In cultured oligodendrocytes isolated from perinatal rat optic nerves, we have analyzed the expression of ionotropic glutamate receptor subunits as well as the effect of the activation of these receptors on oligodendrocyte viability. Reverse transcription-PCR, in combination with immunocytochemistry, demonstrated that most oligodendrocytes differentiated in vitro express the ␣-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor subunits GluR3 and GluR4 and the kainate receptor subunits GluR6, GluR7, KA1 and KA2. Acute and chronic exposure to kainate caused extensive oligodendrocyte death in culture. This effect was partially prevented by the AMPA receptor antagonist GYKI 52466 and was completely abolished by the non-N-methyl-Daspartate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), suggesting that both AMPA and kainate receptors mediate the observed kainate toxicity. Furthermore, chronic application of kainate to optic nerves in vivo resulted in massive oligodendrocyte death which, as in vitro, could be prevented by coinfusion of the toxin with CNQX. These findings suggest that excessive activation of the ionotropic glutamate receptors expressed by oligodendrocytes may act as a negative regulator of the size of this cell population.The main function of oligodendrocytes is to myelinate axons in the vertebrate central nervous system. This cell type develops mostly soon after the majority of neurons are generated and have extended their axons and, therefore, it is likely that neurons play an important role in regulating oligodendrocyte development. In the rat optic nerve, the first fully differentiated oligodendrocytes appear after birth, and the definitive mature population of oligodendrocytes is reached around 6 weeks later (1, 2). The proliferation and the survival of oligodendrocytes depend, respectively, on the electrical activity of neighboring axons and in the reciprocal contacts they establish (for a recent review, see ref.3). Axon to oligodendrocyte signaling results in the generation of the precise number of oligodendrocytes necessary to myelinate entirely a given population of axons. Unfortunately, little information is available about the molecules participating in this signaling process.Oligodendrocytes express neurotransmitter receptors including those activated by glutamate (4). This excitatory amino acid acts at various types of receptors, which can be grouped into two major categories: ionotropic receptors, which gate membrane ion channels permeable to cations; and metabotropic receptors, which are coupled to G proteins. Ionotropic receptors are classified into ␣-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), kainate, and N-methyl-Daspartate (NMDA) subtypes, according to their preferred agonist. Molecular cloning has revealed that each receptor subtype is composed of several subunits with high homology within each receptor class (reviewed in ref. 5). Thus, AMPA receptors are formed by GluR1-4, kainate receptors by GluR5-7 and KA1-2, and NMDA rece...

The system of tangential connections was studied in area 17 of normally reared (NR), binocularly deprived (BD) and dark-reared (DR) kittens and adult cats. Connections were labelled antero- and retrogradely by intracortical micro-injections of several fluorescent markers and horseradish peroxidase conjugated with wheat-germ agglutinin (WGA-HRP). In 5-day-old kittens tangential connections consist of homogeneously distributed fibres extending maximally over 2.7 mm. Around postnatal day (pnd) ten these connections start to express the patchy pattern characteristic of the adult. Retrogradely stained somata and anterogradely labelled terminals become organized in individual 300 to 350 microm wide clusters with a centre-to-centre spacing of about 500 microm. During the first three postnatal weeks the horizontal connections increase their span to up to 10.5 mm and the spacing between individual patches increases to about 700 microm. Over the following 4 weeks these projections become reduced in length and number. In adult NR cats, tangential connections span a distance of up to 3 mm and form a lattice of 200 - 500 microm wide clusters, which have an average centre-to-centre spacing of 1050 microm. Tangential connections originate and terminate in all cortical laminae except layer I and they are organized in register. The distances spanned are largest in supragranular, intermediate in infragranular and shortest in granular layers. In BD and DR cats older than 10 weeks, the length of intracortical tangential fibres becomes reduced to the same extent as in NR animals, but individual clusters are less numerous. The authors conclude that the lattice-like structure of lateral connections evolves independently of visual experience, and that the selectivity of interactions results from pruning of initially exuberant connections. It is suggested that this pruning process is dependent on activity and influenced by visual experience.

The expression of brain derived neurotrophic factor (BDNF) and its preferred receptor (TrkB) in rat retinal ganglion cells (RGCs) have been determined in the present study. To identify RGCs retrograde labelling was performed with fluorogold (FG). Subsequently, retinas were immunostained with antibodies to BDNF and TrkB. We found that all RGCs labelled with FG express both BDNF and its preferred receptor, TrkB. Moreover, displaced amacrine cells were also found to be immunolabelled by both antibodies. Thus BDNF/TrkB signalling in RGCs probably involves endogenous BDNF produced by the RGCs themselves.

New subventricular zone (SVZ)-derived neuroblasts that migrate via the rostral migratory stream are continuously added to the olfactory bulb (OB) of the adult rodent brain. Anosmin-1 (A1) is an extracellular matrix protein that binds to FGF receptor 1 (FGFR1) to exert its biological effects. When mutated as in Kallmann syndrome patients, A1 is associated with severe OB morphogenesis defects leading to anosmia and hypogonadotropic hypogonadism. Here, we show that A1 over-expression in adult mice strongly increases proliferation in the SVZ, mainly with symmetrical divisions, and produces substantial morphological changes in the normal SVZ architecture, where we also report the presence of FGFR1 in almost all SVZ cells. Interestingly, for the first time we show FGFR1 expression in the basal body of primary cilia in neural progenitor cells. Additionally, we have found that A1 over-expression also enhances neuroblast motility, mainly through FGFR1 activity. Together, these changes lead to a selective increase in several GABAergic interneuron populations in different OB layers. These specific alterations in the OB would be sufficient to disrupt the normal processing of sensory information and consequently alter olfactory memory. In summary, this work shows that FGFR1-mediated A1 activity plays a crucial role in the continuous remodelling of the adult OB.

No abstract

As the interest in the neuroprotective possibilities of docosahexaenoic acid (DHA) for brain injury has grown in the recent years, we aimed to investigate the long-term effects of this fatty acid in an experimental model of perinatal hypoxia-ischemia in rats. To this end, motor activity, aspects of learning, and memory function and anxiety, as well as corticofugal connections visualized by using tracer injections, were evaluated at adulthood. We found that in the hours immediately following the insult, DHA maintained mitochondrial inner membrane integrity and transmembrane potential, as well as the integrity of synaptic processes. Seven days later, morphological damage at the level of the middle hippocampus was reduced, since neurons and myelin were preserved and the astroglial reactive response and microglial activation were seen to be diminished. At adulthood, the behavioral tests revealed that treated animals presented better long-term working memory and less anxiety than non-treated hypoxic-ischemic animals, while no difference was found in the spontaneous locomotor activity. Interestingly, hypoxic-ischemic injury caused alterations in the anterograde corticofugal neuronal connections which were not so evident in rats treated with DHA. Thus, our results indicate that DHA treatment can lead to long-lasting neuroprotective effects in this experimental model of neonatal hypoxia-ischemic brain injury, not only by mitigating axonal changes but also by enhancing cognitive performance at adulthood.

The effects of neonatal or adult enucleation on the final adult pattern of the rat visual corticocollicular (C-Co) connection were studied using the anterograde tracer biotinylated dextranamine 10,000 (BDA) iontophoretically injected in the primary visual cortex. In control animals, column-shaped terminal fields limited to a small portion of the collicular surface were observed. Synaptic boutons were present in all superficial strata of the superior colliculus (SC), with the highest density in the ventral part of the stratum griseum superficiale (SGS). Neonatal enucleation caused a considerable expansion of the contralateral visual C-Co terminal fields, which occupied almost the entire collicular surface, suggesting that axonal sprouting had occurred. In addition, terminal boutons tended to localize more dorsally in these cases compared with controls. Following enucleation in adult animals, no changes were observed with respect to the extension of the terminal fields, although a plastic reaction leading to an increase in the bouton density in the stratum zonale (SZ) and upper SGS was found, reflecting a process of reactive synaptogenesis at these levels. These results show that both neonatal and adult visual C-Co fibers react in response to retinal ablation, although this reaction shows distinct characteristics. Molecular factors, such as growth-associated cytoskeletal proteins operating in the cortical origin, and extracellular matrix components and myelin-associated axonal growth inhibitors acting on the collicular target very likely account for these differences.

The aim of this study has been to determine the neuronal types (pyramidal and nonpyramidal) within the rat's visual cortex, which project through the corpus callosum. To this end, the morphology and laminar distribution of callosal cells have been investigated by combining Diamidino Yellow retrograde tracing with intracellular injection of Lucifer Yellow in slightly fixed tissue slices. The visual callosal projection arises from pyramidal cells of diverse morphology in layers II to VIb, as well as from several modified pyramids located mainly in layers II, IV (star pyramids) and VIb (horizontal or inverted pyramids and related forms of spiny stellate cells). Our results indicate that in rats, as in other mammals, several types of nonpyramidal neurons also contribute to the contralateral projection. Bitufted cells in layers II-III and V were found to project contralaterally. Moreover, a spine-free layer V cell and a sparsely spiny multipolar neuron of layer IV were also labeled. In both stellate cells, partial axonal labeling reveals that these callosal cells display a local axonal arborization. Finally, our results of retrograde transport with Diamidino Yellow and with another sensitive retrograde tracer, the beta subunit of the cholera toxin, demonstrate for the first time that the two main neuronal types of layer I participate in the callosal projection. In layer I, several small horizontal cells of the inner half of layer I and a large subpial cell displaying long radiating dendrites were also injected. The latter cell may correspond to the Cajal-Retzius cell of the adult rat. In spite of the important differences in the organization of the visual system between rodents and cats, the callosal projection in both mammals is composed of a large variety of pyramidal cells and several nonpyramidal neurons. This high morphological diversity suggests that the callosal projection is much more physiologically complex than the extracortical efferents of the visual cortex, resembling other cortico-cortical connections. The roles that the different callosal cells may play in the processing of visual information are discussed in relation to the known functions of the corpus callosum.