Morphofunctional plasticity in the adult hypothalamus induces regulation of polysialic acid-neural cell adhesion molecule through changing activity and expression levels of polysialyltransferases.

Polysialic acid-neural cell adhesion molecule (PSA-NCAM) expression in the adult nervous system is restricted to regions retaining a capacity for morphological plasticity. For the female rat hypothalamoneurohypophysial system (HNS), we have previously shown that lactation induces a dramatic decrease in PSA-NCAM, while leaving the level of total NCAM protein unchanged. Here, we wanted to elucidate the molecular mechanisms leading to a downregulation of PSA, thereby stabilizing newly established synapses and neurohemal contacts that accompany the increased activity of oxytocinergic neurons. First, we show that the overall specific activity of polysialyltransferases present in tissue extracts from supraoptic nuclei decreases by approximately 50% during lactation. So far, two polysialyltransferase enzymes, STX and PST, have been characterized for their capacity to transfer PSA onto NCAM in vitro. Using a competitive RT-PCR on RNA extracts from the HNS, we demonstrate furthermore a significant decrease in the expression levels of both STX and PST mRNAs in lactating versus virgin animals. Interestingly, this downregulation of NCAM polysialylation is not correlated with the post-transcriptional regulation of variable alternative spliced exon splicing, in contrast to neural development. The control of polysialylation via a regulation of both enzyme activity and expression underlines the important role of this post-translational modification of NCAM in morphofunctional plasticity in adult brain.

Induction of MAP1B phosphorylation in target-deprived afferent fibers after kainic acid lesion in the adult rat.

We have previously shown that the phosphorylated form of microtubule-associated protein 1B (MAP1B-P), which is located in growing axons during development and regeneration, remains detectable in the adult central nervous system only in areas that undergo morphologic plasticity (Nothias et al. [1996] J. Comp. Neurol. 368:317-334). Our objective in the present study was to determine whether lesion-induced axonal remodeling, in the adult rat, is associated with reinduction of MAP1B phosphorylation. MAP1B-P was not detectable in intact adult thalamic ventrobasal complex (VB), although low levels of MAP1B and its mRNA were present. A neuron-depletion of VB by in situ injection of kainic acid was followed by an induction of MAP1B phosphorylation by 24 hours postlesion. MAP1B-P was detected in fibers originating from undamaged neurons that were not located in the lesion, as demonstrated by the absence of hybridized MAP1B-mRNA. Ultrastructural analysis confirmed the exclusive location of MAP1B-P in axons in a proximodistal gradient. MAP1B phosphorylation appeared to be regulated by posttranslational modification of existing protein because the levels of MAP1B-mRNA did not change. The number of MAP1B-P-labeled fibers increased during the first month postlesion and remained high for a long period. Double staining by using axonal tracing with dextran-biotin and tyrosine hydroxylase immunohistochemistry, showed the presence of MAP1B-P in VB afferents from somatosensory relays and the locus coeruleus. This study supports the hypothesis that MAP1B, at a particular state of phosphorylation, is correlated with axonal remodeling in the adult central nervous system (CNS). We suggest that the interaction of MAP1B-P with microtubules allows the modulation of their dynamic properties during periods of increased axonal plasticity.

Central projections and motor nuclei of the facial, glossopharyngeal, and vagus nerves in the mormyrid fish Gnathonemus petersii.

Most of the information about the anatomy of the fish's cranial nerves was collected in the first two decades of this century. Experimental analysis of the VIIth, IXth, and Xth cranial nerves by modern tract tracing techniques started about 20 years ago. Several species have been investigated to date, including one species of Agnatha (Myxinoidea), two species of elasmobranchs, and species of some orders of Teleostei like Cyprinidae, Siluriformes, Perciformes, and Gadidae. The sensory and motor nuclei of the VIIth, IXth, and Xth cranial nerves of Gnathonemus petersii were studied by anterograde and retrograde axoplasmatic transport of horseradish peroxidase and cobaltous lysine complex. The sensory nuclei form a continuous column of cells in the brain stem extending caudal to the obex. The rostral one-fourth of this column is occupied by the overlapping terminals of the VIIth and IXth nerves. The vagus nerve has 5 roots. The first 4 of these innervate the gills and the fifth supplies viscera. Afferents from the gills terminate ipsilaterally rostral to the obex in topographic order and their terminal fields overlap. Viscerosensory fibers terminate ipsilaterally in the obex region and bilaterally in the commissural nucleus of Cajal. The facial motor nucleus is located rostral to the sensory nucleus. Facial motoneurons have pear-shaped and multipolar perikarya. Their axons form a rostrally directed knee before leaving the brain. The motoneurons of the IXth and Xth nerves have a common cell column. The vagal motoneurons form a periventricular, a medial, and an intermediate cell group rostral to the obex. In the obex region and also caudal to it, a lateral and a caudal group can be distinguished. Vagal motoneurons show a topographic arrangement that is similar to that of the sensory vagal projections. The majority of motoneurons have pear-shaped perikary and ventrolaterally oriented dendrites. In the caudal nucleus the dendrites extend dorsally and overlap the terminals of sensory fibers. The axons form a dorsolaterally directed arch before joining the sensory roots. Since G. petersii uses its electrosensory system primarily for detection of food, its gustatory system is less developed than in other fishes, which possess a large number of taste buds.

The central connections of the olfactory bulbs in cod, Gadus morhua L.

The central projections of the "classical" olfactory system of the cod, Gadus morhua were examined with horseradish peroxidase and cobalt tracing techniques. Label was applied to the olfactory bulb or selectively to central stumps of sectioned individual olfactory tract bundlets. The olfactory bulb projects bilaterally to restricted areas of the dorsolateral, ventromedial and basolateral telencephalon, anterior commissural and preoptic areas, habenular nuclei, dorsal thalamus and to the nucleus posterior tuberis in the diencephalon. An interbulbar connection courses in the medial olfactory tract (MOT). Contralateral projections were less pronounced than on the ipsilateral side. More specifically, the lateral olfactory tract (LOT) projects ipsilaterally to the telencephalon into the Dlv, Dc, Vs and Dp areas. The lateral bundlet of the medial olfactory tract (IMOT) terminates in the Dlv and Dc areas. The medial bundlet of the medial olfactory tract (mMOT) terminates in Vv and Vd. The fused lMOT and mMOT project to the caudal telencephalon in the Vs and Dp. Neurons projecting to the olfactory bulb were located bilaterally in the telencephalon. The majority of the bulbopetal fibers course via the lateral part of the MOT; a few neurons also project to the bulb through the other bundlets of the olfactory tract. The results are compared with previous studies on the olfactory projections of other teleost species and discussed with respect to the reported functional differentiation of the olfactory system in teleosts.

The olfactoretinalis system = terminal nerve?

Immunohistochemistry revealed a distinct terminal nerve (nT) from the olfactoretinalis system (ORS) in several gymnotid species. The ganglion (NOR) of the ORS is a cluster of FMRF-amide immunoreactive neurones located on the olfactory nerve and bulb. The NOR projects to the olfactory bulb and optic nerve but not to the olfactory epithelium. In contrast, the nT is a small substance P or GnRH immunoreactive fibre bundle which originates from a ganglion located rostral to the olfactory epithelium, courses caudally with the olfactory nerve and terminates as glomeruli in the olfactory bulb. Peripherally the ganglion projects to the epithelium of the anterior nostril. Thus, the terminal nerve appears to be a cranial nerve, clearly distinct from the ORS which consequently should no longer be considered as the terminal nerve.

[Extra-bulbar primary olfactory projection in teleost fishes]. / Projection olfactive primaire extrabulbaire chez certains poissons téléostéens.

Immunohistochemical investigations with anti-substance P antiserum demonstrate the existence of an extensive extrabulbar primary olfactory projection in several gymnotid teleost fish. This projection, never described before, originates in particular primary olfactory bundles which enter with the olfactory nerve into the olfactory bulb. While the bulk of the olfactory fibers end with glomeruli in the glomerular layer of the olfactory bulb, two particular bundles penetrate into the telencephalon and end, without forming glomeruli, in several telencephalic and diencephalic regions. A few fibers run as far as to the hypothalamus. In the light of these findings, the general notion that the primary olfactory projection is limited to the olfactory bulb and forms only glomeruli-like terminals, should be reconsidered.

[Long ascending fibers in the dorsal column of a teleost fish: a disynaptic pathway connecting sense organs to cerebellum]. / Fibres longues ascendantes dans les colonnes dorsales d'un poisson téléostéen: une voie disynaptique reliant des organes sensoriels au cervelet.

In spite of the generally accepted opinion that long ascending proprioceptive and tactile fibers do not occur in the spinal dorsal columns of teleost fish, it was demonstrated with degeneration and axonal transport tracing methods that such dorsal column fibers exist in the teleost fish Gnathonemus petersii. These fibers are in fact common spinal afferent fibers originating in spinal ganglion cells. They connect the peripheral sense organs with the lateral funicular nuclei (Fl2) in which the dorsal column fibers terminate, directly through the dorsal columns. In contrast to the dorsal column nuclei of higher vertebrates, the Fl2 nuclei do not project to the diencephalic thalamus but to the caudal lobe and the second lobe (C2) of the corpus cerebelli. Thus, sense organs and cerebellum are connected by a disynaptic pathway. Since the caudal lobe projects directly to the electrosensory lobe, that is, to the target of electrosensory afferents, the presence of a disynaptic pathway in G. petersii suggests the existence of a proprioceptive control of the electrosensory input.

HRP labeling and ultrastructural localization of prepacemaker terminals within the medullary pacemaker nucleus of the weakly electric gymnotiform fish Apteronotus leptorhynchus.

The prepacemaker nucleus (PPN) in the midbrain of gymnotiform electric fish projects to the pacemaker nucleus (PN) in the medulla and modulates its rhythmic discharges in the context of social electric communication. Anterograde labeling of PPN axons with HRP and ultrastructural studies of their terminations in the PN have shown that PPN neurons form synaptic contacts with both types of neurons in the PN, pacemaker cells, and relay cells.

Convergence of electrotonic club endings, GABA- and serotoninergic terminals on second order neurons of the electrosensory pathway in mormyrid fish, Gnathonemus petersii and Brienomyrus niger (Teleostei).

Previous electrophysiological data indicate that the afferent electrosensory impulses conveyed towards the mesencephalon are blocked in the rhombencephalic electrosensory lateral line nucleus (nELL) by the concomitantly occurring EOD (electric organ discharge) command-associated (corollary) discharge. Electron-microscopic observations and anterograde labeling with horseradish peroxidase show that the primary electrosensory fibers terminate with club endings on the adendritic soma of the nELL cells and form gap junctions with the postsynaptic membrane. The remaining part of the soma and the initial segment membrane of nELL cells are covered with a large number of boutons showing chemical synaptic profiles. The GABA-ergic (gamma-aminobutyric-acid) nature of the majority of the boutons is revealed immunocytochemically by anti-GABA and anti-glutamic acid decarboxylase (anti-GAD) antisera, as seen in the light microscope. Electron-microscopic examination confirms the GABAergic nature of most of the bouton-like terminals, whereas club endings show negative immunoreactivity. In addition, serotonin-immunoreactive fibers and boutons are found in the same nucleus, between and next to the nELL cells. It is suggested that the GABAergic endings are the morphological basis for the inhibition that occurs in the nELL and that is mediated by the corollary discharge.