Neurotransmitters as early signals for central nervous system development

During brain ontogenesis, the temporal and spatial generation of the different types of neuronal and glial cells from precursors occurs as a sequence of successive progenitor stages whose proliferation, survival and cell-fate choice are controlled by environmental and cellular regulatory molecules. Neurotransmitters belong to the chemical microenvironment of neural cells, even at the earliest stages of brain development. It is now established that specific neurotransmitter receptors are present on progenitor cells of the developing central nervous system and could play, during neural development, a role that has remained unsuspected until recently. The present review focuses on the occurrence of neurotransmitters and their corresponding ligand-gated ion channel receptors in immature cells, including neural stem cells of specific embryonic and neonatal brain regions. We summarize in vitro and in vivo data arguing that neurotransmitters could regulate morphogenetic events such as proliferation, growth, migration, differentiation and survival of neural precursor cells. The understanding of neurotransmitter function during early neural maturation could lead to the development of pharmacological tools aimed at improving adult brain repair strategies.

[Clinical case of the month. Report of a case of antiphospholipid syndrome]

This article reports a case of Anton-Babinski syndrome, due to right middle cerebral artery thrombosis and attributed to a likely primary antiphospholipid syndrome. It is always difficult to diagnose the latter, especially in the case of our patient who had a past history of multiple venous thromboses but also a heterozygosity for the mutation of the factor V of Leyden. We reviewed the literature dedicated to the prothrombotic events linked to the presence of these antiphospholipid antibodies: the lupus anticoagulant and the anticardiolipin antibodies.

Glycine triggers an intracellular calcium influx in oligodendrocyte progenitor cells which is mediated by the activation of both the ionotropic glycine receptor and Na+-dependent transporters

Using fluo-3 calcium imaging, we demonstrate that glycine induces an increase in intracellular calcium concentration ([Ca2+]i) in cortical oligodendrocyte progenitor (OP) cells. This effect results from a calcium entry through voltage-gated calcium channels (VGCC), as it is observed only in OP cells expressing such channels, and it is abolished either by removal of calcium from the extracellular medium or by application of an L-type VGCC blocker. Glycine-triggered Ca2+ influx in OP cells actually results from an initial depolarization that is the consequence of the activation of both the ionotropic glycine receptor (GlyR) and Na+-dependent transporters, most probably the glycine transporters 1 (GLYT1) and/or 2 (GLYT2) which are colocalized in these cells. Through this GlyR- and transporter-mediated effect on OP intrcellular calcium concentration [Ca2+]i, glycine released by neurons may, as well as other neurotransmitters, serve as a signal between neurons and OP during development.

Identification of PSF, the polypyrimidine tract-binding protein-associated splicing factor, as a developmentally regulated neuronal protein

The polypyrimidine tract-binding protein-associated splicing factor (PSF), which plays an essential role in mammalian spliceosomes, has been found to be expressed by differentiating neurons in developing mouse brain. The sequence of a fragment of mouse PSF was found to be remarkably similar to that of human PSF. Both the expression of PSF mRNA in cortex and cerebellum and PSF immunoreactivity in all brain areas were high during embryonic and early postnatal life and almost disappeared in adult tissue, except in the hippocampus and olfactory bulb where various neuronal populations remained PSF-immunopositive. Double-labeling experiments with anti-PSF antibody and anti-neurofilaments or anti-glial fibrillary acidic protein antibodies on sections of cortex, hippocampus, and cerebellum indicate that PSF is expressed by differentiating neurons but not by astrocytic cells. In vitro, mouse PSF was found to be expressed by differentiating cortical and cerebellar neurons. Radial glia or astrocyte nuclei were not immunopositive; however, oligodendrocytes differentiating in vitro were found to express PSF. The restricted expression of PSF suggests that this splicing factor could be involved in the control of neuronal-specific splicing events occurring at particular stages of neuronal differentiation and maturation.

Oligodendrocytes: from development to demyelinated lesion repair

Spontaneous but incomplete remyelination is observed after a demyelinating lesion. We know since ten years now that oligodendrocyte progenitors, (OP) and totipotent neural stem cells remain present in the central nervous system of adult mammals. Moreover, these OP are more likely the cellular source of remyelinating cells. Here, we intend to demonstrate that the understanding of the myelination process that occurs during development might lead to new treatments aimed at inducing remyelination: stimulation of resident OP or grafting of purified perinatal OP.

Neurotransmitter-mediated regulation of CNS myelination: a review

In addition to treatments aimed at preventing or limiting damage to myelin and oligodendrocytes, there is a crucial need for repair strategies in human demyelinating disorders. There is increasing evidence that besides growth factors, neurotransmitters can regulate different steps of the oligodendrogliogenesis. The present review on neurotransmitter receptor expression and function in the oligodendrocyte lineage emphasizes the concept that in this lineage cell proliferation and differentiation can be controlled through the modulation of the functional state of channel proteins and receptors, such as the delayed K+ rectifier, the AMPA/kainate, dopamine or muscarinic receptors, and, most likely, others yet to be found. We anticipate that a better understanding of the neurotransmitter-mediated neuronal oligodendroglial communication network opens prospects in the field of central nervous system (CNS) myelin repair, allowing the recruitment of the myelinating machinery that is known to remain present but quiescent in the CNS of multiple sclerosis patients.

Cultured oligodendrocyte progenitors derived from cerebral cortex express a glycine receptor which is pharmacologically distinct from the neuronal isoform

Using the whole-cell patch-clamp technique, we demonstrate glycine-induced currents in oligosphere-derived oligodendrocyte progenitors cultured from newborn rats. Similar inward currents are also triggered by beta-alanine and taurine, two established glycine receptor agonists. In our recording conditions, glycine-gated currents in oligodendrocyte progenitors reverse about 0 mV and are reversibly inhibited by the glycine competitive antagonist strychnine, the Cl- channel blocker picrotoxinin and the non-competitive antagonist cyanotriphenylborate. The oligodendrocyte progenitors glycine receptor (GlyR) differs from the corresponding neuronal receptor: [3H]strychnine binding data and the strychnine inhibition curve of glycine-induced currents in oligodendrocyte progenitor cultures suggest the existence of two strychnine binding sites on the oligodendroglial GlyR. Using total RNA isolated from oligodendrocyte progenitors cultures, reverse transcription-polymerase chain reaction analysis of glycine receptor subunit expression shows the presence of alpha2 and beta subunits and immunocytochemical stainings confirm that this GlyR contains an alpha subunit which is not alpha1. The molecular structure of the oligodendroglial GlyR could be either homopentameric alpha2 or heteromeric alpha2beta but in both cases, the sequence of the alpha2 or beta subunits have to be different from the known neuronal sequences in order to explain, respectively, the cyanotriphenylborate (alpha2) and picrotoxinin (beta) sensitivities. This work thus demonstrates that GlyR are expressed by oligodendrocytes obtained not only from spinal cord but also from supraspinal structures. The pharmacological properties and presumably the molecular structure of oligodendroglial GlyR are original. The physiological meaning of the presence of such receptors on developing and mature oligodendrocytes remains unknown.

Developmental regulation of neuroligand-induced responses in cultured oligodendroglia

Using whole-cell patch-clamp techniques, we show that oligosphere-derived oligodendrocyte progenitor cells (OP) display GABA-, glutamate-, 5-HT-, glycine- and acetylcholine-gated inward currents. When OP differentiate into oligodendrocytes (ODC), the amplitude of peak currents elicited by saturating concentrations of these transmitters decreases except for 5-HT. Intracellular Ca2+ concentration changes induced by microperfusion of glutamate, 5-HT, TRH, met-enkephalin and substance P were monitored using a fluo-3-based calcium imaging system. When OP cells differentiate into ODC, a global decrease of the proportion of responding cells is observed. During type-2 astrocytes commitment, this proportion decreases for 5-HT, TRH- and metenkephalin stimulations whereas it remains constant for substance P and glutamate. These data demonstrate a development regulation of neurotransmitter- and neuropeptide-induced responses within the oligodendroglial lineage.

Diazepam-insensitive GABAA receptors on postnatal spiral ganglion neurones in culture

Using dissociated spiral ganglion cell cultures obtained from 3-day-old rat cochlea, we investigated the response of auditory neurones to gamma-aminobutyric acid (GABA) using patch-clamp techniques. In our recording conditions, GABA elicited inward currents in > 95% of the neurones which reversed around 0 mV. Similar inward currents were measured using isoguvacin, a specific agonist of GABAA receptors. GABA-gated currents were reversibly inhibited by the channel blocker picrotoxin and the GABA competitive antagonist bicuculline. These functional GABAA receptors are characterized by an insensitivity to benzodiazepines and a relatively high sensitivity to beta-carbolines and barbiturates. These results show that the GABAA receptor pharmacological properties of spiral ganglion neurones are close to those of cerebellar granule cells.

Beta-carbolines induce apoptotic death of cerebellar granule neurones in culture

Apart from its role in fast inhibitory transmission, only neurotrophic effects have been reported following activation of the GABAA receptor. Here, we show that n-butyl-beta-carboline-3-carboxylate and n-methyl-beta-carboline-3-carboxamide, which are negative allosteric modulators of the GABAA receptor acting at the benzodiazepine site, are neurotoxic for cerebellar granule neurones in culture. The beta-carboline-induced neuronal death is apoptotic since DNA internucleosomal fragmentation was induced and the neurotoxicity could be prevented by inhibitors of mRNA or protein synthesis. As GABA and benzodiazepine ligands (diazepam and Ro 15-1788) protect cerebellar granule cells against beta-carboline-induced toxicity, these data raise the possibility that the interaction between the beta-carbolines and the GABAA receptor is the triggering event leading to neuronal apoptosis.

Astroglia-released factor with negative allosteric modulatory properties at the GABA A receptor

We have previously shown, using whole-cell patch-clamp techniques, that astrocytes release a negative allosteric modulator of the gamma-aminobutyric acid type A receptor (GABAA receptor) with beta-carboline-like properties, thus, likely to act at the benzodiazepine site. Here, using patch-clamp and binding techniques, we confirm that the low-molecular-weight fraction of astroglia-conditioned medium (ACM lmf) contains a factor(s) that negatively modulates GABAA-receptor function. This factor, like beta-carbolines, enhances the specific binding of [35S]t-butyl bicyclophosphorothionate (TBPS) to adult rat cortical membranes in the presence of GABA. However, it fails to interact with various ligands of the benzodiazepine (BZD) site of the GABAA receptor ([3H]flunitrazepam, [3H]Ro 15-1788 and [3H]Ro 15-4513). The question of the actual binding site of the astroglia-derived factor on the GABAA receptor, thus, remains open and can be addressed only after the purification of the active molecule(s) of ACM Imf has been completed, and a labeled form of the endogenous ligand becomes available. Taken together, however, the data suggest that type 1 astrocytes are able to modulate the effects of the main inhibitory neurotransmission in the central nervous system.

Astroglia-released factor shows similar effects as benzodiazepine inverse agonists

Media conditioned by cultured neonatal cerebral cortex microexplants (CCM) or astrocytes (ACM) contain low molecular weight (< 1,000 Da) substance(s) which inhibits the gamma aminobutyric acid (GABA)-induced inward current recorded in cerebellar granule cells and hippocampal neurons in culture using the whole-cell patch-clamp technique. This effect is specific for CCM and ACM, as medium conditioned by PC12 cells (PC12CM) does not affect the GABA response of these cells. It is also specific for GABA-induced currents because glutamate-induced currents do not change either in amplitude or in shape in the presence of CCM or ACM. The inhibitory effect on the GABA response in cerebellar granule cells of both ACM and CCM could be suppressed by flumazenil, a specific benzodiazepine (BZD) antagonist and could be mimicked by two BZD inverse agonists. These data thus demonstrate the presence of a BZD inverse agonist-like activity in CCM and ACM. This effect of ACM on different neuronal cell types was heterogenous since no detectable effect could be observed on the GABA-induced current in GABA-responsive dorsal root ganglion (DRG) neurons, presumably reflecting a functional heterogeneity of the GABAA receptors present in these different neuronal subsets. By the release of such an endogenous BZD inverse agonist-like activity, glia cells could possibly modulate GABAA receptor-mediated responses.