Gene expression of glutamate receptors GluR1 and NR1 is differentially modulated in striatal neurons in rats after 6-hydroxydopamine lesion

Endocytosis of AMPA receptors: Role in neurological conditions

AMPA receptors are glutamate-gated ion channels, present in a wide range of neuron types and in glial cells. Their main role is to mediate fast excitatory synaptic transmission, and therefore, they are critical for normal brain function. In neurons, AMPA receptors undergo constitutive and activity-dependent trafficking between the synaptic, extrasynaptic and intracellular pools. The kinetics of AMPA receptor trafficking is crucial for the precise functioning of both individual neurons and neural networks involved in information processing and learning. Many of the neurological diseases evoked by neurodevelopmental and neurodegenerative malfunctions or traumatic injuries are caused by impaired synaptic function in the central nervous system. For example, attention-deficit/hyperactivity disorder (ADHD), Alzheimer's disease (AD), tumors, seizures, ischemic strokes, and traumatic brain injury are all characterized by impaired glutamate homeostasis and associated neuronal death, typically caused by excitotoxicity. Given the important role of AMPA receptors in neuronal function, it is not surprising that perturbations in AMPA receptor trafficking are associated with these neurological disorders. In this book chapter, we will first introduce the structure, physiology and synthesis of AMPA receptors, followed by an in-depth description of the molecular mechanisms that control AMPA receptor endocytosis and surface levels under basal conditions or synaptic plasticity. Finally, we will discuss how impairments in AMPA receptor trafficking, particularly endocytosis, contribute to the pathophysiology of various neurological disorders and what efforts are being made to therapeutically target this process.

Effects of Combined Bushen Zhichan Recipe and Levodopa in a Rodent Model of Parkinson Disease: Potential Mechanisms

BACKGROUND Parkinson disease is characterized by the loss of neurons in the substantia nigra, and under pathological conditions, glutamate can produce excitotoxic effects on nerve cells. The astrocytic excitatory amino acid transporter (EAAT) 1 can be functionally upregulated and targeted to functional compartments, resulting in reduced excitotoxicity. levodopa is the gold standard for the treatment of Parkinson disease, but prolonged levodopa treatment often leads to the development of abnormal involuntary movements. Numerous studies suggest the potential beneficial effects of traditional Chinese medicine on Parkinson disease. MATERIAL AND METHODS We validated the efficacy of a Bushen Zhichan recipe combined with levodopa in a rodent Parkinson disease model and explored its possible mechanisms. RESULTS Rats in the combined levodopa and Bushen Zhichan recipe group performed significantly better than the control group in the open field and forelimb function experiments. The number of midbrain dopaminergic neurons in rats in the levodopa and Bushen Zhichan recipe group was greater compared to controls. The levodopa and Bushen Zhichan recipe group exhibited decreased glutamate receptors and increased γ-aminobutyric acid receptors in the striatum. At the same time, EAAT1 was increased and EAAT2 was synchronized with the number of glutamate receptors. CONCLUSIONS Our results indicate that levodopa combined with Bushen Zhichan recipe significantly improves behavior and protects dopaminergic neurons in a rodent Parkinson disease model, and suggest that the mechanism involves the decrease of excitatory amino acid toxicity and the increase in the expression of EAAT1.

Enhanced AMPA receptor-mediated excitatory transmission in the rodent rostromedial tegmental nucleus following lesion of the nigrostriatal pathway

The GABAergic rostromedial tegmental nucleus (RMTg) has reciprocal connections with the dopaminergic ventral tegmental area and substantia nigra pars compacta (SNc), and is involved in inhibitory control of monoaminergic nuclei. At present, it is not clear whether unilateral 6-hydroxydopamine lesions of the SNc in rats affect AMPA receptor-mediated excitatory transmission in the RMTg. Here we found that lesions of the SNc in rats increased the firing rate of GABAergic neurons and the level of glutamate in the RMTg compared to sham-operated rats. Intra-RMTg injection of AMPA receptor agonist (S)-AMPA increased the firing rate of the GABAergic neurons in both sham-operated and the lesioned rats, while AMPA receptor antagonist NBQX decreased the firing rate of the neurons. Further, intra-RMTg injection of (S)-AMPA decreased the levels of dopamine and serotonin in the medial prefrontal cortex (mPFC) in the two groups of rats; conversely, NBQX increased the levels of dopamine and serotonin. Compared to sham-operated rats, the duration of (S)-AMPA and NBQX action on the firing rate of GABAergic neurons in the RMTg and release of doapmine and serotonin in the mPFC was prolonged in the lesioned rats. In addition, lesions of the SNc in rats increased protein expression of t-GluR1 and p-GluR1-S831 subunits compared to sham-operated rats. Therefore, these changes in the lesioned rats are associated with increased release of glutamate and up-regulated expression of GluR1 subunit-containing AMPA receptors in the RMTg, which suggest that degeneration of the nigrostriatal pathway enhances AMPA receptor-mediated excitatory transmission in the RMTg.

Electroacupuncture restores spatial learning and downregulates phosphorylated N-methyl-D-aspartate receptors in a mouse model of Parkinson's disease

OBJECTIVE

Parkinson's disease (PD) is a degenerative disorder of the central nervous system. PD can be classified as idiopathic, acquired or hereditary and may be caused by various factors such as oxidative stress, loss of mitochondrial function, neuronal excitotoxicity or calcium imbalance.

METHODS

We hypothesised that electroacupuncture (EA) at KI3 would reduce neuronal excitotoxicity by regulating N-methyl-D-aspartate (NMDA) receptor function and may represent a novel therapeutic approach for PD.

RESULTS

Our results showed that deficits in spatial learning (reflected by the escape latency time in the Morris water maze task) and long-term potentiation (LTP) caused by systemic 6-hydroxydopamine (6-OHDA) administration (that damages dopaminergic neurons) could be rescued by EA on day 3. In PD mice, phosphorylated NMDA receptor subunits NR1 and NR2B were elevated (134.03±10.17% and 123.46±3.47% of baseline levels, respectively) but total NR1 and NR2B was unaffected (101.37±3.87% and 102.61±4.22% of baseline, respectively). Elevated levels of pNR1 and pNR2B, and phosphorylated forms of protein kinase A, protein kinase C, α Ca²⁺/calmodulin-dependent protein kinase extracellular signal-regulated kinases (pERK), and cAMP response element-binding protein were also reduced following EA.

CONCLUSIONS

These novel findings suggest that EA can rescue learning and LTP deficits in a rodent model of PD. The results point to a possible role for EA-based approaches in the clinical treatment of learning deficits associated with PD.

Cell type-specific plasticity of striatal projection neurons in parkinsonism and L-DOPA-induced dyskinesia

The striatum is widely viewed as the fulcrum of pathophysiology in Parkinson's disease (PD) and L-DOPA-induced dyskinesia (LID). In these disease states, the balance in activity of striatal direct pathway spiny projection neurons (dSPNs) and indirect pathway spiny projection neurons (iSPNs) is disrupted, leading to aberrant action selection. However, it is unclear whether countervailing mechanisms are engaged in these states. Here we report that iSPN intrinsic excitability and excitatory corticostriatal synaptic connectivity were lower in PD models than normal; L-DOPA treatment restored these properties. Conversely, dSPN intrinsic excitability was elevated in tissue from PD models and suppressed in LID models. Although the synaptic connectivity of dSPNs did not change in PD models, it fell with L-DOPA treatment. In neither case, however, was the strength of corticostriatal connections globally scaled. Thus, SPNs manifested homeostatic adaptations in intrinsic excitability and in the number but not strength of excitatory corticostriatal synapses.

Changes in surface expression of N-methyl-D-aspartate receptors in the striatum in a rat model of Parkinson's disease

BACKGROUND

N-methyl-D-aspartate (NMDA) receptors play a central role in glutamatergic synaptic transmission in the mammalian brain and are linked to the pathophysiology and symptomatology of Parkinson's disease (PD). However, changes in NMDA receptor expression in distinct subcellular compartments in PD have not been elucidated. In this study, we investigated changes in subcellular expression of NMDA receptors in striatal neurons in a rodent PD model.

METHODS

Intracranial injection of the neurotoxin 6-hydroxydopamine (6-OHDA) was selectively lesioned into the nigrostriatal dopaminergic pathway in adult Sprague Dawley rats, which is a common rat model of PD. A surface receptor crosslinking assay was conducted to examine the response of individual NMDA receptor subunits to dopamine depletion in isolated and confined surface and intracellular compartments of striatal neurons.

RESULTS

In PD rats where 6-OHDA was selectively lesioned, surface expression of NMDA receptor GluN1 subunits as detected by surface protein crosslinking assays was increased in the striatum. In contrast, intracellular levels of GluN1 were decreased in the lesioned region. The NMDA receptor GluN2B subunit was elevated in its abundance in the surface pool of the lesioned striatum, while intracellular GluN2B levels were not altered. GluN2A subunits in both surface and intracellular fractions remained stable. In addition, total cellular levels of striatal GluN1 and GluN2A were not changed in lesioned tissue, while total GluN2B proteins showed an increase.

CONCLUSION

These results demonstrate the differential sensitivity of principal NMDA receptor subunits to dopamine depletion. GluN1 and GluN2B expression in the distinct surface compartment underwent upregulation in striatal neurons after selective lesions of the dopaminergic pathway by 6-OHDA.

A network view on Parkinson's disease

Network-based systems biology tools including Pathway Studio 9.0 were used to identify Parkinson's disease (PD) critical molecular players, drug targets, and underlying biological processes. Utilizing several microarray gene expression datasets, biomolecular networks such as direct interaction, shortest path, and microRNA regulatory networks were constructed and analyzed for the disease conditions. Network topology analysis of node connectivity and centrality revealed in combination with the guilt-by-association rule 17 novel genes of PD-potential interest. Seven new microRNAs (miR-132, miR-133a1, miR-181-1, miR-182, miR-218-1, miR-29a, and miR-330) related to Parkinson's disease were identified, along with more microRNA targeted genes of interest like RIMS3, SEMA6D and SYNJ1. David and IPA enrichment analysis of KEGG and canonical pathways provided valuable mechanistic information emphasizing among others the role of chemokine signaling, adherence junction, and regulation of actin cytoskeleton pathways. Several routes for possible disease initiation and neuro protection mechanisms triggered via the extra-cellular ligands such as CX3CL1, SEMA6D and IL12B were thus uncovered, and a dual regulatory system of integrated transcription factors and microRNAs mechanisms was detected.

Peripheral ammonia as a mediator of methamphetamine neurotoxicity

Ammonia is metabolized by the liver and has established neurological effects. The current study examined the possibility that ammonia contributes to the neurotoxic effects of methamphetamine (METH). The results show that a binge dosing regimen of METH to the rat increased plasma and brain ammonia concentrations that were paralleled by evidence of hepatotoxicity. The role of peripheral ammonia in the neurotoxic effects of METH was further substantiated by the demonstration that the enhancement of peripheral ammonia excretion blocked the increases in brain and plasma ammonia and attenuated the long-term depletions of dopamine and serotonin typically produced by METH. Conversely, the localized perfusion of ammonia in combination with METH, but not METH alone or ammonia alone, into the striatum recapitulated the neuronal damage produced by the systemic administration of METH. Furthermore, this damage produced by the local administration of ammonia and METH was blocked by the GYKI 52466 [4-(8-methyl-9H-1,3-dioxolo[4,5-h][2,3]benzodiazepin-5-yl)-benzamine hydrochloride], an AMPA receptor antagonist. These findings highlight the importance of ammonia derived from the periphery as a small-molecule mediator of METH neurotoxicity and more broadly emphasize the importance of peripheral organ damage as a possible mechanism that mediates the neuropathology produced by drugs of abuse and other neuroactive molecules.

Unilateral lesion of the nigrostriatal pathway decreases the response of GABA interneurons in the dorsal raphe nucleus to 5-HT(1A) receptor stimulation in the rat

This study examined the firing rate and pattern of electrophysiologically and chemically identified GABA interneurons in the dorsal raphe nucleus (DRN), and role of 5-HT(1A) receptor agonist 8-OH-DPAT and the medial prefrontal cortex (mPFC) in the firing activity in rats with 6-hydroxydopamine lesions of the substantia nigra pars compacta (SNc). The interneurons in rats with lesions of the SNc showed a more burst-firing, while having no change in the firing rate; the mPFC and combined mPFC and SNc lesions in rats decreased the firing rate of the interneurons and firing pattern shifted towards a more burst-firing compared to rats with sham lesions of the SNc, respectively. In rats with sham lesions of the SNc, administration of 8-OH-DPAT (1-243 μg/kg, i.v.) produced excitatory-inhibitory, excitatory and inhibitory effects in the firing rate of individual interneurons. However, when these effects were averaged over the group, 8-OH-DPAT had no significant effect on firing rate. In rats with lesions of the SNc, mPFC and the paired lesions, 8-OH-DPAT, at the same doses, inhibited all interneurons tested, respectively. Cumulative doses producing inhibition in rats with the paired lesions were higher than that of rats with lesions of the mPFC. In contrast to rats with sham lesions of the SNc, SNc lesion reduced expression of 5-HT(1A) receptor on parvalbumin positive neurons in the DRN, a subpopulation of GABA interneurons. Our results indicate that the SNc and mPFC regulate the firing activity of GABA interneurons in the DRN. Furthermore, response of likely GABA interneurons to systemic administration of 8-OH-DPAT is altered by lesion of the SNc and mPFC.

Recovery from glycerol-induced acute kidney injury is accelerated by suramin

Acute kidney injury (AKI) is a common and potentially life-threatening complication after ischemia/reperfusion and exposure to nephrotoxic agents. In this study, we examined the efficacy and mechanism(s) of suramin in promoting recovery from glycerol-induced AKI, a model of rhabdomyolysis-induced AKI. After intramuscular glycerol injection (10 ml of 50% glycerol per kilogram) into male Sprague-Dawley rats, serum creatinine maximally increased at 24 to 72 h and then decreased at 120 h. Creatinine clearance (CrCl) decreased 75% at 24 to 72 h and increased at 120 h. Suramin (1 mg/kg i.v.) administered 24 h after glycerol accelerated recovery of renal function as demonstrated by increased CrCl, decreased renal kidney injury molecule-1, and improved histopathology 72 h after glycerol injection. Suramin treatment decreased interleukin-1β (IL-1β) mRNA, transforming growth factor-β(1) (TGF-β(1)), phospho-p65 of nuclear factor-κB (NF-κB), and cleaved caspase-3 at 48 h compared with glycerol alone. Suramin treatment also decreased glycerol-induced activation of intracellular adhesion molecule-1 (ICAM-1) and leukocyte infiltration at 72 h. Urinary/renal neutrophil gelatinase-associated lipocalin 2 (NGAL) levels, hemeoxygenase-1 expression, and renal cell proliferation were increased by suramin compared with glycerol alone at 72 h. Mechanistically, suramin decreases early glycerol-induced proinflammatory (IL-1β and NF-κB) and growth inhibitory (TGF-β(1)) mediators, resulting in the prevention of late downstream inflammatory effects (ICAM-1 and leukocyte infiltration) and increasing compensatory nephrogenic repair. These results support the hypothesis that delayed administration of suramin is effective in abrogating apoptosis, attenuating inflammation, and enhancing nephrogenic repair after glycerol-induced AKI.

GPI-1046 increases presenilin-1 expression and restores NMDA channel activity

BACKGROUND

Previously we showed that 6-hydroxydopamine lesions of the substantia nigra eliminate corticostriatal LTP and that the neuroimmunolophilin ligand (NIL), GPI-1046, restores LTP.

METHODS

We used cDNA microarrays to determine what mRNAs may be over- or under-expressed in response to lesioning and/or GPI-1046 treatment. Patch clamp recordings were performed to investigate changes in NMDA channel function before and after treatments.

RESULTS

We found that 51 gene products were differentially expressed. Among these we found that GPI-1046 treatment up-regulated presenilin-1 (PS-1) mRNA abundance. This finding was confirmed using QPCR. PS-1 protein was also shown to be over-expressed in the striatum of lesioned/GPI-1046-treated rats. As PS-1 has been implicated in controlling NMDA-receptor function and LTP is reduced by lesioning we assayed NMDA mediated synaptic activity in striatal brain slices. The lesion-induced reduction of dopaminergic innervation was accompanied by the near complete loss of NDMA receptor-mediated synaptic transmission between the cortex and striatum. GPI-1046 treatment of the lesioned rats restored NMDA-mediated synaptic transmission but not the dopaminergic innervation. Restoration of NDMA channel function was apparently specific as the sodium channel current density was also reduced due to lesioning but GPI-1046 did not reverse this effect. We also found that restoration of NMDA receptor function was also not associated with either an increase in NMDA receptor mRNA or protein expression.

CONCLUSION

As it has been previously shown that PS-1 is critical for normal NMDA receptor function, our data suggest that the improvement of excitatory neurotransmission occurs through the GPI-1046-induced up-regulation of PS-1.

Changes of AMPA receptors in MPTP monkeys with levodopa-induced dyskinesias

Overactivity of glutamate neurotransmission is suspected to be implicated in Parkinson's disease and levodopa-induced dyskinesia. The fast glutamatergic transmission in the striatum from the cortex is mediated mainly by non-n-methyl-d-aspartate (non-NMDA) receptors. Animal models of Parkinson's disease reveal conflicting data concerning striatal glutamate AMPA receptors. The present study thus sought to shed light on the relationship of striatal AMPA receptors to the development of levodopa-induced dyskinesia. [(3)H]Ro 48-8587, a highly potent and selective-specific antagonist ligand for AMPA receptors, was used to investigate, by autoradiography, striatal AMPA receptors in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-lesioned monkeys treated for 1 month with levodopa alone, levodopa+CI-1041 (NMDA receptor antagonist) or levodopa+cabergoline (D2 receptor agonist). Levodopa-treated MPTP monkeys developed dyskinesias while those that received levodopa+CI-1041 or levodopa+cabergoline did not. In the anterior caudate nucleus and putamen, specific binding of [(3)H]Ro 48-8587 was reduced in all MPTP-treated monkeys compared to control monkeys, but no significant effect of MPTP was measured in the posterior striatum. In dyskinetic monkeys, specific binding of [(3)H]Ro 48-8587 was elevated in subregions of the posterior caudate nucleus and putamen as compared to saline-treated MPTP monkeys. Levodopa+CI-1041 treatment left unchanged specific binding of [(3)H]Ro 48-8587 whereas levodopa+cabergoline treatment reduced it in subregions of the posterior caudate nucleus and putamen compared to control and levodopa-treated MPTP monkeys. Specific binding of [(3)H]Ro 48-8587 was low in the globus pallidus and remained unchanged following both lesion and treatments. In conclusion, the elevated values of AMPA receptors in dyskinetic monkeys (and their prevention through treatments) were only observed in subregions of the striatum.

Assemblies of glutamate receptor subunits with post-synaptic density proteins and their alterations in Parkinson's disease

N-methyl-D-aspartate (NMDA) receptors have been implicated as a mediator of neuronal injury associated with many neurological disorders including ischemia, epilepsy, brain trauma, dementia and neurodegenerative disorders such as Parkinson's disease (PD) and Alzheimer's disease. To this, non-selective NMDA receptor antagonists have been tried and have been shown to be effective in many experimental animal models of disease, and some of these compounds have moved into clinical trials. However, the initial enthusiasm for this approach has waned, because the therapeutic index for most NMDA antagonists is quite poor, with significant adverse effects at clinically effective doses, thus limiting their utility. More recently, the concept that the exact pathways downstream NMDA receptor activation could represent a key variable element among neurological disorders has been put forward. In particular, variations in NMDA receptor subunit composition could be important in different disorders, both in the pathophysiological mechanisms of cell death and in the application of specific symptomatic therapies. As to PD, NMDA receptor complex has been shown to be altered in experimental models of parkinsonism and in PD in humans. Further, it has become increasingly evident that the NMDA receptor complex is intimately involved in the regulation of corticostriatal long-term potentiation, which is altered in experimental parkinsonism. The following sections will examine the modifications of specific NMDA receptor subunits as well as post-synaptic associated signalling complex including kinases and scaffolding proteins in experimental parkinsonism. These findings may allow the identification of specific molecular targets whose pharmacological or genetic manipulation might lead to innovative therapies for PD.

MPTP-induced deficits in striatal synaptic plasticity are prevented by glial cell line-derived neurotrophic factor expressed via an adeno-associated viral vector

This study determined the consequences of dopamine denervation of the striatum on synaptic plasticity and prevention of these changes with gene therapy using an adeno-associated viral vector (AAV) expressing glial cell line-derived neurotrophic factor (GDNF). C57BL6/J mice were injected with the neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine(MPTP); long-term depression (LTD) or potentiation (LTP) were measured in vitro. Fast-scan cyclic voltammetry measured electrically released dopamine from a functionally relevant pool in these same striatal slices. After MPTP, dopamine release and uptake were greatly diminished, and LTP and LTD were blocked in the striatal slices. The loss of plasticity resulted directly from the loss of dopamine since its application rescued synaptic plasticity. Striatal GDNF expression via AAV, before MPTP, significantly protected against the loss of dopamine and prevented the blockade of corticostriatal LTP. These data demonstrate that dopamine plays a role in supporting several forms of striatal plasticity and that GDNF expression via AAV prevents the loss of dopamine and striatal plasticity caused by MPTP. We propose that impairment of striatal plasticity after dopamine denervation plays a role in the symptomology of Parkinson's disease and that AAV expression of neurotrophic factors represents a tenable approach to protecting against or slowing these neurobiological deficits.

Changes in mGlu5 receptor expression in the basal ganglia of reserpinised rats

Dopamine depletion in Parkinson's disease results in a series of pathophysiological changes in the basal ganglia circuitry. Increased release of glutamate plays an important role in this motor disorder, therefore, agents interacting with glutamatergic transmission may have therapeutic potential. In this study we investigated changes in both mRNA expression and the number of binding sites of the mGlu5 receptor in a reserpinised rat model of Parkinson's disease. The in situ hybridisation demonstrated that acute reserpine treatment caused a significant decrease in the expression of mGlu5 receptor mRNA in the rostral and caudal parts of the rat striatum. At the same time, tritium-labelled 2-ethyl-6-(phenylethynyl)-pyridine ([(3)H]MPEP) ligand binding experiments detected a significant increase in the total number of mGlu5 receptors in the same region of the motor loop. These apparently contradictory data can be explained by mGlu5 receptor turnover being down-regulated in reserpinised rats, due possibly to an imbalance in the rates of synthesis/insertion and internalisation/degradation of the receptor. These findings suggest that changes such as these affecting mGlu5 receptors may be involved in the pathophysiological consequences of dopamine depletion in the brain.

Ablation of gene expression of N-methyl-D-aspartate receptor one by antisense oligonucleotides in striatal neurons in culture

In the present study, a twenty-mer antisense oligonucleotide specific for N-methyl-D-aspartate receptor one (ANR1) was applied to striatal neurons in primary cell culture. The ANR1 was found to be specific and nontoxic. Significant reductions in expression of NR1 mRNA and proteins were resulted after a single dose of ANR1 transcripts. Interestingly, there were reductions in total NR1 proteins but two phosphorylated forms of NR1 proteins at serine 896 and 897 residues were not reduced. There was also no change in the pattern of distribution of NR1 immunoreactivity in the striatal neurons. In addition, significant reductions of NMDA-mediated peak inward current were found after application of a higher concentration of ANR1 (20-100 microM) by patch clamp recordings. The present results indicate that ANR1 is a useful agent in reducing NMDA receptor functions. The present data thus provide detailed cellular and molecular mechanisms to explain our previous findings of amelioration of motor symptoms in a rat model of Parkinson's disease. More importantly, application of ANR1 was also found to display neuroprotective effects of striatal neurons against NMDA-induced excitotoxic cell death. The findings have implications in development of new approach in prevention of cell death in neurodegenerative diseases and new treatments for these diseases.

L-3,4-dihydroxyphenylalanine-induced c-Fos expression in the CNS under inhibition of central aromatic L-amino acid decarboxylase

L-3,4-dihydroxyphenylalanine (DOPA) is a neurotransmitter candidate. To map the DOPAergic system functionally, DOPA-induced c-Fos expression was detected under inhibition of central aromatic L-amino acid decarboxylase (AADC). In rats treated with a central AADC inhibitor, DOPA significantly increased the number of c-Fos-positive nuclei in the paraventricular nuclei (PVN) and the nucleus tractus solitarii (NTS), and showed a tendency to increase in the supraoptic nuclei (SON), but not in the striatum. On the other hand, DOPA with a peripheral AADC inhibitor elevated the level of c-Fos-positive nuclei in the four regions, suggesting that DOPA itself induces c-Fos expression in the SON, PVN and NTS. In rats treated with 6-hydroxydopamine (6-OHDA) to lesion the nigrostriatal dopamine (DA) pathway, DOPA significantly induced c-Fos expression in the four regions under the inhibition of peripheral AADC. However, under the inhibition of central AADC, DOPA did not significantly increase the number of c-Fos-positive nuclei in the four regions, suggesting that DOPA at least in part induces c-Fos expression through its conversion to DA. It was likely that the 6-OHDA lesion enhanced the response to DA, but attenuated that to DOPA itself. In conclusion, we proposed that the SON, PVN and NTS include target sites for DOPA itself.

Methamphetamine-induced spectrin proteolysis in the rat striatum

Methamphetamine (METH) is a widely abused psychostimulant. Multiple high doses of METH cause long-term toxicity to dopamine (DA) and serotonin (5-HT) nerve terminals in the brain, as evidenced by decreases in DA and 5-HT content, decreases in tyrosine and tryptophan hydroxylase activities, decreases in DA and 5-HT re-uptake sites, and nerve terminal degeneration. Multiple high doses of METH are known to elicit a rapid increase in DA release and hyperthermia. Although METH also produces a delayed and sustained rise in glutamate, no studies have shown whether METH produces structural evidence of excitotoxicity in striatum, or identified the receptors that mediate this toxicity directly, independent of alterations in METH-induced hyperthermia. These experiments investigated whether METH can cause excitotoxicity as evidenced by cytoskeletal protein breakdown in a glutamate receptor-dependent manner. METH increased calpain-mediated spectrin proteolysis in the rat striatum 5 and 7 days after METH administration without affecting caspase 3-dependent spectrin breakdown. This effect was completely blocked with the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor antagonist, GYKI 52466, but not the NMDA receptor antagonist, MK-801. However, AMPA or NMDA receptor antagonism did not attenuate the METH-induced depletions of the dopamine transporter (DAT). Independent mechanisms involved in mediating spectrin proteolysis and DAT protein loss are discussed.

Striatal plasticity and extrapyramidal motor dysfunction

Knowledge of molecular events contributing to motor dysfunction in Parkinson's disease has advanced rapidly during the past decade. Studies in animal models as well as in patients afflicted by this disorder suggest that the nonphysiologic stimulation of striatal dopamine receptors, first as a result of dopaminergic denervation and later as a consequence of the intermittent high-intensity stimulation produced by standard therapeutic regimens, leads to plastic changes in striatal medium spiny neurons. The clinical appearance of parkinsonism and subsequently of motor response complications is associated with the aberrant activation of signaling cascades within medium spiny neurons that modify the phosphorylation state of their ionotropic glutamatergic receptors. Resultant NMDA and AMPA receptor sensitization augments cortical excitatory input to these spiny efferent neurons, thus altering striatal output in ways that compromise motor function. These findings have already yielded new insight into mechanisms subserving motor memory and synaptic integration as well as accelerated development of novel approaches to the improved treatment of motor disability.