throughout the time course. 5-HT(2C) receptor mRNA expression reached young adult levels at PND 7 in the A-STR and by PND 3 in the P-STR. At each PND age 5-HT(2C) receptor mRNA levels within the P-STR were significantly less (6.23 +/- 1.02-12.32 +/- 0.427) than the A-STR (7.31 +/- 1.65-26.84 +/- 2.24). 5-HT content increased across the developmental time course within the P-STR (5.01 +/- 0.327-15.7 +/- 1.03 ng/mg protein) and A-STR (2.97 +/- 0. 223-11.2 +/- 0.701 ng/mg protein). Four hours following injection (i. p.) of pCA (10 mg/kg), preprotachykinin (PPT) mRNA levels increased 89% in the P-STR but not the anterior (A-STR) striatum of the 3-week-old rat, which were prevented by preinjection (30 min, i.p.) of the 5-HT(2) receptor antagonist ritanserin (1 mg/kg). Together, these data suggest that faster maturity of 5-HT(2A) receptor expression in the P-STR may be sufficient to convey the region-specific acute stimulatory effects of pCA on PPT mRNA transcription in the developing rodent striatum. These results provide further evidence that the influence of 5-HT on neuropeptide gene expression is far stronger in caudal vs. rostral striatal regions during postnatal development.

Stimulated serotonin release from hyperinnervated terminals subsequent to neonatal dopamine depletion regulates striatal tachykinin, but not enkephalin gene expression

Dopamine (DA) depletion in neonatal rodents results in depressed tachykinin and elevated enkephalin gene expression in the adult striatum (STR). Concurrently, serotonin (5-HT) fibers sprout to hyperinnervate the DA-depleted anterior striatum (A-STR). The present study was designed to determine if increased 5-HT release from sprouted terminals influences dysregulated preprotachykinin (PPT) and preproenkephalin (PPE) mRNA expression in the DA-depleted STR. Three-day-old Sprague-Dawley rat pups received bilateral intracerebroventricular injections of vehicle or the DA neurotoxin 6-hydroxydopamine (6-OHDA, 100 microg). Two months later, rats received a single intraperitoneal injection of vehicle or the acute 5-HT releasing agent p-chloroamphetamine (PCA; 10 mg/kg). Rats were killed 4 h later and striata processed for monoamine content by HPLC-ED and mRNA expression by in situ hybridization within specific subregions of the A-STR and posterior striatum (P-STR). 6-OHDA treatment severely (>98%) reduced striatal DA levels, while 5-HT content in the A-STR was significantly elevated (doubled), indicative of 5-HT hyperinnervation. Following 6-OHDA, PPT mRNA levels were depressed 60-66% across three subregions of the A-STR and 52-59% across two subregions of the P-STR, while PPE mRNA expression was elevated in both the A-STR (50-62%) and P-STR (55-82%). PCA normalized PPT mRNA levels in all regions of the DA-depleted A-STR and P-STR, yet did not alter PPE levels in either dorsal central or medial regions from 6-OHDA alone, but reduced PPE to control levels in the dorsal lateral A-STR. These data indicate that increased 5-HT neurotransmission, following neonatal 6-OHDA treatment, primarily influences PPT-containing neurons of the direct striatal output pathway.