Co-culture with the striatum attenuates N-methyl-D-aspartate cytotoxicity in dopaminergic neurons of rat mesencephalic slice cultures

Corticosterone enhances N-methyl-D-aspartate receptor signaling to promote isolated ventral tegmental area activity in a reconstituted mesolimbic dopamine pathway

Elevations in circulating corticosteroids during periods of stress may influence activity of the mesolimbic dopamine reward pathway by increasing glutamatergic N-methyl-D-aspartate (NMDA) receptor expression and/or function in a glucocorticoid receptor-dependent manner. The current study employed organotypic co-cultures of the ventral tegmental area (VTA) and nucleus accumbens (NAcc) to examine the effects of corticosterone exposure on NMDA receptor-mediated neuronal viability. Co-cultures were pre-exposed to vehicle or corticosterone (CORT; 1 μM) for 5 days prior to a 24 h co-exposure to NMDA (200 μM). Co-cultures pre-exposed to a non-toxic concentration of corticosterone and subsequently NMDA showed significant neurotoxicity in the VTA only. This was evidenced by increases in propidium iodide uptake as well as decreases in immunoreactivity of the neuronal nuclear protein (NeuN). Co-exposure to the NMDA receptor antagonist 2-amino-7-phosphonovaleric acid (APV; 50 μM) or the glucocorticoid receptor (GR) antagonist mifepristone (10 μM) attenuated neurotoxicity. In contrast, the combination of corticosterone and NMDA did not produce any significant effects on either measure within the NAcc. Cultures of the VTA and NAcc maintained without synaptic contact showed no response to CORT or NMDA. These results demonstrate the ability to functionally reconstitute key regions of the mesolimbic reward pathway ex vivo and to reveal a GR-dependent enhancement of NMDA receptor-dependent signaling in the VTA.

Reconstruction and quantitative evaluation of dopaminergic innervation of striatal neurons in dissociated primary cultures

Repairing the nigrostriatal pathway is expected to become a future treatment strategy for Parkinson disease. Our aim is to establish an in vitro model for the quantitative analysis of the nigrostriatal projections of dopaminergic neurons using primary dissociated neruons. To form the mesencephalic cell region, mesencephalic cells derived from rat embryos were plated within an isolation wall, which was removed after cell adhesion to the coverslip. After incubation for 11 days, the dopaminergic neurites extending to the outside of the mesencephalic cell region were mainly axons. Treatment with glial cell line-derived neurotrophic factor for 11 days significantly promoted the outgrowth of dopaminergic axons from the mesencephalic cell region in a concentration-dependent manner. When striatal cells were plated outside the mesencephalic cell region, dopaminergic neurites were remarkably extended to the striatal cell region. Moreover, immunocytochemistry for tyrosine hydroxylase and synaptophysin revealed that dopaminergic axons formed synapses with striatal neurons. By contrast, spinal cells did not increase dopaminergic neurite outgrowth. These results indicate that the present method is valuable for evaluating nigrostriatal projections in vitro.

Effects of R-(-)-BPAP on the expressions of neurotrophins and their receptors in mesencephalic slices

R-(-)-1-(Benzofuran-2-yl)-2-propylaminopentane [R-(-)-BPAP] enhances electric field stimulation-induced release of catecholamine from isolated brain stem and ameliorates motor deficits in rats. We evaluated the effects of R-(-)-BPAP on the expression of brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3), and their receptors, trkB and p75(NTR) in rat mesencephalic slice cultures. Levels of mRNA and protein were measured at 48 h after R-(-)-BPAP treatment by reverse transcription-polymerase chain reaction (RT-PCR) and enzyme-linked immunosorbent assay (ELISA), respectively. R-(-)-BPAP significantly increased the mRNA and protein levels of BDNF, without affecting the level of NT-3 mRNA. In addition, R-(-)-BPAP significantly increased the mRNA level of trkB, but not that of p75(NTR). These effects of R-(-)-BPAP may result in enhanced BDNF/trkB signaling, and could thus underlie the potential neurotrophic and antidepressant actions of this drug.

Electrophysiological characteristic of corticoaccumbens synapses in rat mesolimbic system reconstructed using organotypic slice cultures

The nucleus accumbens (NAc) is a component of the mesolimbic system involved in drug dependence. Activity of nucleus accumbens neurons is modulated by glutamatergic afferents from the prefrontal cortex and by dopaminergic afferents from the ventral tegmental area (VTA). In the present study, we reconstructed the mesolimbic system using organotypic slice cultures and examined the effects of dopaminergic agents on synaptic activity in the prefrontal cortex-nucleus accumbens synapses. A slice of each of the prefrontal cortex, nucleus accumbens and ventral tegmental area in newborn rat, was arranged on a multi-electrode dish (MED) filled with culture medium so that they contacted each other, termed a 'triple culture'. Extracellular recording using microelectrodes on the multi-electrode dish showed that a single electrical stimulation of the prefrontal cortex slice evoked field excitatory postsynaptic potential, and that population spikes occurred spontaneously in the nucleus accumbens area of the triple culture. The amplitude of evoked field excitatory postsynaptic potentials and the frequency of spontaneous population spikes were decreased by glutamatergic antagonists, D(-)-2-amino-5-phosphonovaleric acid and 6-cyano-7-nitroquinoxaline-2,3-dione. The D1-like receptor agonist SKF38393, but not the D2-like receptor agonist quinpirole, reduced both the amplitude of field excitatory postsynaptic potential and frequency of spontaneous population spikes. Cocaine depressed field excitatory postsynaptic potential and this depression was reversed by D1-like receptor antagonist SCH23390, but not by D2-like receptor antagonist sulpiride. These results suggest that evoked field excitatory postsynaptic potentials and spontaneous population spikes were driven by glutamatergic neurons and were subject to exogenous and endogenous dopaminergic modulation in the triple culture that was similar to that shown in in vivo.

Lipopolysaccharide-induced dopaminergic cell death in rat midbrain slice cultures: role of inducible nitric oxide synthase and protection by indomethacin

Glial cell activation associated with inflammatory reaction may contribute to pathogenic processes of neurodegenerative disorders, through production of several cytotoxic molecules. We investigated the consequences of glial activation by interferon-gamma (IFN-gamma)/lipopolysaccharide (LPS) in rat midbrain slice cultures. Application of IFN-gamma followed by LPS caused dopaminergic cell death and accompanying increases in nitrite production and lactate dehydrogenase release. Aminoguanidine, an inhibitor of inducible nitric oxide synthase (iNOS), or SB203580, an inhibitor of p38 mitogen-activated protein kinase, prevented dopaminergic cell loss as well as nitrite production. SB203580 also suppressed expression of iNOS and cyclooxygenase-2 (COX-2) induced by IFN-gamma/LPS. A COX inhibitor indomethacin protected dopaminergic neurons from IFN-gamma/LPS-induced injury, whereas selective COX-2 inhibitors such as NS-398 and nimesulide did not. Notably, indomethacin was able to attenuate neurotoxicity of a nitric oxide (NO) donor. Neutralizing antibodies against tumour necrosis factor-alpha and interleukin-1beta did not inhibit dopaminergic cell death caused by IFN-gamma/LPS, although combined application of these antibodies blocked lactate dehydrogenase release and decrease in the number of non-dopaminergic neurons. These results indicate that iNOS-derived NO plays a crucial role in IFN-gamma/LPS-induced dopaminergic cell death, and that indomethacin exerts protective effect by mechanisms probably related to NO neurotoxicity rather than through COX inhibition.

Paraquat leads to dopaminergic neural vulnerability in organotypic midbrain culture

Paraquat (1,1'-dimethyl-4,4'-bipyridinium, PQ) is a herbicide to possibly induce Parkinson's disease (PD), since a strong correlation has been found between the incidence of the disease and the amount of PQ used. In this study, we examined PQ toxicity in rat organotypic midbrain slice cultures. PQ dose dependently reduced the number of dopaminergic neurons in cultured slices. Since this damage was prevented by GBR-12909, the dopamine transporter could be an initial step of the PQ induced dopaminergic neurotoxicity. The sequential treatments with lower PQ and 1-methyl-4-phenyl pyridinium (MPP+) doses, where each dose alone was not lethal, markedly killed dopamine neurons, suggesting that the exposure of a lower dose of PQ could lead to the vulnerability of dopaminergic neurons. This cell death was prevented by the inhibitors of NMDA, nitric oxide synthase (NOS), cycloheximide and caspase cascade. Neurons expressing NOS were identified inside and around the regions where dopamine neurons were packed. The cell death induced by the sequential treatments with PQ and MPP+ was also rescued by L-deprenyl and dopamine D2/3 agonists. These results strongly support that the constant exposure to low levels of PQ would lead to the vulnerability of dopaminergic neurons in the nigrostriatal system by the excitotoxic pathway, and might potentiate neurodegeneration caused by the exposure of other substances and aging.

Neurotoxicity of an endogenous brain amine, 1-benzyl-1,2,3,4-tetrahydroisoquinoline, in organotypic slice co-culture of mesencephalon and striatum

Organotypic slice co-culture of the ventromedial portion of the mesencephalon and striatum was used to evaluate the neurotoxicity of 1-benzyl-1,2,3,4-tetrahydroisoquinoline, an endogenous brain amine related to Parkinson's disease. 1-Benzyl-1,2,3,4-tetrahydroisoquinoline is specifically increased in the cerebrospinal fluid of patients with Parkinson's disease and induces parkinsonian features in the monkey and mouse. Here, it decreased the dopamine content of the cultured mesencephalon in both dose- (10-100 microM) and time- (24 h to 7 days) dependent manners. This result suggests that the neurotoxicity of 1-benzyl-1,2,3,4-tetrahydroisoquinoline is correlated with the overall exposure (concentration multiplied by exposure time). Culture with 100 microM 1-benzyl-1,2,3,4-tetrahydroisoquinoline for 24 h irreversibly reduced the dopamine content. Furthermore, culture with 100 microM 1-benzyl-1,2,3,4-tetrahydroisoquinoline for 10 days caused morphological changes, including cell body shrinkage and distortion of dendritic morphology, in tyrosine hydroxylase-positive cells in the mesencephalon and reduced the number of cells by half. The increase in lactate dehydrogenase activity in the media produced by 1-benzyl-1,2,3,4-tetrahydroisoquinoline was significant in culture of the mesencephalon alone or its co-culture with striatum, but not in cultures of other brain regions. We suggest that 1-benzyl-1,2,3,4-tetrahydroisoquinoline is toxic to tyrosine hydroxylase-positive cells in the ventral mesencephalon and that it is correlated with the integral of the concentration by time of exposure. Thus a low concentration of 1-benzyl-1,2,3,4-tetrahydroisoquinoline may first induce a decrease in the dopamine content then shrinkage of the cell body, followed by the slow death of dopaminergic neurons over a long period. This is the first report that indicates 1-benzyl-1,2,3,4-tetrahydroisoquinoline exerts neurotoxicity at the cellular level, and reveals in part the character of its neurotoxicity.

Dopaminergic neurons develop axonal projections to their target areas in organotypic co-cultures of the ventral mesencephalon and the striatum/prefrontal cortex

Mesencephalic dopaminergic neurons are known to project to the prefrontal cortex (PFC) and the striatum (STR). Organotypic slice co-cultures of the ventral tegmental area/substantia nigra (VTA/SN)-complex and the PFC or STR, respectively, were used to analyze the cytoarchitectural organization of the VTA/SN-complex and the innervation pattern of the target slices by dopaminergic fibers. After 10-28 days of culturing immunocytochemistry with antibodies against tyrosine hydroxylase (TH) was performed. The VTA/SN-complex revealed in vitro an organization of TH-positive cells similar to those observed in rat brains of comparable age. TH-immunoreactive cells exhibited their typical morphology and formed long processes. No TH-immunolabeled elements were found in single cultures of PFC and STR. Tracing of VTA/SN fibers with biocytin as well as TH-immunostaining showed numerous labeled fibers in the co-cultured slices. Extensive fiber crossing was observed in the co-cultures of the VTA/SN-complex and STR but only a sparse fiber bridge in the co-cultured slices of VTA/SN-complex and PFC. The VTA/SN-complex-PFC system obviously retained several of its in vivo characteristics, e.g. the fiber network in the prefrontal cortical subareas. Our results demonstrate that TH-immunoreactive neurons develop their typical innervation pattern in slice co-cultures of VTA/SN-complex and PFC or STR, respectively. This in vitro approach may be useful for investigations of the dopaminergic function in the VTA/SN-prefrontal pathway.

[Role of nitric oxide in survival and death of neurons]

The prominent pathological feature of the brain in Parkinson's disease is selective degeneration of dopaminergic neurons in the substantia nigra of the midbrain. Glutamate and nitric oxide (NO) are the major effectors of the radical stress that may induce selective loss of dopaminergic neurons. It has been postulated that neurotoxicity induced by glutamate and NO in dopaminergic neurons is regulated by certain endogenous factors. We have reported that estradiol protects dopaminergic neurons against NO-mediated glutamate neurotoxicity by reducing intracellular reactive oxygen species (ROS) levels. We further searched for a candidate for neuroprotective substances with unique structure. From the ether extract of fetal calf serum (FCS), we isolated a novel substance possessing protective activity against neurotoxicity induced by glutamate NO. The compound was a sulfur-containing diterpenoid and showed hydroxyl radical scavenging activity. We further analyzed the change of resistance to excitotoxicity in midbrain dopaminergic neurons in co-culture with the striatum by using a slice culture technique. The results suggested that the generation of NO is involved in NMDA cytotoxicity on dopaminergic neurons and that increased activity of SOD in co-culture renders dopaminergic neurons resistant to NMDA cytotoxicity by preventing peroxynitrite formation. Those findings suggest that regulation of intracellular ROS levels plays a critical role in protecting neurons against NO-mediated radical stress in neurodegenerative disorders.

Requirement of neural activity for the maintenance of dopaminergic neurons in rat midbrain slice cultures

Chronic treatment of organotypic midbrain slice cultures with L-type Ca(2+) channel blocker nicardipine (3-10 microM) or verapamil (10 microM) for 18 days resulted in a drastic decrease in the number of dopaminergic neurons. A voltage-dependent Na(+) channel blocker tetrodotoxin (1 microM) was also effective in decreasing the number of dopaminergic neurons. Concurrent application of forskolin (20 microM) or dibutyryl cyclic AMP (1 mM) counteracted the effects of nicardipine and tetrodotoxin. These results suggest that spontaneous neuronal activity within midbrain slice cultures, causing Ca(2+) influx through L-type Ca(2+) channels that maintains intracellular cyclic AMP levels, is required for the maintenance of dopaminergic neurons.

Superoxide dismutase activity in organotypic midbrain-striatum co-cultures is associated with resistance of dopaminergic neurons to excitotoxicity

We have previously demonstrated that dopaminergic neurons in midbrain-striatum slice co-cultures are more resistant to NMDA cytotoxicity than the same neuronal population in single midbrain slice cultures. Here, we show that dopaminergic neurons in midbrain-striatum co-cultures also exhibit resistance to the cytotoxicity of nitric oxide donors, 2,2'-(hydroxynitrosohydrazono)bis-ethanamine (NOC-18) and 3-morpholinosydnonimine (SIN-1). The cytotoxicity of NMDA (30 microM) in single cultures was significantly attenuated by the nitric oxide synthase (NOS) inhibitor N(omega)-nitro-L-arginine (100 microM), whereas the toxicity in co-cultures was not. The levels of tyrosine residue nitration of tyrosine hydroxylase, a hallmark of the occurence of peroxynitrite anion in dopaminergic neurons, were lower in co-cultures than those in single cultures. Single cultures and co-cultures did not show appreciable differences in the number or distribution of NOS-containing neurons as assessed by NADPH diaphorase histochemistry. On the other hand, midbrain slices cultured with striatal slices showed higher levels of superoxide dismutase (SOD) activity as well as increased protein levels of Cu,Zn-SOD, than midbrain slices cultured alone. These results suggested that the generation of NO is involved in NMDA cytotoxicity on dopaminergic neurons, and that increased activity of SOD in co-cultures renders dopaminergic neurons resistant to NMDA cytotoxicity by preventing the formation of peroxynitrite.

The parkinsonian models: invertebrates to mammals

In contrast to Alzheimer's disease, effective therapeutic options are available for Parkinson's disease. Therapy of dopamine replacement such as levodopa improves the symptoms of this disease, but does not inhibit neurodegeneration in the substantia nigra. Numerous studies have suggested that endogenous and environmental neurotoxins, and oxidative stress may participate in this disease, but the detailed mechanisms are still unclear. Recent genetic studies in familial Parkinson's disease and parkinsonism show several gene mutations. This new information regarding pathogenesis offers novel prospects for therapy. To develop novel neuroprotective drugs, it is necessary to have a model for each type of parkinsonism. This review summarizes current findings regarding parkinsonian models in vertebrates and invertebrates and discusses their value.

sigma receptor ligands attenuate N-methyl-D-aspartate cytotoxicity in dopaminergic neurons of mesencephalic slice cultures

We investigated the potential neuroprotective effects of several sigma receptor ligands in organotypic midbrain slice cultures as an excitotoxicity model system. When challenged with 100-microM N-methyl-D-aspartate (NMDA) for 24 h, dopaminergic neurons in midbrain slice cultures degenerated, and this was prevented by (5R, 10S)-(+)-5-methyl-10,11-dihydro-5H-dibenzo[a,b]-cyclohepten-5, 10-imine (MK-801; 1-10 microM). Concomitant application of ifenprodil (1-10 microM) or haloperidol (1-10 microM), both of which are high-affinity sigma receptor ligands, significantly attenuated the neurotoxicity of 100 microM NMDA. The sigma(1) receptor-selective ligand (+)-N-allylnormetazocine ((+)-SKF 10047; 1-10 microM) was also effective in attenuating the toxicity of NMDA. The effect of R(-)-N-(3-phenyl-1-propyl)-1-phenyl-2-aminopropane hydrochloride ((-)-PPAP), a sigma receptor ligand with negligible affinity for the phencyclidine site of NMDA receptors, was also examined. (-)-PPAP (3-100 microM) caused a concentration-dependent reduction of NMDA cytotoxicity, with significant protection at concentrations of 30 and 100 microM. In contrast, (+)-SKF 10047 (10 microM) and (-)-PPAP (100 microM) showed no protective effects against cell death induced by the Ca(2+) ionophore ionomycin (1-3 microM). These results indicate that sigma receptor ligands attenuate the cytotoxic effects of NMDA on midbrain dopaminergic neurons, possibly via inhibition of NMDA receptor functions.

Deprenyl rescues dopaminergic neurons in organotypic slice cultures of neonatal rat mesencephalon from N-methyl-D-aspartate toxicity

The potential neuroprotective effect of (-)-deprenyl (R-N,alpha-dimethyl-N-2-propynylbenzeneethanamine) against N-methyl-D-aspartate (NMDA) excitotoxicity was investigated on rat mesencephalic dopaminergic neurons in organotypic slice cultures. While 24 h application of NMDA (100 microM) caused a marked decrease in the number of surviving dopaminergic neurons, simultaneous application of (-)-deprenyl significantly attenuated the cytotoxic effect of NMDA. (+)-Deprenyl showed a less potent but still significant protective effect against NMDA insult. Pre-treatment of cultures with (-)-deprenyl conferred no protection against subsequent NMDA insult, suggesting that the protective effect of (-)-deprenyl may be independent of its irreversible inhibitory action on monoamine oxidase B. (-)-Deprenyl was also ineffective in preventing cell death induced by H2O2. These results indicated that (-)-deprenyl protects dopaminergic neurons from NMDA excitotoxicity through a mechanism distinct from monoamine oxidase inhibition or detoxification of reaction oxygen species.