Expression of N-methyl-D-aspartate receptor subunit NR1 messenger RNA by identified striatal somatostatin cells

Long-Term Dynamic Changes of NMDA Receptors Following an Excitotoxic Challenge

Excitotoxicity is a form of neuronal death characterized by the sustained activation of N-methyl-D-aspartate receptors (NMDARs) triggered by the excitatory neurotransmitter glutamate. NADPH-diaphorase neurons (also known as nNOS (+) neurons) are a subpopulation of aspiny interneurons, largely spared following excitotoxic challenges. Unlike nNOS (−) cells, nNOS (+) neurons fail to generate reactive oxygen species in response to NMDAR activation, a critical divergent step in the excitotoxic cascade. However, additional mechanisms underlying the reduced vulnerability of nNOS (+) neurons to NMDAR-driven neuronal death have not been explored. Using functional, genetic, and molecular analysis in striatal cultures, we indicate that nNOS (+) neurons possess distinct NMDAR properties. These specific features are primarily driven by the peculiar redox milieu of this subpopulation. In addition, we found that nNOS (+) neurons exposed to a pharmacological maneuver set to mimic chronic excitotoxicity alter their responses to NMDAR-mediated challenges. These findings suggest the presence of mechanisms providing long-term dynamic regulation of NMDARs that can have critical implications in neurotoxic settings.

Corticostriatal Plastic Changes in Experimental L-DOPA-Induced Dyskinesia

In Parkinson's disease (PD), alteration of dopamine- (DA-) dependent striatal functions and pulsatile stimulation of DA receptors caused by the discontinuous administration of levodopa (L-DOPA) lead to a complex cascade of events affecting the postsynaptic striatal neurons that might account for the appearance of L-DOPA-induced dyskinesia (LID). Experimental models of LID have been widely used and extensively characterized in rodents and electrophysiological studies provided remarkable insights into the inner mechanisms underlying L-DOPA-induced corticostriatal plastic changes. Here we provide an overview of recent findings that represent a further step into the comprehension of mechanisms underlying maladaptive changes of basal ganglia functions in response to L-DOPA and associated to development of LID.

Somatostatin receptor 1 and 5 double knockout mice mimic neurochemical changes of Huntington's disease transgenic mice

BACKGROUND

Selective degeneration of medium spiny neurons and preservation of medium sized aspiny interneurons in striatum has been implicated in excitotoxicity and pathophysiology of Huntington's disease (HD). However, the molecular mechanism for the selective sparing of medium sized aspiny neurons and vulnerability of projection neurons is still elusive. The pathological characteristic of HD is an extensive reduction of the striatal mass, affecting caudate putamen. Somatostatin (SST) positive neurons are selectively spared in HD and Quinolinic acid/N-methyl-D-aspartic acid induced excitotoxicity, mimic the model of HD. SST plays neuroprotective role in excitotoxicity and the biological effects of SST are mediated by five somatostatin receptor subtypes (SSTR1-5).

METHODS AND FINDINGS

To delineate subtype selective biological responses we have here investigated changes in SSTR1 and 5 double knockout mice brain and compared with HD transgenic mouse model (R6/2). Our study revealed significant loss of dopamine and cAMP regulated phosphoprotein of 32 kDa (DARPP-32) and comparable changes in SST, N-methyl-D-aspartic acid receptors subtypes, calbindin and brain nitric oxide synthase expression as well as in key signaling proteins including calpain, phospho-extracellular-signal-regulated kinases1/2, synapsin-IIa, protein kinase C-α and calcineurin in SSTR1/5(-/-) and R6/2 mice. Conversely, the expression of somatostatin receptor subtypes, enkephalin and phosphatidylinositol 3-kinases were strain specific. SSTR1/5 appears to be important in regulating NMDARs, DARPP-32 and signaling molecules in similar fashion as seen in HD transgenic mice.

CONCLUSIONS

This is the first comprehensive description of disease related changes upon ablation of G- protein coupled receptor gene. Our results indicate that SST and SSTRs might play an important role in regulation of neurodegeneration and targeting this pathway can provide a novel insight in understanding the pathophysiology of Huntington's disease.

Inhibition of phosphodiesterases rescues striatal long-term depression and reduces levodopa-induced dyskinesia

The aim of the present study was to evaluate the role of the nitric oxide/cyclic guanosine monophosphate pathway in corticostriatal long-term depression induction in a model of levodopa-induced dyskinesia in experimental parkinsonism. Moreover, we have also analysed the possibility of targeting striatal phosphodiesterases to reduce levodopa-induced dyskinesia. To study synaptic plasticity in sham-operated rats and in 6-hydroxydopamine lesioned animals chronically treated with therapeutic doses of levodopa, recordings from striatal spiny neurons were taken using either intracellular recordings with sharp electrodes or whole-cell patch clamp techniques. Behavioural analysis of levodopa-induced abnormal involuntary movements was performed before and after the treatment with two different inhibitors of phosphodiesterases, zaprinast and UK-343664. Levodopa-induced dyskinesia was associated with the loss of long-term depression expression at glutamatergic striatal synapses onto spiny neurons. Both zaprinast and UK-343664 were able to rescue the induction of this form of synaptic plasticity via a mechanism requiring the modulation of intracellular cyclic guanosine monophosphate levels. This effect on synaptic plasticity was paralleled by a significant reduction of abnormal movements following intrastriatal injection of phosphodiesterase inhibitors. Our findings suggest that drugs selectively targeting phosphodiesterases can ameliorate levodopa-induced dyskinesia, possibly by restoring physiological synaptic plasticity in the striatum. Future studies exploring the possible therapeutic effects of phosphodiesterase inhibitors in non-human primate models of Parkinson's disease and the involvement of striatal synaptic plasticity in these effects remain necessary to validate this hypothesis.

Somatostatin in medium-sized aspiny interneurons of striatum is responsible for their preservation in quinolinic acid and N-methyl-D-asparate-induced neurotoxicity

Somatostatin (SST) is a multifunctional peptide and involves in several neurodegenerative diseases. N-Methyl-D-asparate (NMDA) receptor agonist quinolinic acid (QUIN)-induced neurotoxicity mimics an experimental model of Huntington's disease that is characterized by the selective preservation of medium-sized aspiny interneurons and degeneration of medium-sized spiny projection neurons in striatum. In QUIN- and NMDA-induced neurotoxicity, increased expression of SST and messenger RNA levels along with SST release in culture medium is generally observed. However, the molecular mechanisms and the functional consequences of increased SST are still obscure. In the present study, the role of SST was determined using immunoneutralization and immunoblockade of SST in cultured striatal neurons upon QUIN- and NMDA-induced neurotoxicity. The immunoblockade of SST with antisense oligonucleotides and immunoabsorption of released SST with specific antibodies potentiate QUIN- and NMDA-induced neuronal cell death. NADPH-diaphorase positive neurons that are selectively spared in several processes of neurodegeneration result in severe damage upon immunoblockade or immunoabsorption of SST. In addition, exogenous SST along with QUIN and NMDA provides selective preservation of projection neurons, which are selectively susceptible in excitotoxicity. Neuroprotective effect of SST is completely blocked by pertussis toxins, suggesting the role of somatostatin receptors. Taken together, these results provide first evidence that the presence of SST is a unique feature for the selective sparing of medium sized aspiny interneurons in excitotoxicity.

Transcriptional dysregulation in striatal projection- and interneurons in a mouse model of Huntington's disease: neuronal selectivity and potential neuroprotective role of HAP1

Transcriptional dysregulation has been described as a central mechanism in the pathogenesis of Huntington's disease (HD), in which medium spiny projection neurons (MSN) selectively degenerate whereas neuronal nitric-oxide-synthase-positive interneurons (nNOS-IN) survive. In order to begin to understand this differential vulnerability we compared mRNA levels of selected genes involved in N-methyl-D-aspartate (NMDA) glutamate receptor and calcium (Ca2+) signaling pathways in MSN and nNOS-IN from 12-week-old R6/2 mice, a transgenic mouse model of HD and wild-type littermates. We undertook a laser capture microdissection (LCM) study to examine the contribution of transcriptional dysregulation in candidate genes involved in these two signaling pathways in discrete populations of striatal neurons. The use of LCM in combination with quantitative real-time polymerase chain reaction (Q-PCR) allowed us to quantify the neuronal abundance of candidate mRNAs. We found different transcriptional alterations in R6/2 neurons for both MSN and nNOS-IN, indicating that global transcriptional dysregulation alone does not account for selective vulnerability. Further, we observed a striking enrichment of several mRNAs in the nNOS-IN population, including that for the NMDA receptor subunit NR2D, the postsynaptic density protein 95 (PSD-95) and the huntingtin-associated protein 1 (HAP1) as well as nitric-oxide-synthase (nNOS) mRNA itself. The higher expression levels of these molecules in nNOS-IN when compared with MSN together with an association of nNOS, NR2D and HAP1 in a protein complex with PSD-95 suggest that these proteins may be involved in protective pathways that contribute to the resistance of this interneuron population to neurodegeneration in HD.

Characterization of striatal cultures with the effect of QUIN and NMDA

The degeneration of selective and specific types of neurons is a characteristic feature in several neurodegenerative disorders. N-methyl-D-aspartate receptor (NMDAR) agonist quinolinic acid (QUIN)-induced excitotoxicity has been implicated in neurodegeneration and mimics Huntington's disease (HD) by the loss of medium-sized spiny projection neurons while sparing medium-sized aspiny interneurons in the striatum. Previous work suggests that somatostatin/neuropeptide Y (SST/NPY)-containing neurons are selectively preserved in HD due to the presence of nicotinamide adenine dinucleotide phosphate diaphorase (NADPH-d) and the lack of NMDAR. In the present study, the distribution of somatostatin (SST), neuropeptide Y (NPY), nitric oxide synthase (nNOS), NMDA receptor type-1 (NR1), and the enzyme NADPH-d was determined in cultured striatal neurons with the effect of QUIN and N-methyl-D-aspartate (NMDA). SST/NPY-positive neurons, which constitute approximately 8-10% of striatal neurons, are selectively spared in QUIN/NMDA-treated cultures. nNOS and NADPH-d-positive neurons, comprising 3.8% of the neuronal population, also exhibit selective resistance to excitotoxicity. Most NR1-positive neurons, which constitute >80% of the total neuronal population, are lost in majority upon treatment with QUIN and NMDA. SST and NADPH-d-positive neurons also colocalize with Cu/Zn superoxide dismutase (Cu/Zn SOD). In conclusion, our results thus demonstrate that SST/NPY/nNOS-positive neurons are selectively spared in NMDA agonist-induced excitotoxicity, which could be attributed to the presence of Cu/Zn SOD and NADPH-d in addition to the low abundance of NMDAR on these neurons.

Involvement of nitric oxide, cyclic GMP and phosphodiesterase 5 in excitatory amino acid and GABA release in the nucleus accumbens evoked by activation of the hippocampal fimbria

It is known that the nucleus accumbens contains all elements of the nitric oxide (NO)-cyclic GMP (cGMP) system but the role of NO in this nucleus is not well understood. We investigated the contribution of the NO-cGMP system in the neurotransmission elicited by hippocampal nerve signals which are propagated to the nucleus accumbens via the fornix/fimbria. This glutamatergic hippocampus-accumbens projection was electrically stimulated for short periods in the urethane-anaesthetized rat. The nucleus accumbens was simultaneously superfused by the push-pull technique with compounds that influence the NO system and the released glutamate, aspartate and GABA were determined in the superfusate. Superfusion of the nucleus accumbens with the NO donor, PAPA/NO, enhanced basal release of the investigated amino acids with a complex concentration dependency. The release of glutamate and aspartate was also increased by the inhibitor of phosphodiesterase 5, UK-114,542. The PAPA/NO-elicited release of glutamate and aspartate was diminished by superfusion with the inhibitor of guanylyl cyclase, NS 2028. Basal release of amino acid transmitters was not influenced by NS 2028 and the NO synthase inhibitor, 7-NINA.Electrical stimulation of the fornix/fimbria increased the outflow of aspartate, glutamate and GABA in the nucleus accumbens. The stimulation-evoked release was abolished by superfusion of the nucleus with tetrodotoxin and strongly diminished by NS 2028, 7-NINA and N(G)-nitro-L-arginine methyl ester (L-name), while PAPA/NO facilitated stimulation-evoked release of these neurotransmitters. UK-114,542 also enhanced the evoked release of glutamate and aspartate while evoked GABA release was not influenced by the phosphodiesterase inhibitor. These findings indicate that NO plays the role of an excitatory transmitter in the nucleus accumbens and that nerve signals from the hippocampus propagated via fornix/fimbria induce NO synthesis in the nucleus accumbens. NO does not exert a tonic influence on basal release but facilitates release of aspartate, glutamate and GABA through increased cGMP synthesis. Phosphodiesterase 5 seems to be involved in the termination of the NO effect in glutamatergic but not in GABAergic neurons.

The nitric oxide system modulates the in vivo release of acetylcholine in the nucleus accumbens induced by stimulation of the hippocampal fornix/fimbria-projection

Nerve signals from the hippocampus to the nucleus accumbens (NAc) are transmitted through a glutamatergic pathway via the fornix/fimbria fibres. The aim of the present study was to investigate whether cholinergic neurons are activated by this projection and whether the nitric oxide (NO) system is also involved in the signal transduction within this nucleus. For this purpose, the NAc of urethane-anaesthetized rats was superfused, by the push-pull technique, with compounds that influence the NO system while the fornix/fimbria was electrically stimulated for short periods. The amount of acetylcholine (ACh) released in the superfusate was then determined. Electrical stimulation of the fornix/fimbria increased the ACh output in the NAc. This effect was abolished by superfusion with tetrodotoxin and decreased by superfusion with the glutamate receptor antagonists AP-5 and DNQX indicating the involvement of action potentials and glutamate. Superfusion with the inhibitor of neuronal NO synthase, NS 2028 also diminished stimulation-evoked ACh release. The NO donor PAPA/NO increased basal release. Simultaneous application of PAPA/NO and electrical stimulation led to an over-additive increase of ACh release. The effect of PAPA/NO on stimulation-evoked release was also abolished by NS 2028. The selective inhibitor of phosphodiesterase type 5 (PDE 5), 5-[2-ethoxy-5-(morpholinylacetyl)phenyl]-1,6-dihydro-1-methyl-3-propyl-7H-pyrazolo[4,3-d]pyrimidin-7-one methanesulphanate monohydrate also enhanced stimulation-induced release of ACh. Our findings indicate, that action potentials propagated by the fornix/fimbria to the NAc release glutamate which increases ACh release predominantly via NMDA receptors. In addition, nitrergic neurons are activated to enhance NO synthesis. The released NO seems to exert, via cGMP, a potent facilitatory role in the transduction and processing of signals from the hippocampus within the NAc, while the PDE 5 decreases the effects of NO.

Changes in NMDA-receptor gene expression are associated with neurotoxicity induced neonatally by glutamate in the rat brain

The N-methyl-D-aspartate receptor (NMDA-R) is fully functional in the rat early in embryogenesis, and diverse neuronal plasticity events are regulated through its activation later in postnatal development. On the other hand, systemic administration of glutamate (Glu) to rats at birth induces neuronal degeneration in glutamatergic central nervous system regions via Glu receptor activation. However, it is not known whether an increase in neonatal Glu levels modifies the gene expression of NMDA-R subunits, or if these putative changes are related to gamma-aminobutyric acid-mediated (GABAergic) neurotransmission. We measured, by means of semi-quantitative reverse transcriptase polymerase chain reaction, changes in gene expression of the NMDA-R subunits: NMDA-R1, NMDA-R 2A and NMDA-R 2B in cerebral cortex (CC), striatum (ST) and hippocampus (HP) in the brains of rats treated neonatally with monosodium L-glutamate (MSG). These studies were supported by histological and quantitative analysis of the glia. Our results showed histological evidence of neuronal damage, and increased glial cell number and activity were detected. This was seen mainly in the ST and HP of MSG-treated animals. Significant increases in NMDA-R1, 2A and 2B subunits gene expression was also observed in ST and HP but not in CC, where only NMDA-R 2B was increased in MSG-treated rats. Our data suggest that increases in Glu levels and activation of Glu-receptors after neonatal administration of MSG induce an increase in glial cell reactivity and important changes in NMDA-R molecular composition, with signs of neuronal damage.

Expression of NMDA receptor subunit mRNAs in neurochemically identified projection and interneurons in the human striatum

N-methyl-D-aspartate (NMDA) receptors are composed of subunits from two families: NR1 and NR2. We used a dual-label in situ hybridization technique to assess the levels of NR1 and NR2A-D messenger ribonucleic acid (mRNA) expressed in projection neurons and interneurons of the human striatum. The neuronal populations were identified with digoxigenin-tagged complementary RNA probes for preproenkephalin (ENK) and substance P (SP) targeted to striatal projection neurons, and somatostatin (SOM), glutamic acid decarboxylase 67 kD (GAD(67)), and choline acetyltransferase (ChAT) targeted to striatal interneurons. Intense NR1 signals were found over all striatal neurons. NR2A signals were high over GAD(67)-positive neurons and intermediate over SP-positive neurons. ENK-positive neurons displayed low NR2A signals, whereas ChAT- and SOM-positive neurons were unlabeled. NR2B signals were intense over all neuronal populations in striatum. Signals for NR2C and NR2D were weak. Only ChAT-positive neurons displayed moderate signals, whereas all other interneurons and projection neurons were unlabeled. Moderate amounts of NR2D signal were detected over SOM- and ChAT-positive neurons; GAD(67)- and SP-positive striatal neurons displayed low and ENK-positive neurons displayed no NR2D hybridization signal. These data suggest that all human striatal neurons have NMDA receptors, but different populations have different subunit compositions that may affect function as well as selective vulnerability.

Ultrastructural and metabolic changes in the neuropeptide Y-containing striatal neuronal network after thermocoagulatory cortical lesion in adult rat

This study examined the effects of unilateral thermocoagulatory cortical lesion on the pattern of neuropeptide Y immunostaining in the rat ipsilateral striatum at 4 and 21 days post-lesion. Light microscopic analysis showed a significant increase in the number of neuropeptide Y-positive neurons vs. control at both time points; paradoxically, the intraneuronal level of labelling significantly decreased at 4 days post-lesion but increased at 21 days post-lesion. Ultrastructural analysis in control condition showed a higher proportion of dendritic versus axonal labelled processes (3.5 ratio); all the neuropeptide Y synaptic terminals formed symmetrical contacts, mostly onto unlabelled dendrites. At 4 days post-lesion, the neuropeptide Y-positive axon density dramatically increased (+576%) without significant change in the labelled dendrite density, vs. control values; the density of neuropeptide Y synaptic terminals increased in parallel by 233%. In addition, a significant proportion of large neuropeptide Y boutons forming asymmetrical synapses onto unlabelled spines were observed. At 21 days post-lesion, densities of neuropeptide Y dendrites, axons, and synaptic terminals increased by 68, 246 and 125%, respectively, vs. control. But, the morphological features of the neuropeptide Y axonal processes and synaptic specializations of the boutons were similar to those observed in control condition. These data (1) raise an important issue regarding the origin of the terminals forming asymmetrical synapses in the striatum, (2) suggest that adaptative changes in the neuropeptide Y neuronal network may be a main component of striatal remodelling resulting from the progressive loss of cortical inputs, and (3) reinforce the view that neuropeptide Y and excitatory amino acid functions may be tightly linked in the striatum.

NMDA and non-NMDA receptor-mediated excitotoxicity are potentiated in cultured striatal neurons by prior chronic depolarization

The excitatory input from cortex and/or thalamus to striatum appears to promote the maturation of glutamate receptors on striatal neurons, but the mechanisms by which it does so have been uncertain. To explore the possibility that the excitatory input to striatum might influence glutamate receptor maturation on striatal neurons, at least in part, by its depolarizing effect on striatal neurons, we examined the influence of chronic KCl depolarization on the development of glutamate receptor-mediated excitotoxic vulnerability and glutamate receptors in cultured striatal neurons. Dissociated striatal neurons from E17 rat embryos were cultured for 2 weeks in Barrett's medium containing either low (3 mM) or high (25 mM) KCl. The vulnerability of these neurons to NMDA receptor agonists (NMDA and quinolinic acid), non-NMDA receptor agonists (AMPA and KA), and a metabotropic glutamate receptor agonist (trans-ACPD) was examined by monitoring cell loss 24 h after a 1-h agonist exposure. We found that high-KCl rearing potentiated the cell loss observed with 500 microM NMDA or 250 microM KA and yielded cell loss with 250 microM AMPA that was not evident under low KCl rearing. In contrast, neither QA up to 5 mM nor trans-ACPD had a significant toxic effect in either KCl group. ELISA revealed that chronic high KCl doubled the abundance of NMDA NR2A/B, AMPA GluR2/3, and KA GluR5-7 receptor subunits on cultured striatal neurons and more than doubled AMPA GluR1 and GluR4 subunits, but had no effect on NMDA NR1 subunit levels. These receptor changes may contribute to the potentiation of NMDA and non-NMDA receptor-mediated excitotoxicity shown by these neurons following chronic high-KCl rearing. Our studies suggest that membrane depolarization produced by corticostriatal and/or thalamostriatal innervation may be required for maturation of glutamate receptors on striatal neurons, and such maturation may be important for expression of NMDA and non-NMDA receptor-mediated excitotoxicity by striatal neurons. Striatal cultures raised under chronically depolarized conditions may, thus, provide a more appropriate culture model to study the role of NMDA or non-NMDA receptor subtypes in excitotoxicity in striatum.

The parafascicular thalamic nucleus but not the prefrontal cortex facilitates the nitric oxide/cyclic GMP pathway in rat striatum

We investigated whether the parafascicular thalamic nucleus and the prefrontal cortex, the two major excitatory inputs to the striatum, modulate the nitric oxide/cyclic GMP pathway in rat striatum. Electrical stimulation (10 pulses of 0.5 ms, 10 V applied at 10 Hz, 140 microA) delivered bilaterally to the parafascicular thalamic nucleus for a total of 4, 10 and 20 min, time-dependently facilitated cyclic GMP output in the dorsal striatum of freely moving rats, assessed by trans-striatal microdialysis. Electrical stimulation to the prefrontal cortex for a total duration of 20 min did not affect striatal cyclic GMP levels. The facilitatory effect observed after electrical stimulation of the parafascicular thalamic nucleus was blocked by co-perfusion with tetrodotoxin, suggesting that the effect is mediated by neuronal process(es). The non-competitive N-methyl-D-aspartate receptor antagonist, dizocilpine maleate (30 microM infused into the dorsal striatum), and the competitive one, 3-[(R)-carboxypiperazin-4-yl]-propyl-phosphonic acid (50 microM infused), but not local perfusion of the alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionic acid antagonist, 6-nitro-7-sulphamoylbenzo(f)quinoxaline-2,3-dione (15 microM perfused locally), abolished the cyclic GMP response in the striatum. The nitric oxide synthase inhibitor, 7-nitroindazole, applied locally (1 mM), blocked the electrically evoked increase in striatal extracellular cyclic GMP. This increase was also prevented by local application (100 and 300 microM) of 1H-(1,2,4)-oxadiazolo-(4,3a)-quinoxalin-1-one, a selective inhibitor of soluble guanylyl cyclase. The results provide direct functional evidence of selective thalamic facilitation of the nitric oxide/cyclic GMP pathway in the dorsal striatum, through activation of N-methyl-D-aspartate receptors.

Modulation of quinolinic acid-induced depletion of striatal NADPH diaphorase and enkephalinergic neurons by inhibition of nitric oxide synthase

The present study was designed to examine the role of nitric oxide (NO) in quinolinic acid (QUIN)-induced depletion of rat striatal nicotinamide adenine dinucleotide phosphate (NADPH) diaphorase and enkephalinergic neurons. Intrastriatal injection of QUIN produced a dose-dependent decrease in NADPH diaphorase and enkephalin positive cells, with cell loss being evident following the injection of 6 and 18 nmol QUIN, respectively. To evaluate the role of NO in QUIN-induced toxicity, animals were pretreated with the non-specific nitric oxide synthase (NOS) inhibitor, Nomega-nitro-l-arginine (l-NAME) or the selective neuronal NOS inhibitor, 7-nitro indazole (7-NI). l-NAME (2x250 mg/kg, i.p. 8 h apart) maximally inhibited striatal NOS activity by 85%, while 7-NI (50 mg/kg, i.p.) maximally inhibited striatal NOS activity by 60%. Pretreatment with l-NAME or 7-NI potentiated the loss of NADPH diaphorase neurons resulting from intrastriatal injection of low doses of QUIN (18 nmol). Neither NOS inhibitor had any effect on the loss of striatal NADPH diaphorase neurons induced by a higher dose of QUIN (24 nmol). In contrast, 7-NI partially prevented the QUIN (18 and 24 nmol)-induced loss of enkephalinergic neurons, while l-NAME had no effect. These results indicate that NO formation may play a role in QUIN-induced loss of enkephalinergic neurons, but not in the loss of NADPH diaphorase neurons.

NMDAR1 glutamate receptor subunit isoforms in neostriatal, neocortical, and hippocampal nitric oxide synthase neurons

Nitric oxide (NO), an unconventional and diffusible neurotransmitter, is synthesized by nitric oxide synthase (NOS). NMDA glutamate receptors are potent regulators of NO synthesis. We have used dual-label immunofluorescence and confocal microscopy to examine forebrain neurons in the rat that contain high levels of neuronal NOS (nNOS) for the presence of the NMDAR1 receptor subunit protein and regions of this protein encoded by three alternative spliced segments of the NMDAR1 mRNA: N1, C1, and C2. In the neostriatum, neocortex, and hippocampus, nNOS-labeled neurons exhibit strong NMDAR1 immunoreactivity (-ir). In all three of these regions, nNOS-positive neurons are characterized by the absence of immunoreactivity for the C1 segment of NMDAR1, whereas C1-ir is abundant in most nNOS-negative neurons. In addition, nNOS-ir neurons exhibit selective staining for the alternative C2' terminus of NMDAR1 that is produced when the C2 segment is absent. These results demonstrate directly that neurons with abundant nNOS-ir contain NMDAR1 receptor subunit proteins and that the NMDAR1 isoforms present in these cells differ from those of most other neurons in these regions. The distinct NMDA receptor phenotype of these nNOS-positive neurons is likely to contribute to both the physiological regulation of NO release by glutamate as well as to NO-mediated excitotoxic injury.

Role of glutamate receptor subtypes in the differential release of somatostatin, neuropeptide Y, and substance P in primary serum-free cultures of striatal neurons

The spiny and aspiny neuronal populations of the striatum display differential vulnerability to the toxic effects of glutamatergic agonists. Substance P-containing spiny neurons appear to be more vulnerable to NMDA-receptor-mediated toxicity and less susceptible to kainate toxicity than the somatostatin- and neuropeptide Y (NPY)-containing aspiny population. We studied whether selective glutamatergic agonists might have similar differential effects on neuropeptide release from the substance P- and somatostatin/NPY-containing neuronal populations. After collection of a baseline sample, striatal neurons in primary culture were treated with one of the following: phosphate-buffered saline, 56 mM potassium chloride (KCl), 100 microM N-methyl-D-aspartate (NMDA), 100 microM quisqualate, 100 microM kainate, or 100 microM glutamate. Baseline and treatment samples were measured by radioimmunoassay for somatostatin, NPY, and substance P. KCl and kainate provoked a selective release of somatostatin and NPY, whereas substance P measured in the same samples showed no response. By contrast, NMDA elicited a selective release of substance P without a similar increase of either somatostatin or NPY. Quisqualate evoked comparable responses in the three peptides. These results indicate that the glutamatergic regulation of somatostatin and NPY release from aspiny striatal neurons in primary culture is preferentially mediated by the kainate receptor, whereas substance P release is selectively mediated by the NMDA receptor. These findings suggest a preferential expression of functional kainate receptors on the aspiny somatostatin/NPY neurons and of NMDA receptors on the substance-P-containing spiny neurons.

Dopamine D1 and D2 receptor gene expression in the striatum in Huntington's disease

The cellular expression of dopamine D1 and D2 receptor mRNAs was investigated in the postmortem human caudate nucleus of control cases and genetically and pathologically confirmed cases of Huntington's disease (HD) by using quantitative in situ hybridization. The HD cases were categorized (0-4) by severity of striatal neuropathology according to the Vonsattel scale. For the HD grade 0 case, a pronounced reduction in the number of D1 and D2 mRNA-positive cells was observed compared with controls; however, the abundance of both receptor mRNAs per remaining cell was within the control range. For D2 receptor mRNA, the number of detectable D2-positive medium-sized cells decreased with increasing pathology; this decrease was accompanied by a gradual reduction in the intensity of D2 signal per cell. By contrast, for D1 receptor mRNA, despite a decrease in the number of D1 mRNA-positive cells detected, the average cellular expression of D1 mRNA was markedly reduced in the HD grade 1 case and then increased (relative to the grade 1 case) with increasing pathology, presumably reflecting the relative survival of D1-expressing striatal interneurons. The implications of these findings for providing further information on the neurodegenerative process in HD are discussed.

Expression of NMDA receptor-1 (NR1) and huntingtin in striatal neurons which colocalize somatostatin, neuropeptide Y, and NADPH diaphorase: a double-label histochemical and immunohistochemical study

The subset of striatal neurons which colocalize SS/NPY/NADPH-d are selectively resistant to neurodegeneration in Huntington's Disease (HD) and to excitotoxic cell death induced experimentally with NMDA receptor (NMDAR) agonists. Here we have analyzed the expression of immunoreactive NMDAR-1 (NR1) subunit (as an index of NMDAR protein) and of huntingtin (the normal product of the HD gene) in primary cultures of rat striatum to see if differential expression of the two antigens in the subset of SS/NPY/NADPH-d and other striatal neurons can explain their selective resistance or vulnerability. Double-label histochemical and immunocytochemical studies were carried out using conventional and confocal laser scanning microscopy to characterize the cellular and subcellular expression of NR1 and SS, or NPY or bNOS, together with NADPH-d histochemistry. The percentages of cultured striatal neurons that were positive for NADPH-d, SS, NPY, bNOS, and NRI were, respectively, 3.8, 8.4, 10.2, 5.1, and 80%. The majority of striatal NADPH-d neurons coexpressed SS and NPY; 17% of SS-producing neurons were strongly positive for NR1; the remaining cells (approximately 80%) exhibited only weak NR1 expression. Comparable data were obtained for NPY-positive neurons, 15% of which colocalized NR1 strongly and 70-80% weakly. By double-label immunofluorescence, huntingtin was nonselectively expressed in virtually all striatal neurons including SS/NPY/NADPH-d neurons. These results show that the majority of striatal SS/NPY/NADPH-d neurons express NR1. The relative abundance of NR1 in SS/NPY/NADPH-d neurons, however, varies between a small subset of neurons that are receptor rich and the remainder that express low levels only and may determine susceptibility to NMDAR-mediated neurotoxicity. Huntingtin is nonselectively expressed in virtually all striatal neurons and does not appear to be a determinant of the selective resistance of normal striatal SS/NPY/NADPH-d neurons to NMDA toxicity.

Phenotypic characterization of neurotensin messenger RNA-expressing cells in the neuroleptic-treated rat striatum: a detailed cellular co-expression study

The chemical phenotype of proneurotensin messenger RNA-expressing cells was determined in the acute haloperidol-treated rat striatum using a combination of (35S)-labelled and alkaline phosphatase-labelled oligonucleotides. Cellular sites of proneurotensin messenger RNA expression were visualized simultaneously on tissue sections processed to reveal cellular sites of preproenkephalin A messenger RNA or the dopamine and adenylate cyclase phosphoprotein-32, messenger RNA. The cellular co-expression of preproenkepahlin A (enkephalin) and preprotachykinin (substance P) messenger RNA was also examined within forebrain structures. Cellular sites of enkephalin (substance P) and dopamine and adenylate cyclase phosphoprotein-32 messenger RNAs were visualized using alkaline phosphatase-labelled oligonucleotides whilst sites of substance P and proneurotensin messenger RNA expression were detected using (35S)-labelled oligos. Cellular sites of enkephalin and dopamine and adenylate cyclase phosphoprotein-32 gene expression were identified microscopically by the concentration of purple alkaline phosphatase reaction product within the cell cytoplasm, whereas sites of substance P and proneurotensin gene expression were identified by the dense clustering of silver grains overlying cells. An intense hybridization signal was detected for all three neuropeptide messenger RNAs in the striatum, the nucleus accumbens and septum. Dopamine and adenylate cyclase phosphoprotein-32 messenger RNA was detected within the neostriatum but not within the septum. In all forebrain regions examined, with the exception of the islands of Calleja, the cellular expression of enkephalin messenger RNA and substance P messenger RNA was discordant; the two neuropeptide messenger RNAs were detected essentially in different cells, although in the striatum and nucleus accumbens occasional isolated cells were detected which contained both hybridization signals; dense clusters of silver grains overlay alkaline phosphatase-positive cells, demonstrating clearly that these dual-labelled cells expressed both messenger RNAs. By contrast, the hybridization signals for proneurotensin and enkephalin, and proneurotensin and dopamine and adenylate cyclase phosphoprotein-32 were generally coincident, at least within the neostriatum; most proneurotensin messenger RNA-positive cells expressed enkephalin messenger RNA and were also positive for dopamine and adenylate cyclase phosphoprotein-32 messenger RNA. However, occasional proneurotensin messenger RNA-positive striatal cells were identified that were single-labelled and did not express enkephalin messenger RNA. Within the septal nucleus, enkephalin messenger RNA and substance P messenger RNA were expressed essentially within segregated cell populations. These studies illustrate further the utility of co-expression techniques for investigating the chemical phenotype of cells within the CNS and demonstrate that the distribution of neuropeptide co-expressing cells is different within different brain regions. That several populations of proneurotensin messenger RNA-positive striatal cells may exist, of which one population is sensitive to haloperidol, co-expresses enkephalin messenger RNA and is positive for dopamine and adenylate cyclase phosphoprotein-32 messenger RNA may be of some significance in neuropsychiatric/neurological disorders given that the translated peptide, neurotensin, is known to influence and interact closely with the dopamine systems.

Ionotropic glutamate receptors and nitric oxide synthesis in the rat striatum

A population of interneurones in the rat striatum contain neuronal nitric oxide synthase (nNOS). The nitric oxide (NO)-cGMP pathway may modulate neurotransmission in this brain area but little is known about the physiological stimulator(s) of nNOS in striatal neurones. Exposure of intact striatal slices to N-methyl-D-aspartate (NMDA) (100 microM, 8 min) caused an almost two-fold increase in cGMP levels (EC50 approximately 25 microM), which was inhibited by the nNOS inhibitor N-omega-L-nitroarginine (IC50 approximately 70 nM) and by removing external Ca2+. Neither (S)-alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) nor the kainate receptor agonist (2S,4R)-4-methylglutamate increased basal cGMP levels. Thus, glutamatergic striatal inputs could stimulate a NO-cGMP pathway in the striatum via NMDA receptors but not via AMPA or kainate receptors.

NMDA and kainate-evoked release of nitric oxide and classical transmitters in the rat striatum: in vivo evidence that nitric oxide may play a neuroprotective role

The effects of N-methyl-D-aspartate (NMDA), kainate, S-alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) and KCl on striatal nitric oxide (NO), acetylcholine (ACh), dopamine (DA), serotonin (5-HT), aspartate (ASP), glutamate (GLU) and gamma-aminobutyric acid (GABA) release were measured in anaesthetized rats in vivo by microdialysis and in vitro in organotypic slice cultures. Local NMDA (1-100 microM) infusion by retrodialysis dose-dependently increased levels of classical transmitters, NO2-, NO3-, citrulline and arginine at similar thresholds (10 microM). Similar patterns of NMDA-evoked (50 microM) release were seen in striatal cultures. NMDA-evoked changes were all calcium-dependent and blocked by NMDA (APV or MK-801) but not AMPA/kainate (DNQX) receptor antagonists, excepting DA which could be prevented by both. In vivo, kainate increased NO2-, NO3-, CIT and ARG levels at 50 and 100 microM but was less potent than NMDA. Kainate also evoked significant ACh, DA and GLU release dose-dependently starting at 1-10 microM whereas 5-HT, ASP and GABA required 50 or 100 microM doses. Kainate effects were inhibited by DNQX, but not by APV, and were calcium-dependent, AMPA failed to alter NO2-, NO3-, CIT or ARG levels at 50 or 100 microM doses but dose-dependently increased ACh and DA. Similar results were seen with kainate (50 microM) and AMPA (50 microM) in vitro. KCl evoked NO2-, NO3-, CIT and ARG release as well as that of the classical transmitters in vivo and in vitro. In vivo administration of the NO synthase inhibitor L-nitroarginine (L-NARG; 100 microM) significantly reduced NO2-, NO3- and CIT levels and prevented NMDA, kainate or KCl-evoked increases. It also potentiated ACh, ASP, GLU and GABA release and reduced that of DA in response to 50 microM NMDA whereas treatment with an NO-donor (SNAP; 10 microM) significantly reduced evoked ACh, ASP and GLU release. The NO synthase inhibitor L-NARG potentiated kainate-evoked ACh release and reduced that of DA, although less potently than NMDA, but it had no effect on KCl-evoked transmitter release. Overall, these results show that both NMDA and kainate increase striatal NO release at similar dose-thresholds as for classical transmitter release suggesting that NO is dynamically released under physiological and not just pathological conditions. Reductions of striatal NO levels also potentiates calcium-dependent transmitter release in response to NMDA and, to a lesser extent, kainate, whereas increasing them reduces it. This is consistent with a role for NO as a neuroprotective agent in this region acting to desensitize NMDA receptors.

Somatostatin receptors in the central nervous system

Somatostatin was first identified chemically in 1973, since when much has been established about its synthesis, storage and release. It has important physiological actions, including a tonic inhibitory effect on growth hormone release from the pituitary. It has other central actions which are not well understood but recent cloning studies have identified at least five different types of cell membrane receptor for somatostatin. The identification of their genes has allowed studies on the distribution of the receptor transcripts in the central nervous system where they show distinct patterns of distribution, although there is evidence to indicate that more than one receptor type can co-exist in a single neuronal cell. Receptor selective radioligands and antibodies are being developed to further probe the exact location of the receptor proteins. This will lead to a better understanding of the functional role of these receptors in the brain and the prospect of determining the role, if any, of somatostatin in CNS disorders and the identification of potentially useful medicines.

Cellular expression of ionotropic glutamate receptor subunits on specific striatal neuron types and its implication for striatal vulnerability in glutamate receptor-mediated excitotoxicity

Glutamate receptors are composed of subtype-specific subunits. Variation in the precise subunit composition of a receptor may result in significant functional differences. Thus, a precise knowledge of subunit composition on striatal neurons is a prerequisite for understanding the selective vulnerability of striatal neurons to excitatory amino acids. In the present study, we used an immunohistochemical double-labelling approach to localize ionotropic glutamate receptor subunits (NMDAR1, GluR1, GluR2/3, GluR4 and GluR5/6/7) on specific striatal neuron populations. Our results showed that striatal cholinergic and somatostatin interneurons were not labelled for the alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionate, receptor subunits GluR1, GluR2/3 and GluR4. Most cholinergic and somatostatin interneurons (83.3% to 100%), however, were double-labelled for the N-methyl-D-aspartate receptor subunit NR1 and kainic acid receptor subunits GluR5/6/7. All parvalbumin interneurons were labelled for GluR1 and GluR4, and 96% GluR1 positive and 95% GluR4 positive neurons were also double-labelled as parvalbumin interneurons. About half of all parvalbumin interneurons co-localized with GluR2/3, and over 97% were labelled for NR1 and GluR5/6/7. Among striatal projection neurons, enkephalin-positive (mainly striatopallidal) neurons, striatonigral neurons (mainly substance P-positive) and calbindin-positive matrix neurons were not immunostained for GluR1 or GluR4. In contrast, 95% to 100% of each of these types of projection neurons were double-labelled for NR1, GluR2/3 and GluR5/6/7. Our results demonstrate that striatal neuron types differ in their expression of ionotropic glutamate receptor subunits and subtypes. The clear difference between striatal interneurons and projection neurons in ionotropic glutamate receptor subtypes/subunits supports the idea that differential glutamate receptor expression mechanism may account for the selective vulnerability of striatal projection neurons to excitotoxicity, and that glutamate receptor-mediated excitotoxicity may be involved in the striatal neurodegenerative diseases.

Decreased neuronal nitric oxide synthase messenger RNA and somatostatin messenger RNA in the striatum of Huntington's disease

The cellular abundance of neuronal nitric oxide synthase and somatostatin messenger RNAs was compared in the caudate nucleus, putamen and sensorimotor cortex of Huntington's disease and control cases. Neuronal nitric oxide synthase messenger RNA was significantly decreased in the caudate nucleus and putamen, but not in the sensorimotor cortex in Huntington's disease; the decrease in neuronal nitric oxide synthase messenger RNA became more pronounced with the severity of the disease. Somatostatin gene expression was significantly decreased in the dorsal putamen in Huntington's disease, but was essentially unchanged in all other regions examined. The density of neurons expressing detectable levels of neuronal nitric oxide synthase messenger RNA was reduced in the striata of Huntington's disease cases with advanced pathology; the density of neurons expressing detectable levels of somatostatin messenger RNA was similar in control and Huntington's disease cases. Neuropeptide Y-, somatostatin- and NADPH-diaphorase-positive neurons were consistently present throughout the striatum across all the grades of the disease. Neuronal nitric oxide synthase and NADPH-diaphorase activity (a histochemical marker for nitric oxide synthase-containing neurons) co-localize with somatostatin and neuropeptide Y in interneurons in the human striatum and cerebral cortex. Although the neurodegeneration associated with Huntington's disease is most evident in the striatum (particularly the dorsal regions), neuronal nitric oxide synthase/neuropeptide Y/somatostatin interneurons are relatively spared. Nitric oxide released by neuronal nitric oxide synthase-containing neurons may mediate glutamate-induced excitotoxic cell death, a mechanism proposed to be instrumental in causing the neurodegeneration seen in Huntington's disease. The results described here suggest that although the population of interneurons containing somatostatin, neuropeptide Y and neuronal nitric oxide synthase do survive in the striatum in Huntington's disease they are damaged during the course of the disease. The results also show that the reduction in neuronal nitric oxide synthase and somatostatin messenger RNAs is most pronounced in the more severely affected dorsal regions of the striatum. Furthermore, the loss of neuronal nitric oxide messenger RNA becomes more pronounced with the severity of the disease; thus implying a down-regulation in neuronal nitric oxide synthase messenger RNA synthesis, and potentially neuronal nitric oxide synthase protein levels, in Huntington's disease.

Striatal interneurones: chemical, physiological and morphological characterization

The neostriatum is the largest component of the basal ganglia, and the main recipient of afferents to the basal ganglia from the cerebral cortex and thalamus. Studies of the cellular organization of the neostriatum have focused upon the spiny projection neurones, which represent the vast majority of neurones, but the identity and functions of interneurones in this structure have remained enigmatic despite decades of study. Recently, the discovery of cytochemical markers that are specific for each of the major classes of striatal interneurones, and the combination of this with intracellular recording and staining, has revealed the identities of interneurones and some of their functional characteristics in a way that could not have been imagined by the classical morphologists. These methods also suggest some possible modes of action of interneurones in the neostriatal circuitry.

Effects of striatal excitotoxicity on huntingtin-like immunoreactivity

The relationship between the specific neuronal loss observed in Huntington's disease and the mutation in the IT15 gene responsible for this disease remains obscure. Using an antipeptide antibody against amino acids 3114-3141 of the human huntington protein, we demonstrate that striatal injection of quinolinic acid in mice induces increased immunoreactivity for huntington in some remaining neurons but not in glial cells. This increase is apparent in both neuronal cell bodies and in cell processes in the white matter six hours after excitotoxic challenge. This finding suggests that huntington may be involved in the response to excitotoxic stress in these neurons.

Differential expression of mGluR5 metabotropic glutamate receptor mRNA by rat striatal neurons

Metabotropic glutamate receptors (mGluRs) mediate the effects of glutamate neurotransmission on intracellular second messenger systems. Among the seven distinct mGluR receptor isoforms currently identified, the mGluR5 isoform is expressed particularly prominently in the striatum, where it may contribute to neuronal plasticity, motor behaviors, and excitotoxic injury. mGluR5 mRNA expression in striatal enkephalinergic, somatostatinergic, and cholinergic neurons was examined using double label in situ hybridization techniques. mGluR5 expression is abundant in a large number of medium-sized striatal cells but is absent in a significant minority of neurons. Double label in situ hybridization with 35S-dATP- and digoxygenin-dUTP-tailed oligonucleotide probes demonstrated that mGluR5 message is highly expressed by enkephalinergic striatal neurons but is not detectable in cholinergic or somatostatin interneurons. In addition, some nonenkephalin, presumably substance P, neurons were also strongly labeled for mGluR5. The differential expression of mGluR5 in striatal projection neurons vs. interneurons may contribute to the selective vulnerability of these neurons to disease processes.

Repeated asphyxia causes loss of striatal projection neurons in the fetal sheep brain

Repeated episodes of cerebral hypoxia-ischemia can cause primarily striatal neuronal loss in the developing brain. We investigated the effect of repeated episodes of asphyxia on specific neuronal sub-populations of the basal ganglia in late-gestation fetal sheep. Asphyxia was induced in 10 fetal sheep (118-126 days gestation) by occluding the umbilical cord for 5 min. This procedure was repeated four times at 30 min intervals and the brains were fixed 3 days later for histopathology. Immunohistochemical markers were used to identify various populations of neurons in the striatum. Antibodies to calbindin were used to stain the GABAergic medium-sized striatal projection neurons and antibodies to somatostatin and parvalbumin to identify striatal interneurons. Striatal projection neurons to the globus pallidus were recognized by enkephalin immunoreactivity, while the striatonigral terminals were identified in the substantia nigra pars reticulata by substance P immunohistochemical labelling. The results showed a marked loss of calbindin staining in the striatum, evident by both reduced cell numbers and a decrease in neuropil staining. The number of parvalbumin immunoreactive cells was also reduced in the striatum, while somatostatin interneurons were selectively preserved. In addition, immunostaining for enkephalin in the globus pallidus and for substance P in the substantia nigra was markedly reduced. These results show that the stiatal GABAergic medium-sized projection neurons are severely affected by recurrent episodes of asphyxia. These findings are confirmed and extended by the results demonstrating that both the enkephalin/GABA striatopallidal and the substance P/GABA stiatonigral pathways are affected. The results of this study therefore suggest that the efferent striatal projections to the globus pallidus and to the substantia nigra may be involved in asphyxial episodes resulting in cerebral palsy.

Expression patterns of a glutamate-binding protein in the rat central nervous system: comparison with N-methyl-D-aspartate receptor subunit 1 in rat

Using radioactive in situ hybridization histochemistry, we examined the topographical patterns of expression of the messenger RNA encoding a glutamate-binding protein (N-methyl-D-aspartate receptor glutamate-binding protein in rat; NMDARgbs) in the central nervous system of the rat. Expression patterns of N-methyl-D-aspartate receptor glutamate-binding protein were compared with those of N-methyl-D-aspartate receptor subunit 1 (NMDAR1) of the N-methyl-D-aspartate receptor on adjacent sections. N-methyl-D-aspartate receptor glutamate-binding protein is not expressed in glial cells. The expression of both N-methyl-D-aspartate receptor glutamate-binding protein and N-methyl-D-aspartate receptor subunit 1 was observed in virtually all neurons throughout the central nervous system. The mean level of N-methyl-D-aspartate receptor subunit 1 expression was higher than that of N-methyl-D-aspartate receptor glutamate-binding protein. Similar topographical patterns of expression of N-methyl-D-aspartate receptor glutamate-binding protein and N-methyl-D-aspartate receptor were observed in most regions, except in discrete thalamic, hypothalamic and brainstem nuclei. Concomitantly for N-methyl-D-aspartate receptor glutamate-binding protein and N-methyl-D-aspartate receptor subunit 1, the highest expression levels were distributed in the mitral layer of main and accessory olfactory bulbs, granule cell layer of the dentate gyrus, polymorphic and pyramidal layers of CA1-3 fields of Ammon's horn. A slightly less prominent expression was observed in the glomerular and granule cell layers of main and accessory olfactory bulbs, anterior olfactory nucleus, layer 2 of piriform cortex, olfactory tubercle and taenia tecta. In the cerebellum, the prominent level of N-methyl-D-aspartate receptor glutamate-binding protein expression was slightly higher in the Purkinje cell layer than in the granule cell layer, an opposite pattern being observed for N-methyl-D-aspartate receptor subunit 1. A moderately high expression level of both messenger RNAs was observed in the medial septal nucleus, nucleus of the diagonal band of Broca, dorsal part of the endopiriform nucleus, and in the anteroventral and anterolateral parts of the bed nucleus of the stria terminalis. In the neocortex, the mean expression level of N-methyl-D-aspartate receptor glutamate-binding protein is moderate, while the mean level of N-methyl-D-aspartate receptor subunit 1 expression is high. With both probes, layer IV is slightly less labeled than the other layers.(ABSTRACT TRUNCATED AT 400 WORDS)