Christopher Bishop

The partial 5-HT1A agonist buspirone reduces the expression and development of l-DOPA-induced dyskinesia in rats and improves l-DOPA efficacy

Dopamine (DA) replacement therapy with l-DOPA remains the standard pharmacotherapy for Parkinsons disease (PD). Unfortunately, chronic l-DOPA treatment is accompanied by development of motor fluctuations and l-DOPA-induced dyskinesia (LID). While serotonin (5-HT)(1A) agonists acutely reduce these complications, their prophylactic and long-term effects are not well-delineated. To test this, male Sprague-Dawley rats received unilateral 6-hydroxydopamine (6-OHDA) lesions. In experiment 1, l-DOPA-primed rats were pre-treated with Vehicle (0.9% NaCl), various doses of the partial 5-HT(1A) agonist, buspirone (0.25, 1.0 or 2.5 mg/kg, ip) or buspirone (2.5 mg/kg, ip)+the 5-HT(1A) antagonist, WAY100635 (0.5 mg/kg, ip) 5 min prior to l-DOPA (12 mg/kg+15 mg/kg benserazide, ip). Rats were tested for LID using the abnormal involuntary movements (AIMs) scale and motor performance using the forepaw adjusting steps test (FAS). In experiment 2, l-DOPA-naïve rats received co-administration of l-DOPA+buspirone (1.0 or 2.5 mg/kg, ip) for 2 weeks. AIMs and FAS were measured throughout. In l-DOPA-primed rats, buspirone dose-dependently reduced LID and improved l-DOPA-related motor performance due to action at the 5-HT(1A) receptor. In l-DOPA-naïve rats, buspirone delayed LID development while improving l-DOPAs anti-parkinsonian efficacy indicating the potential long-term benefits of 5-HT(1A) agonists for reduction of l-DOPA-related side effects.

The role of the dorsal raphe nucleus in the development, expression, and treatment of L-dopa-induced dyskinesia in hemiparkinsonian rats.

Convergent evidence indicates that in later stages of Parkinsons disease raphestriatal serotonin neurons compensate for the loss of nigrostriatal dopamine neurons by converting and releasing dopamine derived from exogenous administration of the pharmacotherapeutic L‐3,4‐dihydroxyphenyl‐L‐alanine (L‐dopa). Because the serotonin system is not equipped with dopamine autoregulatory mechanisms, it has been postulated that raphe‐mediated striatal dopamine release may fluctuate dramatically. These fluctuations may portend the development of abnormal involuntary movements called L‐dopa‐induced dyskinesia (LID). As such, it has been hypothesized that reducing the activity of raphestriatal neurons could dampen supraphysiological stimulation of striatal dopamine receptors thereby alleviating LID. To directly address this, the current study employed the rodent model of LID to investigate the contribution of the rostral raphe nuclei (RRN) in the development, expression and treatment of LID. In the first study, dual serotonin/dopamine selective lesions of the RRN and medial forebrain bundle, respectively, verified that the RRN are essential for the development of LID. In a direct investigation into the neuroanatomical specificity of these effects, microinfusions of ±8‐OH‐DPAT into the intact dorsal raphe nucleus dose‐dependently attenuated the expression of LID without affecting the antiparkinsonian efficacy of L‐dopa. These current findings reveal the integral contribution of the RRN in the development and expression of LID and implicate a prominent role for dorsal raphe 5‐HT1AR in the efficacious properties of 5‐HT1AR agonists. Synapse 63:610–620, 2009.

Local modulation of striatal glutamate efflux by serotonin 1A receptor stimulation in dyskinetic, hemiparkinsonian rats

Serotonin 1A receptor (5-HT(1A)R) agonists reduce both L-DOPA- and D1 receptor (D1R) agonist-mediated dyskinesia, but their anti-dyskinetic mechanism of action is not fully understood. Given that 5-HT(1A)R stimulation reduces glutamatergic neurotransmission in the dopamine-depleted striatum, 5-HT(1A)R agonists may diminish dyskinesia in part through modulation of pro-dyskinetic striatal glutamate levels. To test this, rats with unilateral medial forebrain bundle dopamine or sham lesions were primed with L-DOPA (12 mg/kg+benserazide, 15 mg/kg, sc) or the D1R agonist SKF81297 (0.8 mg/kg, sc) until abnormal involuntary movements (AIMs) stabilized. On subsequent test days, rats were treated with vehicle or the 5-HT(1A)R agonist ±8-OH-DPAT (1.0 mg/kg, sc), followed by L-DOPA or SKF81297, or intrastriatal ±8-OH-DPAT (7.5 or 15 mM), followed by L-DOPA. In some cases, the 5-HT(1A)R antagonist WAY100635 was employed to determine receptor-specific effects. In vivo microdialysis was used to collect striatal samples for analysis of extracellular glutamate levels during AIMs assessment. Systemic and striatal ±8-OH-DPAT attenuated L-DOPA-induced dyskinesia and striatal glutamate efflux while WAY100635 reversed ±8-OH-DPATs effects. Interestingly, systemic ±8-OH-DPAT diminished D1R-mediated AIMs without affecting glutamate. These findings indicate a novel anti-dyskinetic mechanism of action for 5-HT(1A)R agonists with implications for the improved treatment of Parkinsons disease.

The differential effects of 5-HT1A 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 Parkinsons 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.

Potential mechanisms underlying anxiety and depression in Parkinson's disease: consequences of L-DOPA treatment

Though the most recognizable symptoms of Parkinsons disease (PD) are motor-related, many patients also suffer from debilitating affective symptoms that deleteriously influence quality of life. Dopamine (DA) loss is likely involved in the onset of depression and anxiety in PD. However, these symptoms are not reliably improved by DA replacement therapy with l-3,4-dihydroxyphenylalanine (l-DOPA). In fact, preclinical and clinical evidence suggests that l-DOPA treatment may worsen affect. Though the neurobiological mechanisms remain unclear, recent research contends that l-DOPA further perturbs the function of the norepinephrine and serotonin systems, already affected by PD pathology, which have been intimately linked to the development and expression of anxiety and depression. As such, this review provides an overview of the clinical characteristics of affective disorders in PD, examines the utility of animal models for the study of anxiety and depression in PD, and finally, discusses potential mechanisms by which DA loss and subsequent l-DOPA therapy influence monoamine function and concomitant affective symptoms.

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 Parkinsons disease.

Role of the primary motor cortex in l-DOPA-induced dyskinesia and its modulation by 5-HT1A receptor stimulation

While serotonin 5-HT1A receptor (5-HT1AR) agonists reduce L-DOPA-induced dyskinesias (LID) by normalizing activity in the basal ganglia neurocircuitry, recent evidence suggests putative 5-HT1AR within the primary motor cortex (M1) may also contribute. To better characterize this possible mechanism, c-fos immunohistochemistry was first used to determine the effects of systemic administration of the full 5-HT1AR agonist ±8-OH-DPAT on L-Dopa-induced immediate early gene expression within M1 and the prefrontal cortex (PFC) of rats with unilateral medial forebrain bundle (MFB) dopamine (DA) lesions. Next, in order to determine if direct stimulation of 5-HT1AR within M1 attenuates the onset of LID, rats with MFB lesions were tested for L-Dopa-induced abnormal involuntary movements (AIMs) and rotations following M1 microinfusions of ±8-OH-DPAT with or without coadministration of the 5-HT1AR antagonist WAY100635. Finally, ±8-OH-DPAT was infused into M1 at peak dyskinesia to determine if 5-HT1AR stimulation attenuates established L-Dopa-induced AIMs and rotations. While no treatment effects were seen within the PFC, systemic ±8-OH-DPAT suppressed L-Dopa-induced c-fos within M1. Intra-M1 5-HT1AR stimulation diminished the onset of AIMs and this effect was reversed by WAY100635 indicating receptor specific effects. Finally, continuous infusion of ±8-OH-DPAT into M1 at peak dyskinesia alleviated L-Dopa-induced AIMs. Collectively, these findings support an integral role for M1 in LID and its modulation by local 5-HT1AR.

Contribution of the Striatum to the Effects of 5-HT1A Receptor Stimulation in L-DOPA-treated Hemiparkinsonian Rats

Clinical and experimental studies implicate the use of serotonin (5‐HT)1A receptor agonists for the reduction of L‐3,4‐dihydroxyphenylalanine (L‐DOPA)‐induced dyskinesia (LID). Although raphe nuclei likely play a role in these antidyskinetic effects, an unexplored population of striatal 5‐HT1A receptors (5‐HT1AR) may also contribute. To better characterize this mechanism, L‐DOPA‐primed hemiparkinsonian rats received the 5‐HT1AR agonist ±8‐OH‐DPAT (0, 0.1, 1.0 mg/kg, i.p.) with or without cotreatment with the 5‐HT1AR antagonist WAY100635 (0.5 mg/kg, i.p.) 5 min after L‐DOPA, after which abnormal involuntary movements (AIMs), rotations, and forelimb akinesia were quantified. To establish the effects of 5‐HT1AR stimulation on L‐DOPA‐induced c‐fos and preprodynorphin (PPD) mRNA within the dopamine‐depleted striatum, immunohistochemistry and real‐time reverse transcription polymerase chain reaction, respectively, were used. Finally, to determine the contribution of striatal 5‐HT1AR to these effects, L‐DOPA‐primed hemiparkinsonian rats received bilateral intrastriatal microinfusions of ±8‐OH‐DPAT (0, 5, or 10 μg/side), WAY100635 (5 μg/side), or both (10 μg + 5 μg/side) 5 min after L‐DOPA, after which AIMs and rotations were examined. Systemic ±8‐OH‐DPAT dose‐ and receptor‐dependently attenuated L‐DOPA‐mediated AIMs and improved forelimb akinesia. Striatal c‐fos immunoreactivity and PPD mRNA ipsilateral to the lesion were strongly induced by L‐DOPA, while ±8‐OH‐DPAT suppressed these effects. Finally, intrastriatal infusions of ±8‐OH‐DPAT reduced AIMs while coinfusion of WAY100635 reversed its antidyskinetic effect. Collectively, these results support the hypothesis that the cellular and behavioral properties of 5‐HT1AR agonists are conveyed in part via a population of functional 5‐HT1AR within the striatum.

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

Chronic l‐3,4‐dihydroxyphenylalanine (L‐DOPA) pharmacotherapy in Parkinsons 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.

Behavioral and neurochemical effects of chronic L-DOPA treatment on nonmotor sequelae in the hemiparkinsonian rat.

Depression and anxiety are the prevalent nonmotor symptoms that worsen quality of life for Parkinsons disease (PD) patients. Although dopamine (DA) cell loss is a commonly proposed mechanism, the reported efficacy of DA replacement therapy with L-DOPA on affective symptoms is inconsistent. To delineate the effects of DA denervation and chronic L-DOPA treatment on affective behaviors, male Sprague–Dawley rats received unilateral 6-hydroxydopamine or sham lesions and were treated daily with L-DOPA (12 mg/kg+benserazide, 15 mg/kg, subcutaneously) or vehicle (0.9% NaCl, 0.1% ascorbic acid) for 28 days before commencing investigations into anxiety (locomotor chambers, social interaction) and depression-like behaviors (forced swim test) during the OFF phase of L-DOPA. One hour after the final treatments, rats were killed and striatum, prefrontal cortex, hippocampus, and amygdala were analyzed through high-performance liquid chromatography for monoamine levels. In locomotor chambers and social interaction, DA lesions exerted mild anxiogenic effects. Surprisingly, chronic L-DOPA treatment did not improve these effects. Although DA lesion reduced climbing behaviors on day 2 of exposure to the forced swim test, chronic L-DOPA treatment did not reverse these effects. Neurochemically, L-DOPA treatment in hemiparkinsonian rats reduced norepinephrine levels in the prefrontal cortex, striatum, and hippocampus. Collectively, these data suggest that chronic L-DOPA therapy in severely DA-lesioned rats does not improve nonmotor symptoms and may impair nondopaminergic processes, indicating that long-term L-DOPA therapy does not exert necessary neuroplastic changes for improving affect.