Lesion of subthalamic or motor thalamic nucleus in 6-hydroxydopamine-treated rats: Effects on striatal glutamate and apomorphine-induced contralateral rotations

Recovery of motor function is associated with rescue of glutamate biomarkers in the striatum and motor cortex following treatment with Mucuna pruriens in a murine model of Parkinsons disease

There is growing interest in the use of natural products for the treatment of Parkinson's disease (PD). Mucuna pruriens has been used in the treatment of humans with PD. The goal of this study was to determine if daily oral treatment with an extract of Mucuna pruriens, starting after the MPTP-induced loss of nigrostriatal dopamine in male mice, would result in recovery/restoration of motor function, tyrosine hydroxylase (TH) protein expression in the nigrostriatal pathway, or glutamate biomarkers in both the striatum and motor cortex. Following MPTP administration, resulting in an 80 % loss of striatal TH, treatment with Mucuna pruriens failed to rescue either striatal TH or the dopamine transporter back to the control levels, but there was restoration of gait/motor function. There was an MPTP-induced loss of TH-labeled neurons in the substantia nigra pars compacta and in the number of striatal dendritic spines, both of which failed to be recovered following treatment with Mucuna pruriens. This Mucuna pruriens-induced locomotor recovery following MPTP was associated with restoration of two striatal glutamate transporter proteins, GLAST (EAAT1) and EAAC1 (EAAT3), and the vesicular glutamate transporter 2 (Vglut2) within the motor cortex. Post-MPTP treatment with Mucuna pruriens, results in locomotor improvement that is associated with recovery of striatal and motor cortex glutamate transporters but is independent of nigrostriatal TH restoration.

Exendin-4 for Parkinson's disease

This review article discusses the preclinical evidence and clinical trials testing the use of a peptide agonist of the glucagon-like peptide (GLP) receptor that promotes insulin secretion in the animal models of and patient with Parkinson's disease (PD). In particular, we focus on the therapeutic effects of the GLP receptor agonist exendin-4, also called exenatide, in PD. The ultimate goal of this article is to provide a critical assessment of the laboratory and clinical data toward guiding the translation of exendin-4 as a clinically relevant therapeutic for PD.

PT320, Sustained-Release Exendin-4, Mitigates L-DOPA-Induced Dyskinesia in a Rat 6-Hydroxydopamine Model of Parkinson's Disease

Background

We previously demonstrated that subcutaneous administration of PT320, a sustained-release (SR) form of exendin-4, resulted in the long-term maintenance of steady-state exenatide (exendin-4) plasma and target levels in 6-hydroxydopamine (6-OHDA)-pretreated animals. Additionally, pre- or post-treatment with PT320 mitigated the early stage of 6-OHDA-induced dopaminergic neurodegeneration. The purpose of this study was to evaluate the effect of PT320 on L-3,4-dihydroxyphenylalanine (L-DOPA)-induced abnormal involuntary movements (AIMs) in the rat 6-OHDA model of Parkinson’s disease.

Methods

Adult male Sprague–Dawley rats were unilaterally lesioned in the right medial forebrain bundle by 6-OHDA. L-DOPA and benserazide were given daily for 22 days, starting from 4 weeks after lesioning. PT320 was co-administered weekly for 3 weeks. AIM was evaluated on days 1, 16, and 22 after initiating L-DOPA/benserazide + PT320 treatment. Brain tissues were subsequently collected for HPLC measurements of dopamine (DA) and metabolite concentrations.

Results

L-DOPA/benserazide increased AIMs of limbs and axial as well as the sum of all dyskinesia scores (ALO) over 3 weeks. PT320 significantly reduced the AIM scores of limbs, orolingual, and ALO. Although PT320 did not alter DA levels in the lesioned striatum, PT320 significantly attenuated 6-OHDA-enhanced DA turnover.

Conclusion

PT320 attenuates L-DOPA/benserazide-induced dyskinesia in a 6-OHDA rat model of PD and warrants clinical evaluation to mitigate Parkinson’s disease in humans.

Dietary therapy restores glutamatergic input to orexin/hypocretin neurons after traumatic brain injury in mice

Study Objectives

In previous work, dietary branched-chain amino acid (BCAA) supplementation, precursors to de novo glutamate and γ-aminobutyric acid (GABA) synthesis, restored impaired sleep-wake regulation and orexin neuronal activity following traumatic brain injury (TBI) in mice. TBI was speculated to reduce orexin neuronal activity through decreased regional excitatory (glutamate) and/or increased inhibitory (GABA) input. Therefore, we hypothesized that TBI would decrease synaptic glutamate and/or increase synaptic GABA in nerve terminals contacting orexin neurons, and BCAA supplementation would restore TBI-induced changes in synaptic glutamate and/or GABA.

Methods

Brain tissue was processed for orexin pre-embed diaminobenzidine labeling and glutamate or GABA postembed immunogold labeling. The density of glutamate and GABA immunogold within presynaptic nerve terminals contacting orexin-positive lateral hypothalamic neurons was quantified using electron microscopy in three groups of mice (n = 8 per group): Sham/noninjured controls, TBI without BCAA supplementation, and TBI with BCAA supplementation (given for 5 days, 48 hr post-TBI). Glutamate and GABA were also quantified within the cortical penumbral region (layer VIb) adjacent to the TBI lesion.

Results

In the hypothalamus and cortex, TBI decreased relative glutamate density in presynaptic terminals making axodendritic contacts. However, BCAA supplementation only restored relative glutamate density within presynaptic terminals contacting orexin-positive hypothalamic neurons. BCAA supplementation did not change relative glutamate density in presynaptic terminals making axosomatic contacts, or relative GABA density in presynaptic terminals making axosomatic or axodendritic contacts, within either the hypothalamus or cortex.

Conclusions

These results suggest TBI compromises orexin neuron function via decreased glutamate density and highlight BCAA supplementation as a potential therapy to restore glutamate density to orexin neurons.

Intervention with exercise restores motor deficits but not nigrostriatal loss in a progressive MPTP mouse model of Parkinson's disease

Many studies have investigated exercise therapy in Parkinson's disease (PD) and have shown benefits in improving motor deficits. However, exercise does not slow down the progression of the disease or induce the revival of lost nigrostriatal neurons. To examine the dichotomy of behavioral improvement without the slowing or recovery of dopaminergic cell or terminal loss, we tested exercise therapy in an intervention paradigm where voluntary running wheels were installed half-way through our progressive PD mouse model. In our model, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) is administered over 4 weeks with increased doses each week (8, 16, 24, 32-kg/mg). We found that after 4 weeks of MPTP treatment, mice that volunteered to exercise had behavioral recovery in several measures despite the loss of 73% and 53% tyrosine hydroxylase (TH) within the dorsolateral (DL) striatum and the substantia nigra (SN), respectively which was equivalent to the loss seen in the mice that did not exercise but were also administered MPTP for 4 weeks. Mice treated with 4 weeks of MPTP showed a 41% loss of vesicular monoamine transporter II (VMAT2), a 71% increase in the ratio of glycosylated/non-glycosylated dopamine transporter (DAT), and significant increases in glutamate transporters including VGLUT1, GLT-1, and excitatory amino acid carrier 1. MPTP mice that exercised showed recovery of all these biomarkers back to the levels seen in the vehicle group and showed less inflammation compared to the mice treated with MPTP for 4 weeks. Even though we did not measure tissue dopamine (DA) concentration, our data suggest that exercise does not alleviate motor deficits by sparing nigrostriatal neurons, but perhaps by stabilizing the extraneuronal neurotransmitters, as evident by a recovery of DA and glutamate transporters. However, suppressing inflammation could be another mechanism of this locomotor recovery. Although exercise will not be a successful treatment alone, it could supplement other pharmaceutical approaches to PD therapy.

Subthalamotomy in the treatment of Parkinson's disease: clinical aspects and mechanisms of action

Parkinson's disease (PD) is a neurodegenerative condition that can be pharmacologically treated with levodopa. However, important motor and nonmotor symptoms appear with its long-term use. The subthalamic nucleus (STN) is known to be involved in the pathophysiology of PD and to contribute to levodopa-induced complications. Surgery is considered in patients who have advanced PD that is refractory to pharmacotherapy and who display disabling dyskinesia. Deep brain stimulation of the STN is currently the main surgical procedure for PD, but lesioning is still performed. This review covers the clinical aspects and complications of subthalamotomy as one of the lesion-based options for PD patients with levodopa-induced dyskinesias. Moreover, the authors discuss the possible effects of subthalamic lesioning.

Effects of subthalamic nucleus lesions and stimulation upon corticostriatal afferents in the 6-hydroxydopamine-lesioned rat

Abnormalities of striatal glutamate neurotransmission may play a role in the pathophysiology of Parkinson's disease and may respond to neurosurgical interventions, specifically stimulation or lesioning of the subthalamic nucleus (STN). The major glutamatergic afferent pathways to the striatum are from the cortex and thalamus, and are thus likely to be sources of striatal neuronally-released glutamate. Corticostriatal terminals can be distinguished within the striatum at the electron microscopic level as their synaptic vesicles contain the vesicular glutamate transporter, VGLUT1. The majority of terminals which are immunolabeled for glutamate but are not VGLUT1 positive are likely to be thalamostriatal afferents. We compared the effects of short term, high frequency, STN stimulation and lesioning in 6-hydroxydopamine (6OHDA)-lesioned rats upon striatal terminals immunolabeled for both presynaptic glutamate and VGLUT1. 6OHDA lesions resulted in a small but significant increase in the proportions of VGLUT1-labeled terminals making synapses on dendritic shafts rather than spines. STN stimulation for one hour, but not STN lesions, increased the proportion of synapses upon spines. The density of presynaptic glutamate immuno-gold labeling was unchanged in both VGLUT1-labeled and -unlabeled terminals in 6OHDA-lesioned rats compared to controls. Rats with 6OHDA lesions+STN stimulation showed a decrease in nerve terminal glutamate immuno-gold labeling in both VGLUT1-labeled and -unlabeled terminals. STN lesions resulted in a significant decrease in the density of presynaptic immuno-gold-labeled glutamate only in VGLUT1-labeled terminals. STN interventions may achieve at least part of their therapeutic effect in PD by normalizing the location of corticostriatal glutamatergic terminals and by altering striatal glutamatergic neurotransmission.

In vivo brain microdialysis: advances in neuropsychopharmacology and drug discovery

INTRODUCTION: Microdialysis is an important in vivo sampling technique, useful in the assay of extracellular tissue fluid. The technique has both pre-clinical and clinical applications but is most widely used in neuroscience. The in vivo microdialysis technique allows measurement of neurotransmitters such as acetycholine (ACh), the biogenic amines including dopamine (DA), norepinephrine (NE) and serotonin (5-HT), amino acids such as glutamate (Glu) and gamma aminobutyric acid (GABA), as well as the metabolites of the aforementioned neurotransmitters, and neuropeptides in neuronal extracellular fluid in discrete brain regions of laboratory animals such as rodents and non-human primates. AREAS COVERED: In this review we present a brief overview of the principles and procedures related to in vivo microdialysis and detail the use of this technique in the pre-clinical measurement of drugs designed to be used in the treatment of chemical addiction, neurodegenerative diseases such as Alzheimer's disease (AD), Parkinson's disease (PD) and as well as psychiatric disorders such as attention-deficit/hyperactivity disorder (ADHD) and schizophrenia. This review offers insight into the tremendous utility and versatility of this technique in pursuing neuropharmacological investigations as well its significant potential in rational drug discovery. EXPERT OPINION: In vivo microdialysis is an extremely versatile technique, routinely used in the neuropharmacological investigation of drugs used for the treatment of neurological disorders. This technique has been a boon in the elucidation of the neurochemical profile and mechanism of action of several classes of drugs especially their effects on neurotransmitter systems. The exploitation and development of this technique for drug discovery in the near future will enable investigational new drug candidates to be rapidly moved into the clinical trial stages and to market thus providing new successful therapies for neurological diseases that are currently in demand.

Lesion of the ventromedial nucleus of the thalamus blocks acute cocaine-induced changes in striatal glutamate

A single injection of cocaine increases extracellular glutamate in the rat dorsolateral striatum 1 day after the acute cocaine was administered (McKee and Meshul, 2005). However, the nuclei that facilitate this increase in striatal glutamate remain unknown. We hypothesized that the cocaine-induced increase in striatal glutamate was produced by activation of the ventromedial (VM) nucleus of the thalamus via the thalamo-corticostriatal or thalamostriatal pathways. First, rats received an electrolytic lesion of the VM. One day after a single cocaine or vehicle injection, extracellular glutamate was measured in the dorsolateral striatum using in vivo microdialysis. The motor thalamus lesion blocked the cocaine-induced increase in striatal glutamate and reduced extracellular glutamate to the level of the vehicle-treated group. This study shows a critical role for the VM nucleus of the thalamus in mediating the effects of cocaine on extracellular glutamate levels in the rat dorsolateral striatum.

L-DOPA-INDUCED DYSKINESIA AND STEREOTACTIC SURGERY FOR PARKINSON'S DISEASE

OBJECTIVE

To assess the impact of different surgical targets and techniques, such as ablation and deep brain stimulation, to treat patients with L-dopa-induced dyskinesia (LID), a major therapeutic complication of Parkinson's disease.

METHODS

This review analyzes the effects of early surgical procedures to treat hyperkinesia and the current methods and targets used to combat LID in Parkinson's disease, which are mainly thalamotomy, pallidotomy, and deep brain stimulation of the globus pallidus internus and the subthalamic nucleus.

RESULTS

Available information indicates that surgery of the globus pallidus internus and thalamus (the pallidal receiving area) and of the subthalamic nucleus has a pronounced antidyskinetic effect. This effect is associated with a concomitant improvement in the parkinsonian (“off”-medication) state. Although it is more profound with pallidal and subthalamic surgery, such an effect can also be observed to some extent with thalamic surgery. The latter is attributable to the fact that surgery of the ventralis intermedius is primarily effective for treating tremor. An integral pallidothalamic pathway is needed for dyskinesia to be expressed. Thus, LID is less frequent after subthalamotomy or deep brain stimulation of the subthalamic nucleus through a functional effect mediated by the physiological normalization of the motor system and by an indirect effect associated with a reduction in the daily dose of L-dopa.

CONCLUSION

Surgery is the only treatment available for Parkinson's disease that can predictably improve both the parkinsonian motor syndrome and LID. The exact mechanisms involved in these effects are not well understood. Pallidal and thalamic surgery affecting pallidal relays reduce LID frequency by disrupting the pallidothalamic circuit, probably eliminating the neuronal activity associated with dyskinesia. Alternatively, the antidyskinetic effect of subthalamic nucleus surgery may in part be attributable to a reduction in the L-dopa dose as well as to the stabilization of the basal ganglia circuits after the surgical procedure.

Human neural progenitor cell transplants into the subthalamic nucleus lead to functional recovery in a rat model of Parkinson’s disease

Despite the success of foetal nigral transplantation for the treatment of Parkinson's disease, supply limitations of tissue means that alternative sources must be found. Transplantation of human neural progenitor cells (HNPCs) may offer a solution, however few studies have shown functional recovery in animal models of PD without cell modification. Here we show that unmodified HNPC grafted into the subthalamic nucleus (STN) show excellent survival of up to 5 months and induce significant functional recovery following amphetamine-induced rotations within 4 weeks. For the first time we also show that HNPCs, which remain in an immature nestin-positive state, produce VEGF in vivo allowing further modification of the host brain. This suggests that even in the absence of cell replacement strategies utilising immature progenitor cells could be of real therapeutic value.

Deficits of glutamate transmission in the striatum of experimental hemiballism

Hemiballism (HB) is a quite rare disorder, generally secondary to stroke, neoplasms or demyelinating plaques, classically considered as almost pathognomonic of a lesion in the subthalamic nucleus (STN). This alteration causes involuntary movements in the chorea-ballism spectrum. One theory is that the output nuclei of the basal ganglia are overinhibited in HB, while little is known about the physiological state of the striatum, the major input structure of the basal ganglia. In the present study, we recorded spontaneous and miniature excitatory and inhibitory postsynaptic currents (sEPSCs, mEPSCs, sIPSCs, mIPSCs) from projection neurons of the striatum of experimental HB. We found a selective reduction of striatal sEPSC and mEPSC frequency following chemical lesion of the STN of the rat, suggesting that reduced synaptic excitation of the input structure of the basal ganglia represents a physiological correlate of HB.

Deficits of glutamate transmission in the striatum of toxic and genetic models of Huntington's disease

Altered glutamate transmission in the striatum has been proposed to play a critical role in the pathophysiology of Huntington's disease (HD), a genetic disorder associated with impaired activity of the mitochondrial complex II (succinate dehydrogenase, SD). In the present study, we recorded spontaneous (sEPSCs) and miniature excitatory postsynaptic currents (mEPSCs) from striatal neurons of both toxic (systemic administration of 3-nitropropionic acid in rats) and genetic models of HD (R6/2 transgenic mice). In both models, we found a significant down-regulation of glutamate transmission, suggesting that reduced synaptic excitation of the input structure of the basal ganglia represents a physiological correlate of HD.

l-dopa-induced reversal in striatal glutamate following partial depletion of nigrostriatal dopamine with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine

We have reported that 1 month following acute (20mg/kg x 4) or subchronic (30 mg/kg/day x 7d) administration of the neurotoxin, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine, there is an increase or decrease, respectively, in the extracellular level of striatal glutamate as determined by in vivo microdialysis [Robinson S, Freeman P, Moore C, Touchon JC, Krentz L, Meshul CK (2003) Acute and subchronic MPTP administration differentially affects striatal glutamate synaptic function. Exp Neurol 180:73-86]. The goal of this study was to determine the effects of treatment with l-dopa (15 mg/kg) for 21 days on striatal glutamate starting on day 8 after the first dose of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine was administered to mice. Following acute administration of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine, the increase in extracellular striatal glutamate due to lesion of the nigrostriatal pathway was completely reversed to a level below that found in the vehicle-treated group after l-dopa treatment. Subchronic 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine treatment resulted in a decrease in striatal extracellular glutamate that was reversed to the level close to that observed in the vehicle-treated group. There was no change in the density of nerve terminal glutamate immunolabeling associated with the synaptic vesicle pool, suggesting that the alterations in extracellular glutamate most likely originated from the calcium-independent pool. There was a similar decrease in the relative density of tyrosine hydroxylase immunolabeling, a marker for dopamine terminals, within the dorsolateral striatum in both the acute and subchronic 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-treated groups that had been administered l-dopa. There was a decrease in the relative density of immunolabeling within the dorsolateral striatum for the glutamate transporter, GLT-1, following acute 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine treatment in the groups administered either vehicle or l-dopa. There was no change in GLT-1 immunolabeling following subchronic 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine. The results demonstrate that the reversal in the extracellular level of striatal glutamate following l-dopa treatment in both the acute and subchronic 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-treated groups is not due to changes in either striatal dopamine nerve terminals or in the density of the glutamate transporter, GLT-1.

Differential survival patterns among midbrain dopaminergic cells of MPTP-treated monkeys and 6OHDA-lesioned rats

We explore the patterns of survival among dopaminergic cells of the midbrain in MPTP-treated macaque monkeys and 6OHDA-lesioned Sprague-Dawley rats. For the monkeys, animals were injected intramuscularly with MPTP for 8 days consecutively and then allowed to survive for 21 days. For the rats, 6OHDA was injected into the midbrain and then allowed to survive for either 7, 28 or 84 days. Brains were processed for tyrosine hydroxylase (TH) and calbindin immunocytochemistry to label populations in the ventral and dorsal tiers of midbrain dopaminergic cells. In monkeys, while there was a decrease in the TH+ cell number in the ventral tier of MPTP-treated cases (65%), there was an overall increase (22%) in the TH+ and calbindin+ cell number in the dorsal tier. Double labelling studies indicate that approximately 50% of TH+ cells of the dorsal tier contain calbindin also. In rats, there was a decrease in TH+ cell number in the ventral tier of 6OHDA-lesioned cases (97%), and to a lesser extent, in the TH+ and calbindin+ cell number in the dorsal tier ( approximately 40%). In conclusion, we show a surprising increase in TH+ and calbindin+ cell number in the dorsal tier in response to MPTP insult; such an increase was not evident after 6OHDA insult. We suggest that the increase in antigen expression relates to the dopaminergic reinnervation of the striatum in MPTP-treated cases. We also suggest that the greater loss of dopaminergic cells in the ventral tier when compared to the dorsal tier relates to glutamate toxicity.

Apomorphine-induced alterations in striatal and substantia nigra pars reticulata glutamate following unilateral loss of striatal dopamine

We have reported time-dependent changes in extracellular glutamate within the striatum at 1 and 3 months following a unilateral lesion of the nigrostriatal pathway using the neurotoxin, 6-hydroxydopamine (6-OHDA) (Meshul, C.K., Emre, N., Nakamura, C.M., Allen, C., Donohue, M.K., Buckman, J.F., 1999. Time-dependent changes in striatal glutamate synapses following a 6-hydroxydopamine lesion. Neurosci. 88, 1-16.). The aim of the present study was to determine the effects of such a lesion on glutamate within the substantia nigra pars reticulata (SN-PR) and the effect of subchronic administration of the dopamine D-1/D-2 agonist, apomorphine, on extracellular glutamate within both the striatum and the SN-PR using in vivo microdialysis. One month after the lesion, there is an increase in extracellular glutamate within the striatum and apomorphine treatment leads to a further increase. Within the SN-PR, a loss of striatal dopamine leads to a decrease in extracellular glutamate, while apomorphine treatment leads to a further decrease in nigral glutamate. Three months after a 6-OHDA lesion, there is a decrease in extracellular striatal glutamate, with apomorphine administration leading to essentially no further change in glutamate. The loss of striatal dopamine increased extracellular glutamate within the SN-PR while apomorphine administration resulted in a decrease in extracellular glutamate back to the value observed in the control group. The data suggests that the increase in striatal glutamate 1 month following a 6-OHDA lesion alone or following subchronic apomorphine is consistent with the hypothesis that a decrease in glutamate within the SN-PR leads to activation of the thalamo-cortico-striatal pathway. The decrease in striatal glutamate 3 months after a nigrostriatal lesion is also consistent with the observed increase in extracellular glutamate within the SN-PR, thus leading to a decrease in output of the thalamo-cortico-striatal pathway.

Subthalamic nucleus lesion reverses motor abnormalities and striatal glutamatergic overactivity in experimental parkinsonism

Subthalamic nucleus (STN) is a target of choice for the neurosurgical treatment of Parkinson's disease (PD). The therapeutic effect of STN lesion in PD is classically ascribed to the rescue of physiological activity in the output structures of the basal ganglia, and little is known about the possible involvement of the striatum. In the present study, therefore, we electrophysiologically recorded in vitro single striatal neurons of DA-depleted rats unilaterally lesioned by 6-hydroxydopamine, treated or not with therapeutic doses of levodopa (l-DOPA), or with a consecutive ipsilateral STN lesion. We show that the beneficial motor effects produced in parkinsonian rats by STN lesion or l-DOPA therapy were paralleled by the normalization of overactive frequency and amplitude of striatal glutamate-mediated spontaneous excitatory postsynaptic currents (sEPSCs). Since neither l-DOPA treatment nor STN lesion affected sEPSCs kinetic properties, the reversal of these abnormalities in striatal excitatory synaptic transmission can be attributable to the normalization of glutamate release.