Optimal policies for Bayesian olfactory search in turbulent flows

In many practical scenarios, a flying insect must search for the source of an emitted cue which is advected by the atmospheric wind. On the macroscopic scales of interest, turbulence tends to mix the cue into patches of relatively high concentration over a background of very low concentration, so that the insect will detect the cue only intermittently and cannot rely on chemotactic strategies which simply climb the concentration gradient. In this work we cast this search problem in the language of a partially observable Markov decision process and use the Perseus algorithm to compute strategies that are near-optimal with respect to the arrival time. We test the computed strategies on a large two-dimensional grid, present the resulting trajectories and arrival time statistics, and compare these to the corresponding results for several heuristic strategies, including (space-aware) infotaxis, Thompson sampling, and QMDP. We find that the near-optimal policy found by our implementation of Perseus outperforms all heuristics we test by several measures. We use the near-optimal policy to study how the search difficulty depends on the starting location. We also discuss the choice of initial belief and the robustness of the policies to changes in the environment. Finally, we present a detailed and pedagogical discussion about the implementation of the Perseus algorithm, including the benefits-and pitfalls-of employing a reward-shaping function.

Effect of Sodium Hypochlorite 0.05% on MMP-9 Extracellular Release in Chronic Wounds

BACKGROUND

In chronic wounds, high concentrations of matrix metalloproteinases (MMPs) can cause excessive proteolysis and slow wound healing. Consequently, restoring a proper MMP balance can help reduce the risk of a chronic wound. An antiseptic solution containing 0.05% sodium hypochlorite (Amukine Med 0.05%, Angelini S.p.A.; hereafter termed NaClO solution) is available on the market. The NaClO solution was proven effective and safe in managing infected skin wounds. To further characterize its activity, this study evaluated the in vitro activity of the NaClO solution on the monocyte release of MMPs.

METHODS

Human monocytic THP-1 (ATCC^® TIB-202™) cell lines were differentiated into macrophages and treated with different concentrations of NaClO (from 0.05% to 5 × 10^-7%). In addition, the THP-1 cell line was stimulated with wound fluid (WF) from patients with active venous leg ulcers in the inflammatory phase. The effect of NaClO (0.025-0.0062%) was also evaluated on healthy human peripheral blood serum samples. The effects of treatments on the gelatinolytic activity of MMP-9 were evaluated by gelatin zymography. The effects on MMPs release were evaluated through the Pro™ Human MMP 9-plex Assay. An exploratory scratch wound healing assay was also performed.

RESULTS

The NaClO solution reduced the gelatinolytic activity of MMP-9 and its activated form. The downregulation of MMP-9 gelatinolytic activity was also observed in peripheral blood serum. The MMPs profile showed a reduction in MMP-1 release (p < 0.05) and a slight reduction of the release of MMP-9 and MMP-12 after the treatment with LPS and the NaClO solution. A slight improvement in wound healing was observed after macrophage activation and treatment with the NaClO solution.

CONCLUSIONS

The results obtained suggest a possible ability of the NaClO solution to modulate the proteolytic pathways in the wound microenvironment, further characterizing its activity and use in clinical practice during wound care.

Design and validation of neuronal exocytosis blocking peptides as potential novel antiperspirants

Thermoregulation and heat dissipation by sweat production and evaporation are vital for human survival. However, hyperhidrosis or excessive perspiration might affect people's quality of life by causing discomfort and stress. The prolonged use of classical antiperspirants, anticholinergic medications or botulinum toxin injections for persistent hyperhidrosis might produce diverse side effects that limit their clinical use. Inspired by botox molecular mode of action, we used an in silico molecular modelling approach to design novel peptides to target neuronal acetylcholine exocytosis by interfering with the Snapin-SNARE complex formation. Our exhaustive design rendered the selection of 11 peptides that decreased calcium-dependent vesicle exocytosis in rat DRG neurons, reducing αCGRP release and TRPV1 inflammatory sensitization. The most potent peptides were palmitoylated peptides SPSR38-4.1 and SPSR98-9.1 that significantly suppressed acetylcholine release in vitro in human LAN-2 neuroblastoma cells. Noteworthy, local acute and chronic administration of SPSR38-4.1 peptide significantly decreased, in a dose-dependent manner, pilocarpine-induced sweating in an in vivo mouse model. Taken together, our in silico approach lead to the identification of active peptides able to attenuate excessive sweating by modulating neuronal acetylcholine exocytosis, and identified peptide SPSR38-4.1 as a promising new antihyperhidrosis candidate for clinical development.

Benzydamine plays a role in limiting inflammatory pain induced by neuronal sensitization

Benzydamine is an active pharmaceutical compound used in the oral care pharmaceutical preparation as NSAID. Beside from its anti-inflammatory action, benzydamine local application effectively reliefs pain showing analgesic and anaesthetic properties. Benzydamine mechanism of action has been characterized on inflammatory cell types and mediators highlighting its capacity to inhibit pro-inflammatory mediators' synthesis and release. On the other hand, the role of benzydamine as neuronal excitability modulator has not yet fully explored. Thus, we studied benzydamine's effect over primary cultured DRG nociceptors excitability and after acute and chronic inflammatory sensitization, as a model to evaluate relative nociceptive response. Benzydamine demonstrated to effectively inhibit neuronal basal excitability reducing its firing frequency and increasing rheobase and afterhyperpolarization amplitude. Its effect was time and dose-dependent. At higher doses, benzydamine induced changes in action potential wavelength, decreasing its height and slightly increasing its duration. Moreover, the compound reduced neuronal acute and chronic inflammatory sensitization. It inhibited neuronal excitability mediated either by an inflammatory cocktail, acidic pH or high external KCl. Notably, higher potency was evidenced under inflammatory sensitized conditions. This effect could be explained either by modulation of inflammatory and/or neuronal sensitizing signalling cascades or by direct modulation of proalgesic and action potential firing initiating ion channels. Apparently, the compound inhibited Nav1.8 channel but had no effect over Kv7.2, Kv7.3, TRPV1 and TRPA1. In conclusion, the obtained results strengthen the analgesic and anti-inflammatory effect of benzydamine, highlighting its mode of action on local pain and inflammatory signalling.

α-glyceryl-phosphoryl-ethanolamine protects human hippocampal neurons from aging-induced cellular alterations

Brain ageing has been related to a decrease in cellular metabolism, to an accumulation of misfolded proteins and to an alteration of the lipid membrane composition. These alterations act as contributive aspects of age‐related memory decline by reducing membrane excitability and neurotransmitter release. In this sense, precursors of phospholipids (PLs) can restore the physiological composition of cellular membranes and ameliorate the cellular defects associated with brain ageing. In particular, phosphatidylcholine (PC) and phosphatidylethanolamine (PE) have been shown to restore mitochondrial function, reduce the accumulation of amyloid beta (Aβ) and, at the same time, provide the amount of acetylcholine needed to reduce memory deficit. Among PL precursors, alpha‐glycerylphosphorylethanolamine (GPE) has shown to protect astrocytes from Aβ injuries and to slow‐down ageing of human neural stem cells. GPE has been evaluated in aged human hippocampal neurons, which are implicated in learning and memory, and constitute a good in vitro model to investigate the beneficial properties of GPE. In order to mimic cellular ageing, the cells have been maintained 21 days in vitro and challenged with GPE. Results of the present paper showed GPE ability to increase PE and PC content, glucose uptake and the activity of the chain respiratory complex I and of the GSK‐3β pathway. Moreover, the nootropic compound showed an increase in the transcriptional/protein levels of neurotrophic and well‐being related genes. Finally, GPE counteracted the accumulation of ageing‐related misfolded proteins (a‐synuclein and tau). Overall, our data underline promising effects of GPE in counteracting cellular alterations related to brain ageing and cognitive decline.

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.

Pharmacological activation of mGlu5 receptors with the positive allosteric modulator VU0360172, modulates thalamic GABAergic transmission

Previous studies have shown that injection of the mGlu5 receptor positive allosteric modulator (PAM) VU0360172 into either the thalamus or somatosensory cortex markedly reduces the frequency of spike-and-wave discharges (SWDs) in the WAG/Rij model of absence epilepsy. Here we have investigated the effects of VU0360172 on GABA transport in the thalamus and somatosensory cortex, as possible modes of action underlying the suppression of SWDs. Systemic VU0360172 injections increase GABA uptake in thalamic synaptosomes from epileptic WAG/Rij rats. Consistent with this observation, VU0360172 could also enhance thalamic GAT-1 protein expression, depending on the dosing regimen. This increase in GAT-1 expression was also observed in the thalamus from non-epileptic rats (presymptomatic WAG/Rij and Wistar) and appeared to occur selectively in neurons. The tonic GABA_A receptor current present in ventrobasal thalamocortical neurons was significantly reduced by VU0360172 consistent with changes in GAT-1 and GABA uptake. The in vivo effects of VU0360172 (reduction in tonic GABA current and increase in GAT-1 expression) could be reproduced in vitro by treating thalamic slices with VU0360172 for at least 1 h and appeared to be dependent on the activation of PLC. Thus, the effects of VU0360172 do not require an intact thalamocortical circuit. In the somatosensory cortex, VU0360172 reduced GABA uptake but did not cause significant changes in GAT-1 protein levels. These findings reveal a novel mechanism of regulation mediated by mGlu5 receptors, which could underlie the powerful anti-absence effect of mGlu5 receptor enhancers in animal models.

The Trace Kynurenine, Cinnabarinic Acid, Displays Potent Antipsychotic-Like Activity in Mice and Its Levels Are Reduced in the Prefrontal Cortex of Individuals Affected by Schizophrenia

Cinnabarinic acid (CA) is a kynurenine metabolite that activates mGlu4 metabotropic glutamate receptors. Using a highly sensitive ultra-performance liquid chromatography/tandem mass spectrometry (UPLC/MS-MS) method, we found that CA is present in trace amounts in human brain tissue. CA levels were largely reduced in the prefrontal cortex (PFC) of individuals affected by schizophrenia. This reduction did not correlate with age, sex, duration of the disease, and duration and type of antipsychotic medication and might, therefore, represent a trait of schizophrenia. Interestingly, systemic treatment with low doses of CA (<1 mg/kg, i.p.) showed robust efficacy in several behavioral tests useful to study antipsychotic-like activity in mice and rats and attenuated MK-801-evoked glutamate release. CA failed to display antipsychotic-like activity and inhibit excitatory synaptic transmission in mice lacking mGlu4 receptors. These findings suggest that CA is a potent endogenous antipsychotic-like molecule and reduced CA levels in the PFC might contribute to the pathophysiology of schizophrenia.

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.

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.

5-HT2A-mGlu2/3 receptor complex in rat spinal cord glutamatergic nerve endings: A 5-HT2A to mGlu2/3 signalling to amplify presynaptic mechanism of auto-control of glutamate exocytosis

Presynaptic mGlu2/3 autoreceptors exist in rat spinal cord nerve terminals as suggested by the finding that LY379268 inhibited the 15 mM KCl-evoked release of [³H]D-aspartate ([³H]D-Asp) in a LY341495-sensitive manner. Spinal cord glutamatergic nerve terminals also possess presynaptic release-regulating 5-HT_2A heteroreceptors. Actually, the 15 mM KCl-evoked [³H]D-Asp exocytosis from spinal cord synaptosomes was reduced by the 5-HT_2A agonist (±)DOI, an effect reversed by the 5-HT_2A antagonists MDL11,939, MDL100907, ketanserin and trazodone (TZD). We investigated whether mGlu2/3 and 5-HT_2A receptors colocalize and cross-talk in these terminals and if 5-HT_2A ligands modulate the mGlu2/3-mediated control of glutamate exocytosis. Western blot analysis and confocal microscopy highlighted the presence of mGlu2/3 and 5-HT_2A receptor proteins in spinal cord VGLUT1 positive synaptosomes, where mGlu2/3 and 5-HT_2A receptor immunoreactivities largely colocalize. Furthermore, mGlu2/3 immunoprecipitates from spinal cord synaptosomes were also 5-HT_2A immunopositive. Interestingly, the 100 pM LY379268-induced reduction of the 15 mM KCl-evoked [³H]D-Asp overflow as well as its inhibition by 100 nM (±)DOI became undetectable when the two agonists were concomitantly added. Conversely, 5-HT_2A antagonists (MDL11,939, MDL100907, ketanserin and TZD) reinforced the release-regulating activity of mGlu2/3 autoreceptors. Increased expression of mGlu2/3 receptor proteins in synaptosomal plasmamembranes paralleled the gain of function of the mGlu2/3 autoreceptors elicited by 5-HT_2A antagonists. Based on these results, we propose that in spinal cord glutamatergic terminals i) mGlu2/3 and 5-HT_2A receptors colocalize and interact one each other in an antagonist-like manner, ii) 5-HT_2A antagonists are indirect positive allosteric modulator of mGlu2/3 autoreceptors controlling glutamate exocytosis.

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.

Complete coverage of space favors modularity of the grid system in the brain

Grid cells in the entorhinal cortex fire when animals that are exploring a certain region of space occupy the vertices of a triangular grid that spans the environment. Different neurons feature triangular grids that differ in their properties of periodicity, orientation, and ellipticity. Taken together, these grids allow the animal to maintain an internal, mental representation of physical space. Experiments show that grid cells are modular, i.e., there are groups of neurons which have grids with similar periodicity, orientation, and ellipticity. We use statistical physics methods to derive a relation between variability of the properties of the grids within a module and the range of space that can be covered completely (i.e., without gaps) by the grid system with high probability. Larger variability shrinks the range of representation, providing a functional rationale for the experimentally observed comodularity of grid cell periodicity, orientation, and ellipticity. We obtain a scaling relation between the number of neurons and the period of a module, given the variability and coverage range. Specifically, we predict how many more neurons are required at smaller grid scales than at larger ones.

Repulsion and metabolic switches in the collective behavior of bacterial colonies

Bacteria inoculated on surfaces create colonies that spread out, forming patterns shaped by their mutual interactions. Here, by a combination of experiments and modeling, we address two striking phenomena observed when colonies spread out circularly, without dendritic instabilities. First, the velocity of spreading is generically found to decrease as levels of nutrients initially deposited on the surface increase. We demonstrate that the slowdown is due to phenomena of differentiation, leading to the coexistence of bacteria in different states of motility and we model their dynamics. Second, colonies spreading out from different inocula on the same surface are observed to merge or repel (halting at a finite distance), depending on experimental conditions. We identify the parameters that determine the fate of merging versus repulsion, and predict the profile of arrest in the cases of repulsion.

Inferring maps of forces inside cell membrane microdomains

Mapping of the forces on biomolecules in cell membranes has spurred the development of effective labels, e.g., organic fluorophores and nanoparticles, to track trajectories of single biomolecules. Standard methods use particular statistics, namely the mean square displacement, to analyze the underlying dynamics. Here, we introduce general inference methods to fully exploit information in the experimental trajectories, providing sharp estimates of the forces and the diffusion coefficients in membrane microdomains. Rapid and reliable convergence of the inference scheme is demonstrated on trajectories generated numerically. The method is then applied to infer forces and potentials acting on the receptor of the toxin labeled by lanthanide-ion nanoparticles. Our scheme is applicable to any labeled biomolecule and results show its general relevance for membrane compartmentation.

Intermittency in two-dimensional Ekman-Navier-Stokes turbulence

We study the statistics of the vorticity field in two-dimensional Navier-Stokes turbulence with linear Ekman friction. We show that the small-scale vorticity fluctuations are intermittent, as conjectured by Bernard [Europhys. Lett. 50, 333 (2000)] and Nam et al. [Phys. Rev. Lett. 84, 5134 (2000)]. The small-scale statistics of vorticity fluctuations coincide with that of a passive scalar with finite lifetime transported by the velocity field itself.

Statistical conservation laws in turbulent transport

We address the statistical theory of fields that are transported by a turbulent velocity field, both in forced and in unforced (decaying) experiments. With very few provisos on the transporting velocity field, correlation functions of the transported field in the forced case are dominated by statistically preserved structures. In decaying experiments we identify infinitely many statistical constants of the motion, which are obtained by projecting the decaying correlation functions on the statistically preserved functions. We exemplify these ideas and provide numerical evidence using a simple model of turbulent transport. This example is chosen for its lack of Lagrangian structure, to stress the generality of the ideas.

Universal decay of scalar turbulence

The asymptotic decay of passive scalar fields is solved analytically for the Kraichnan model, where the velocity has a short correlation time. At long times, two universality classes are found, both characterized by a distribution of the scalar-generally non-Gaussian-with global self-similar evolution in time. Analogous behavior is found numerically with a more realistic flow resulting from an inverse energy cascade.

Statistical geometry in scalar turbulence

A general link between geometry and intermittency in passive scalar turbulence is established. The anomalous part of the scalar correlation functions is shown to be dominated by special functions of particle configurations. Their major property is that those functions calculated along the particle trajectories remain statistically constant in time. Those conservation laws qualitatively imply the persistence of scalar particles in strongly clustered geometries.

Universality and saturation of intermittency in passive scalar turbulence

The statistical properties of a scalar field advected by the nonintermittent Navier-Stokes flow arising from a two-dimensional inverse energy cascade are investigated. The universality properties of the scalar field are probed by comparing the results obtained with two types of injection mechanisms. Scaling properties are shown to be universal, even though anisotropies injected at large scales persist down to the smallest scales and local isotropy is not fully restored. Scalar statistics is strongly intermittent and scaling exponents saturate to a constant for sufficiently high orders. This is observed also for the advection by a velocity field rapidly changing in time, pointing to the genericity of the phenomenon.

Inverse energy cascade in two-dimensional turbulence: deviations from gaussian behavior

High-resolution numerical simulations of stationary inverse energy cascade in two-dimensional turbulence are presented. Deviations from Gaussian behavior of velocity differences statistics are quantitatively investigated. The level of statistical convergence is pushed enough to permit reliable measurement of the asymmetries in the probability distribution functions of longitudinal increments and odd-order moments, which bring the signature of the inverse energy flux. No measurable intermittency corrections could be found in their scaling laws. The seventh order skewness increases by almost two orders of magnitude with respect to the third, thus becoming of order unity.