Antagonism of metabotropic glutamate receptor type 5 prevents levodopa‐induced dyskinesia development in a male rat model of Parkinson's disease: Electrophysiological evidence

Levodopa‐induced dyskinesia (LID) is a common complication in patients with advanced Parkinson's disease (PD) undergoing treatment with levodopa. Glutamate receptor antagonists can suppress LID; however, the underlying mechanisms remain unclear. Here, we aimed to evaluate the effect of 3‐((2‐methyl‐1,3‐thiazol‐4‐yl)ethynyl)pyridine (MTEP), a metabotropic glutamate receptor 5 (mGluR5) antagonist, on dyskinesia. We recorded the neuronal activity of the entopeduncular nucleus and examined responses to cortical electric stimulation in the control group (n = 6) and three groups of rats (male PD model). Saline was intraperitoneally administered to dopamine lesioned (DL) rats (n = 6), levodopa/benserazide (L/B) was administered to LID rats (n = 8), and L/B combined with MTEP was administered to MTEP rats (n = 6) twice daily for 14 days. We administered L/B to LID and MTEP rats 48 h after the final administration of MTEP to examine the chronic effect of MTEP. The control and DL groups did not have LID. The MTEP group had less LID than the LID group (p < .01) on day 1 and day 18. The control group had a typical triphasic pattern consisting of early excitation (early‐Ex), inhibition, and late excitation (late‐Ex). However, the inhibition phase disappeared, was partially observed, and was fully suppressed in the DL, LID, and MTEP groups, respectively. The cortico‐striato‐entopeduncular pathway is important in the pathophysiology of LID. mGluR5 antagonism suppresses LID progression by preventing physiological changes in the cortico‐striato‐entopeduncular pathway. Future studies are required to validate these results.


| INTRODUC TI ON
Parkinson's disease (PD) is a progressive neurodegenerative disease characterized by dopamine deficiency.Levodopa is the gold standard for PD symptomatic treatment; however, levodopainduced dyskinesia (LID) is a common complication in the advanced stages of PD, with 35% patients affected within 10 years of initiating levodopa therapy in Japan (Sato et al., 2006).The pathophysiological mechanisms of LID remain unclear.However, denervation of dopaminergic neurons increases the sensitivity of dopamine (D) 1 receptors in the striatum and reduces negative feedback from gamma-aminobutyric acid (GABA) B receptors (Borgkvist et al., 2018).In addition, denervation of the dopaminergic nerves causes serotonergic nerves to metabolize levodopa to dopamine and release it into the striatum.The absence of D2 cells or dopamine transporters in serotonergic nerves, which control dopamine levels in the synaptic cleft, causes a rapid increase in dopamine levels after levodopa administration (Cheshire & Williams, 2012;Tanaka et al., 1999).As a result, GABAergic input from the striatum to the internal globus pallidus (GPi) becomes excessive and significantly suppresses GPi activity, resulting in dyskinesia (Papa et al., 1999;Tomiyama et al., 2004).Glutamatergic receptors also play an important role in the pathogenesis of LID (Campanelli et al., 2022;Klawans et al., 1977).Increased glutamatergic signals are observed in the striatum due to hypersensitivity to glutaminergic input from the cortex in LID animal models (Chase et al., 2003;Gerfen, 2003).Glutamatergic projections from the subthalamic nucleus (STN) to the GPi are involved in the pathogenesis of dyskinesia (Stefani et al., 2011).Amantadine is an N-methyld-aspartate receptor antagonist used in the clinical treatment of dyskinesia (Crosby et al., 2003;Metman et al., 1999;Wang et al., 2022).
Additionally, the anti-dyskinetic effects of metabotropic glutamate receptor type 5 (mGluR5) blocking drugs have been reported in rat models of PD administered 6-hydroxydopamine (6-OHDA) (Lin et al., 2007;Rascol et al., 2017).Although several studies have explored the therapeutic mechanisms of the anti-dyskinetic effect of glutamate receptor blockers (Sgambato-Faure & Cenci, 2012), no study has simultaneously explored the pathophysiological mechanisms of LID and the therapeutic mechanisms of glutamate receptor blockers.
Therefore, in the present study, we aimed to evaluate the effects of mGluR5 antagonism on LID and explore the underlying physiological mechanisms.We generated dopamine lesioned (DL) rats (PD and dyskinesia models) using 6-OHDA and attempted to induce or treat LID by administering levodopa alone or in combination with 3-((2-methyl-1,3-thiazol-4-yl)ethynyl)pyridine (MTEP).
In addition, we aimed to determine whether the therapeutic effect of MTEP on dyskinesia is symptomatic or prophylactic.Since repeated administration of levodopa induces both dyskinesia and electrophysiological changes, we hypothesized that such a treatment would suppress dyskinesia as well as electrophysiological changes.

| Animals
Male Long-Evans rats were purchased from Japan Clare (Japan) and habituated to human handling.All rats used in the experiments were 8 weeks old.The rats were maintained on a 12-h light/dark cycle and provided with ad libitum access to food and water.The room temperature and humidity were maintained at 25°C and 50%, respectively.
A maximum of two rats were placed in each cage.The cages were cleaned twice daily to maintain internal cleanliness and additional cleanings were performed as required.No animals other than rats were present in the environment.All experimental procedures were approved by the Institutional Animal Care and Use Committee of the National Institute of Natural Sciences (approval number: 2022204).
Two weeks later, dopaminergic lesions were evaluated using the apomorphine-induced rotation test.Rats who rotated more than 20 times in 5 min at 15-20 min after apomorphine administration were considered successful PD models (Iancu et al., 2005).Saline (4 μL, .9%)was intraperitoneally administered to rats in the control and DL groups, levodopa/benserazide (L/B) was intraperitoneally administered to the LID group, and L/B + MTEP was intraperitoneally administered to the MTEP group twice daily for 14 consecutive days.The abnormal involuntary movement (AIM) scores were assessed on days 1, 4, and 11 (Lindgren et al., 2007).At the end of the 14 days after a 48-h washout period, L/B was intraperitoneally administered to the LID and MTEP groups and saline was administered to the control and DL groups; AIM scores were then reassessed on day 17.A head chamber was then attached to each

Significance
This study elucidates the electrophysiological mechanisms of levodopa-induced dyskinesia and demonstrates the potential for dyskinesia treatment with a metabotropic glutamate receptor 5 (mGluR5) antagonist.Furthermore, this study demonstrates the therapeutic mechanisms of the mGluR5 antagonist for dyskinesia.The results suggest that mGluR5 antagonist treatment is an option for dyskinesia development prevention.rat's skull, with a stimulating electrode implanted in the motor cortex (M1).Single-unit potential responses of the entopeduncular nucleus (EP) to electrical stimulation of M1 were recorded within 1 week of the last drug administration.

| Apomorphine-induced rotation test to evaluate the motor features of PD
After 14 days of 6-OHDA administration, each rat was placed in an acrylic cylinder (inner diameter: 30 cm; height: 30 cm).Apomorphine was intraperitoneally administered, and rats that rotated to the left side more than 20 times over 5 min at 15-20 min after administration were considered successful models of PD (Tanaka et al., 1999) and were divided into three groups (DL, LID, and MTEP).Rats that did not qualify as PD model rats were euthanized in a painless manner using PB (100 mg/kg; Nacalai-Tesque).
F I G U R E 1 Experimental design and histological and behavioral examination results.(a) Saline or 6-OHDA was injected to the right MFB, and the apomorphine-induced rotation test was performed 14 days later.Saline, levodopa/benserazide, and/or MTEP were administered twice daily for 14 days, and AIM scores were assessed.Two days after the washout, AIM scores were reassessed.After the head chamber operation and implantation of the stimulus electrode, single-unit recording was performed, followed by euthanization.

| Levodopa or saline treatment and dyskinesia evaluation
One week after the apomorphine test, saline (1.0 mL/kg) was administered to the control and DL groups; levodopa (50 mg/kg; Sigma-Aldrich) and benserazide (12.5 mg/kg; Wako, Japan) were administered to the LID group; and levodopa (50 mg/kg; Sigma-Aldrich), benserazide (12.5 mg/kg; Wako), and MTEP (5 mg/kg; Abcam, UK) were administered to the MTEP group twice daily for 2 weeks.MTEP was administered 20 min before every L/B administration to the MTEP group only.The AIM score was measured every 20 min for a total of 3 h on days 1, 4, and 11 after the first administration of L/B to evaluate dyskinesia (Lindgren et al., 2007).At the end of the 14 days, saline was administered to the control and DL groups and L/B was administered to the LID and MTEP groups, and AIM scores were reassessed on day 17.

| Surgery for head chamber attachment and stimulation electrode implantation
The rats were anesthetized with isoflurane (Pfizer) and PB (5 mg/kg; Nacalai-Tesque) and held in a stereotaxic apparatus.Six microscrews (Nabeya Bi-tech, Japan) and a head chamber (Narishige) were fixed to the skull using a resin cement (Sun Medical, Japan).A silver wire was wrapped around one of the screws to serve as a reference electrode.
On the day after head chamber attachment, the stimulating electrode was fixed to M1 for recording.The rats were re-anesthetized with isoflurane (Pfizer) and held in the stereotaxic apparatus.A small hole was drilled to access the motor cortex.Two pairs of bipolar stimulating electrodes (diameter: .15mm, cashew-coated tungsten electrode, distance between electrodes: .5 mm, exposed tip: .25 mm; Unique Medical, Japan) were implanted into the forelimb regions of the M1 and fixed using a resin cement (Sun Medical, Japan).Somatotopy of these regions was confirmed by intracortical microstimulation (a train of 10 pulses in 200 μs, .7 Hz, 20 mA).

| Neuronal activity recording
The rats were held in a stereotaxic apparatus in an awake state.
A glass-coated tungsten microelectrode (1.0 MΩ at 1 kHz; Alpha Omega, Israel) was inserted vertically from the dura mater to the EP (target area: 2.0-2.5 mm posterior, 2.0-2.5 mm lateral, and 7.0-7.5 mm ventral from the bregma) with a hydraulic microdrive.
Neuroelectrical activity was recorded using an OmniPlex (Plexon, TX, USA).Spontaneous discharges were recorded for 30 s.The responses to cortical electrical stimulation (monophasic single pulse in 200 ms, .7 Hz, 50 mA) were recorded during 100 stimulation trials.

| Analysis of electrophysiological data
The spontaneous discharge rates and coefficient of variation (CV) of interspike intervals (ISIs) were calculated from the first 30 s of the recordings without cortical stimulation (Chiken et al., 2015;Legéndy & Salcman, 1985).Responses to cortical stimulation were analyzed using PSTHs.The mean (μ baseline ) and standard deviation (SD baseline ) of the discharge rates during the 30 s before the onset of stimulation were considered the baseline discharge rate, and the statistical significance was set at μ baseline ± 1.65 SD baseline (corresponding to p = .01,two-tailed t-test).If two consecutive bins during 3-8 ms for early-Ex, three consecutive bins during 5-19 ms for inhibition, and two consecutive bins during 12-30 ms for late-Ex exceeded the significance level, the response was considered statistically significant (corresponding to p < .01 after Bonferroni correction, two-tailed t-test).
Once significant bins were detected, the response was considered continuous unless two consecutive bins fell below the significance level (within μ baseline ± 1.65 SD baseline ).The starting and ending points were defined as the first and last bins of the response, respectively.

| Location of the recordings
After the final recording, the rats were deeply anesthetized with isoflurane (Pfizer), and PB (50 mg/kg, Nacalai-Tesque) was intraperitoneally administered to suppress respiration.An abdominal incision was made, and the heart was exposed; 200 mL saline followed by 200 mL of 10% paraformaldehyde in phosphate buffer solution (Nacalai-Tesque) were perfused into the left ventricle using an EYELA-Perista Pump (Rikakikai, Japan).Rats were sacrificed under deep anesthesia with isoflurane to avoid any pain.The brain was removed, fixed in 10% paraformaldehyde, and cryoprotected in graded sucrose with phosphatebuffered saline (10% sucrose with azide in .01MPBS) at 4°C overnight.
Rat brain sections (40 μm thick) were obtained using a freezing microtome.The sections were washed three times with PBS and incubated with a blocking buffer (2% bovine serum albumin) for 30 min.Goat primary antibody against rat TH (1:1000; Merck, Germany) was added and the sections were incubated at 4°C overnight.The following day, the sections were washed with PBS three times and incubated with a secondary antibody at 37°C for 1 h.Sections were mounted on MAS-coated glass slides (Matsunami Glass, Japan), covered with VECTASHIELD mounting medium with 4′,6-diamidino-2-phenylindole (Vector Laboratories, CA, USA) and cover glass (Matsunami Glass), and examined under an electron microscope (Olympus, Japan).

| Statistical analysis
All experimental data are expressed as median (range).If a value was >1.5 times the interquartile range (IQR), it was considered an outlier.
Statistical analyses were performed using NeuroExplorer (Plexon), SAS 9.4 (SAS Inc, Cary, NC), and R.4.2.2.The AIM scores of the control and DL groups were "0."Therefore, it was impossible to compare the control or DL group and the LID or MTEP group.We used the nparLD package in R to determine the differences between the AIM scores of the four groups (control, DL, LID, and MTEP) over time.The package provides various rank-based nonparametric methods for analyzing longitudinal data in factorial experiments (Noguchi et al., 2012).We also evaluated the treatment and time effects, as well as their interactions, using the robust ANOVA-type statistic (ATS) and classical Wald-type statistic (WTS).The ATS accurately controls the Type I error rate, even for small samples.The WTS facilitates accurate and reliable nonparametric analysis of longitudinal data in factorial experiments with minimal conditions for the available data.The spontaneous activity of EP neurons including the firing rate and CV of ISIs and electrophysiological parameters such as the duration and amplitude of each response to cortical stimulation were examined using the Kruskal-Wallis test because the scores were not normally distributed.Dunn's multiple comparison test was performed for pairwise comparisons between each independent group as an ad hoc analysis.The proportions of neurons exhibiting regular firing and cortical-evoked response patterns were examined using Fisher's exact test.p-value <.05 were considered statistically significant.

| AIM scores
AIM scores were significantly higher in the LID group compared with the MTEP group during the evaluated period (p < .01,nparL method).

| Spontaneous activity of EP neurons
A total of 35 neurons from eight rats in the control group, 61 neurons from six rats in the DL group, 41 neurons from eight rats in the LID group, and 56 neurons from eight rats in the MTEP group were analyzed.The mean firing rate of EP neurons was 94.3 ± 42.5 Hz in the control group and was comparable in the DL, LID, and MTEP groups (89.7 ± 34.6 Hz, DL; 90.0 ± 4 2.6 Hz, LID; 84.5 ± 35.5 Hz, MTEP; p = 1.00;Kruskal-Wallis test with Dunn's multiple comparison test).

| Changes in the responses of EP neurons to cortical stimulation
The typical response pattern of EP neurons in the control group was triphasic, composed of early-Ex, inhibition, and late-Ex (Figure 2b, control), as previously reported (Dwi Wahyu et al., 2021).Each component of the triphasic response represents a hyper-direct pathway through the cortex-STN-EP, a direct pathway through the cortex-striatum-EP, and an indirect pathway through the cortex-striatum-external globus pallidus (GPe)-STN-EP, respectively (Nambu et al., 2002).This triphasic response was also dominant in the PSTH population (Figure 2c) and was the dominant neuronal pattern (Figure 2d).In contrast, monophasic excitation replaced the triphasic pattern in the DL group (Figure 2b) and was also dominant in the PSTH population (Figure 2c) (48/61; 78.3%) (Figure 2d).The response containing the inhibition phase was significantly reduced in the DL group compared to that in the control group (6/61; 9.8%; p < .01)(Figure 2d).This trend was confirmed by quantitative analysis (Table .1).However, monophasic excitation remained dominant in the LID group (Figure 2b), even in the PSTH population (Figure 2c,d Monophasic excitation to cortical stimulation was dominant in the MTEP group (Figure 2b).The PSTH population also showed monophasic excitation (Figure 2c), which remained the dominant response pattern (42/57, 73.7%) (Figure 2d), as in the DL group.
The inhibition containing response was significantly reduced in the MTEP group compared with the control group (9/57, 15.8%, p < .01,Fisher's exact test).
According to the quantitative analysis, the amplitude and duration of early-Ex and late-Ex were comparable among all groups; however, the duration and amplitude of the inhibition phase were significantly shorter and lower in the DL, LID, and MTEP groups than in the control group (p < .01;Kruskal-Wallis test with Dunn's multiple comparison test; Table 1).

| Location of recorded EP neurons and immunohistochemistry results
The recording site was confirmed using tissue sections, plotted in the representative frontal planes of the EP with symbols (Figure 1d).

| DISCUSS ION
Currently, there is no disease-modifying therapy for PD, and patients with PD are prescribed levodopa for their entire life to maintain their quality of life (QOL).At present, the alleviation or prevention of motor complications including LID during PD therapy is an emerging field of research (Espay et al., 2018;Lennert et al., 2012;Wu et al., 2019).In this study, we found that MTEP administered with L/B suppressed LID.Furthermore, LID was suppressed when MTEP was administered and when L/B alone was administered to the MTEP group after MTEP washout.Together, these findings indicate that MTEP exerts therapeutic and prophylactic effects on LID.

| Spontaneous activity of EP in different animal models
The cerebral cortex and basal ganglia comprise three segregated parallel circuits.The "direct" pathway is a monosynaptic connection between the cerebral cortex and D1 receptor-expressing striatal neurons.D1 receptor-expressing neurons directly send axons to the GPi or substantia nigra pars reticulata (SNr).The "indirect" pathway is composed of the cortex-striatum-GPe-STN-GPi/SNr.In this pathway, striatal neurons express D2 receptors.The "hyper-direct" pathway is composed of the cortex-STN and GPi/SNr.Reduced dopaminergic input from the substantia nigra pars compacta to the striatum decreases GABAergic input from the striatum to the GPi/ SNr of the direct pathway and increases glutaminergic input from the STN to the GPi/SNr following decreased GABAergic input from the striatum to the GPe and GPe to the STN in the indirect pathway.This occurs due to GPi/SNr neuronal activity induction and leads to increased inhibitory inputs to the thalamus (Albin et al., 1989;DeLong, 1990;Gerfen et al., 1990).LID is hypothesized to occur when the GABAergic input from the direct pathway neurons in the striatum exceeds the glutaminergic input from the STN, resulting in excessive inhibition of GPi neurons (Alexander et al., 1990).
Moreover, GABA release is increased in the GPi/SNr at dyskinesia F I G U R E 2 Analysis of spontaneous neural activity and neural responses to cortical stimulation in EP neurons.(a) Dot plots (median, first, and third quartiles, and the minimum and maximum excluding any outliers, which were values >1.5 times the interquartile range, of the upper and lower quartiles).Dot plots of the firing rate (left).No significant differences were noted among the control, DL, LID, and MTEP groups.The CV of the ISIs was significantly higher in the DL, LID, and MTEP groups than that in the control group (right).No significant differences were noted among the DL, LID, and MTEP groups.*Significantly higher compared with the control group <. onset (Yamamoto et al., 2006).In PD, D1 receptors become hypersensitive due to dopamine denervation, whereas GABA release at the GPi/SNr increases due to dysfunction of GABA B receptors in the direct pathway, which prevents negative feedback on GABA release (Borgkvist et al., 2018).Furthermore, as dopamine denervation progresses, serotonergic neurons convert administered levodopa to dopamine and release it into the striatum.Dopaminergic neurons, but not serotonergic neurons, can buffer dopamine concentration in the striatal synaptic clefts, which results in the rapid and pulsatile non-physiological stimulation of D1 receptors after levodopa administration.
In our study, the spontaneous firing rate of the EP was comparable among the groups, as previously reported (Dwi Wahyu et al., 2021).In contrast, the CV of the ISIs was significantly higher in the DL, LID, and MTEP groups compared with the control group and was comparable among the DL, LID, and MTEP groups.
Alterations in firing patterns are considered to be more reliable features of parkinsonism and dyskinesia than the firing rate (Lee et al., 2007;Ryu et al., 2011).The firing patterns of neurons in the basal ganglia change to irregular, mixed, or burst firing in the PD model (Hassani et al., 1996;Ryu et al., 2011).Moreover, the regularity of firing in GP and SN neurons is modulated at the onset of dyskinesia, although no consensus has been reached on its underlying causes (Dwi Wahyu et al., 2021;Lee et al., 2007).Our results demonstrated that MTEP treatment did not lead to a change in ISI compared with the LID group.Although the firing pattern, rather than the firing rate, is an important pathophysiological mechanisms of PD and LID, our results suggest that factors other than the spontaneous firing rate or irregularity of output nucleus (GPi/ SNr) neuron activity may also contribute to PD.

| Changes in the responses of EP neurons to cortical stimulation
In our study, cortical stimulation induced early-Ex, inhibition, and late-Ex, which represent hyper-direct, direct, and indirect pathways, respectively, in the EP neurons of the control group.The inhibition phase disappeared in the DL group, partially recovered in the LID group, and disappeared in the MTEP (Figure 2b).This trend was also confirmed in the PSTH population (Figure 2c).Dopamine depletion reduces the activity of striatal neurons in the direct pathway (Chiken et al., 2015;Mallet et al., 2006;Parker et al., 2018;Ryan et al., 2018).
Reduced input from the striatum to the GPi in the direct pathway plays an important role in decreasing the inhibition phase (Dwi Wahyu et al., 2021).After repeated levodopa administration and consequent LID induction, the response of D1-positive striatal neurons to synaptic inputs increases even when dyskinesia is not clinically evident.Moreover, direct pathway neuronal activity in the striatum is increased during dyskinesia (Parker et al., 2018;Ryan et al., 2018;Suarez et al., 2016).In our study, the inhibition phase observed in the LID group disappeared with combined MTEP and L/B administration.Several studies have indicated that mGluR5 potentiates ionotropic glutamate N-methyl-D-aspartate receptors and dopamine D1/D5 receptors in the direct pathway (Pourmirbabaei et al., 2019).
Patients with LID have higher specific binding of mGluR5 to the basal ganglia than patients without dyskinesia (Ouattara et al., 2011).Moreover, mGluR5 gene expression is upregulated in the striatum of animals with LID (Luján et al., 1997;Rylander et al., 2009).Among  synaptic plasticity (Scarduzio et al., 2022).Several studies have demonstrated the effectiveness of mGluR5 antagonists in LID; however, the physiological mechanisms of their effect on LID remain unclear.
Our results demonstrate that the functional changes in the direct pathway, which are induced by repeated administration of levodopa, play an important role in the pathophysiological mechanisms of LID.
Additionally, the co-administration of mGluR5 antagonists prevent LID development by preventing the functional change of the direct pathway.Since both human and animal studies have reported that mGluR5 antagonists have no effect on PD motor symptoms or antiparkinsonian drugs (Lin et al., 2007;Rascol et al., 2017), MTEP may be one of the treatment option for LID.

| Limitations
This study has several limitations.First, the mGluR5 gene is mainly expressed in the cerebral cortex, hippocampus, striatum, and basal ganglia including the striatum and EP (Valerio et al., 1997;Wong et al., 2013).In addition, mGluR5 is expressed in glial cells, including astrocytes and microglia (Loane et al., 2012).Systemic administration of MTEP blocks mGluR5 in these regions.Therefore, the specific area or pathways contributing to the changes in the electrophysiological properties of the direct pathway neurons observed in this study remain unknown.Second, only male rats were included to rule out any differences attributed to sex, which are yet to be reported for rat models of PD.Third, a previous study showed decreased mGluR5 binding potential in 6-OHDA-lesioned rats and increased mGluR5 binding potential in rat models of LID using positron emission tomography (Crabbé et al., 2018); we did not explore the effects of these findings on our electrophysiological results.
Further research is warranted to clarify these points.

| Conclusions
Our study suggests that blocking mGluR5 may have therapeutic and

ACK N OWLED G M ENTS
This work was supported by JSPS KAKENHI grant numbers 18K07374 and 21K07282.The funding company did not influence data interpretation.
The black arrows indicate the time points when AIM score evaluation was performed (days 1, 4, 11, and 17 from the first administration of each drug or saline).(b) Histological results confirm TH-positive neurons in the striatum.A rat model of PD was created by administering 6-OHDA.The dopaminergic cells in the striatum degenerated.(c) The AIM score was significantly higher in the LID and MTEP groups compared with the control and DL groups, where the score was 0. The AIM score was significantly lower in the MTEP group compared with the LID group.(d) Neural distribution of recorded EP neurons.The neurons recorded in representative frontal sections in the EP (posterior −2.25, 2.50, and 2.75 mm from bregma) of all rats in the control, DL, LID, and MTEP groups are shown.*Significantly higher compared with the control, DL, and MTEP groups (p < .01).6-OHDA, 6-hydroxydopamine; AIM, abnormal involuntary movement; DL, dopamine lesioned; EP, entopeduncular nucleus; LID, levodopa-induced dyskinesia; MFB, medial forebrain bundle; MTEP, 3-([2-methyl-1,3-thiazol-4-yl]ethynyl) pyridine; PD, Parkinson's disease.
striatal neurons, mGluR5 is abundant in the postsynaptic membranes of GABAergic projection neurons, termed medium spiny neurons (MSNs), and modulates dopamine-dependent signaling and TA B L E 1 Quantitative analysis of the responses of EP neurons to cortical stimulation.
prophylactic effects on LID.Electrophysiological findings suggest that blocking mGluR5 may cause a change in the neuronal responsiveness of direct pathway neurons to the cortical input.Overall, our findings will help to elucidate the pathophysiological mechanisms underlying LID and aid in the development of therapeutics to prevent motor complications in PD.DECL AR ATION OF TR ANS PAREN C YThe authors, reviewers, and editors affirm that in accordance with the policies set by the Journal of Neuroscience Research, this manuscript presents an accurate and transparent account of the study being reported and that all critical details describing the methods and results are present.AUTH O R CO NTR I B UTI O N S Hikaru Kamo was involved in conceptualization; data curation; formal analysis; investigation; visualization; writing-original draft.Hirokazu Iwamuro was involved in data curation; methodology; validation.Ryota Nakamura was involved in data curation; resources; validation.Shuko Nojiri: was involved in formal analysis; visualization; writing-review & editing.Ayami Okuzumi: was involved in writingreview & editing.Takashi Ogawa was involved in methodology; software; validation.Asuka Nakajima was involved in writing-review & editing.Nobutaka Hattori was involved in conceptualization; funding acquisition; project administration; supervision; writing-review & editing.Yasushi Shimo was involved in conceptualization; funding acquisition; project administration; supervision; writing-review & editing.
Note:The Kruskal-Wallis test with Dunn's multiple comparison test was performed to compare the duration and amplitude between the control and DL, LID, or MTEP groups.The amplitude and duration of early excitation were not significantly different between all groups.The duration and amplitude of the inhibition phase were significantly shorter and lower in the DL, LID, and MTEP groups compared with the control group (p < .01;Kruskal-Wallis test with Dunn's multiple comparison test).The amplitude of late excitation was significantly lower in the LID group compared with the control, DL, and MTEP groups (p < .01;Kruskal-Wallis test with Dunn's multiple comparison test).Values indicate the median and interquartile range; *p < .01,significantly different from the control group.