Disturbance of Metabotropic Glutamate Receptor-Mediated Long-Term Depression (mGlu-LTD) of Excitatory Synaptic Transmission in the Rat Hippocampus After Prenatal Immune Challenge.

Maternal immune challenge has proved to induce moderate to severe behavioral disabilities in the offspring. Cognitive/behavioral deficits are supported by changes in synaptic plasticity in different brain areas. We have reported previously that prenatal exposure to bacterial LPS could induce inhibition of hippocampal long-term potentiation (LTP) in the CA1 area of the juvenile/adult male offspring associated with spatial learning inabilities. Nevertheless, deficits in plasticity could be observed at earlier stages as shown by the early loss of long-term depression (LTD) in immature animals. Moreover, aberrant forms of plasticity were also evidenced such as the transient occurrence of LTP instead of LTD in 15-25 day-old animals. This switch from LTD to LTP seemed to involve the activation of metabotropic glutamate receptor subtype 1 and 5 (mGlu1/5). We have thus investigated here whether the long-term depression elicited by the direct activation of these receptors (mGlu-LTD) with a selective agonist was also disturbed after prenatal stress. We find that in prenatally stressed rats, mGlu1/5 stimulation elicits long-term potentiation (mGlu-LTP) independently of N-methyl-D-aspartate receptors. Both mGlu5 and mGlu1 receptors are involved in this switch of plasticity. Moreover, this mGlu-LTP is still observed at later developmental stages than previously reported, i.e. after 25 day-old. In addition, increasing synaptic GABA with tiagabine tends to inhibit mGlu-LTP occurrence. By contrast, long-term depression induced with the activation of CB1 cannabinoid receptor is unaffected by prenatal stress. Therefore, prenatal stress drastically alters mGlu1/5-associated plasticity throughout development. MGlu-mediated plasticity is an interesting parameter to probe the long-lasting deficits reported in this model.

Long-Lasting Impairment of mGluR5-Activated Intracellular Pathways in the Striatum After Withdrawal of Cocaine Self-Administration.

Background: Cocaine addiction continues to be a major heath concern, and despite public health intervention there is a lack of efficient pharmacological treatment options. A newly identified potential target are the group I metabotropic glutamate receptors, with allosteric modulators showing particular promise. Methods: We evaluated the capacity of group I metabotropic glutamate receptors to induce functional responses in ex vivo striatal slices from rats with (1) acute cocaine self-administration, (2) chronic cocaine self-administration, and (3) 60 days cocaine self-administration withdrawal by Western blot and extracellular recordings of synaptic transmission. Results: We found that striatal group I metabotropic glutamate receptors are the principal mediator of the mGluR1/5 agonist (RS)-3,5-dihydroxyphenylglycine-induced cAMP responsive-element binding protein phosphorylation. Both acute and chronic cocaine self-administration blunted group I metabotropic glutamate receptor effects on cAMP responsive-element binding protein phosphorylation in the striatum, which correlated with the capacity to induce long-term depression, an effect that was maintained 60 days after chronic cocaine self-administration withdrawal. In the nucleus accumbens, the principal brain region mediating the rewarding effects of drugs, chronic cocaine self-administration blunted group I metabotropic glutamate receptor stimulation of extracellular signal-regulated protein kinases 1/2 and cAMP responsive-element binding protein. Interestingly, the group I metabotropic glutamate receptor antagonist/inverse-agonist, 2-methyl-6-(phenylethynyl)pyridine hydrochloride, led to a specific increase in cAMP responsive-element binding protein phosphorylation after chronic cocaine self-administration, specifically in the nucleus accumbens, but not in the striatum. Conclusions: Prolonged cocaine self-administration, through withdrawal, leads to a blunting of group I metabotropic glutamate receptor responses in the striatum. In addition, specifically in the accumbens, group I metabotropic glutamate receptor signaling to cAMP responsive-element binding protein shifts from an agonist-induced to an antagonist-induced cAMP responsive-element binding protein phosphorylation.

MT5-MMP is a new pro-amyloidogenic proteinase that promotes amyloid pathology and cognitive decline in a transgenic mouse model of Alzheimer's disease.

Membrane-type 5-matrix metalloproteinase (MT5-MMP) is a proteinase mainly expressed in the nervous system with emerging roles in brain pathophysiology. The implication of MT5-MMP in Alzheimer's disease (AD), notably its interplay with the amyloidogenic process, remains elusive. Accordingly, we crossed the genetically engineered 5xFAD mouse model of AD with MT5-MMP-deficient mice and examined the impact of MT5-MMP deficiency in bigenic 5xFAD/MT5-MMP(-/-) mice. At early stages (4 months) of the pathology, the levels of amyloid beta peptide (Aß) and its amyloid precursor protein (APP) C-terminal fragment C99 were largely reduced in the cortex and hippocampus of 5xFAD/MT5-MMP(-/-), compared to 5xFAD mice. Reduced amyloidosis in bigenic mice was concomitant with decreased glial reactivity and interleukin-1ß (IL-1ß) levels, and the preservation of long-term potentiation (LTP) and spatial learning, without changes in the activity of α-, ß- and γ-secretases. The positive impact of MT5-MMP deficiency was still noticeable at 16 months of age, as illustrated by reduced amyloid burden and gliosis, and a better preservation of the cortical neuronal network and synaptophysin levels in bigenic mice. MT5-MMP expressed in HEKswe cells colocalized and co-immunoprecipitated with APP and significantly increased the levels of Aß and C99. MT5-MMP also promoted the release of a soluble APP fragment of 95 kDa (sAPP95) in HEKswe cells. sAPP95 levels were significantly reduced in brain homogenates of 5xFAD/MT5-MMP(-/-) mice, supporting altogether the idea that MT5-MMP influences APP processing. MT5-MMP emerges as a new pro-amyloidogenic regulator of APP metabolism, whose deficiency alleviates amyloid pathology, neuroinflammation and cognitive decline.

Involvement of PKA and ERK pathways in ghrelin-induced long-lasting potentiation of excitatory synaptic transmission in the CA1 area of rat hippocampus.

Acute effects of ghrelin on excitatory synaptic transmission were evaluated on hippocampal CA1 synapses. Ghrelin triggered an enduring enhancement of synaptic transmission independently of NMDA receptor activation and probably via postsynaptic modifications. This ghrelin-mediated potentiation resulted from the activation of GHS-R1a receptors as it was mimicked by the selective agonist JMV1843 and blocked by the selective antagonist JMV2959. This potentiation also required the activation of PKA and ERK pathways to occur as it was inhibited by KT5720 and U0126, respectively. Moreover it most probably involved Ca(2+) influxes as both ghrelin and JMV1843 elicited intracellular Ca(2+) increases, which were dependent on the presence of extracellular Ca(2+) and mediated by L-type Ca(2+) channels opening. In addition, ghrelin potentiated AMPA receptor-mediated [Ca(2+) ]i increases while decreasing NMDA receptor-mediated ones. Thus the potentiation of synaptic transmission by GHS-R1a at hippocampal CA1 excitatory synapses probably results from postsynaptic mechanisms involving PKA and ERK activation, which are producing long-lasting enhancement of AMPA receptor-mediated responses.

Tiagabine improves hippocampal long-term depression in rat pups subjected to prenatal inflammation.

Maternal inflammation during pregnancy is associated with the later development of cognitive and behavioral impairment in the offspring, reminiscent of the traits of schizophrenia or autism spectrum disorders. Hippocampal long-term potentiation and long-term depression of glutamatergic synapses are respectively involved in memory formation and consolidation. In male rats, maternal inflammation with lipopolysaccharide (LPS) led to a premature loss of long-term depression, occurring between 12 and 25 postnatal days instead of after the first postnatal month, and aberrant occurrence of long-term potentiation. We hypothesized this would be related to GABAergic system impairment. Sprague Dawley rats received either LPS or isotonic saline ip on gestational day 19. Male offspring's hippocampus was studied between 12 and 25 postnatal days. Morphological and functional analyses demonstrated that prenatal LPS triggered a deficit of hippocampal GABAergic interneurons, associated with presynaptic GABAergic transmission deficiency in male offspring. Increasing ambient GABA by impairing GABA reuptake with tiagabine did not interact with the low frequency-induced long-term depression in control animals but fully prevented its impairment in male offspring of LPS-challenged dams. Tiagabine furthermore prevented the aberrant occurrence of paired-pulse triggered long-term potentiation in these rats. Deficiency in GABA seems to be central to the dysregulation of synaptic plasticity observed in juvenile in utero LPS-challenged rats. Modulating GABAergic tone may be a possible therapeutic strategy at this developmental stage.

Area-specific alterations of synaptic plasticity in the 5XFAD mouse model of Alzheimer's disease: dissociation between somatosensory cortex and hippocampus.

Transgenic mouse models of Alzheimer's disease (AD) that overproduce the amyloid beta peptide (Aß) have highlighted impairments of hippocampal long-term synaptic plasticity associated with the progression of the disease. Here we examined whether the characteristics of one of the hallmarks of AD, i.e. Aß deposition, in both the somatosensory cortex and the hippocampus, correlated with specific losses of synaptic plasticity in these areas. For this, we evaluated the occurrence of long-term potentiation (LTP) in the cortex and the hippocampus of 6-month old 5xFAD transgenic mice that exhibited massive Aß deposition in both regions but with different features: in cortical areas a majority of Aß deposits comprised a dense core surrounded by a diffuse corona while such kind of Aß deposition was less frequently observed in the hippocampus. In order to simultaneously monitor synaptic changes in both areas, we developed a method based on the use of Multi-Electrode Arrays (MEA). When compared with wild-type (WT) mice, basal transmission was significantly reduced in both areas in 5xFAD mice, while short-term synaptic plasticity was unaffected. The induction of long-term changes of synaptic transmission by different protocols revealed that in 5xFAD mice, LTP in the layer 5 of the somatosensory cortex was more severely impaired than LTP triggered in the CA1 area of the hippocampus. We conclude that cortical plasticity is deficient in the 5xFAD model and that this deficit could be correlated with the proportion of diffuse plaques in 5xFAD mice.

N-acetyl-cysteine prevents pyramidal cell disarray and reelin-immunoreactive neuron deficiency in CA3 after prenatal immune challenge in rats.

BACKGROUND: Prenatal infection is a major risk factor for the occurrence of neuropsychiatric disorders. These have been associated with hippocampal neuroanatomical and functional abnormalities. In the present study, we evaluated the occurrence of pyramidal cell disarray and reelin neuronal deficit in the hippocampus, and the protective role of N-acetyl-cysteine (NAC) in a rodent experimental model of prenatal immune challenge. METHODS: Sprague-Dawley rats received either 500 µg/kg of endotoxin (lipopolysaccharide, LPS) or 2 ml/kg of isotonic saline by i.p. injection on day 19 of gestation. After LPS injection, rats were or were not maintained on a preventive treatment of NAC (5 g/l in tap water), up to delivery. The pyramidal cell orientation and the number and type of reelin-expressing neurons were determined in male offspring. RESULTS: Prenatal LPS challenge led to permanent pyramidal cell disarray and to an early and transient decreased density of reelin-immunoreactive neurons. These disorders, more pronounced in the CA3 area, were prevented by NAC. CONCLUSION: Hippocampal cytoarchitectural alterations and reelin deficiency may be involved in the development of remote cognitive impairments in this model. The antioxidant NAC is an efficient neuroprotective drug that underlines the role of oxidative stress in prenatal infection and associated neurodevelopmental damage.

Synthesis and characterization of a cyclooctapeptide analogue of ω-agatoxin IVB enhancing the activity of Cav2.1 calcium channels activity in cultured hippocampal neurons.

The structure of the toxin ω-agatoxin IVB, extracted from the venom of funnel-web spider Agelenopsis aperta, is an important lead structure when considering the design of modulators of synaptic transmission which largely involves P/Q-type (CaV2.1) voltage gated calcium channels (VGCC) at central synapses. Focusing on the loop 2 of the ω-agatoxin IVB that seems to be the most preeminent interacting domain of the toxin with the CaV2.1 VGCC, cyclooctapeptides mimicking this loop were synthesized. While (14)Trp is essential for the binding of the neurotoxin to the CaV2.1 VGCC, the substitution of the (12)Cys for a glycidyl residue led to a cyclooctapeptide named EP14 able to enhance CaV2.1 VGCC-associated currents measured with patch-clamp recordings and to evoke ω-agatoxin IVA-sensitive intracellular Ca(2+) increase as measured by fura-2 spectrofluoroimaging. Furthermore, this cyclooctapeptide was able to potentiate spontaneous excitatory synaptic transmission in a network of cultured hippocampal neurons, consistent with the activation of presynaptic VGCC by EP14. In addition, this peptide did not affect cell survival measured with the MTT assay. Therefore, such new cyclopeptidic structures are potential good candidates for synthesis of new agents aimed at the restoration deficient excitatory synaptic transmission.

α-tocopherol and α-tocopheryl phosphate interact with the cannabinoid system in the rodent hippocampus.

α-Tocopherol (α-TOH), a dietary component of vitamin E, is well known for its antioxidant capacity. Nevertheless, recent studies have pointed out non-anti-radical properties including cellular and genomic actions. Decreased levels of α-tocopherol in the brain are associated with neuronal dysfunctions ranging from mood disorders to neurodegeneration. All these behavioral effects of α-tocopherol deficiency probably do not rely simply on its anti-radical properties, but could also be reminiscent of a not-yet characterized neuromodulatory action. We have thus measured the direct actions of α-tocopherol and of its natural phosphate derivative, α-tocopheryl phosphate (α-TP), on synaptic transmission in rodent hippocampus. These compounds had opposite actions on both glutamatergic and GABAergic transmission: whereas α-TOH potentiated these transmissions, α-TP inhibited them. Interestingly, these effects were both mediated by cannabinoid receptors (CB1Rs), because they were blocked by the CB1R antagonist AM251. Although α-tocopherol and α-tocopheryl phosphate did not directly bind CB1R, both α-TP and CB1R agonists inhibited forskolin-evoked Erk1/2 phosphorylation in a nonadditive manner. Furthermore, both α-tocopherol and α-tocopheryl phosphate attenuated depolarization-induced suppression of excitation and CB1R agonist-mediated hypothermia. Therefore, we identify α-tocopherol as new lipid modulator of the cannabinoid system in the rodent hippocampus, i.e., a novel "non-anti-radical" action of vitamin E, which may have some preeminent impact in neuronal disorders associated with vitamin E deficiency.

Early, time-dependent disturbances of hippocampal synaptic transmission and plasticity after in utero immune challenge.

BACKGROUND: Maternal infection during pregnancy is a recognized risk factor for the occurrence of a broad spectrum of psychiatric and neurologic disorders, including schizophrenia, autism, and cerebral palsy. Prenatal exposure of rats to lipopolysaccharide (LPS) leads to impaired learning and psychotic-like behavior in mature offspring, together with an enduring modification of glutamatergic excitatory synaptic transmission. The question that arises is whether any alterations of excitatory transmission and plasticity occurred at early developmental stages after in utero LPS exposure. METHODS: Electrophysiological experiments were carried out on the CA1 area of hippocampal slices from prenatally LPS-exposed male offspring from 4 to 190 days old to study the developmental profiles of long-term depression (LTD) triggered by delivering 900 shocks either single- or paired-pulse (50-msec interval) at 1 Hz and the N-methyl-D-aspartate receptor (NMDAr) contribution to synaptic transmission. RESULTS: The age-dependent drop of LTD is accelerated in prenatally LPS-exposed animals, and LTD is transiently converted into a slow-onset long-term potentiation between 16 and 25 days old. This long-term potentiation depends on Group I metabotropic glutamate receptors and protein kinase A activations and is independent of NMDArs. Maternal LPS challenge also leads to a rapid developmental impairment of synaptic NMDArs. This was associated with a concomitant reduced expression of GluN1, without any detectable alteration in the developmental switch of NMDAr GluN2 subunits. CONCLUSIONS: Aberrant forms of synaptic plasticity can be detected at early developmental stages after prenatal LPS challenge concomitant with a clear hypo-functioning of the NMDAr in the hippocampus. This might result in later-occurring brain dysfunctions.

Synthesis of C5-tetrazole derivatives of 2-amino-adipic acid displaying NMDA glutamate receptor antagonism.

Five derivatives of 2-amino-adipic acid bearing a tetrazole-substituted in C5 position were synthesized. These compounds displayed selective antagonism towards N-methyl-D: -aspartate (NMDA) receptors compared with AMPA receptors, and they were devoid of any neurotoxicity. Among these five analogues, one exhibited a higher affinity for synaptic NMDA responses than the other four. Therefore, C5 tetrazole-substituted of 2-amino-adipic acid represent an interesting series of new NMDA receptor antagonists. This approach may be considered as a new strategy to develop ligands specifically targeted to synaptic or extra-synaptic NMDA receptors.

Neuroprotection induced by vitamin E against oxidative stress in hippocampal neurons: involvement of TRPV1 channels.

Pretreatment of cultured hippocampal neurons with a low concentration of alpha-tocopherol (alpha-TP), the major component of vitamin E, results in a long-lasting protection against oxidative damages, via genomic effects. This neuroprotection is associated with the attenuation of a calcium influx triggered by oxidative agents such as Fe(2+) ions. This Ca(2+) influx is supported by a TRP-like channel, also partly involved in capacitive calcium entry within neurons. Here, we evidence the contribution of TRPV1 channels in this mechanism. TRPV1 channels are activated by various agents including capsaicin, the pungent component of hot chili peppers and blocked by capsazepine (CPZ) or 5'-iodo-resiniferatoxin. Both TRPV1 inhibitors strongly reduced Fe(2+) ion-mediated toxicity and Ca(2+) influx, in the same way as to alpha-TP pretreatment. Moreover, CPZ also decreased capacitive calcium entry in hippocampal neurons. Finally, both CPZ and 5'-iodo-resiniferatoxin reduced spontaneous excitatory synaptic transmission; this depression of synaptic transmission being largely occluded in alpha-TP-pretreated neurons. In conclusion, in our experimental model, TRPV1 channels are involved in the Fe(2+) ion-induced neuronal death and a negative modulation of this channel activity by alpha-TP pretreatment may account, at least in part, for the long-lasting neuroprotection against oxidative stress.

Unveiling novel forms of hippocampal synaptic plasticity with microelectrode arrays.

In this report, we elucidate by use of microelectrode arrays novel forms of long-term depression and potentiation in the hippocampus which are triggered by low frequency afferent stimulation and which rely on the activation of metabotropic glutamate receptor of the fifth subtype (mGlu5 receptor).

Alpha-tocopherol-mediated long-lasting protection against oxidative damage involves an attenuation of calcium entry through TRP-like channels in cultured hippocampal neurons.

We have reported that a transient treatment of hippocampal neurons with alpha-tocopherol induced a long-lasting protection against oxidative damage mediated by Fe(2+) ions. This protection required protein synthesis. Here, we have studied whether this "hyposensitivity" to oxidative stress could be linked to an altered Ca(2+) homeostasis. Fe(2+) ions triggered a Ca(2+) entry which was required for Fe(2+) ion-induced toxicity. This influx was sensitive to blockers of TRP-like nonspecific Ca(2+) channels, including Ruthenium Red, La(3+), and Gd(3+) ions which also prevented the Fe(2+) ion-induced toxicity and oxidative stress as revealed by protein carbonylation status. The pretreatment with alpha-tocopherol resulted in a reduction of the Ca(2+) increase induced by Fe(2+) ions and masked the blocking effect of La(3+) ions. Moreover, such a pretreatment reduced the capacitive Ca(2+) entries (CCE) observed after metabotropic glutamate receptor stimulation, which are known to involve TRP-like channels. By contrast, in a model of "hypersensitivity" to oxidative stress obtained by chronic stimulation of glucocorticoid receptors, we observed an exacerbation of the various effects of Fe(2+) ions, i.e., cellular toxicity and Ca(2+) increase, and the glutamate-stimulated CCE. Therefore, we conclude that the long-lasting neuroprotection induced by alpha-tocopherol pretreatment likely results from an attenuation of Ca(2+) entries via TRP-like channels.

A transient treatment of hippocampal neurons with alpha-tocopherol induces a long-lasting protection against oxidative damage via a genomic action.

Neuroprotection exerted by alpha-tocopherol against oxidative stress was investigated in cultured rat hippocampal neurons. In addition to its direct action as a radical scavenger revealed at concentrations above 10 microM, a transient application of 1 microM alpha-tocopherol phosphate (alpha-TP) to neurons induced a complete delayed long-lasting protection against oxidative insult elicited by exposure to Fe2+ ions, but not against excitotoxicity. A minimal 16-h application of alpha-TP was required to observe the protection against subsequent oxidative stress. This delayed protection could last up to a week after the application of alpha-TP, even when medium was changed after the alpha-TP treatment. Cycloheximide, added either 2 h before or together with alpha-TP, prevented the delayed neuroprotection, but not the acute. However, cycloheximide applied after the 16-h alpha-TP pretreatment did not alter the delayed neuroprotection. Neither Trolox, a cell-permeant analogue of alpha-tocopherol, nor other antioxidants, such as epigallocatechin-gallate and N-acetyl-L-cysteine, elicited a similar long-lasting protection. Only tert-butylhydroquinone could mimic the alpha-TP effect. Depletion of glutathione (GSH) by L-buthionine sulfoximine did not affect the delayed alpha-TP protection. Thus, in addition to its acute anti-radical action, alpha-TP induces a long-lasting protection of neurons against oxidative damage, via a genomic action on antioxidant defenses apparently unrelated to GSH biosynthesis.