Regulation of N-methyl-D-aspartate-induced toxicity in the neostriatum: a role for metabotropic glutamate receptors?

Activation of mGluR1 negatively modulates glutamate-induced phase shifts of the circadian pacemaker in the mouse suprachiasmatic nucleus

In mammals, photic information delivered to the suprachiasmatic nucleus (SCN) via the retinohypothalamic tract (RHT) plays a crucial role in synchronizing the master circadian clock located in the SCN to the solar cycle. It is well known that glutamate released from the RHT terminals initiates the synchronizing process by activating ionotropic glutamate receptors (iGluRs) on retinorecipient SCN neurons. The potential role of metabotropic glutamate receptors (mGluRs) in modulating this signaling pathway has received less attention. In this study, using extracellular single-unit recordings in mouse SCN slices, we investigated the possible roles of the G_q/11 protein-coupled mGluRs, mGluR1 and mGluR5, in photic resetting. We found that mGluR1 activation in the early night produced phase advances in neural activity rhythms in the SCN, while activation in the late night produced phase delays. In contrast, mGluR5 activation had no significant effect on the phase of these rhythms. Interestingly, mGluR1 activation antagonized phase shifts induced by glutamate through a mechanism that was dependent upon Ca_V1.3 L-type voltage-gated Ca²⁺ channels (VGCCs). While both mGluR1-evoked phase delays and advances were inhibited by knockout (KO) of Ca_V1.3 L-type VGCCs, different signaling pathways appeared to be involved in mediating these effects, with mGluR1 working via protein kinase G in the early night and via protein kinase A signaling in the late night. We conclude that, in the mouse SCN, mGluR1s function to negatively modulate glutamate-evoked phase shifts.

Circadian rhythm disruption in a mouse model of Rett syndrome circadian disruption in RTT

Disturbances in the sleep/wake cycle are prevalent in patients with Rett syndrome (RTT). We sought to determine whether the circadian system is disrupted in a RTT model, Mecp2(-/y) mice. We found that MeCP2 mutants showed decreased strength and precision of daily rhythms of activity coupled with extremely fragmented sleep. The central circadian clock (suprachiasmatic nucleus) exhibited significant reduction in the number of neurons expressing vasoactive intestinal peptide (VIP) as well as compromised spontaneous neural activity. The molecular clockwork was disrupted both centrally in the SCN and in peripheral organs, indicating a general disorganization of the circadian system. Disruption of the molecular clockwork was observed in fibroblasts of RTT patients. Finally, MeCP2 mutant mice were vulnerable to circadian disruption as chronic jet lag accelerated mortality. Our finds suggest an integral role of MeCP2 in the circadian timing system and provides a possible mechanistic explanation for the sleep/wake distrubances observed in RTT patients. The work raises the possibility that RTT patients may benefit from a temporally structured environment.

Microglia-associated granule cell death in the normal adult dentate gyrus

Microglial cells are constantly monitoring the central nervous system for sick or dying cells and pathogens. Previous studies showed that the microglial cells in the dentate gyrus have a heterogeneous morphology with multipolar cells in the hilus and fusiform cells apposed to the granule cell layer both at the hilar and at the molecular layer borders. Although previous studies showed that the microglia in the dentate gyrus were not activated, the data in the present study show dying granule cells apposed by Iba1-immunolabeled microglial cell bodies and their processes both at hilar and at molecular layer borders of the granule cell layer. Initially, these Iba1-labeled microglial cells surround individual, intact granule cell bodies. When small openings in the plasma membrane of granule cells are observed, microglial cells are apposed to these openings. When larger openings in the plasma membrane occur at this site of apposition, the granule cells display watery perikaryal cytoplasm, watery nucleoplasm and damaged organelles. Such morphological features are characteristic of neuronal edema. The data also show that following this localized disintegration of the granule cell’s plasma membrane, the Iba1-labeled microglial cell body is found within the electron-lucent perikaryal cytoplasm of the granule cell, where it is adjacent to the granule cell’s nucleus which is deformed. We propose that granule cells are dying by a novel microglia-associated mechanism that involves lysis of their plasma membranes followed by neuronal edema and nuclear phagocytosis. Based on the morphological evidence, this type of cell death differs from either apoptosis or necrosis.

Symptomatology, pathophysiology, diagnostic work-up, and treatment of Hirschsprung disease in infancy and childhood

In the majority of infants and children with constipation, no obvious cause can be identified. A rare cause of constipation is Hirschsprung disease (HD). HD is characterized by the absence of ganglion cells from the anorectum for a variable length up to the duodenum. The extent of the aganglionic segment varies, but in most patients the lesion does not extend beyond the rectum and sigmoid colon. This review focuses on the passage of meconium, the recognition of HD, and new insights in its pathophysiology and genetics. The authors also provide a summary of the diagnostic evaluation and treatment of HD in infancy and childhood.

Modulation of NMDA receptors in the cerebellum. II. Signaling pathways and physiological modulators regulating NMDA receptor function

NMDA receptors in cerebellum have specific characteristics that make their function and modulation different from those of NMDA receptors in other brain areas. The properties of the NMDA receptor that modulate its function: Subunit composition, post-translational modifications and synaptic localization are summarized in an accompanying article. In this review we summarize how different signaling molecules modulate the function of NMDA receptors. The function of the receptors is modulated by the co-agonists glycine and serine and this modulation is different in cerebellum than in other areas. The NMDA receptor also has binding sites for polyamines that regulate its function. Other signaling molecules that modulate NMDA receptors function are: cAMP, neurotrophic factors such as BDNF, FGF-2 or neuregulins. These and other molecules allow an interplay between NMDA receptors and other receptors for neurotransmitters that may in this way modulate NMDA receptor function. This has been reported, for example, for metabotropic glutamate receptors. The expression and function of NMDA receptor is also modulated by synaptic activity, allowing an adaptation of the receptors function to the external inputs. NMDA receptors modulate important cerebral processes. NMDA receptors in different brain areas seem to modulate different processes. Cerebellar NMDA receptors play a special role in the modulation of motor learning and coordination. This is also briefly reviewed.

Seizures induced in immature rats by homocysteic acid and the associated brain damage are prevented by group II metabotropic glutamate receptor agonist (2R,4R)-4-aminopyrrolidine-2,4-dicarboxylate

The present study has examined the anticonvulsant and neuroprotective effect of group II metabotropic glutamate receptor (mGluR) agonist (2R,4R)-4-aminopyrrolidine-2,4-dicarboxylate (2R,4R-APDC) in the model of seizures induced in immature 12-day-old rats by bilateral intracerebroventricular infusion of dl-homocysteic acid (DL-HCA, 600 nmol/side). For biochemical analyses, rat pups were sacrificed during generalized clonic-tonic seizures, approximately 45-50 min after infusion. Comparable time intervals were used for sacrificing the pups which had received 2R,4R-APDC. Low doses of 2R,4R-APDC (0.05 nmol/side) provided a pronounced anticonvulsant effect which was abolished by pretreatment with a selective group II mGluR antagonist LY341495. Generalized clonic-tonic seizures were completely suppressed and cortical energy metabolite changes which normally accompany these seizures were either normalized (decrease of glucose and glycogen) or markedly reduced (an accumulation of lactate). EEG recordings support the marked anticonvulsant effect of 2R,4R-APDC, nevertheless, this was only partial. In spite of the absence of obvious motor phenomena, isolated spikes or even short periods of partial ictal activity could be observed. Isolated spikes could also be seen in some animals after application of 2R,4R-APDC alone, reflecting most likely subclinical proconvulsant activity of this agonist. The neuroprotective effect of 2R,4R-APDC was evaluated after 24 h and 6 days of survival following DL-HCA-induced seizures. Massive neuronal degeneration, as revealed by Fluoro-Jade B staining, was observed in a number of brain regions following infusion of DL-HCA alone (seizure group), whereas 2R,4R-APDC pretreatment provided substantial neuroprotection. The present findings support the possibility that group II mGluRs are a promising target for a novel approach to treating epilepsy.

DCG-IV but not other group-II metabotropic receptor agonists induces microglial BDNF mRNA expression in the rat striatum. Correlation with neuronal injury

We have previously described a neuroprotective action of (2S,2'R,3'R)-2-(2'3'-dicarboxycyclopropyl)glycine (DCG-IV), an agonist for group-II metabotropic receptors, on dopaminergic nerve terminals against the degeneration induced by 1-methyl-4-phenylpyridinium (MPP+). This effect was accompanied by an up-regulation of brain-derived neurotrophic factor (BDNF) mRNA expression in the rat striatum. We have now analyzed the phenotypic nature of the BDNF mRNA-expressing cells in response to intrastriatal injection of DCG-IV. Dual in situ hybridization and immunohistochemistry revealed that microglial cells but not astrocytes were responsible for this induction. Subsequent analysis demonstrated that this effect was accompanied by striking loss of striatal glutamic acid decarboxylase (GAD) mRNA and massive appearance of internucleosomal DNA fragmentation, a hallmark of apoptosis. A dose-response study demonstrated that doses of DCG-IV as low as 5 nmol was very toxic in terms GAD mRNA and apoptosis. 0.5 nmol of DCG-IV did not induce toxicity at all in terms of GAD mRNA and apoptosis. Activation of group-II metabotropic receptors in striatum with N-Acetyl-Asp-Glu (NAAG; a mGlu3 agonist) and (2R,4R)-4-aminopyrrolidine-2,4-dicarboxylate (a mGlu2 and mGlu3 agonist) did not induce neither loss of GAD mRNA nor appearance of apoptosis (doses up to 20 nmol). In additional experiments, NAAG, in contrast to DCG-IV, failed to protect the striatal dopaminergic system against the degeneration induced by MPP+ as studied by microdialysis. Finally, we studied the mechanism by which DCG-IV is highly toxic. For that, selective antagonists of either metabotropic--(R,S)-alpha-methyl-4-carboxyphenylglycine and LY 341495--or ionotropic (N-methyl-D-aspartate, NMDA)--DL-2-amino-5-phosphonovaleric acid (AP-5) glutamate receptors --were co-administered with DCG-IV. Only AP-5 highly protected the striatum against the degeneration induced by DCG-IV. Since DCG-IV also activates the NMDA receptor at concentrations higher than 3 microM, it is conceivable that a intrastriatal concentration equal or higher than 3 microM after a single striatal injection of 5-20 nmol of DCG-IV. Our findings suggest that much caution must be exerted when testing the numerous neuroprotective effects ascribed to group-II metabotropic receptor activation, in particular when using DCG-IV. We conclude that the neuroprotectant capability of a given compound on a specific system does not exclude the possibility of inducing toxicity on a different one.

Delayed postnatal development of NMDA receptor function in medium-sized neurons of the rat striatum

During early postnatal development, the N-methyl-D-aspartate (NMDA) subtype of glutamate receptor plays a dominant role in excitatory amino acid-mediated synaptic transmission in essentially every brain region that has been examined. In contrast, we have found that in the rat striatum, NMDA receptor-mediated current develops later in the medium-sized neurons (MSNs) than currents mediated by activation of non-NMDA receptors. MSNs were identified using infrared video microscopy, and voltage-clamped in a slice preparation using the whole-cell patch-clamp technique. Intrastriatal stimulation was used to evoke excitatory synaptic currents from slices in animals ranging in age from postnatal day (PND) 5 to 60. Though most cells from animals younger than PND 10 failed to respond to synaptic stimulation, postsynaptic responses were occasionally evoked in cells as young as PND 5. Synaptic currents from cells between PNDs 5 and 7 had a significant contribution due to activation of non- NMDA receptors, as evidenced by sensitivity to the non-NMDA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione and rapidly rising and falling response components. The relative contribution of NMDA receptors increased approximately twofold from the first to the third postnatal week; no further change was observed through PND 60. At the same ages that the NMDA receptors contributed maximally to the synaptic current, the decay time constant of the NMDA receptor-mediated current decreased significantly. The increasing weight of NMDA receptor-mediated current may reflect a change in the number of functional receptors at the synapse since there was no apparent change in the voltage dependence of the current. To more completely examine receptor function early in postnatal development, NMDA and kainate were applied either iontophoretically or in the bath. Iontophoretic application of NMDA onto cells obtained from rats between PNDs 3 and 5 only occasionally evoked current, provided that the membrane was held at depolarized potentials to remove the Mg(2+) block. In contrast, application of kainate consistently evoked a response from cells of the same age group. Bath application of the same agonists provided similar results. Taken together, the present experiments demonstrate that striatal non-NMDA receptor-mediated currents are more mature than NMDA receptor-mediated currents early in development.

Differential sensitivity of medium- and large-sized striatal neurons to NMDA but not kainate receptor activation in the rat

Infrared videomicroscopy and differential interference contrast optics were used to identify medium- and large-sized neurons in striatal slices from young rats. Whole-cell patch-clamp recordings were obtained to compare membrane currents evoked by application of N-methyl-d-aspartate (NMDA) and kainate. Inward currents and current densities induced by NMDA were significantly smaller in large- than in medium-sized striatal neurons. The negative slope conductance for NMDA currents was greater in medium- than in large-sized neurons and more depolarization was required to remove the Mg2+ blockade. In contrast, currents induced by kainate were significantly greater in large-sized neurons whilst current densities were approximately equal in both cell types. Spontaneous excitatory postsynaptic currents occurred frequently in medium-sized neurons but were relatively infrequent in large-sized neurons. Excitatory postsynaptic currents evoked by electrical stimulation were smaller in large- than in medium-sized neurons. A final set of experiments assessed a functional consequence of the differential sensitivity of medium- and large-sized neurons to NMDA. Cell swelling was used to examine changes in somatic area in both neuronal types after prolonged application of NMDA or kainate. NMDA produced a time-dependent increase in somatic area in medium-sized neurons whilst it produced only minimal changes in large interneurons. In contrast, application of kainate produced significant swelling in both medium- and large-sized cells. We hypothesize that reduced sensitivity to NMDA may be due to variations in receptor subunit composition and/or the relative density of receptors in the two cell types. These findings help define the conditions that put neurons at risk for excitotoxic damage in neurological disorders.

Group I metabotropic glutamate receptor activation increases phosphorylation of cAMP response element-binding protein, Elk-1, and extracellular signal-regulated kinases in rat dorsal striatum

Cyclic AMP response element-binding protein (CREB) is a major transcriptional activator at the calcium and cAMP response-element (CaCRE). Phosphorylated (p)CREB facilitates gene expression in striatal neurons. Elk-1 is another transcriptional regulator at the serum response element in the upstream promoter region of the CaCRE. Elk-1 is phosphorylated by extracellular signal-regulated kinases (ERK) and may also contribute to the regulation of gene expression. To evaluate putative roles of group I metabotropic glutamate receptors (mGluRs) in CREB, Elk-1, and ERK phosphorylation, the group I selective agonist, 3,5-dihydroxyphenylglycine (DHPG), was infused into the dorsal striatum at doses of 125, 250, or 500 nmol in freely moving rats. Semi-quantitative immunohistochemistry demonstrated that DHPG significantly increased levels of pCREB, pElk-1, and pERK immunoreactivity of ipsilateral dorsal striatum in a dose dependent manner. The increased immunoreactivity by 500 nmol DHPG was significantly blocked by intrastriatal infusion of the group I selective antagonist, n-phenyl-7-(hydroxyimino)cyclopropa[b]chromen-1a-carboxamide (PHCCC, 25 nmol), but not by the group II/III antagonist, (RS)-alpha-methylserine-o-phosphate monophenyl ester (MSOPPE, 25 nmol). These data suggest that group I mGluR activation is positively linked to signaling cascades resulting in CREB, Elk-1, and ERK phosphorylation in the striatum in vivo.

Cyclic AMP and mitogen-activated protein kinases are required for glutamate-dependent cyclic AMP response element binding protein and Elk-1 phosphorylation in the dorsal striatum in vivo

Dopaminergic and glutamatergic signalling cascades are integrated in striatal medium spiny neurones by cyclic AMP response-element binding protein and Elk-1 phosphorylation. Phosphorylated cyclic AMP response-element binding protein and phosphorylated Elk-1 contribute to c-fos expression by binding to the calcium and cyclic AMP response-element and the serum response element, respectively, in the c-fos promoter. The role of cyclic AMP and mitogen-activated protein kinase signalling cascades in glutamate-induced cyclic AMP response-element binding protein and Elk-1 phosphorylation and Fos expression was investigated using semiquantitative immunocytochemistry in vivo. Intracerebroventricular infusion of the sodium channel blocker, tetrodotoxin, decreased the glutamate-induced increase in phosphorylated cyclic AMP response-element binding protein, phosphorylated Elk-1, and Fos immunoreactivity. Intracerebroventricular infusion of the mitogen-activated and extracellular signal-regulated kinase inhibitor, PD98059, the p38 mitogen-activated protein kinase inhibitor, SB203580, or the cyclic AMP inhibitor, Rp-8-Br-cAMPS, decreased glutamate-induced phosphorylated cyclic AMP response-element binding protein, phosphorylated Elk-1, and Fos immunoreactivity. Simultaneous infusion of glutamate and Sp-8-Br-cAMPS, a cyclic AMP analogue, augmented induction of Fos immunoreactivity but not phosphorylated cyclic AMP response-element binding protein or phosphorylated Elk-1 immunoreactivity. These data indicate that cyclic AMP and mitogen-activated protein kinase signalling cascades are necessary for glutamate to induce cyclic AMP response-element binding protein and Elk-1 phosphorylation and Fos expression in the striatum. Furthermore, neuronal activity plays an important role in glutamate-induced signalling cascades in vivo.

N-methyl-D-aspartate but not glutamate induces the release of hydroxyl radicals in the neonatal rat: modulation by group I metabotropic glutamate receptors

Although they likely involve activation of N-methyl-D-aspartate (NMDA) receptors, the mechanisms giving rise to perinatal hypoxic-ischemic-induced damages remained unclear. The purpose of the present study was to investigate in vivo the mechanisms regulating the glutamate-induced release of toxic hydroxyl radicals (.OH) in neonatal rat. Anesthetized 7-day-old Wistar rat pups bearing a microdialysis cannula implanted in the striatum were perfused with a solution containing salicylate as an.OH trap. Hydroxyl radicals formation was evaluated, after a 3 hr postoperative delay, by measuring the 2,3-DHBA levels by HPLC/EC before, during and over 3 hr after the administration of glutamatergic agonists or antagonists. Administration of NMDA and of ibotenate dramatically increased the efflux of.OH, 17-fold and sixfold, respectively. Glutamate, used at the same concentration did not produce any significant increase in the.OH release and may even decrease this efflux when given at larger concentrations. The NMDA-induced.OH response was partially but progressively reduced by glutamate coinjection and completely blunted by DHPG [(RS)-3, 5-dihydroxyphenylglycine], a group I metabotropic glutamate receptor agonist. Conversely, AIDA [(RS)-1-aminoindan-1,5-dicarboxylic acid], an antagonist of the same receptors, unmasked an.OH response to glutamate. These results are evidence that the glutamate-induced activation of a group I metabotropic glutamate receptor normally protected the neonatal brain from any glutamate activation of NMDA receptor, which otherwise would produce the release of toxic hydroxyl radicals. Targeting group I metabotropic glutamate receptors and/or.OH might contribute to protecting the neonatal brain against perinatal hypoxic-ischemic induced lesions.

Differential distribution of metabotropic glutamate receptors 1a, 1b, and 5 in the rat spinal cord

Metabotropic glutamate receptors (mGluRs) modulate somatosensory, autonomic, and motor functions at spinal levels. mGluR postsynaptic actions over spinal neurons display the pharmacologic characteristics of type I mGluRs; however, the spinal distribution of type I mGluR isoforms remains poorly defined. In this study, the authors describe a differential distribution of immunoreactivity to various type I mGluR isoforms (mGluR1a, mGluR5a,b, and mGluR1b) that suggests a correlation between specific isoforms and particular aspects of spinal cord function. Two different antisera raised against mGluR5a,b detected intense immunoreactivity within nociceptive afferent terminal fields (laminae I and II) and also in autonomic regions (parasympathetic and sympathetic). In contrast, two of three anti-mGluR1a antibodies did not immunostain lamina I or II. Laminae I and II immunostaining by a third anti-mGluR1a antibody was competed by a peptide sequence obtained from a homologous region in mGluR5, suggesting possible cross reactivity in fixed tissue. Autonomic neurons did not express mGluR1a immunoreactivity. All anti-mGluR1a antibodies strongly and specifically immunolabeled dendritic and somatic membranes of neurons in the deep dorsal horn (lamina III-V) and the ventral horn (lamina VI-IX). Somatic motoneurons expressed mGluR1a immunoreactivity but little or no mGluR5 immunoreactivity. Phrenic and pudendal motoneurons expressed the highest level of mGluR1a immunoreactivity in the spinal cord. Intense mGluR1b immunoreactivity was restricted to a few scattered neurons and a prominent group of neurons in lamina X. Lamina II neurons expressed low levels of mGluR1b immunoreactivity. Ultrastructurally, type I mGluR immunoreactivity was found mostly at extrasynaptic sites on the plasma membrane, but it was also found perisynaptically, in the body of the postsynaptic regions or in relation to intracytoplasmic structures.

mGluR7-like receptor and GABA(B) receptor activation enhance neurotoxic effects of N-methyl-D-aspartate in cultured mouse striatal GABAergic neurones

Presynaptic metabotropic glutamate receptors (mGluRs) of group III constitute possible targets for putative neuroprotective drugs acting against glutamate excitotoxic insults. Indeed, in glutamatergic cerebellar granule neurones in culture, high concentrations of L-2-amino-4-phosphonobutyrate (L-AP4, above 0.3 mM, thus activating mGluR7) inhibit NMDA-induced cell death. In contrast, in striatal cultures which are enriched in GABAergic neurones, we show that high concentrations of L-AP4 increased neuronal death in control as well as in NMDA-stimulated cultures. Moreover, similar results were obtained with the GABA(B)R agonist. baclofen. Both the neuroprotective effects in cerebellar granule cells and the neurotoxic effects in striatal neurones were mediated via Gi-Go-coupled mGluRs, suggesting that these effects were probably mediated by mGluR7a or b and GABA(B)R expressed in these neurones. In striatal neurones, we found that L-AP4 and baclofen inhibited both basal and NMDA-stimulated GABA release. These inhibitions of GABA release may be responsible for the increase in basal and NMDA-stimulated neuronal death. Indeed, blockade of GABA(A) receptors with bicuculline increased neuronal death of control and NMDA-treated striatal cultures. Taken together, these results suggest that L-AP4 and baclofen, via mGluR7 and GABA(B)R, reduced the neuroprotective effect of GABA present in striatal cultures acting via GABA(A) receptors. Although caution must be taken when extrapolating from in vitro to in vivo situations, the present experiments and the recent observations that mGluR7 and GABA(B)R are expressed in heterologous synapses, should be taken into consideration when evaluating the neuroprotective action of future mGluR7 specific agonists or GABA(B)R specific antagonists.

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.

Metabotropic glutamate receptor modulation of excitotoxicity in the neostriatum: role of calcium channels

We have previously shown that metabotropic glutamate receptor (mGluR) activation can attenuate N-methyl-d-aspartate (NMDA)-induced excitotoxic injury in the neostriatum both in vivo and in vitro. Our earlier studies made use of the non-subtype selective mGluR agonist 1-amino-cyclopentane-1,3-dicarboxylic acid (tACPD). In the present study, we extended these observations by identifying the subtype of mGluR involved. Using selective mGluR agonists, we provide evidence that the Group II mGluRs are responsible for inhibition of NMDA excitotoxicity in the neostriatum. In addition, we provide evidence that the inhibitory effects of tACPD on excitotoxicity are dependent upon calcium influx as they are blocked by a low calcium solution as well as the broad-spectrum calcium channel blocker cadmium. The tACPD-induced attenuation was also blocked by omega-conotoxin GVIA suggesting participation of N-type calcium channels. Whole cell voltage clamp recordings were made to directly determine the effects of mGluRs on voltage-gated calcium channels in neostriatal neurons. As predicted, both tACPD and the Group II agonist 3C4HPG inhibited calcium currents in neostriatal neurons. Again this effect was blocked by omega-conotoxin GVIA. Overall the results suggest that mGluR regulation of voltage-gated calcium channels can limit NMDA toxicity in the neostriatum.

mGluR7-like metabotropic glutamate receptors inhibit NMDA-mediated excitotoxicity in cultured mouse cerebellar granule neurons

Glutamate-induced glutamate release may be involved in the delayed neuronal death induced by N-methyl-D-aspartate (NMDA). In order to examine a possible modulatory effect of the presynaptic group III mGluRs on glutamate excitotoxicity, the effect of L-2-amino-4-phosphonobutyrate (L-AP4) was examined on NMDA-induced delayed death of mouse cerebellar granule neurons in culture. We found that L-AP4, at high concentration (in the millimolar range), inhibited in a non-competitive manner the NMDA-induced toxicity. This effect was mimicked by high concentration of L-serine-o-phosphate (L-SOP), and was inhibited by pertussis toxin (PTX) indicating the involvement of a Gi/o protein. This suggests the involvement of mGluR7 in the L-AP4 effect, and this was consistent with the detection of both mGluR7 protein and mRNA in these cultured neurons. To examine the mechanism of the L-AP4-induced protection from excitotoxic damage, the effect of L-AP4 on glutamate release was examined. L-AP4 (> or = 1 mM) noncompetitively inhibited by more than 60% the glutamate release induced by NMDA during the insult. We also observed that the 10-min NMDA receptor stimulation resulted in a dramatic increase in the extracellular glutamate concentration reaching 6000% of the control value 24 h after the insult. This large increase was also inhibited when NMDA was applied in the presence of > or = 1 mM L-AP4. Part of the L-AP4-induced protection from excitotoxic damage of granule neurons may therefore result from the inhibition of the vicious cycle: dying cells release glutamate, glutamate induced cell death. The present results add to the hypothesis that presynaptic mGluRs, probably mGluR7, may be the targets of drugs decreasing glutamate release and then neuronal death observed in some pathological situations.

Dopaminergic modulation of early signs of excitotoxicity in visualized rat neostriatal neurons

Cell swelling induced by activation of excitatory amino acid receptors is presumably the first step in a toxic cascade that may ultimately lead to cell death. Previously we showed that bath application of N-methyl-D-aspartate (NMDA) or kainate (KA) produces swelling of neostriatal cells. The present experiments examined modulation of NMDA and KA-induced cell swelling by dopamine (DA) and its receptor agonists. Nomarski optics and infra-red videomicroscopy were utilized to visualize neostriatal medium-sized neurons in thick slices from rat pups (12-18 postnatal days). Increase in somatic cross-sectional area served as the indicator of swelling induced by bath application of glutamate receptor agonists. NMDA induced cell swelling in a dose-dependent manner. Activation of DA receptors in the absence of NMDA did not produce swelling. DA and the D1 receptor agonist SKF 38393, increased the magnitude of swelling produced by NMDA. This effect was reduced in the presence of the D1 receptor antagonist, SCH 23390. In contrast, activation of D2 receptors by quinpirole decreased the magnitude of NMDA-induced cell swelling. DA slightly attenuated cell swelling induced by activation of KA receptors. Quinpirole produced a significant concentration-dependent reduction in KA-induced swelling while SKF38393 increased KA-induced swelling, but only at a low concentration of KA. Together, these results provide additional support for the hypothesis that the direction of DA modulation depends on the glutamate receptor subtype, as well as the DA receptor subtype activated. One possible consequence of these observations is that endogenous DA may be an important contributing factor in the mechanisms of cell death in Huntington's disease.

Antagonists for group I mGluRs attenuate excitotoxic neuronal death in cortical cultures

Activation of ion channel-linked glutamate receptors, especially N-methyl-D-aspartate (NMDA) receptors, mediates the excitotoxic effects of glutamate upon central neurons. We examined the hypothesis that activation of group I metabotropic glutamate receptors (mGluRs) would increase NMDA receptor-mediated cortical neuronal death. Addition of the selective group I mGluR agonists, dihydroxyphenylglycine (DHPG) or trans-azetidine-2,4-dicarboxylic acid (t-ADA) potentiated NMDA-induced neuronal death, and application of the group I mGluR-selective antagonist, aminoindan-1,5-dicarboxylic acid (AIDA), as well as the non-selective antagonists methyl-4-carboxyphenylglycine (MCPG) or 4-carboxyphenylglycine (4CPG) reduced NMDA- and kainate-induced neuronal death in murine cortical cultures. The pro-excitotoxic effect of group I mGluR activation may be mediated largely by enhancement of glutamate release, as DHPG potentiated high potassium-stimulated glutamate release, and the protective effects of both AIDA and MCPG were abolished when NMDA and alpha-amino-3-hydroxy-5-methyl-4-isoxazole proprionic acid (AMPA) receptors were blocked immediately after toxic NMDA receptor overstimulation. The present data support the possibility that antagonizing group I mGluRs may be a useful strategy for attenuating excitotoxic neuronal death in certain disease states.

Enhanced expression of metabotropic glutamate receptor 3 messenger RNA in the rat spinal cord during ultraviolet irradiation induced peripheral inflammation

Metabotropic glutamate receptors are thought to play a role in the development and maintenance of spinal hyperexcitability resulting in hyperalgesia and pain. In this study we have used in situ hybridization to investigate the distribution of metabotropic glutamate receptors mGluR1-7 messenger RNA in the rat spinal cord in a model of inflammatory hyperalgesia. Hyperalgesia was induced in nine-day-old rats by exposure of the left hindpaw to an ultraviolet light source. Lumbar portions of spinal cords were removed from control and ultraviolet-treated animals. In situ hybridization with specific oligonucleotide probes was used to localize metabotropic glutamate receptor messenger RNAs. mGluR1, 3-5 and 7 subtype messenger RNA was detected in the gray matter of the spinal cord with distribution being specific for the different subtypes. A significant increase in the expression of mGluR3 messenger RNA was seen in cells of the dorsal laminae in both sides of the lumbar spinal cord. This increase was most pronounced in laminae II, III and IV but gradually decreased and disappeared by the third day of inflammation. In parallel with this, behavioural experiments revealed mechanical hyperalgesia in both hindlimbs after ultraviolet irradiation. There was no change in mGluR3 messenger RNA expression in the thoracic segments. No changes have been detected in the levels of expression of mGluR 1,2,4,5,7 subtype messenger RNA in spinal cords taken from hyperalgesic animals. These observations show that during ultraviolet irradiation induced inflammation, the synthesis of mGluR3 messenger RNA is altered suggesting that regulation of metabotropic glutamate receptor expression may be instrumental in plastic changes within the spinal cord during the development of hyperalgesia and pain.

The phosphoprotein DARPP-32 mediates cAMP-dependent potentiation of striatal N-methyl-D-aspartate responses

The signal transduction pathway underlying the cAMP-dependent modulation of rat striatal N-methyl-D-aspartate (NMDA) responses was investigated by using the two-electrode voltage-clamp technique. In oocytes injected with rat striatal poly(A)+ mRNA, activation of cAMP-dependent protein kinase (PKA) by forskolin potentiated NMDA responses. Inhibition of protein phosphatase 1 (PP1) and/or protein phosphatase 2A (PP2A) by the specific inhibitor calyculin A occluded the PKA-mediated potentiation of striatal NMDA responses, suggesting that the PKA effect was mediated by inhibition of a protein phosphatase. Coinjection of oocytes with striatal mRNA and antisense oligodeoxynucleotides directed against the protein phosphatase inhibitor DARPP-32 dramatically reduced the PKA enhancement of NMDA responses. NMDA responses recorded from oocytes injected with rat hippocampal poly(A)+ mRNA were not affected by stimulation of PKA. When oocytes were coinjected with rat hippocampal poly(A)+ mRNA plus complementary RNA coding for DARPP-32, NMDA responses were potentiated after stimulation of PKA. The results provide evidence that DARPP-32, which is enriched in the striatum, may participate in the signaling between the two major afferent striatal pathways, the glutamatergic and the dopaminergic projections, by the cAMP-dependent regulation of striatal NMDA currents.

Glutamate metabotropic receptor agonist 1S,3R-ACPD induces internucleosomal DNA fragmentation and cell death in rat striatum

Glutamate metabotropic receptor mediated mechanisms have been implicated in both neuroprotection and neurotoxicity. To characterize these mechanisms further in vivo, the effects of an intrastriatally injected metabotropic receptor agonist, trans-(1S,3R)-1-amino-1,3-cyclopentanedicarboxylic acid (1S,3R-ACPD), were studied alone and together with N-methyl-D-aspartate (NMDA) or kainic acid (KA) receptor agonists on DNA fragmentation and nerve cell death. 1S,3R-ACPD induced internucleosomal DNA fragmentation of striatal cells in a dose-dependent manner. TUNEL and propidium iodide staining showed DNA fragmentation and profound nuclear condensation around the injection site. Fragmented nuclei were occasionally seen under light microscopy. Internucleosomal DNA fragmentation induced by 1S,3R-ACPD was attenuated by the protein synthesis inhibitor cycloheximide as well as by the non-selective and selective metabotropic receptor antagonists L-(+)-2-amino-3-phosphonopionic acid (L-AP3), (RS)-aminoindan-1,5-dicarboxylic acid and (RS)-alpha-methylserine-o-phosphate monophenyl ester, respectively. The 1S,3R-ACPD (100-900 nmol) induced death of striatal neurons was suggested by the reduction in NMDA and D1 dopamine receptors by up to 13% (P < 0.05) and 20% (P < 0.05) as well as by the decline in GAD67 mRNA (25%, P < 0.01) and proenkephalin mRNA levels (35%, P < 0.01). Interestingly, 1S,3R-ACPD attenuated internucleosomal DNA fragmentation induced by NMDA, but potentiated that induced by KA. These results suggest that metabotropic receptor stimulation leads to the death of striatal neurons by a mechanism having the biochemical stigmata of apoptosis. Moreover, metabotropic receptor stimulation evidently exerts opposite effects on pre- or postsynaptic mechanisms contributing to the NMDA and KA-induced apoptotic-like death of these neurons.

Histamine modulates NMDA-dependent swelling in the neostriatum

In the present study, infrared differential interference contrast videomicroscopy was used to examine the effect of histamine on N-methyl-D-aspartate-induced swelling in neostriatal neurons in a brain slice preparation. Histamine caused a concentration-dependent increase in swelling evoked by N-methyl-D-aspartate. By itself, histamine did not cause swelling. Electrical stimulation also caused N-methyl-D-aspartate-dependent swelling which was enhanced by histamine. In addition, histamine was found to enhance N-methyl-D aspartate-induced swelling from postnatal day 7 to 28 but not at postnatal day 3. Finally, this histamine-induced enhancement was prevented by treatment with either the H2 receptor antagonist cimetidine or with the potassium channel blocker tetraethylammonium chloride. Overall, these findings suggest that histamine modulates N-methyl-D-aspartate receptor function in the neostriatum through a H2 receptor-mediated regulation of potassium channels.

Glutamate, but not dopamine, stimulates stress-activated protein kinase and AP-1-mediated transcription in striatal neurons

Drugs that stimulate dopamine and glutamate receptors have been shown to induce the expression of AP-1 proteins (such as c-Fos and c-Jun) in the striatum and to induce binding of these proteins to AP-1 sites on DNA, leading to the hypothesis that AP-1-mediated transcription contributes to the long-term effects of these drugs. To examine this hypothesis, we compared the regulation of AP-1-mediated transcription to the inductions of AP-1-binding activity and genes encoding AP-1 proteins in primary cultures of striatal neurons. Although glutamate, dopamine, and forskolin (an activator of adenylate cyclase) all induce c-fos mRNA and AP-1 binding, we found, surprisingly, that only glutamate induces transcription of a transfected AP-1-driven fusion gene. To explore the basis for this discrepancy, we investigated the possibility that the phosphorylation of c-Jun may also be required for AP-1-mediated transcription in striatal neurons. Glutamate, but neither dopamine nor forskolin, raises the levels of phosphorylated c-Jun as well as the activity of a Jun kinase (SAPK/JNK) in striatal cultures. Both the glutamatergic induction of AP-1-mediated transcription and activation of SAPK/JNK appear to be mediated, at least in part, via NMDA receptors. In striatal neurons, the phosphorylation of AP-1 proteins produced by glutamate may be required to convert AP-1 protein expression and binding to transcriptional activation.

Glutamate, but not dopamine, stimulates stress-activated protein kinase and AP-1-mediated transcription in striatal neurons

Drugs that stimulate dopamine and glutamate receptors have been shown to induce the expression of AP-1 proteins (such as c-Fos and c-Jun) in the striatum and to induce binding of these proteins to AP-1 sites on DNA, leading to the hypothesis that AP-1-mediated transcription contributes to the long-term effects of these drugs. To examine this hypothesis, we compared the regulation of AP-1-mediated transcription to the inductions of AP-1-binding activity and genes encoding AP-1 proteins in primary cultures of striatal neurons. Although glutamate, dopamine, and forskolin (an activator of adenylate cyclase) all induce c-fos mRNA and AP-1 binding, we found, surprisingly, that only glutamate induces transcription of a transfected AP-1-driven fusion gene. To explore the basis for this discrepancy, we investigated the possibility that the phosphorylation of c-Jun may also be required for AP-1-mediated transcription in striatal neurons. Glutamate, but neither dopamine nor forskolin, raises the levels of phosphorylated c-Jun as well as the activity of a Jun kinase (SAPK/JNK) in striatal cultures. Both the glutamatergic induction of AP-1-mediated transcription and activation of SAPK/JNK appear to be mediated, at least in part, via NMDA receptors. In striatal neurons, the phosphorylation of AP-1 proteins produced by glutamate may be required to convert AP-1 protein expression and binding to transcriptional activation.

Glutamate receptor-induced toxicity in neostriatal cells

Infrared differential interference contrast (IR DIC) videomicroscopy was used to measure and characterize cell swelling induced by activation of glutamate receptors (GluR) in a neostriatal brain slice preparation. This swelling is, in many cases, a prelude to necrotic cell death. Activation of N-methyl-D-aspartate (NMDA) and non-NMDA ionotropic GluRs caused cell swelling. The concentration-response relationships and the time courses of the onset of agonist-induced swelling were very similar for NMDA and kainate (KA). However, cells were able to recover from KA but not NMDA-induced swelling. Results from ion substitution experiments suggest that sodium, chloride and to a lesser extent calcium ions play critical roles in this swelling. Heterogeneity in the response to NMDA occurred within cells of the neostriatum. Approximately 15% of the cells did not swell when exposed to NMDA. The magnitude of the NMDA-induced swelling also varied depending on the region of the nervous system. Swelling was greater in the neostriatum and neocortex than in the hippocampus and it did not occur in the suprachiasmatic nucleus. In conclusion, IR DIC videomicroscopy can be used to follow quantitatively the dynamics of GluR-evoked responses in single cells and should be instrumental in determining the factors capable of modifying excitotoxicity.