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

Effect of -NBOMe Compounds on Sensorimotor, Motor, and Prepulse Inhibition Responses in Mice in Comparison With the 2C Analogs and Lysergic Acid Diethylamide: From Preclinical Evidence to Forensic Implication in Driving Under the Influence of Drugs

In the last decade, the market for new psychoactive substances has been enriched by numerous psychedelic phenethylamines, which mimic the psychoactive effect of lysergic acid diethylamide (LSD). In particular, the -NBOMe series, which are more potent than their 2C compounds analogs, are considered worthy substitutes for LSD by users. The purpose of this study was to assess the effects of 25H-NBOMe and its halogenated derivatives (25I-NBOMe and 25B-NBOMe) in comparison to their 2C compounds analogs and LSD on the sensorimotor (visual, acoustic, and overall tactile), reaction time, spontaneous (total distance traveled) and stimulated (drag, accelerod test) motor activity, grip strength test, and prepulse inhibition (PPI) responses in mice. Systemic administration of -NBOMe, 2C compounds analogs, and LSD (0.001-10 mg/kg) differently impaired the sensorimotor, reaction time, motor, and PPI responses in mice. In particular, halogenated (25I and 25B)-NBOMe derivatives appear to be more effective than the entire class of 2C compounds analogs in altering visual and acoustic responses, affecting reaction time, and motor and sensory gating in PPI test. In fact, the specific rank order of compounds potency for nearly all of the experiments showed that (25I and 25B)-NBOMe were more potent than 2C compounds analogs and LSD. -NBOMe and 2C compounds analogs impaired not only the reception of incoming sensory stimuli (visual and acoustic), but their correct brain processing (PPI) in an equal and sometimes stronger way than LSD. This sensory impairment directly affected the spontaneous motor response and reaction time of mice, with no change in performance in stimulated motor activity tests. These aspects should be carefully considered to better understand the potential danger that psychedelic phenethylamines, in particular -NBOMe, may pose to public health, with particular reference to decreased performance in driving and hazardous works that require special sensorimotor skills.

Serotonergic targets for the treatment of L-DOPA-induced dyskinesia

Dopamine (DA) replacement therapy with L-3,4-dihydroxyphenylalanine (L-DOPA) continues to be the gold-standard treatment for Parkinson's disease (PD). Despite clear symptomatic benefit, long-term L-DOPA use often results in the development of L-DOPA-induced dyskinesia (LID), significantly reducing quality of life and increasing costs for PD patients and their caregivers. Accumulated research has demonstrated that several pre- and post-synaptic mechanisms contribute to LID development and expression. In particular, raphe-striatal hyperinnervation and unregulated DA release from 5-HT terminals is postulated to play a central role in LID manifestation. As such, manipulation of the 5-HT system has garnered considerable attention. Both pre-clinical and clinical research has supported the potential of modulating the 5-HT system for LID prevention and treatment. This review discusses the rationale for continued investigation of several potential anti-dyskinetic strategies including 5-HT stimulation of 5-HT1A and 5-HT1B receptors and blockade of 5-HT2A receptors and SERT. We present the latest findings from experimental and clinical investigations evaluating these 5-HT targets with the goal of identifying those with translational promise and the challenges associated with each.

Serotoninergic and dopaminergic modulation of cortico-striatal circuit in executive and attention deficits induced by NMDA receptor hypofunction in the 5-choice serial reaction time task

Executive functions are an emerging propriety of neuronal processing in circuits encompassing frontal cortex and other cortical and subcortical brain regions such as basal ganglia and thalamus. Glutamate serves as the major neurotrasmitter in these circuits where glutamate receptors of NMDA type play key role. Serotonin and dopamine afferents are in position to modulate intrinsic glutamate neurotransmission along these circuits and in turn to optimize circuit performance for specific aspects of executive control over behavior. In this review, we focus on the 5-choice serial reaction time task which is able to provide various measures of attention and executive control over performance in rodents and the ability of prefrontocortical and striatal serotonin 5-HT_1A, 5-HT_2A, and 5-HT_2C as well as dopamine D₁- and D₂-like receptors to modulate different aspects of executive and attention disturbances induced by NMDA receptor hypofunction in the prefrontal cortex. These behavioral studies are integrated with findings from microdialysis studies. These studies illustrate the control of attention selectivity by serotonin 5-HT_1A, 5-HT_2A, 5-HT_2C, and dopamine D₁- but not D₂-like receptors and a distinct contribution of these cortical and striatal serotonin and dopamine receptors to the control of different aspects of executive control over performance such as impulsivity and compulsivity. An association between NMDA antagonist-induced increase in glutamate release in the prefrontal cortex and attention is suggested. Collectively, this review highlights the functional interaction of serotonin and dopamine with NMDA dependent glutamate neurotransmission in the cortico-striatal circuitry for specific cognitive demands and may shed some light on how dysregulation of neuronal processing in these circuits may be implicated in specific neuropsychiatric disorders.

Serotonin hyperinnervation and upregulated 5-HT2A receptor expression and motor-stimulating function in nigrostriatal dopamine-deficient Pitx3 mutant mice

The striatum receives serotonin (5-hydroxytryptamine, 5-HT) innervation and expresses 5-HT2A receptors (5-HT2ARs) and other 5-HT receptors, raising the possibility that the striatal 5-HT system may undergo adaptive changes after chronic severe dopamine (DA) loss and contribute to the function and dysfunction of the striatum. Here we show that in transcription factor Pitx3 gene mutant mice with a selective, severe DA loss in the dorsal striatum mimicking the DA denervation in late Parkinson's disease (PD), both the 5-HT innervation and the 5-HT2AR mRNA expression were increased in the dorsal striatum. Functionally, while having no detectable motor effect in wild type mice, the 5-HT2R agonist 2,5-dimethoxy-4-iodoamphetamine increased both the baseline and l-dopa-induced normal ambulatory and dyskinetic movements in Pitx3 mutant mice, whereas the selective 5-HT2AR blocker volinanserin had the opposite effects. These results demonstrate that Pitx3 mutant mice are a convenient and valid mouse model to study the compensatory 5-HT upregulation following the loss of the nigrostriatal DA projection and that the upregulated 5-HT2AR function in the DA deficient dorsal striatum may enhance both normal and dyskinetic movements.

Effects of dopamine and serotonin antagonist injections into the striatopallidal complex of asymptomatic MPTP-treated monkeys

The cardinal symptoms of Parkinson's disease (PD), akinesia, rigidity and tremor, are only observed when the striatal level of dopamine (DA) is decreased by 60-80%. It is likely that compensatory mechanisms during the early phase of DA depletion delay the appearance of motor symptoms. In a previous study, we proposed a new PD monkey model with progressive MPTP intoxication. Monkeys developed all of the motor symptoms and then fully recovered despite a large DA cell loss in the substantia nigra (SN). Compensatory mechanisms certainly help to offset the dysfunction induced by the DA lesion, facilitating motor recovery in this model. Neurotransmitter measurements in the striatal sensorimotor and associative/limbic territories of these monkeys subsequently revealed that DA and serotonin (5-HT) could play a role in recovery mechanisms. To try to determine the involvement of these neurotransmitters in compensatory mechanisms, we performed local injections of DA and 5-HT antagonists (cis-flupenthixol and mianserin, respectively) into these two striatal territories and into the external segment of the globus pallidus (GPe). Injections were performed on monkeys that were in an asymptomatic state after motor recovery. Most parkinsonian motor symptoms reappeared in animals with DA antagonist injections either in sensorimotor, associative/limbic striatal territories or in the GPe. In contrast to the effects with DA antagonist, there were mild parkinsonian effects with 5-HT antagonist, especially after injections in sensorimotor territories of the striatum and the GPe. These results support a possible, but slight, involvement of 5-HT in compensatory mechanisms and highlight the possible participation of 5-HT in some behavioural disorders. Furthermore, these results support the notion that the residual DA in the different striatal territories and the GPe could be involved in important mechanisms of compensation in PD.

Dorsal-striatal 5-HT₂A and 5-HT₂C receptors control impulsivity and perseverative responding in the 5-choice serial reaction time task

RATIONAL

Prefrontal cortex (PFC) and dorsal striatum are part of the neural circuit critical for executive attention. The relationship between 5-HT and aspects of attention and executive control is complex depending on experimental conditions and the level of activation of different 5-HT receptors within the nuclei of corticostriatal circuitry.

OBJECTIVE

The present study investigated which 5-HT(2A) and 5-HT(2C) receptors in the dorsomedial-striatum (dm-STR) contribute to executive attention deficit induced by blockade of NMDA receptors in the PFC.

MATERIALS AND RESULTS

Executive attention was assessed by the five-choice serial reaction time task (5-CSRTT), which provides indices of attention (accuracy) and those of executive control over performance such as premature (an index of impulsivity) and perseverative responding. The effects of targeted infusion in dm-STR of 100 and 300 ng/μl doses of the selective 5-HT(2A) antagonist M100907 and 1 and 3 μg/μl doses of 5-HT(2C) agonist Ro60-0175 was examined in animals injected with 50 ng/μl dose of a competitive NMDA receptor antagonist 3-(R)-2-carboxypiperazin-4-phosphonic acid (CPP) in the mPFC. Blockade of NMDA receptors impaired accuracy as well as executive control as shown by increased premature and perseverative responding. The CPP-induced premature and perseverative over-responding were dose-dependently prevented by both M100907 and Ro60-0175. Both drugs partially removed the CPP-induced accuracy deficit but only at the highest dose tested.

CONCLUSIONS

It is suggested that in the dorsal striatum, 5-HT by an action on 5-HT(2A) and 5-HT(2C) receptors may integrate the glutamate corticostriatal inputs critical for different aspects of the 5-CSRT task performance.

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.

Brain 5-HT(2A) receptors in MPTP monkeys and levodopa-induced dyskinesias

Levodopa-induced dyskinesias (LIDs) are abnormal involuntary movements induced by the chronic use of levodopa (l-Dopa) limiting the quality of life of Parkinson's disease (PD) patients. We evaluated changes of the serotonin 5-HT(2A) receptors in control monkeys, in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-lesioned monkeys and in l-Dopa-treated MPTP monkeys, without or with adjunct treatments to inhibit the expression of LID: CI-1041, a selective NR1A/2B subunit antagonist of glutamate N-methyl-d-aspartic acid (NMDA) receptor, or Cabergoline, a long-acting dopamine D(2) receptor agonist. All treatments were administered for 1 month and animals were killed 24 h after the last dose of l-Dopa. Striatal concentrations of serotonin were decreased in all MPTP monkeys investigated, as measured by high-performance liquid chromatography. [(3) H]Ketanserin-specific binding to 5-HT(2A) receptors was measured by autoradiography. l-Dopa treatment that induced dyskinesias increased 5-HT(2A) receptor-specific binding in the caudate nucleus and the anterior cingulate gyrus (AcgG) compared with control monkeys. Moreover, [(3) H]Ketanserin-specific binding was increased in the dorsomedial caudate nucleus in l-Dopa-treated MPTP monkeys compared with saline-treated MPTP monkeys. Nondyskinetic monkeys treated with CI-1041 or Cabergoline showed low 5-HT(2A) -specific binding in the posterior dorsomedial caudate nucleus and the anterior AcgG compared with dyskinetic monkeys. No significant difference in 5-HT(2A) receptor binding was observed in any brain regions examined in saline-treated MPTP monkeys compared with control monkeys. These results confirm the involvement of serotonergic pathways and the glutamate/serotonin interactions in LID. They also support targeting 5-HT(2A) receptors as a potential treatment for LID.

Antidepressant-like effect of Salvia sclarea is explained by modulation of dopamine activities in rats

AIM OF THE STUDY

The purpose of the present study was to screen aromatic essential oils that have antidepressant effects to identify the regulatory mechanisms of selected essential oils.

MATERIALS AND METHODS

The antidepressant effects of essential oils of Anthemis nobilis (chamomile), Salvia sclarea (clary sage; clary), Rosmarinus officinalis (rosemary), and Lavandula angustifolia (lavender) were assessed using a forced swim test (FST) in rats. Rats were treated with essential oils by intraperitoneal injection or inhalation. Serum levels of corticosterone were assessed by enzyme-linked immunosorbent assay (ELISA).

RESULTS

Among the essential oils tested, 5% (v/v) clary oil had the strongest anti-stressor effect in the FST. We further investigated the mechanism of clary oil antidepression by pretreatment with agonists or antagonists to serotonin (5-HT), dopamine (DA), adrenaline, and GABA receptors. The anti-stressor effect of clary oil was significantly blocked by pretreatment with buspirone (a 5-HT(1A) agonist), SCH-23390 (a D(1) receptor antagonist) and haloperidol (a D(2), D(3), and D(4) receptor antagonist).

CONCLUSIONS

Our findings indicate that clary oil could be developed as a therapeutic agent for patients with depression and that the antidepressant-like effect of clary oil is closely associated with modulation of the DAnergic pathway.

5-HT2A receptor antagonists improve motor impairments in the MPTP mouse model of Parkinson's disease

Clinical observations have suggested that ritanserin, a 5-HT(2A/C) receptor antagonist may reduce motor deficits in persons with Parkinson's Disease (PD). To better understand the potential antiparkinsonian actions of ritanserin, we compared the effects of ritanserin with the selective 5-HT(2A) receptor antagonist M100907 and the selective 5-HT(2C) receptor antagonist SB 206553 on motor impairments in mice treated with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). MPTP-treated mice exhibited decreased performance on the beam-walking apparatus. These motor deficits were reversed by acute treatment with L-3,4-dihydroxyphenylalanine (levodopa). Both the mixed 5-HT(2A/C) antagonist ritanserin and the selective 5-HT(2A) antagonist M100907 improved motor performance on the beam-walking apparatus. In contrast, SB 206553 was ineffective in improving the motor deficits in MPTP-treated mice. These data suggest that 5-HT(2A) receptor antagonists may represent a novel approach to ameliorate motor symptoms of Parkinson's disease.

The nigrostriatal dopamine system of aging GFRalpha-1 heterozygous mice: neurochemistry, morphology and behavior

Given the established importance of glial cell line-derived neurotrophic factor (GDNF) in maintaining dopaminergic neurotransmitter systems, the nigrostriatal system and associated behaviors of mice with genetic reduction of its high-affinity receptor, GDNF receptor (GFR)alpha-1 (GFRalpha-1(+/-)), were compared with wild-type controls. Motor activity and the stimulatory effects of a dopamine (DA) D1 receptor agonist (SKF 82958) were assessed longitudinally at 8 and 18 months of age. Monoamine concentrations and dopaminergic nerve terminals in the striatum and the number of dopaminergic neurons in the substantia nigra (SN) were assessed. The results support the importance of GFRalpha-1 in maintaining normal function of the nigrostriatal dopaminergic system, with deficits being observed for GFRalpha-1(+/-) mice at both ages. Motor activity was lower and the stimulatory effects of the DA agonist were enhanced for the older GFRalpha-1(+/-) mice. DA in the striatum was reduced in the GFRalpha-1(+/-) mice at both ages, and tyrosine hydroxylase-positive cell numbers in the SN were reduced most substantially in the older GFRalpha-1(+/-) mice. The combined behavioral, pharmacological probe, neurochemical and morphological measures provide evidence of abnormalities in GFRalpha-1(+/-) mice that are indicative of an exacerbated aging-related decline in dopaminergic system function. The noted deficiencies, in turn, suggest that GFRalpha-1 is necessary for GDNF to maintain normal function of the nigrostriatal dopaminergic system. Although the precise mechanism(s) for the aging-related changes in the dopaminergic system remain to be established, the present study clearly establishes that genetic reductions in GFRalpha-1 can contribute to the degenerative changes observed in this system during the aging process.

Striatal 5-HT1A receptor stimulation reduces D1 receptor-induced dyskinesia and improves movement in the hemiparkinsonian rat

Convergent evidence suggests that serotonin 5-HT1A receptor (5-HT1AR) agonists reduce l-DOPA-induced dyskinesia by auto-regulating aberrant release of l-DOPA-derived dopamine (DA) from raphestriatal neurons. However, recent findings indicate that 5-HT1AR stimulation also modifies D1 receptor (D1R)-mediated dyskinesia and rotations implicating a previously unexplored extra-raphe mechanism. In order to characterize the contribution of the striatum to these effects, rats with medial forebrain bundle DA lesions were tested for abnormal involuntary movements (AIMs) and rotations following striatal microinfusions of the 5-HT1AR agonist +/-8-OH-DPAT and systemic D1R agonist treatment with SKF81297. Additional rats with multi-site striatal DA lesions were tested for motor disability following systemic or intrastriatal +/-8-OH-DPAT with or without systemic SKF81297. In rats with medial forebrain bundle lesions, striatal infusions of +/-8-OH-DPAT dose-dependently reduced AIMs while conversely increasing rotations. In rats with striatal lesions, +/-8-OH-DPAT alone, both systemic and intrastriatal administration, optimally reversed motor disability. Collectively, these results support an important functional interaction between 5-HT1AR and D1R in the striatum with implications for the improved treatment of Parkinson's disease.

Serotonin Activates Presynaptic and Postsynaptic Receptors in Rat Globus Pallidus

Although recent histological, behavioral, and clinical studies suggest that serotonin (5-HT) plays significant roles in the control of pallidal activity, only little is known about the physiological action of 5-HT in the pallidum. Our recent unit recording study in monkeys suggested that 5-HT provides both presynaptic and postsynaptic modulations of pallidal neurons. The present study using rat brain slice preparations further explored these presynaptic and postsynaptic actions of 5-HT. Bath application of 5-HT or the 5-HT1A/1B/1D/5/7 receptor (R) agonist 5-carboxamidotryptamine maleate (5-CT) depolarized some and hyperpolarized other pallidal neurons. Pretreatments of slices with blockers of the hyperpolarization–cyclic nucleotide-activated current or with the 5-HT2/7R–selective antagonist mesulergine occluded 5-CT–induced depolarization. The 5-HT1AR–selective blocker N-[2[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]- N-2-pyridinylcyclohex- anecarboxamide maleate occluded the 5-CT–induced hyperpolarization. These results suggested involvement of 5-HT7R and 5-HT1AR in the postsynaptic depolarization and hyperpolarization, respectively. 5-CT presynaptically suppressed both internal capsule stimulation–induced excitatory postsynaptic currents (EPSCs) and striatal stimulation–induced inhibitory postsynaptic currents (IPSCs). The potencies of 5-CT on the presynaptic effects were 20- to 25-fold higher than on postsynaptic effects, suggesting that 5-HT mainly modulates presynaptic sites in the globus pallidus. Experiments with several antagonists suggested involvement of 5-HT1B/DR in the presynaptic suppression of EPSCs. However, the receptor type involved in the presynaptic suppression of IPSCs was inconclusive. The present results provided evidence that 5-HT exerts significant control over the synaptic inputs and the autonomous activity of pallidal neurons.

5-HT 1B receptor-mediated serotoninergic modulation of methylphenidate-induced locomotor activation in rats

Previous studies have shown that the dopamine (DA) uptake blocker methylphenidate, a psychostimulant widely used for the treatment of attention-deficit hyperactivity disorder (ADHD), prevents the neurotoxic effects of the highly abused DA releaser methamphetamine. However, there is a lack of information about the pharmacological interactions of these two drugs at the behavioral level. When systemically administered within an interval of 2 h, previous administration of methylphenidate (10 mg/kg, intraperitoneal (i.p.)) did not modify locomotor activation induced by methamphetamine. On the other hand, previous administration of methamphetamine (1 mg/kg, i.p.) markedly potentiated methylphenidate-induced motor activation. With in vivo microdialysis experiments, methamphetamine and methylphenidate were found to increase DA extracellular levels in the nucleus accumbens (NAs). Methamphetamine, but not methylphenidate, significantly increased the extracellular levels of serotonin (5-HT) in the NAs. Methamphetamine-induced 5-HT release remained significantly elevated for more than 2 h after its administration, suggesting that the increased 5-HT could be responsible for the potentiation of methylphenidate-induced locomotor activation. In fact, previous administration of the 5-HT uptake blocker fluoxetine (10 mg/kg, i.p.) also potentiated the motor activation induced by methylphenidate. A selective 5-HT 1B receptor antagonist (GR 55562; 1 mg/kg), but not a 5-HT2 receptor antagonist (ritanserin; 2 mg/kg, i.p.), counteracted the effects of methamphetamine and fluoxetine on the motor activation induced by methylphenidate. Furthermore, a 5-HT 1B receptor agonist (CP 94253; 1-10 mg/kg, i.p.) strongly and dose-dependently potentiated methylphenidate-induced locomotor activation. The 5-HT 1B receptor-mediated modulation of methylphenidate-induced locomotor activation in rat could have implications for the treatment of ADHD.

Overexpression of human wildtype torsinA and human DeltaGAG torsinA in a transgenic mouse model causes phenotypic abnormalities

Primary torsion dystonia is an autosomal-dominant inherited movement disorder. Most cases are caused by an in-frame deletion (GAG) of the DYT1 gene encoding torsinA. Reduced penetrance and phenotypic variability suggest that alteration of torsinA amino acid sequence is necessary but not sufficient for development of clinical symptoms and that additional factors must contribute to the factual manifestation of the disease. We generated 4 independent transgenic mouse lines, two overexpressing human mutant torsinA and two overexpressing human wildtype torsinA using a strong murine prion protein promoter. Our data provide for the first time in vivo evidence that not only mutant torsinA is detrimental to neuronal cells but that also wildtype torsinA can lead to neuronal dysfunction when overexpressed at high levels. This hypothesis is supported by (i) neuropathological findings, (ii) neurochemistry, (iii) behavioral abnormalities and (iv) DTI-MRI analysis.

The differential effects of 5-HT(1A) receptor stimulation on dopamine receptor-mediated abnormal involuntary movements and rotations in the primed hemiparkinsonian rat

Serotonin 1A receptor (5-HT(1A)R) agonists have emerged as valuable supplements to l-DOPA therapy, demonstrating that they can decrease side effects and enhance motor function in animal models of Parkinson's disease (PD) and human PD patients. The precise mechanism by which these receptors act remains unknown and there is limited information on how 5-HT(1A)R stimulation impacts striatal dopamine (DA) D1 receptor (D1R) and D2 receptor (D2R) function. The current study examined the effects of 5-HT(1A)R stimulation on DA receptor-mediated behaviors. Male Sprague-Dawley rats were rendered hemiparkinsonian by unilateral 6-OHDA lesions and primed with the D1R agonist SKF81297 (0.8 mg/kg, i.p.) in order to sensitize DA receptors. Using a randomized within subjects design, rats received a first injection of: Vehicle (dH(2)O) or the 5-HT(1A)R agonist +/-8-OH-DPAT (0.1 or 1.0 mg/kg, i.p.), followed by a second injection of: Vehicle (dimethyl sulfoxide), the D1R agonist SKF81297 (0.8 mg/kg, i.p.), the D2R agonist quinpirole (0.2 mg/kg, i.p.), or l-DOPA (12 mg/kg+benserazide, 15 mg/kg, i.p.). On test days, rats were monitored over a 2-h period immediately following the second injection for abnormal involuntary movements (AIMs), analogous to dyskinesia observed in PD patients, and contralateral rotations. The present findings indicate that 5-HT(1A)R stimulation reduces AIMs induced by D1R, D2R and l-DOPA administration while its effects on DA agonist-induced rotations were receptor-dependent, suggesting that direct 5-HT(1A)R and DA receptor interactions may contribute to the unique profile of 5-HT(1A)R agonists for the improvement of PD treatment.

MDMA and fenfluramine reduce L-DOPA-induced dyskinesia via indirect 5-HT1A receptor stimulation

Chronic L-3,4-dihydroxyphenylalanine (L-DOPA) pharmacotherapy in Parkinson's disease is often accompanied by the development of abnormal and excessive movements known as dyskinesia. Clinical and experimental studies indicate that indirect serotonin agonists can suppress dyskinesia without affecting the efficacy of L-DOPA. While the mechanism by which these effects occur is not clear, recent research suggests that serotonin 5-HT1A receptors may play a pivotal role. To test this, male Sprague-Dawley rats with unilateral 6-hydroxydopamine medial forebrain bundle lesions received 1 week of daily treatment with L-DOPA (12 mg/kg, i.p.) plus benserazide (15 mg/kg, i.p.). Beginning on the 8th day of treatment and every 3rd or 4th day thereafter, rats were pretreated with vehicle (0.9% NaCl), the serotonin and dopamine releaser 3,4-methylenedioxymethamphetamine (MDMA; 0.25 or 2.5 mg/kg, i.p.) or the serotonin releaser fenfluramine (FEN; 0.25 or 2.5 mg/kg, i.p.) 5 min prior to L-DOPA, after which abnormal involuntary movements (AIMs) and rotations were quantified every 20th minute for 2 h. Pretreatment with 2.5 mg/kg of either MDMA or FEN reduced AIMs. To determine the contribution of the 5-HT1A receptor to these effects, another group of L-DOPA-primed 6-hydroxydopamine-lesioned rats were pretreated with the 5-HT1A antagonist WAY100635 (0.5 mg/kg, i.p.), MDMA + WAY100635 (2.5 + 0.5 mg/kg, i.p.) or FEN + WAY100635 (2.5 + 0.5 mg/kg, i.p.) 5 min prior to L-DOPA and subsequent AIMs and rotation tests. The antidyskinetic effects of MDMA and FEN were reversed by cotreatment with WAY100635. These results suggest that 5-HT-augmenting compounds such as MDMA and FEN probably convey antidyskinetic properties in part via stimulation of 5-HT1A receptors.

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.

Evidence that the effect of 5-HT2 receptor stimulation on temporal differentiation is not mediated by receptors in the dorsal striatum

5-HT2 receptor stimulation alters temporal differentiation in free-operant timing schedules. The anatomical location of the receptor population responsible for this effect is unknown. We examined the effect of a 5-HT2 receptor agonist and antagonists, injected systemically and into the dorsal striatum, a region that is believed to play a major role in interval timing. Rats were trained under the free-operant psychophysical procedure to press levers A and B in 50s trials in which reinforcement was provided intermittently for responding on A in the first half, and B in the second half of the trial. Percent responding on B (%B) was recorded in successive 5s epochs of the trials; logistic functions were fitted to the data from each rat to derive timing indices (T50: time corresponding to %B = 50; Weber fraction: [T75-T25]/2T50, where T75 and T25 are the times corresponding to %B = 75 and %B = 25). Systemic treatment with the 5-HT(2A/2C) receptor agonist 2,5,-dimethoxy-4-iodo-amphetamine (DOI) (0.25 mg/kg, s.c.) reduced T50; the 5-HT2A receptor antagonist MDL-100907 (0.5 mg/kg, i.p.) did not affect performance, but completely blocked the effect of DOI. DOI (1 and 3 microg) injected bilaterally into the dorsal striatum did not alter T50. The effect of systemic treatment with DOI (0.25 mg/kg, s.c.) was not altered by intra-striatal injection of MDL-100907 (0.3 microg) or the 5-HT2C receptor antagonist RS-102221 (0.15 microg). The ability of systemically administered MDL-100907 to reverse DOI's effect on T50 confirms the sensitivity of temporal differentiation to 5-HT2A receptor stimulation. The failure of intra-striatal MDL-100907 to antagonize the effects of DOI suggests that 5-HT2A receptors in the dorsal striatum are unlikely to be primarily responsible for DOI's effects on timing. Furthermore, the results provide no evidence for a role of striatal 5-HT2C receptors in DOI's effect on timing.

Dopamine D1 and D2 receptor contributions to L-DOPA-induced dyskinesia in the dopamine-depleted rat

Using a rat model of L-DOPA-induced dyskinesia (LID), the contributions of dopamine D1 and D2 receptors to axial, limb, and orolingual (ALO) abnormal involuntary movements (AIMs) elicited by L-DOPA were examined. Chronic L-DOPA-treated rats received the D1 receptor antagonist SCH23390 (0.01, 0.1, and 1.0 mg/kg; i.p.), the D2 receptor antagonist Eticlopride (0.01, 0.1, and 1.0 mg/kg; i.p.), a mixture of both antagonists (0.01, 0.1, 1.0 mg/kg each; i.p.), or vehicle 30 min prior to L-DOPA (6 mg/kg; i.p.)+Benserazide (15 mg/kg; i.p.). SCH23390 (0.1 and 1.0 mg/kg) significantly reduced axial and limb AIMs, while the same doses of Eticlopride significantly decreased axial, limb, and orolingual AIMs. Co-administration of SCH23390+Eticlopride significantly reduced axial (0.01, 0.1 and 1.0 mg/kg), limb (0.1 and 1.0 mg/kg), and orolingual (0.1 and 1.0 mg/kg) AIMs. These results indicate the importance of D1 and D2 receptors to LID and further validate the rat AIMs model.

Failure of MK-801 to suppress D1 receptor-mediated induction of locomotor activity and striatal preprotachykinin mRNA expression in the dopamine-depleted rat

N-methyl-D-aspartate receptor antagonism exerts suppressive influences over dopamine D1 receptor-mediated striatal gene expression and locomotor behavior in the intact rat. The present study examined the effects of the N-methyl-D-aspartate receptor antagonist MK-801 on locomotor activity and striatal preprotachykinin mRNA expression stimulated by the D1 agonist (+/-)6-chloro-7, 8-dihydroxy-3-allyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine hydrobromide in rats with bilateral dopamine lesions. Two months after neonatal dopamine lesions with 6-hydroxydopamine, rats were challenged with (+/-)6-chloro-7, 8-dihydroxy-3-allyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine hydrobromide (1.0 mg/kg) 15 min after administration of the N-methyl-D-aspartate receptor antagonist MK-801 (0.1 mg/kg). In the intact rat, MK-801 prevented the induction of striatal preprotachykinin mRNA by D1 agonism. Similarly, direct infusion of (+/-)6-chloro-7, 8-dihydroxy-3-allyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine hydrobromide (3.0 microg) into the intact striatum produced an increase in locomotor activity that was suppressed by MK-801 (1.0 microg) co-infusion. In the dopamine-depleted rat, MK-801 (0.1 mg/kg) administered prior to (+/-)6-chloro-7, 8-dihydroxy-3-allyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine hydrobromide (1.0 mg/kg) increased, rather than suppressed, striatal preprotachykinin mRNA levels. Intrastriatal infusion of MK-801 (1.0 microg) failed to inhibit D1-mediated induction of motor activity in dopamine-depleted animals. Together, these data provide further support that N-methyl-D-aspartate receptor antagonists lose their ability to block D1-mediated behavioral activation following dopamine depletion. The activation, rather than suppression, of tachykinin neurons of the direct striatonigral pathway may play a facilitatory role in this mechanism.

Serotonergic mechanisms in Parkinson’s disease: opposing results from preclinical and clinical data

Parkinson's disease (PD) is a neuropsychiatric disease affecting approximately 1-2% of the general population. The classical triad of symptoms, tremor, rigidity, and bradykinesia is mainly caused by degeneration of dopaminergic neurons from the substantia nigra. However, other neurotransmitter systems also show signs of degeneration, among which the serotonergic system. The exact role of serotonin in PD remains unclear. We present here a review about functional serotonergic interventions and serotonergic imaging studies in PD, and will go into the importance of combining preclinical and clinical research data in order to gain more insight into the role of serotonin in PD. More specifically, the present review is aimed at bridging the gap between data from animal models of PD and data from human research.

New adenosine A2A receptor antagonists: Actions on Parkinson's disease models

The 8-substituted 9-ethyladenine derivatives: 8-bromo-9-ethyladenine (ANR 82), 8-ethoxy- 9-ethyladenine (ANR 94), and 8-furyl-9-ethyladenine (ANR 152) have been characterized in vitro as adenosine receptor antagonists. Adenosine is deeply involved in the control of motor behaviour and substantial evidences indicate that adenosine A(2A) receptor antagonists improve motor deficits in animal models of Parkinson's disease. On this basis, the efficacy of ANR 82, ANR 94, and ANR 152 in rat models of Parkinson's disease was evaluated. All compounds tested reversed the catalepsy induced by haloperidol. However, in unilaterally 6-hydroxydopamine-lesioned rats, only ANR 94 and ANR 152 potentiated l-dihydroxy-phenylalanine (l-DOPA) effect on turning behaviour and induced contralateral turning behaviour in rats sensitised to l-DOPA. Taken together the results of this study indicate that some 8-substituted 9-ethyladenine derivatives ameliorate motor deficits in rat models of Parkinson's disease, suggesting a potential therapeutic role of these compounds.

Intrastriatal dopamine D1 receptor agonist-mediated motor behavior is reduced by local neurokinin 1 receptor antagonism

Recent evidence suggests that striatal neurokinin receptors modulate dopamine (DA)-induced motor behaviors. To further examine this, we studied the effects of intrastriatal neurokinin 1 receptor (NK1R) antagonism on motor behaviors induced by direct infusion of the full DA D1 receptor agonist SKF 82958. Adult male Sprague-Dawley rats received bilateral intrastriatal 0.8-mul infusions of the NK1R receptor antagonist LY 306,740 (0, 27, or 54 nmol/side) followed by intrastriatal infusions of SKF 82958 (0 or 24 nmol/side) into the dorsal striatum. Following each infusion, rats were placed into automated activity monitors for the quantification of horizontal activity, total distance traveled, movement bouts, and stereotypy counts. As expected, SKF 82958 increased motor activity on all behavioral measures. More importantly, whereas 27 nmol was without effect, prior infusion of 54 nmol LY 306,740 significantly reduced most aspects of behavior. The results of this study suggest that functional NK1Rs within the striatum play a permissive role in the motor behaviors induced by D1R stimulation.

Intranigral antagonism of neurokinin 1 and 3 receptors reduces intrastriatal dopamine D1 receptor-stimulated locomotion in the rat

Stimulation of striatal dopamine (DA) D1 receptors increases the activity of the direct striatonigral pathway resulting in movement. While GABA has long been considered the primary effector of this pathway, co-released tachykinin peptides and their respective nigral tachykinin receptors are also in position to influence movement. Therefore, the present studies determined to what extent nigral tachykinin receptor subtypes contribute to striatal D1-mediated locomotion. Adult male Sprague-Dawley rats bearing chronic cannulae in the dorsal striatum and/or substantia nigra (SN) were tested for locomotor responses to various drug infusions. Unilateral intranigral infusions of the neurokinin-1 (NK1) antagonist LY306740 (0 and 50 nmol) but not the neurokinin-3 (NK3) antagonist SR142801 (0 and 50 nmol) led to ipsilateral rotations. Bilateral intrastriatal infusions of the full D1 agonist SKF 82958 (0, 1.2 and 12.0 nmol) dose-dependently increased locomotion. Prior bilateral intranigral infusions of LY306740 or SR142801 (0, 5.0 and 50 nmol) dose-dependently attenuated locomotor activity induced by intrastriatal SKF 82958 (12.0 nmol). These findings indicate that NK1, but not NK3, receptors within the SN may be tonically stimulated. However, activation of both nigral NK1 and NK3 receptors appears to be required for increased locomotion in response to striatal D1 receptor stimulation.

NMDA receptor blockade attenuates locomotion elicited by intrastriatal dopamine D1-receptor stimulation

Previous behavioral studies suggest that the striatum mediates a hyperactive response to systemic NMDA receptor antagonism in combination with systemic D1 receptor stimulation. However, many experiments conducted at the cellular level suggest that inhibition of NMDA receptors should block D1 receptor-mediated locomotor activity. Therefore, we investigated the consequences of NMDA receptor blockade on the ability of striatal D1 receptors to elicit locomotor activity using systemic and intrastriatal injections of the NMDA antagonist MK-801 combined with intrastriatal injections of the D1 full agonist SKF 82958. Following drug treatment locomotor activity was measured via computerized activity monitors designed to quantify multiple parameters of rodent open-field behavior. Both systemic (0.1 mg/kg) and intrastriatal (1.0 microg) MK-801 pretreatments completely blocked locomotor and stereotypic activity elicited by 10 microg of SKF 82958 directly infused into the striatum. Further, increased activity triggered by intrastriatal SKF 82958 was attenuated by a posttreatment with intrastriatal infusion of 1 microg MK-801. These data suggest that D1-stimulated locomotor behaviors controlled by the striatum require functional NMDA channels.

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.