CXCR4 and NMDA Receptors Are Functionally Coupled in Rat Hippocampal Noradrenergic and Glutamatergic Nerve Endings

Metamodulation of presynaptic NMDA receptors: New perspectives for pharmacological interventions

Metamodulation shifted the scenario of the central neuromodulation from a simplified unimodal model to a multimodal one. It involves different receptors/membrane proteins physically associated or merely colocalized that act in concert to control the neuronal functions influencing each other. Defects or maladaptation of metamodulation would subserve neuropsychiatric disorders or even synaptic adaptations relevant to drug dependence. Therefore, this "vulnerability" represents a main issue to be deeply analyzed to predict its aetiopathogenesis, but also to propose targeted pharmaceutical interventions. The review focusses on presynaptic release-regulating NMDA receptors and on some of the mechanisms of their metamodulation described in the literature. Attention is paid to the interactors, including both ionotropic and metabotropic receptors, transporters and intracellular proteins, which metamodulate their responsiveness in physiological conditions but also undergo adaptation that are relevant to neurological dysfunctions. All these structures are attracting more and more the interest as promising druggable targets for the treatment of NMDAR-related central diseases: these substances would not exert on-off control of the colocalized NMDA receptors (as usually observed with NMDAR full agonists/antagonists), but rather modulate their functions, with the promise of limiting side effects that would favor their translation from preclinic to clinic.

Central nervous system interaction and crosstalk between nAChRs and other ionotropic and metabotropic neurotransmitter receptors

Neuronal nicotinic acetylcholine receptors (nAChRs) are widely distributed in both the peripheral and the central nervous systems. nAChRs exert a crucial modulatory influence on several brain biological processes; they are involved in a variety of neuronal diseases including Parkinson's disease, Alzheimer's disease, epilepsy, and nicotine addiction. The influence of nAChRs on brain function depends on the activity of other neurotransmitter receptors that co-exist with nAChRs on neurons. In fact, the crosstalk between receptors is an important mechanism of neurotransmission modulation and plasticity. This may be due to converging intracellular pathways but also occurs at the membrane level, because of direct physical interactions between receptors. In this line, this review is dedicated to summarizing how nAChRs and other ionotropic and metabotropic receptors interact and the relevance of nAChRs cross-talks in modulating various neuronal processes ranging from the classical modulation of neurotransmitter release to neuron plasticity and neuroprotection.

Presynaptic 5-HT2A-mGlu2/3 Receptor–Receptor Crosstalk in the Prefrontal Cortex: Metamodulation of Glutamate Exocytosis

The glutamatergic nerve endings of a rat prefrontal cortex (PFc) possess presynaptic 5-HT_2A heteroreceptors and mGlu2/3 autoreceptors, whose activation inhibits glutamate exocytosis, and is measured as 15 mM KCl-evoked [³H]D-aspartate ([³H]D-asp) release (which mimics glutamate exocytosis). The concomitant activation of the two receptors nulls their inhibitory activities, whereas blockade of the 5-HT_2A heteroreceptors with MDL11,939 (1 μM) strengthens the inhibitory effect elicited by the mGlu2/3 receptor agonist LY329268 (1 μM). 5-HT_2A receptor antagonists (MDL11,939; ketanserin; trazodone) amplify the impact of low (3 nM) LY379268. Clozapine (0.1–10 μM) mimics the 5-HT_2A agonist (±) DOI and inhibits the KCl-evoked [³H]D-asp overflow in a MDL11,939-dependent fashion, but does not modify the (±) DOI-induced effect. mGlu2 and 5-HT_2A proteins do not co-immunoprecipitate from synaptosomal lysates, nor does the incubation of PFc synaptosomes with MDL11,939 (1 μM) or clozapine (10 µM) modify the insertion of mGlu2 subunits in synaptosomal plasma membranes. In conclusion, 5-HT_2A and mGlu2/3 receptors colocalize, but do not physically associate, in PFc glutamatergic terminals, where they functionally interact in an antagonist-like fashion to control glutamate exocytosis. The mGlu2/3-5-HT_2A metamodulation could be relevant to therapy for central neuropsychiatric disorders, including schizophrenia, but also unveil cellular events accounting for their development, which also influence the responsiveness to drugs regimens.

Synaptosomes and Metamodulation of Receptors

Synaptosomes are re-sealed pinched off nerve terminals that maintain all the main structural and functional features of the original structures and that are appropriate to study presynaptic events. Because of the discovery of new structural and molecular events that dictate the efficiency of transmitter release and of its receptor-mediated control in the central nervous system, the interest in this tissue preparation is continuously renewing. Most of these events have been already discussed in previous reviews, but few of them were not and deserve some comments since they could suggest new functional and possibly therapeutic considerations. Among them, the "metamodulation" of receptors represents an emerging aspect that dramatically increased the complexity of the presynaptic compartment, adding new insights to the role of presynaptic receptors as modulators of chemical synapses. Deciphering the mechanism of presynaptic metamodulation would permit indirect approaches to control the activity of presynaptic release-regulating receptors that are currently orphans of direct ligands/modulators, paving the road for the proposal of new therapeutic approaches for central neurological diseases.

Cellular, synaptic, and network effects of chemokines in the central nervous system and their implications to behavior

Accumulating evidence highlights chemokines as key mediators of the bidirectional crosstalk between neurons and glial cells aimed at preserving brain functioning. The multifaceted role of these immune proteins in the CNS is mirrored by the complexity of the mechanisms underlying its biological function, including biased signaling. Neurons, only in concert with glial cells, are essential players in the modulation of brain homeostatic functions. Yet, attempts to dissect these complex multilevel mechanisms underlying coordination are still lacking. Therefore, the purpose of this review is to summarize the current knowledge about mechanisms underlying chemokine regulation of neuron-glia crosstalk linking molecular, cellular, network, and behavioral levels. Following a brief description of molecular mechanisms by which chemokines interact with their receptors and then summarizing cellular patterns of chemokine expression in the CNS, we next delve into the sequence and mechanisms of chemokine-regulated neuron-glia communication in the context of neuroprotection. We then define the interactions with other neurotransmitters, neuromodulators, and gliotransmitters. Finally, we describe their fine-tuning on the network level and the behavioral relevance of their modulation. We believe that a better understanding of the sequence and nature of events that drive neuro-glial communication holds promise for the development of new treatment strategies that could, in a context- and time-dependent manner, modulate the action of specific chemokines to promote brain repair and reduce the neurological impairment.

Presynaptic Release-regulating Metabotropic Glutamate Receptors: An Update

Metabotropic glutamate (mGlu) receptors represent the largest family of glutamate receptors in mammals and act as fine tuners of the chemical transmission in central nervous system (CNS). In the last decade, results concerning the expression and the subcellular localization of mGlu receptors further clarified their role in physio-pathological conditions. Concomitantly, their pharmacological characterization largely improved thanks to the identification of new compounds (chemical ligands and antibodies recognizing epitopic sequences of the receptor proteins) that allowed to decipher the protein compositions of the naive receptors. mGlu receptors are expressed at the presynaptic site of chemical synapses. Here, they modulate intraterminal enzymatic pathways controlling the migration and the fusion of vesicles to synaptic membranes as well as the phosphorylation of colocalized receptors. Both the control of transmitter exocytosis and the phosphorylation of colocalized receptors elicited by mGlu receptors are relevant events that dictate the plasticity of nerve terminals, and account for the main role of presynaptic mGlu receptors as modulators of neuronal signalling. The role of the presynaptic mGlu receptors in the CNS has been the matter of several studies and this review aims at briefly summarizing the recent observations obtained with isolated nerve endings (we refer to as synaptosomes). We focus on the pharmacological characterization of these receptors and on their receptor-receptor interaction / oligo-dimerization in nerve endings that could be relevant to the development of new therapeutic approaches for the cure of central pathologies.

Somatostatin, a Presynaptic Modulator of Glutamatergic Signal in the Central Nervous System

Somatostatin is widely diffused in the central nervous system, where it participates to control the efficiency of synaptic transmission. This peptide mainly colocalizes with GABA, in inhibitory, GABA-containing interneurons from which it is actively released in a Ca²⁺ dependent manner upon application of depolarizing stimuli. Once released in the synaptic cleft, somatostatin acts locally, or it diffuses in the extracellular space through "volume diffusion", a mechanism(s) of distribution which mainly operates in the cerebrospinal fluid and that assures the progression of neuronal signalling from signal-secreting sender structures towards receptor-expressing targeted neurons located extrasynaptically, in a non-synaptic, inter-neuronal form of communication. Somatostatin controls the efficiency of central glutamate transmission by either modulating presynaptically the glutamate exocytosis or by metamodulating the activity of glutamate receptors colocalized and functionally coupled with somatostatin receptors in selected subpopulations of nerve terminals. Deciphering the role of somatostatin in the mechanisms of "volume diffusion" and in the "receptor-receptor interaction" unveils new perspectives in the central role of this fine tuner of synaptic strength, paving the road to new therapeutic approaches for the cure of central disorders.

Presynaptic release-regulating NMDA receptors in isolated nerve terminals: A narrative review

The existence of presynaptic, release‐regulating NMDA receptors in the CNS has been long matter of discussion. Most of the reviews dedicated to support this conclusion have preferentially focussed on the results from electrophysiological studies, paying little or no attention to the data obtained with purified synaptosomes, even though this experimental approach has been recognized as providing reliable information concerning the presence and the role of presynaptic release‐regulating receptors in the CNS. To fill the gap, this review is dedicated to summarising the results from studies with synaptosomes published during the last 40 years, which support the existence of auto and hetero NMDA receptors controlling the release of transmitters such as glutamate, GABA, dopamine, noradrenaline, 5‐HT, acetylcholine and peptides, in the CNS of mammals. The review also deals with the results from immunochemical studies in isolated nerve endings that confirm the functional observations.

NMDA receptors in axons: there's no coincidence

In the textbook view, N-methyl-d-aspartate (NMDA) receptors are postsynaptically located detectors of coincident activity in Hebbian learning. However, controversial presynaptically located NMDA receptors (preNMDARs) have for decades been repeatedly reported in the literature. These preNMDARs have typically been implicated in the regulation of short-term and long-term plasticity, but precisely how they signal and what their functional roles are have been poorly understood. The functional roles of preNMDARs across several brain regions and different forms of plasticity can differ vastly, with recent discoveries showing key involvement of unusual subunit composition. Increasing evidence shows preNMDAR can signal through both ionotropic action by fluxing calcium and in metabotropic mode even in the presence of magnesium blockade. We argue that these unusual properties may explain why controversy has surrounded this receptor type. In addition, the expression of preNMDARs at some synapse types but not others can underlie synapse-type-specific plasticity. Last but not least, preNMDARs are emerging therapeutic targets in disease states such as neuropathic pain. We conclude that axonally located preNMDARs are required for specific purposes and do not end up there by accident.

Environmental Training and Synaptic Functions in Young and Old Brain: A Presynaptic Perspective

BACKGROUND

Aging is an unavoidable, physiological process that reduces the complexity and the plasticity of the synaptic contacts in Central Nervous System (CNS), having profound implications for human well-being. The term "cognitive reserve" refers to central cellular adaptations that augment the resilience of human brain to damage and aging. The term "Cognitive training" indicates the cultural, social and physical stimulations proposed as add-on therapy for the cure of central neurological diseases. "Cognitive training" reinforces the "cognitive reserve" permitting to counteract brain impairments and rejuvenating synaptic complexity. The research has begun investigating the clinical impact of the "cognitive training" in aged people, but additional work is needed to definitively assess its effectiveness. In particular, there is a need to understand, from a preclinical point of view, whether "cognitive training" promotes compensatory effects or, alternatively, if it elicits genuine recovery of neuronal defects. Although the translation from rodent studies to the clinical situation could be difficult, the results from pre-clinical models are of high clinical relevance, since they should allow a better understanding of the effects of environmental interventions in aging-associated chronic derangements in mammals.

CONCLUSION

Data in literature and the recent results obtained in our laboratory concerning the impact of environmental stimulation on the presynaptic release of noradrenaline, glutamate and gamma amino butyric acid (GABA) suggest that these neurotransmitters undergo different adaptations during aging and that they are differently tuned by "cognitive training". The impact of "cognitive training" on neurotransmitter exocytosis might account for the cellular events involved in reinforcement of "cognitive reserve" in young and old animals.

Morphine-Induced Modulation of Endolysosomal Iron Mediates Upregulation of Ferritin Heavy Chain in Cortical Neurons

HIV-associated neurocognitive disorders (HAND) remain prevalent and are aggravated by µ-opioid use. We have previously shown that morphine and other µ-opioids may contribute to HAND by inhibiting the homeostatic and neuroprotective chemokine receptor CXCR4 in cortical neurons, and this novel mechanism depends on upregulation of the protein ferritin heavy chain (FHC). Here, we examined the cellular events and potential mechanisms involved in morphine-mediated FHC upregulation using rat cortical neurons of either sex in vitro and in vivo. Morphine dose dependently increased FHC protein levels in primary neurons through µ-opioid receptor (µOR) and Gαi-protein signaling. Cytoplasmic FHC levels were significantly elevated, but nuclear FHC levels and FHC gene expression were unchanged. Morphine-treated rats also displayed increased FHC levels in layer 2/3 neurons of the prefrontal cortex. Importantly, both in vitro and in vivo FHC upregulation was accompanied by loss of mature dendritic spines, which was also dependent on µOR and Gαi-protein signaling. Moreover, morphine upregulated ferritin light chain (FLC), a component of the ferritin iron storage complex, suggesting that morphine altered neuronal iron metabolism. Indeed, prior to FHC upregulation, morphine increased cytoplasmic labile iron levels as a function of decreased endolysosomal iron. In line with this, chelation of endolysosomal iron (but not extracellular iron) blocked morphine-induced FHC upregulation and dendritic spine reduction, whereas iron overloading mimicked the effect of morphine on FHC and dendritic spines. Overall, these data demonstrate that iron mediates morphine-induced FHC upregulation and consequent dendritic spine deficits and implicate endolysosomal iron efflux to the cytoplasm in these effects.

Acute Functional Adaptations in Isolated Presynaptic Terminals Unveil Synaptosomal Learning and Memory

Synaptosomes are used to decipher the mechanisms involved in chemical transmission, since they permit highlighting the mechanisms of transmitter release and confirming whether the activation of presynaptic receptors/enzymes can modulate this event. In the last two decades, important progress in the field came from the observations that synaptosomes retain changes elicited by both “in vivo” and “in vitro” acute chemical stimulation. The novelty of these studies is the finding that these adaptations persist beyond the washout of the triggering drug, emerging subsequently as functional modifications of synaptosomal performances, including release efficiency. These findings support the conclusion that synaptosomes are plastic entities that respond dynamically to ambient stimulation, but also that they “learn and memorize” the functional adaptation triggered by acute exposure to chemical agents. This work aims at reviewing the results so far available concerning this form of synaptosomal learning, also highlighting the role of these acute chemical adaptations in pathological conditions.

Prolonged activation of CXCR4 hampers the release-regulating activity of presynaptic NMDA receptors in rat hippocampal synaptosomes

We investigated the impact of the prolonged exposure of rat hippocampal synaptosomes to CXCL12 (3 nM) on the NMDA-mediated release of [³H]D-aspartate ([³H]D-Asp) or [³H]noradrenaline ([³H]NA). Synaptosomes were stimulated twice with NMDA/CXCL12 and the amount of the NMDA-evoked tritium release (S1 and S2) quantified to calculate the S2/S1 ratio. The S2/S1 ratio for both transmitters was drastically decreased by 3 nM CXCL12 between the two stimuli (CXCL12-treated synaptosomes) in a AMD3100-sensitive manner. The phosphorylation of the GluN1 subunit in Ser 896 was reduced in CXCL12-treated synaptosomes, while the overall amount of GluN1 and GluN2B proteins as well as the GluN2B insertion in synaptosomal plasmamembranes were unchanged. We conclude that the CXCR4/NMDA cross-talk is dynamically regulated by the time of activation of the CXCR4s. Our results unveil a functional cross-talk that might account for the severe impairments of central transmission that develop in pathological conditions characterized by CXCL12 overproduction.

Immuno-Pharmacological Characterization of Presynaptic GluN3A-Containing NMDA Autoreceptors: Relevance to Anti-NMDA Receptor Autoimmune Diseases

Mouse hippocampal glutamatergic nerve endings express presynaptic release-regulating NMDA autoreceptors (NMDARs). The presence of GluN1, GluN2A, GluN2B, and GluN3A subunits in hippocampal vesicular glutamate transporter type 1-positive synaptosomes was confirmed with confocal microscopy. GluN2C, GluN2D, and GluN3B immunopositivity was scarcely present. Incubation of synaptosomes with the anti-GluN1, the anti-GluN2A, the anti-GluN2B, or the anti-GluN3A antibody prevented the 30 μM NMDA/1 μM glycine-evoked [³H]D-aspartate ([³H]D-ASP) release. The NMDA/glycine-evoked [³H]D-ASP release was reduced by increasing the external protons, consistent with the participation of GluN1 subunits lacking the N1 cassette to the receptor assembly. The result also excludes the involvement of GluN1/GluN3A dimers into the NMDA-evoked overflow. Complement (1:300) released [³H]D-ASP in a dizocilpine-sensitive manner, suggesting the participation of a NMDAR-mediated component in the releasing activity. Accordingly, the complement-evoked glutamate overflow was reduced in anti-GluN-treated synaptosomes when compared to the control. We speculated that incubation with antibodies had favored the internalization of NMDA receptors. Indeed, a significant reduction of the GluN1 and GluN2B proteins in the plasma membranes of anti-GluN1 or anti-GluN2B antibody-treated synaptosomes emerged in biotinylation studies. Altogether, our findings confirm the existence of presynaptic GluN3A-containing release-regulating NMDARs in mouse hippocampal glutamatergic nerve endings. Furthermore, they unveil presynaptic alteration of the GluN subunit insertion in synaptosomal plasma membranes elicited by anti-GluN antibodies that might be relevant to the central alterations occurring in patients suffering from autoimmune anti-NMDA diseases.

Presynaptic mGlu1 Receptors Control GABAB Receptors in an Antagonist-Like Manner in Mouse Cortical GABAergic and Glutamatergic Nerve Endings

Mouse cortical GABAergic synaptosomes possess presynaptic inhibitory GABA_B autoreceptors. Accordingly, (±)baclofen (3 μM) inhibits in a CGP53423-sensitive manner the 12 mM KCl-evoked release of preloaded [³H]GABA. Differently, the existence of presynaptic release-regulating metabotropic glutamate type 1 (mGlu1) heteroreceptors in these terminals is still matter of discussion, although confocal microscopy unveiled the existence of mGlu1α with GABA_B1 or GABA_B2 proteins in cortical VGAT-positive synaptosomes. The group I mGlu agonist 3,5-DHPG failed to modify on its own the 12 mM KCl-evoked [³H]GABA exocytosis from cortical nerve endings, but, when added concomitantly to the GABA_B agonist, it significantly reduced the 3 μM (±)baclofen-induced inhibition of [³H]GABA exocytosis. Conversely, the mGlu1 antagonist LY367385 (0.03–1 μM), inactive on its own on GABA exocytosis, amplified the 3 μM (±)baclofen-induced inhibition of [³H]GABA overflow. The ( ± )baclofen-induced inhibition of [³H]GABA exocytosis was more pronounced in cortical synaptosomes from Grm1^crv4/crv4 mice, which bear a spontaneous mutation of the Grm1 gene leading to the functional inactivation of the mGlu1 receptor. Inasmuch, the expression of GABA_B2 receptor protein in cortical synaptosomal lysates from Grm1^crv4/crv4 mice was increased when compared to controls. Altogether, these observations seem best interpreted by assuming that mGlu1 coexist with GABA_B receptors in GABAergic cortical synaptosomes, where they control GABA receptors in an antagonist-like manner. We then asked whether the mGlu1-mediated control of GABA_B receptors is restricted to GABAergic terminals, or if it occurs also in other subpopulations of nerve endings. Release-regulating GABA_B receptors also exist in glutamatergic nerve endings. (±)baclofen (1 μM) diminished the 12 mM KCl-evoked [³H]D-aspartate overflow. Also in these terminals, the concomitant presence of 1 μM LY367385, inactive on its own, significantly amplified the inhibitory effect exerted by (±)baclofen on [³H]D-aspartate exocytosis. Confocal microscopy confirmed the colocalization of mGlu1 with GABA_B1 and GABA_B2 labeling in vesicular glutamate type1 transporter-positive particles. Our results support the conclusion that mGlu1 receptors modulate in an antagonist-like manner presynaptic release-regulating GABA_B receptors. This receptor–receptor interaction could be neuroprotective in central disease typified by hyperglutamatergicity.

Environmental training is beneficial to clinical symptoms and cortical presynaptic defects in mice suffering from experimental autoimmune encephalomyelitis

The effect of "prophylactic" environmental stimulation on clinical symptoms and presynaptic defects in mice suffering from the experimental autoimmune encephalomyelitis (EAE) at the acute stage of disease (21 ± 1 days post immunization, d.p.i.) was investigated. In EAE mice raised in an enriched environment (EE), the clinical score was reduced when compared to EAE mice raised in standard environment (SE).Concomitantly, gain of weight and increased spontaneous motor activity and curiosity were observed, suggesting increased well-being in mice. Impaired glutamate exocytosis and cyclic adenosine monophosphate (cAMP) production in cortical terminals of SE-EAE mice were evident at 21 ± 1 d.p.i.. Differently, the 12 mM KCl-evoked glutamate exocytosis from cortical synaptosomes of EE-EAE mice was comparable to that observed in SE and EE-control mice, but significantly higher than that in SE-EAE mice. Similarly, the 12 mM KCl-evoked cAMP production in EE-EAE mice cortical synaptosomes recovered to the level observed in SE and EE-control mice. MUNC-18 and SNAP25 contents, but not Syntaxin-1a and Synaptotagmin 1 levels, were increased in cortical synaptosomes from EE-EAE mice when compared to SE-EAE mice. Circulating IL-1β was increased in the spinal cord, but not in the cortex, of SE-EAE mice, and it did not recover in EE-EAE mice. Inflammatory infiltrates were reduced in the cortex but not in the spinal cord of EE-EAE mice. Demyelination was observed in the spinal cord; EE significantly diminished it. We conclude that "prophylactic" EE is beneficial to synaptic derangements and preserves glutamate transmission in the cortex of EAE mice. This article is part of the Special Issue entitled "Neurobiology of Environmental Enrichment".

Immuno-pharmacological characterization of group II metabotropic glutamate receptors controlling glutamate exocytosis in mouse cortex and spinal cord

BACKGROUND AND PURPOSE

We recently proposed the existence of mGlu3 -preferring autoreceptors in spinal cord terminals and of mGlu2 -preferring autoreceptors in cortical terminals. This study aims to verify our previous conclusions and to extend their pharmacological characterization.

EXPERIMENTAL APPROACH

We studied the effect of LY566332, an mGlu2 receptor positive allosteric modulator (PAM), and of LY2389575, a selective mGlu3 receptor negative allosteric (NAM) modulator, on the mGlu2/3 agonist LY379268-mediated inhibition of glutamate exocytosis [measured as KCl-evoked release of preloaded [3 H]-D-aspartate]. The mGlu2 PAM BINA and the mGlu3 NAM ML337, as well as selective antibodies recognizing the N-terminal of the receptor proteins, were used to confirm the pharmacological characterization of the native receptors.

KEY RESULTS

Cortical synaptosomes possess LY566332-sensitive autoreceptors that are slightly, although significantly, susceptible to LY2389575. In contrast, LY566332-insensitive and LY2389575-sensitive autoreceptors are present in spinal cord terminals. BINA and ML337 mimicked LY566332 and LY2389575, respectively, in controlling LY379268-mediated inhibition of glutamate exocytosis from both cortical and spinal cord synaptosomes. Incubation of cortical synaptosomes with anti-mGlu2 antibody prevented the LY379268-induced inhibition of glutamate exocytosis, and this response was partially reduced by the anti-mGlu3 antibody. Incubation of spinal cord synaptosomes with the anti-mGlu3 antibody abolished LY379268-mediated reduction of glutamate exocytosis from these terminals, while the anti-mGlu2 antibody was inactive. Western blot analysis and confocal microscopy data were largely consistent with these functional observations.

CONCLUSIONS AND IMPLICATIONS

We confirmed that mGlu3 -preferring autoreceptors exist in spinal cord terminals. Differently, cortical glutamatergic terminals possess mGlu2 /mGlu3 heterodimers, whose inhibitory effect is largely mediated by mGlu2 receptors.

CCL5-Glutamate Cross-Talk in Astrocyte-Neuron Communication in Multiple Sclerosis

The immune system (IS) and the central nervous system (CNS) are functionally coupled, and a large number of endogenous molecules (i.e., the chemokines for the IS and the classic neurotransmitters for the CNS) are shared in common between the two systems. These interactions are key elements for the elucidation of the pathogenesis of central inflammatory diseases. In recent years, evidence has been provided supporting the role of chemokines as modulators of central neurotransmission. It is the case of the chemokines CCL2 and CXCL12 that control pre- and/or post-synaptically the chemical transmission. This article aims to review the functional cross-talk linking another endogenous pro-inflammatory factor released by glial cells, i.e., the chemokine Regulated upon Activation Normal T-cell Expressed and Secreted (CCL5) and the principal neurotransmitter in CNS (i.e., glutamate) in physiological and pathological conditions. In particular, the review discusses preclinical data concerning the role of CCL5 as a modulator of central glutamatergic transmission in healthy and demyelinating disorders. The CCL5-mediated control of glutamate release at chemical synapses could be relevant either to the onset of psychiatric symptoms that often accompany the development of multiple sclerosis (MS), but also it might indirectly give a rationale for the progression of inflammation and demyelination. The impact of disease-modifying therapies for the cure of MS on the endogenous availability of CCL5 in CNS will be also summarized. We apologize in advance for omission in our coverage of the existing literature.

Prophylactic versus Therapeutic Fingolimod: Restoration of Presynaptic Defects in Mice Suffering from Experimental Autoimmune Encephalomyelitis

Fingolimod, the first oral, disease-modifying therapy for MS, has been recently proposed to modulate glutamate transmission in the central nervous system (CNS) of mice suffering from Experimental Autoimmune Encephalomyelitis (EAE) and in MS patients. Our study aims at investigating whether oral fingolimod recovers presynaptic defects that occur at different stages of disease in the CNS of EAE mice. In vivo prophylactic (0.3 mg/kg for 14 days, from the 7th day post immunization, d.p.i, the drug dissolved in the drinking water) fingolimod significantly reduced the clinical symptoms and the anxiety-related behaviour in EAE mice. Spinal cord inflammation, demyelination and glial cell activation are markers of EAE progression. These signs were ameliorated following oral fingolimod administration. Glutamate exocytosis was shown to be impaired in cortical and spinal cord terminals isolated from EAE mice at 21 ± 1 d.p.i., while GABA alteration emerged only at the spinal cord level. Prophylactic fingolimod recovered these presynaptic defects, restoring altered glutamate and GABA release efficiency. The beneficial effect occurred in a dose-dependent, region-specific manner, since lower (0.1-0.03 mg/kg) doses restored, although to a different extent, synaptic defects in cortical but not spinal cord terminals. A delayed reduction of glutamate, but not of GABA, exocytosis was observed in hippocampal terminals of EAE mice at 35 d.p.i. Therapeutic (0.3 mg/kg, from 21 d.p.i. for 14 days) fingolimod restored glutamate exocytosis in the cortex and in the hippocampus of EAE mice at 35 ± 1 d.p.i. but not in the spinal cord, where also GABAergic defects remained unmodified. These results improve our knowledge of the molecular events accounting for the beneficial effects elicited by fingolimod in demyelinating disorders.

HIV-1 Glycoprotein 120 Enhancement of N-Methyl-D-Aspartate NMDA Receptor-Mediated Excitatory Postsynaptic Currents: Implications for HIV-1-Associated Neural Injury

It is widely accepted that human immunodeficiency virus type 1 (HIV-1) envelope glycoprotein 120 (gp120) plays an important role in HIV-1-induced neural injury and pathogenesis of HIV-1-associated dementia (HAND). Multiple pathways have been proposed for gp120-induced neurotoxicity, amongst is the activation of N-Methyl-D-Aspartate receptors (NMDARs). It has been shown that gp120 causes neuronal injury or death and gp120 transgenic mice exhibit neurological similarity to that of HAND, all of which can be blocked or attenuated by NMDAR antagonists. Several lines of evidence indicate the subtype and location of activated NMDARs are key determinants of the nature of NMDAR physiology. To examine the subtype and the location of NMDARs affected by gp120, we studied gp120 on subtype NMDAR-mediated EPSCs in the CA1 region of rat hippocampal slices through "blind" whole-cell patch recordings. Our results showed bath application of gp120 increased both NR2A- and NR2B-mediated EPSCs possibly via a presynaptic mechanism, with much stronger effect on NR2B-mediated EPSCs. In contrast, gp120 failed on enhancing AMPA receptor-mediated EPSCs. Ca²⁺ imaging studies revealed that gp120 potentiated glutamate-induced increase of intracellular Ca²⁺ concentration in rat hippocampal neuronal cultures which were blocked by a NMDAR antagonist, but not by an AMPA receptor antagonist, indicating gp120 induces Ca²⁺ influx through NMDARs. Further investigations demonstrated that gp120 increased the EPSCs mediated by extrasynaptic NR2BRs. Taken together, these results demonstrate that gp120 interacts with both NR2A and NR2B subtypes of NMDARs with a predominant action on the extrasynaptic NR2B, implicating a role NR2B may play in HIV-1-associated neuropathology.