Biomedical subjects

D A Poulain

Publications and source records attributed to D A Poulain.

At least 19 recordsLinked to original sources

PubMed2005

A study of the role of neuro-glial remodeling in the oxytocin system at lactation.

Under conditions of strong secretion of neurohypophysial hormone, such as during parturition, lactation and dehydration, the hypothalamic oxytocin-system displays a remarkable morphological plasticity such that astrocytic coverage of its neurones diminishes, their surfaces become directly juxtaposed and contacted by an increased number of synapses. A growing body of evidence indicates that these anatomical changes have an impact on glutamatergic neurotransmission in the supraoptic nucleus, and may be therefore of physiological consequence. We here evaluated the consequences of the inhibition of such plasticity on the overall activity of the oxytocin system during lactation. Remodeling was prevented by performing hypothalamic microinjections in gestating rats of endoneuraminidase, an enzyme that removes polysialic acid from the neural cell adhesion molecule. Our earlier studies established that the presence of polysialic acid is a prerequisite for remodeling of the oxytocin system in the supraoptic and paraventricular nuclei. In dams in which polysialic acid was absent in all magnocellular nuclei after bilateral endoneuraminidase injections, parturition was normal and neither the frequency nor the amplitude of suckling-induced reflex milk ejections was different from vehicle-treated dams. The weight gain of pups was also normal as was water intake by the dams. We then assessed the electrical activity of antidromically identified magnocellular neurones in the polysialic acid-free supraoptic nucleus of isoflurane-anesthetized lactating rats. Basal and bursting activity characteristic of oxytocin neurones before each reflex milk ejection was not significantly different from that recorded in the supraoptic nucleus of rats with normal levels of polysialic acid. Our results indicate that neuro-glial remodeling, despite its role on fine modulation of oxytocin neuronal activity, is not essential to parturition and lactation.

Animals↗

PubMed2001

Control of glutamate clearance and synaptic efficacy by glial coverage of neurons.

Analysis of excitatory synaptic transmission in the rat hypothalamic supraoptic nucleus revealed that glutamate clearance and, as a consequence, glutamate concentration and diffusion in the extracellular space, is associated with the degree of astrocytic coverage of its neurons. Reduction in glutamate clearance, whether induced pharmacologically or associated with a relative decrease of glial coverage in the vicinity of synapses, affected transmitter release through modulation of presynaptic metabotropic glutamate receptors. Astrocytic wrapping of neurons, therefore, contributes to the regulation of synaptic efficacy in the central nervous system.

ATP-Binding Cassette Transporters↗

PubMed2001

Maternity leads to morphological synaptic plasticity in the oxytocin system.

The oxytocinergic system, which plays a major role in the control of different aspects of maternity, undergoes extensive synaptic and neuronal-glial remodelling during parturition and lactation and has thus become a remarkable example of activity-dependent morphological synaptic plasticity in the adult mammalian brain. The use of different comparative ultrastructural analyses on the rat supraoptic and paraventricular nuclei, together with identification of pre- and post-synaptic elements, has allowed us to show that there is a significant increase in the number of GABAergic, glutamatergic and noradrenergic synapses impinging on oxytocin neurons, concomitant with a reduction of glial coverage of the neurons. This synaptic plasticity involves axo-dendritic and axo-somatic contacts originating from terminals making one or several synaptic contacts in one plane of section. While noradrenergic afferents arise from medullary catecholaminergic neurons, our recent in vitro observations indicate that GABAergic and glutamatergic afferents derive, at least partly, from local intrahypothalamic neurons, in close proximity to oxytocin neurons. The cellular mechanisms underlying this morphological synaptic plasticity remain to be determined but it is highly likely that they depend on increased activity in both pre- and post-synaptic elements. Moreover, the oxytocin system continues to express 'embryonic' molecular features that may allow the morphological plasticity to occur. In particular, it expresses high levels of cell surface adhesion molecules currently thought to intervene in synaptic remodelling in the developing and lesioned central nervous system, including the weakly adhesive polysialylated isoform of the Neural Cell Adhesion Molecule, the axonal glycoprotein F3 and its ligand, the extracellular matrix glycoprotein, tenascin-C.

Afferent Pathways↗

PubMed2001

The polysialylated neural cell adhesion molecule reaches cell surfaces of hypothalamic neurons and astrocytes via the constitutive pathway.

Understanding how neurons and glia sort and deliver cell adhesion molecules to their cell surface should provide important clues as to how such molecules participate in dynamic neuronal functions in the developing and adult brain. The present study examines translocation of polysialylated neural cell adhesion molecule (PSA-NCAM), a negative regulator of cell adhesion, in cells of the rat hypothalamo-neurohypophysial system in which it is expressed throughout life and which undergo morphological remodelling in response to stimulation. PSA-NCAM expression in this system does not vary markedly in relation to different conditions of regulated neurosecretion, suggesting that the glycoprotein reaches cell surfaces via the constitutive pathway. To study this more directly, we here used immunofluorescence for PSA on NCAM in live, unpermeabilized cells to monitor PSA-NCAM surface expression in organotypic slice cultures from postnatal rat hypothalami. Subsequent immunolabelling for oxytocin confirmed that the cultures included magnocellular oxytocinergic neurons displaying many properties of adult neurosecretory neurons in situ. In the cultures, immunoreaction for PSA-NCAM was visible on the surface of oxytocinergic and non-oxytocinergic axons. This reaction disappeared after exposure of the cultures to endoneuraminidase, an enzyme which specifically cleaves alpha-2-8-linked PSA from NCAM. PSA-NCAM reappeared on axonal surfaces 4h after enzyme washout. Such reexpression was visibly not affected by neuronal activity inhibition (blockade of Ca(2+) channels with Mn(2+), of Na(+) channels with tetrodotoxin, or of glutamate receptors with 6-cyano-7-nitroquinoxaline-2,3-dione or D-2-amino-5-phosphonopentanoic acid) or facilitation (K(+) depolarization or GABA-A receptor blockade with bicuculline). In contrast, PSA-NCAM surface translocation was inhibited reversibly by cooling the cultures at 20 degrees C, a procedure which blocks constitutive secretion and which resulted in accumulation of PSA-NCAM in the cytoplasm of oxytocinergic and non-oxytocinergic neurons. This treatment also revealed PSA-NCAM in the cytoplasm of underlying astrocytes. Our observations provide direct evidence that PSA-NCAM reaches the cell surface of hypothalamic neurons and astrocytes via the constitutive pathway, independently of Ca(2+) entry and enhanced neuronal activity. Thus, PSA-NCAM in the hypothalamo-neurohypophysial system would be continuously available to permit its cells to undergo remodelling whenever the proper stimulus intervenes.

Animals↗

PubMed2001

Mobilization of the cell adhesion glycoprotein F3/contactin to axonal surfaces is activity dependent.

F3/contactin is a cell adhesion/recognition molecule of the immunoglobulin superfamily implicated in axonal growth. We examined its subcellular distribution and mobilization to the cell surface in oxytocin- (OT-) secreting neurons, which express it throughout life and the axons of which undergo activity-dependent remodelling. This was performed in hypothalamic organotypic slice cultures containing OT neurons with properties of adult neurosecretory cells. Immunocytochemistry and immunoblot analysis confirmed that OT neurons express high levels of F3/contactin in vitro. Light and confocal microscopy of cultures that underwent double immunofluorescence after fixation showed F3/contactin immunoreactivity throughout the cytoplasm of OT somata, dendrites and axons, and also in non-OT axons and in putative synaptic boutons which contacted OT neurons. By contrast, after treatment of live cultures with anti-F3/contactin antibodies followed by double immunofluorescence for the glycoprotein and OT, F3/contactin immunoreactivity was visible only on the surface of axons, whether or not OT-immunoreactivity was present. Because of its glycosylphosphatidyl-inositol (GPI) linkage, F3/contactin can occur in a membrane-bound or soluble form. As seen from immunocytochemistry of live cells and immunoblot analysis, treatment of cultures with a GPI-specific phospholipase C (GPI-PLC) resulted in loss of F3/contactin immunoreactivity from all cell surfaces. F3/contactin immunoreactivity reappeared on axonal surfaces within 5 h after enzyme washout. Such re-expression was accelerated by neuronal activity facilitation (by K+ depolarization or gamma-aminobutyric acid (GABA)-A receptor blockade with bicuculline) and inhibited by neuronal activity repression [by blockade of Ca2+ channels with Mn2+, Na+ channels with tetrodotoxin (TTX) or excitatory inputs with glutamate antagonists]. Our observations establish therefore that F3/contactin surface expression in hypothalamic neurons is polarized to the axons where it occurs mainly in a GPI-linked form. We also provide direct evidence that externalization of F3/contactin depends on Ca2+ entry and neuronal electrical activity. Taken together with our earlier finding that the glycoprotein is localized in neurosecretory granules, we demonstrate that F3/contactin is mobilized to the axonal surface via the activity-dependent regulated pathway, thus arriving at the correct place and time to intervene in activity-dependent remodelling of axons.

Action Potentials↗

PubMed2000

17-Oestradiol modulates in vitro electrical properties and responses to kainate of oxytocin neurones in lactating rats.

1. Intracellular current clamp recordings were performed from identified oxytocin (OT) neurones in acute hypothalamic slices taken from lactating Wistar rats at early (5th day: LD-5) and late (21st day: LD-21) lactation. 2. The basic electrophysiological properties of LD-21 OT neurones differed from those of LD-5 OT neurones: their resting membrane potential was more depolarised (-51.5 versus -54.9 mV); their action potential duration was longer (1.6 versus 1.2 ms); their hyperpolarising after-potential (HAP) following single spikes and after-hyperpolarisation (AHP) following a burst of action potentials had smaller amplitudes (-46 and -67 %, respectively); and they lacked spike frequency adaptation during a burst. 3. In LD-21 neurones bath application of 17beta-oestradiol (10-7 M, 6-14 min) reversibly restored all these properties to values observed in LD-5 cells. This treatment had no effect on LD-5 neurones. 4. LD-21 neurones were less sensitive to kainate than LD-5 neurones. 17beta-Oestradiol significantly potentiated the kainate-induced response in LD-21, but not in LD-5 neurones. 5. The effects of 17beta-oestradiol were presumably mediated through a non-genomic mechanism since they occurred within a few minutes of administration, and disappeared within 30-40 min of washout. They were not inhibited by tamoxifen, an antagonist of the nuclear oestrogen receptor ER-alpha. Lastly, cholesterol, a non-active lipophilic molecule, had no effect. 6. Our observations demonstrate that, in the absence of 17beta-oestradiol, the basic electrical properties and sensitivity to kainate of OT neurones become altered between early and late lactation. However, the rise in circulating levels of oestrogens during the late phase of lactation may contribute to maintain OT neurone reactivity as long as suckling continues.

Animals↗

PubMed2000

Differential expression of two adhesion molecules of the immunoglobulin superfamily, F3 and polysialylated NCAM, in hypothalamic magnocellular neurones capable of plasticity.

The adult hypothalamo-neurohypophysial system undergoes activity-dependent, reversible morphological changes which result in reduced astrocytic coverage of its neurones and an increase in their synaptic contacts. Our recent observations show that neurones and glia of the hypothalamo-neurohypophysial system continue to express 'embryonic' molecular features which may underlie their capacity to undergo such plasticity. These include expression of cell surface molecules like the glycosyl phosphatidyl inositol (GPI)-linked glycoprotein F3, which intervenes in axonal outgrowth, and the polysialylated isoform of the neural cell adhesion molecule (PSA-NCAM), which reduces cell adhesion and promotes dynamic cell interactions. F3 is colocalised with vasopressin and oxytocin hormones in neurosecretory granules and follows an activity-dependent, regulated pathway for surface expression on neurohypophysial axons. In contrast, PSA-NCAM appears to follow a constitutive pathway, independent of the activity of the hypothalamo-neurohypophysial system, for expression on axonal and glial surfaces, in the hypothalamic magnocellular nuclei and in the neurohypophysis. The role of F3 remains to be determined but in view of its presumptive functions during development, we propose that it promotes remodelling of neurosecretory terminals. On the other hand, we provide direct evidence that surface expression of PSA on NCAM is essential to morphological plasticity since its specific enzymatic degradation in vivo inhibited the neuronal-glial and synaptic changes normally induced by stimulation of secretion from the hypothalamo-neurohypophysial system.

Animals↗

PubMed2000

Activity-dependent morphological synaptic plasticity in an adult neurosecretory system: magnocellular oxytocin neurons of the hypothalamus.

Oxytocin and vasopressin neurons, located in the supraoptic and paraventricular nuclei of the hypothalamus, send their axons to the neurohypophysis where the neurohormones are released directly into the general circulation. Hormone release depends on the electrical activity of the neurons, which in turn is regulated by different afferent inputs. During conditions that enhance oxytocin secretion (parturition, lactation, and dehydration), these afferents undergo morphological remodelling which results in an increased number of synapses contacting oxytocin neurons. The synaptic changes are reversible with cessation of stimulation. Using quantitative analyses on immunolabelled preparations, we have established that this morphological synaptic plasticity affects both inhibitory and excitatory afferent inputs to oxytocin neurons. This review describes such synaptic modifications, their functional significance, and the cellular mechanisms that may be responsible.

Adult↗

PubMed1999

Cell surface expression of polysialic acid on NCAM is a prerequisite for activity-dependent morphological neuronal and glial plasticity.

Polysialic acid (PSA) on the extracellular domain of the neural cell adhesion molecule (NCAM) reduces cell adhesion and is considered an important regulator of cell surface interactions. The hypothalamo-neurohypophysial system (HNS), whose glia, neurons, and synapses undergo striking, reversible morphological changes in response to physiological stimulation, expresses high levels of PSA-NCAM throughout life. Light and electron microscopic immunocytochemistry in normal rats and rats in which cell transport was blocked with colchicine showed that PSA-NCAM is expressed in both HNS neurons and glia, particularly at the level of astrocytic processes that envelop neuronal profiles and can undergo remodeling. Moreover, we confirmed that the overall levels of PSA-NCAM were not greatly altered by stimulation (lactation and chronic salt ingestion). Nevertheless, PSA is essential to morphological plasticity. Using comparative ultrastructural analysis, we found that, after specific enzymatic removal of PSA from NCAM by microinjection of endoneuraminidase close to the hypothalamic magnocellular nuclei in vivo, there was no apparent withdrawal of astrocytic processes nor any increase in synaptic contacts normally induced by lactation and dehydration. Our observations demonstrate, therefore, that expression of PSA on cell surfaces in the adult HNS is indispensable to its capacity for activity-dependent morphological neuronal-glial and synaptic plasticity. The carbohydrate PSA on NCAM can thus be considered a necessary permissive factor to allow neuronal and glial remodeling to occur whenever the proper inductive stimulus intervenes.

Animals↗

PubMed1999

Adenosine-induced presynaptic inhibition of IPSCs and EPSCs in rat hypothalamic supraoptic nucleus neurones.

1. The effects of adenosine on synaptic transmission in magnocellular neurosecretory cells were investigated using whole-cell patch-clamp recordings in acute rat hypothalamic slices that included the supraoptic nucleus. 2. Adenosine reversibly reduced the amplitude of evoked inhibitory (IPSCs) and excitatory (EPSCs) postsynaptic currents in a dose-dependent manner (IC50 approximately 10 microM for both types of current). 3. Depression of IPSCs and EPSCs by adenosine was reversed by the application of the A1 adenosine receptor antagonist 8-cyclopentyl-1, 3-dimethylxanthine (CPT; 10 microM). 4. When pairs of stimuli were given at short intervals, adenosine inhibitory action was always less effective on the second of the two responses than on the first, resulting in an increased paired-pulse facilitation and suggesting a presynaptic site of action. This observation was confirmed by analysis of spontaneous miniature synaptic currents whose frequency, but not amplitude or kinetics, was reversibly reduced by 100 microM adenosine. 5. CPT had no effect on synaptic responses evoked at a low frequency of stimulation (0.05-0.5 Hz), indicating the absence of tonic activation of A1 receptors under these recording conditions. However, CPT inhibited a time-dependent depression of both IPSCs and EPSCs induced during a 1 Hz train of stimuli. 6. Taken together, these results suggest that adenosine can be released within the supraoptic nucleus at a concentration sufficient to inhibit the release of GABA and glutamate via the activation of presynaptic A1 receptors. By its inhibitory feedback action on the major afferent inputs to oxytocin and vasopressin neurones, adenosine could optimally adjust electrical and secretory activities of hypothalamic magnocellular neurones.

Adenosine↗

PubMed1999

Contribution of astrocytes to activity-dependent structural plasticity in the adult brain.

A striking example of the capacity of adult astrocytes to undergo reversible morphological changes in response to stimuli which enhance neuronal activity is offered by astrocytes of the adult hypothalamo-neurohypophysial system (HNS). The HNS is composed of magnocellular neurons secreting the neurohormones oxytocin and vasopressin from axon terminals in the neurohypophysis. Upon activation of HNS secretion, glial coverage of oxytocin neurons significantly diminishes and their surfaces become extensively juxtaposed. These glial changes are invariably accompanied by structural synaptic remodelling resulting in increased numbers of GABAergic, glutamatergic, and noradrenergic afferents. In the neurohypophysis, they result in an enhanced neurohemal contact area. HNS glia in the adult continue to display "embryonic" features that may allow such activity-dependent structural plasticity. For example, supraoptic astrocytes display a radial glia-like morphology and continue to express vimentin, together with GFAP. All HNS astrocytes secrete extracellular matrix glycoproteins, like tenascin-C; they also express high levels of polysialylated NCAM or PSA-NCAM and the glycoprotein F3, molecules considered essential for neuronal-glial interactions in the developing and lesioned CNS. HNS expression of most of these proteins does not visibly vary under different conditions of neurohormone secretion. We consider them as permissive factors, therefore, allowing HNS cells to undergo remodeling whenever the proper stimuli intervene. In the hypothalamic nuclei, one such stimulus is oxytocin itself which, in synergy with steroids, can induce neuronal-glial remodelling; adrenaline does so in the neurohypophysis.

Animals↗

PubMed1999

Visualization of local afferent inputs to magnocellular oxytocin neurons in vitro.

We recently showed that oxytocin (OT) neurons in organotypic slice cultures obtained from postnatal rat hypothalamus display complex patterns of electrical activity, similar to those of adult magnocellular OT neurons in vivo. Here we used such cultures to investigate the identity and, in particular, the origin of afferent inputs responsible for this activity. Multiple immunostaining with light and confocal microscopy showed that the somata and dendrites of oxytocinergic neurons were contacted by numerous synapses, visualized by their reaction to the synaptic markers, synaptophysin or synapsin. Many were GABAergic, displaying immunoreactivities for glutamic acid decarboxylase or gamma-aminobutyric acid (GABA); others were enriched in glutamate immunoreactivity. Such afferents presumably arose from GABA- or glutamate-immunoreactive neurons, respectively, with distinct and characteristic morphologies and topographies. A few dopaminergic boutons (tyrosine hydroxylase- or dopamine-immunopositive) impinged on OT neurons; they arose from dopamine-positive neurons located along the third ventricle. No noradrenergic profiles were detected. Despite the presence of choline acetyl-transferase (ChAT)-immunoreactive neurons, there were no cholinergic contacts. Lastly, we found oxytocinergic synapses, identified by immunoreaction for OT-related neurophysin and synapsin, contacting OT somata and dendrites. Our observations thus demonstrate that inhibitory and excitatory inputs to OT neurons derive from local intrahypothalamic GABA and glutamate neurons, in close proximity to the neurons. They also reveal that OT neurons are innervated by hypothalamic dopaminergic neurons. Finally, they confirm the existence of homotypic OT synaptic contacts which derive from local OT neurons.

Acetylcholinesterase↗

PubMed1998

Evidence for a hypothalamic oxytocin-sensitive pattern-generating network governing oxytocin neurons in vitro.

During lactation and parturition, magnocellular oxytocin (OT) neurons display a characteristic bursting electrical activity responsible for pulsatile OT release. We investigated this activity using hypothalamic organotypic slice cultures enriched in magnocellular OT neurons. As shown here, the neurons are functional and actively secrete amidated OT into the cultures. Intracellular recordings were made from 23 spontaneously bursting and 28 slow irregular neurons, all identified as oxytocinergic with biocytin and immunocytochemistry. The bursting electrical activity was similar to that described in vivo and was characterized by bursts of action potentials (20.1 +/- 4.3 Hz) lasting approximately 6 sec, over an irregular background activity. OT (0.1-1 microM), added to the medium, increased burst frequency, reducing interburst intervals by 70%. The peptide also triggered bursting in 27% of nonbursting neurons. These effects were mimicked by the oxytocin receptor (OTR) agonist [Thr4, Gly7]-OT and inhibited by the OTR antagonist desGly-NH2d(CH2)5[D-Tyr2,Thr4]OVT. Burst rhythmicity was independent of membrane potential. Hyperpolarization of the cells unmasked volleys of afferent EPSPs underlying the bursts, which were blocked by CNQX, an AMPA/kainate receptor antagonist. Our results reveal that OT neurons are part of a hypothalamic rhythmic network in which a glutamatergic input governs burst generation. OT neurons, in turn, exert a positive feedback on their afferent drive through the release of OT.

Animals↗

PubMed1998

Regulated expression of the cell adhesion glycoprotein F3 in adult hypothalamic magnocellular neurons.

F3, a glycoprotein of the immunoglobulin superfamily implicated in axonal growth, occurs in oxytocin (OT)-secreting and vasopressin (AVP)-secreting neurons of the adult hypothalamo-neurohypophysial system (HNS) whose axons undergo morphological changes in response to stimulation. Immunocytochemistry and immunoblot analysis showed that during basal conditions of HNS secretion, there are higher levels of this glycosylphosphatidyl inositol-anchored protein in the neurohypophysis, where their axons terminate, than in the hypothalamic nuclei containing their somata. Physiological stimulation (lactation, osmotic challenge) reversed this pattern and resulted in upregulation of F3 expression, paralleling that of OT and AVP under these conditions. In situ hybridization revealed that F3 expression in the hypothalamus is restricted to its magnocellular neurons and demonstrated a more than threefold increase in F3 mRNA levels in response to stimulation. Confocal and electron microscopy localized F3 in secretory granules in all neuronal compartments, a localization confirmed by detection of F3 immunoreactivity in granule-enriched fractions obtained by sucrose density gradient fractionation of rat neurohypophyses. F3 was not visible on any cell surface in the magnocellular nuclei. In contrast, in the neurohypophysis, it was present not only in secretory granules but also on the surface of axon terminals and glia and in extracellular spaces. Taken together, our observations reveal that the cell adhesion glycoprotein F3 is colocalized with neurohypophysial peptides in secretory granules. It follows, therefore, the regulated pathway of secretion in HNS neurons to be released by exocytosis at their axon terminals in the neurohypophysis, where it may intervene in activity-dependent structural axonal plasticity.

Animals↗

PubMed1998

Electrophysiological studies of oxytocin neurons in organotypic slice cultures.

We have developed organotypic slice cultures derived from postnatal rat hypothalamus which contain well-differentiated oxytocin neurons. Intracellular recordings of identified neurons show that these cultured oxytocin cells exhibit basal electrical properties closely similar to those of magnocellular cells recorded in vivo and in acute in vitro preparations from adult animals. The cultures also include GABAergic and glutamatergic neurons making connections with the oxytocin cells, which strongly suggests that the rich GABAergic and glutamatergic innervations of adult oxytocin neurons in vivo derive largely from local hypothalamic sources. Pharmacological manipulations indicate that the cultured oxytocin neurons present functional GABAA (but not GABAB) receptors, and ionotropic non-NMDA and NMDA receptors, but no metabotropic receptors for glutamate. These synaptic inputs control to a great extent the electrical activity of oxytocin neurons. Of particular interest is our observation that the cultured oxytocin neurons display a recurrent bursting activity which does not appear to result from an endogenous regenerative activity, but from a patterned glutamatergic input. Our preliminary data show that oxytocin plays a facilitatory role in this bursting activity and suggest that such activity is generated within an hypothalamic circuitry.

Animals↗

PubMed1998

Factors governing activity-dependent structural plasticity of the hypothalamoneurohypophysial system.

1. The adult hypothalamoneurohypophysial system (HNS) undergoes reversible morphological changes in response to physiological stimulation. 2. In the hypothalamus, stimulation of neurohormone secretion results in reduced astrocytic coverage of oxytocinergic somata and dendrites so that their surfaces become directly juxtaposed. Concurrently, there is a significant increase in the number of GABAergic, glutamatergic. and noradrenergic synapses impinging on the neurons. 3. In the neurohypophysis, stimulation induces retraction of pituicyte processes from the perivascular area and enlargement and multiplication of neurosecretory terminals. 4. These neuronal-glial and synaptic changes are reversible with cessation of stimulation, thus rendering the HNS an excellent model to study physiologically linked structural neuronal plasticity in the adult CNS. 5. We still do not know the cellular mechanisms and factors underlying such plasticity. Recent studies indicate, however, that the adult HNS expresses molecular characteristics normally associated with histogenesis and/or tissue reorganization in developing or regenerating neural systems. They include expression of cell adhesion molecules such as the highly sialylated isoform of the neural cell adhesion molecule, PSA-NCAM, and the glycoproteins, F3 and tenascin-C. 6. The expression of PSA-NCAM and tenascin-C does not show striking differences in terms of age, sex or physiological condition but that of F3 varies considerably with neurohypophysial stimulation. 7. We postulate that such molecular features allow magnocellular neurons and their glia to undergo neuronal-glial and synaptic plasticity throughout life, provided the proper stimulus intervenes. 8. Thus, in the hypothalamic nuclei, centrally released oxytocin acting in synergy with steroids can induce such plasticity, while adrenaline, acting through beta-adrenergic mechanisms, does so in the neurohypophysis.

Animals↗

PubMed1997

Activin facilitates neuronal development in the rat amygdala.

The amygdala is one of the richest sources for activin receptor in the central nervous system (CNS) but the function of activin in the amygdala is unknown. An in vitro culture system was developed to study the effect of recombinant human activin-A on neuronal growth. Activin-A (1000 pM) was added continuously from day 1 in vitro and the medium changed every 2 days. Continuous visual assessment revealed that control neurones started to atrophy within 2 days of incubation in serum free N2-MEM. After 6 days in culture, cells were fixed and stained for growth-associated protein (GAP-43), a membrane-bound phosphoprotein involved in axonal elongation and synaptogenesis. Activin-A reduced the number of atrophying neurones and stimulated neuritic growth. The results presented here indicate a possible neurotrophic role for activin-A in the neonatal CNS.

Activins↗

PubMed1997

Expression of high levels of the extracellular matrix glycoprotein, tenascin-C, in the normal adult hypothalamoneurohypophysial system.

Glia and neurons of the hypothalamoneurohypophysial system (HNS) undergo reversible morphological changes, which are concomitant with the remodelling of afferents onto the neurons, under different conditions of neurohormone secretion. Here, we show that the adult rat HNS contains high levels of tenascin-C (TN-C), which is an extracellular matrix glycoprotein whose expression is usually associated with neuronal-glial interactions in the developing and lesioned central nervous system (CNS). By using light and electron microscopic immunocytochemical procedures, we visualized TN-C immunoreactivity in the hypothalamic supraoptic (SON) and paraventricular nuclei, where somata of the neurons are localized; in the median eminence, where their axons transit; and in the neurohypophysis, where they terminate. Hypothalamic areas adjacent to the magnocellular nuclei were devoid of immunoreactivity. Electron microscopy of the neurohypophysis showed immunolabelling of perivascular spaces, glial (pituicyte) and axonal surfaces, a type of labelling that also characterized the median eminence. In the hypothalamic nuclei, there was labelling of extracellular spaces and astrocytic surfaces. In normal animals, we detected no cytoplasmic reaction in glia somata, neurons, or endothelial cells. However, in animals treated with the intracellular transport blocker colchicine, there was intracytoplasmic labelling of all HNS glial cells, indicating a glial source for TN-C. Immunoblot analysis revealed TN-C isoforms of apparent high molecular weight (225, 240, and 260 kD) in the SON and median eminence, whereas lower MW forms (190/200 kD) predominated in the neurohypophysis. By using immunocytochemistry and immunoblot analysis, we found no visible differences in TN-C expression in relation to age, sex, or differing neurohypophysial secretion, which suggests that the expression of TN-C is a permanent feature of the HNS.

Animals↗