Chronic exposure to IL-6 induces a desensitized phenotype of the microglia.

BACKGROUND: When the homeostasis of the central nervous system (CNS) is altered, microglial cells become activated displaying a wide range of phenotypes that depend on the specific site, the nature of the activator, and particularly the microenvironment generated by the lesion. Cytokines are important signals involved in the modulation of the molecular microenvironment and hence play a pivotal role in orchestrating microglial activation. Among them, interleukin-6 (IL-6) is a pleiotropic cytokine described in a wide range of pathological conditions as a potent inducer and modulator of microglial activation, but with contradictory results regarding its detrimental or beneficial functions. The objective of the present study was to evaluate the effects of chronic IL-6 production on the immune response associated with CNS-axonal anterograde degeneration. METHODS: The perforant pathway transection (PPT) paradigm was used in transgenic mice with astrocyte-targeted IL6-production (GFAP-IL6Tg). At 2, 3, 7, 14, and 21 days post-lesion, the hippocampal areas were processed for immunohistochemistry, flow cytometry, and protein microarray. RESULTS: An increase in the microglia/macrophage density was observed in GFAP-IL6Tg animals in non-lesion conditions and at later time-points after PPT, associated with higher microglial proliferation and a major monocyte/macrophage cell infiltration. Besides, in homeostasis, GFAP-IL6Tg showed an environment usually linked with an innate immune response, with more perivascular CD11b+/CD45high/MHCII+/CD86+ macrophages, higher T cell infiltration, and higher IL-10, IL-13, IL-17, and IL-6 production. After PPT, WT animals show a change in microglia phenotype expressing MHCII and co-stimulatory molecules, whereas transgenic mice lack this shift. This lack of response in the GFAP-IL6Tg was associated with lower axonal sprouting. CONCLUSIONS: Chronic exposure to IL-6 induces a desensitized phenotype of the microglia.

Neural injury alters proliferation and integration of adult-generated neurons in the dentate gyrus.

Neural plasticity following brain injury illustrates the potential for regeneration in the central nervous system. Lesioning of the perforant path, which innervates the outer two-thirds of the molecular layer of the dentate gyrus, was one of the first models to demonstrate structural plasticity of mature granule cells (Parnavelas et al., 1974; Caceres and Steward, 1983; Diekmann et al., 1996). The dentate gyrus also harbors a continuously proliferating population of neuronal precursors that can integrate into functional circuits and show enhanced short-term plasticity (Schmidt-Hieber et al., 2004; Abrous et al., 2005). To examine the response of adult-generated granule cells to unilateral complete transection of the perforant path in vivo, we tracked these cells using transgenic POMC-EGFP mice or by retroviral expression of GFP. Lesioning triggered a marked proliferation of newborn neurons. Subsequently, the dendrites of newborn neurons showed reduced complexity within the denervated zone, but dendritic spines still formed in the absence of glutamatergic nerve terminals. Electron micrographs confirmed the lack of intact presynaptic terminals apposing spines on mature cells and on newborn neurons. Newborn neurons, but not mature granule cells, had a higher density of dendritic spines in the inner molecular layer postlesion accompanied by an increase in miniature EPSC amplitudes and rise times. Our results indicate that injury causes an increase in newborn neurons and lamina-specific synaptic reorganization indicative of enhanced plasticity. The presence of de novo dendritic spines in the denervated zone suggests that the postlesion environment provides the necessary signals for spine formation.

Calcium homeostasis of acutely denervated and lesioned dentate gyrus in organotypic entorhino-hippocampal co-cultures.

Denervation of neurons, e.g. upon traumatic injury or neuronal degeneration, induces transneuronal degenerative events, such as spine loss, dendritic pruning, and even cell loss. We studied one possible mechanism proposed to trigger such events, i.e. excess glutamate release from severed axons conveyed transsynaptically via postsynaptic calcium influx. Using 2-photon microscopical calcium imaging in organotypic entorhino-hippocampal co-cultures, we show that acute transection of the perforant path elicits two independent effects on calcium homeostasis in the dentate gyrus: a brief, short-latency elevation of postsynaptic calcium levels in denervated granule cells, which can be blocked by preincubation with tetrodotoxin, and a long-latency astroglial calcium wave, not blocked by tetrodotoxin and propagating slowly through the hippocampus. While neuronal calcium elevations upon axonal transection placed remote from the target area were similar to those elicited by brief trains of electrical stimulation of the perforant path, large-scale calcium signals were observed upon lesions placed close to or within the dendritic field of granule cells. Concordantly, induction of c-fos in denervated neurons coincided spatially with cell populations showing prolonged calcium elevations upon concomitant dendritic damage. Since denervation of dentate granule cells by remote transection of the perforant path induces transsynaptic dendritic reorganization in the utilized organotypic cultures, a generalized breakdown of the cellular calcium homeostasis is unlikely to underlie these transneuronal changes.

Axonal plasticity elicits long-term changes in oligodendroglia and myelinated fibers.

Axons are linked to induction of myelination during development and to the maintenance of myelin and myelinated tracts in the adult CNS. Currently, it is unknown whether and how axonal plasticity in adult CNS impacts the myelinating cells and their precursors. In this article, we report that newly formed axonal sprouts are able to induce a protracted myelination response in adult CNS. We show that newly formed axonal sprouts, induced by lesion of the entorhino-hippocampal perforant pathway, have the ability to induce a myelination response in stratum radiatum and lucidum CA3. The lesion resulted in significant recruitment of newly formed myelinating cells, documented by incorporation of the proliferation marker bromodeoxyuridine into chondroitin sulphate NG2 expressing cells in stratum radiatum and lucidum CA3 early after lesion, and the occurrence of a 28% increase in the number of oligodendrocytes, of which some had incorporated bromodeoxyuridine, 9 weeks post-lesion. Additionally, a marked increase (41%) in myelinated fibres was detected in silver stained sections. Interestingly, these apparently new fibres achieved the same axon diameter as unlesioned mice but myelin thickness remained thinner than normal, suggesting that the sprouting axons in stratum radiatum and lucidum CA3 were not fully myelinated 9 weeks after lesion. Our combined results show that sprouting axons provide a strong stimulus to oligodendrocyte lineage cells to engage actively in the myelination processes in the adult CNS.

Enhanced microglial clearance of myelin debris in T cell-infiltrated central nervous system.

Acute multiple sclerosis lesions are characterized by accumulation of T cells and macrophages, destruction of myelin and oligodendrocytes, and axonal damage. There is, however, limited information on neuroimmune interactions distal to sites of axonal damage in the T cell-infiltrated central nervous system. We investigated T-cell infiltration, myelin clearance, microglial activation, and phagocytic activity distal to sites of axonal transection through analysis of the perforant pathway deafferented dentate gyrus in SJL mice that had received T cells specific for myelin basic protein (TMBP) or ovalbumin (TOVA). The axonal lesion of TMBP-recipient mice resulted in lesion-specific recruitment of large numbers of T cells in contrast to very limited T-cell infiltration in TOVA-recipient and -naïve perforant pathway-deafferented mice. By double immunofluorescence and confocal microscopy, infiltration with TMBP but not TOVA enhanced the microglial response to axonal transection and microglial phagocytosis of myelin debris associated with the degenerating axons. Because myelin antigen-specific immune responses may provoke protective immunity, increased phagocytosis of myelin debris might enhance regeneration after a neural antigen-specific T cell-mediated immune response in multiple sclerosis.

Directed fiber outgrowth from transplanted embryonic cortex-derived neurospheres in the adult mouse brain.

Neural transplantation has emerged as an attractive strategy for the replacement of neurons that have been lost in the central nervous system. Multipotent neural progenitor cells are potentially useful as donor cells to repopulate the degenerated regions. One important aspect of a transplantation strategy is whether transplanted cells are capable of fiber outgrowth with the aim of rebuilding axonal connections within the host brain. To address this issue, we expanded neuronal progenitor from the cortex of embryonic day 15 ubiquitously green fluorescent protein-expressing transgenic mice as neurospheres in vitro and grafted them into the entorhinal cortex of 8-week-old mice immediately after a perforant pathway lesion. After transplantation into a host brain with a lesion of the entorhino-hippocampal projection, the neurosphere-derived cells extended long fiber projections directed towards the dentate gyrus. Our results indicate that transplantation of neurosphere-derived cells might be a promising strategy to replace lost or damaged axonal projections.

Excitability changes within transverse lamellae of dentate granule cells and their longitudinal spread following orthodromic or antidromic activation.

The functional organization of the perforant path input to the dentate gyrus of the exposed hippocampus was studied in adult rabbits anesthetized with urethane and chloralose. Electrical stimulation of perforant path fibers caused excitation of granule cells along narrow, nearly transverse strips (lamellae) of tissue. Stimulation of granule cell axons (mossy fibers) in CA3 caused antidromic activation of granule cells along similar strips. Paired-pulse stimulation revealed marked changes in granule cell excitability both within a lamella (on-line) and for several mm off-line along the septo-temporal axis of the dentate gyrus. After the first pulse, granule cells were inhibited for up to about 100 ms and then facilitated for up to hundreds of ms. Feedback activity along mossy fiber collaterals exciting local inhibitory and excitatory neurons appeared to dominate in producing on- and off-line inhibition and facilitation. Neurons mediating these effects could be inhibitory basket cells and other inhibitory interneurons targeting granule cells on- and off-line. In addition, excitatory mossy cells with far reaching, longitudinally running axons could affect off-line granule cells by exciting them directly or inhibit them indirectly by exciting local inhibitory interneurons. A scheme for dentate gyrus function is proposed whereby information to the dentate gyrus becomes split into interacting transverse strips of neuronal assemblies along which temporal processing occurs. A matrix of neuronal assemblies thus arises within which fragments of events and experiences is stored through the plasticity of synapses within and between the assemblies. Similar fragments may then be recognized at later times allowing memories of the whole to be created by pattern completion at subsequent computational stages in the hippocampus.

Axonal lesion-induced microglial proliferation and microglial cluster formation in the mouse.

Microglia are innate immune cells and form the first line of defense of the CNS. Proliferation is a key event in the activation of microglia in acute pathology, and has been extensively characterized in rats, but not in mice. In this study we investigated axonal-lesion-induced microglial proliferation and surface antigen expression in C57BL/6 mice. Transection of the entorhino-dentate perforant path projection results in an anterograde axonal and a dense terminal degeneration that induces a region-specific activation of microglia in the dentate gyrus. Time-course analysis showed activation of microglial cells within the first week post-lesion and cell counting demonstrated a significant 1.6-fold increase in microglial numbers 24 h post-lesion reaching a maximal 3.8-fold increase 3 days post-lesion compared with controls. Double staining for the microglial macrophage antigen-1 and the proliferation marker bromodeoxyuridine, injected 1 h prior to perfusion, showed that lesion-reactive microglia accounted for the vast majority of proliferating cells. Microglia proliferated as soon as 24 h after lesion and 25% of all microglial cells were proliferating 3 days post-lesion. Immunofluorescence double staining showed that most activated, proliferating microglia occurred in multicellular clusters and co-expressed the intercellular adhesion molecule-1 and the hematopoietic stem cell marker cluster of differentiation 34. In conclusion, this study extends observations of axonal lesion-induced microglial proliferation in rats to mice, and provides new information on early microglial proliferation and microglial cluster formation and surface antigen expression in the mouse.

Tumor necrosis factor and its p55 and p75 receptors are not required for axonal lesion-induced microgliosis in mouse fascia dentata.

Tumor necrosis factor (TNF) is a potent pro-inflammatory and neuromodulatory cytokine. In the CNS it is produced primarily by microglia and considered to regulate microglial activation. On the basis of previous observations of increased microglial TNF mRNA synthesis in areas of anterograde axonal and terminal degeneration in mice, we studied the effect of TNF and its p55 and p75 receptors on axonal lesion-induced microglial activation in fascia dentata following transection of the perforant path (PP) projection. Unexpectedly, cell counting showed that the axonal lesion-induced microglial response in TNF and TNF-p55p75 receptor knock out mice and C57BL/6 mice was similar 5 days after the lesion. In addition, the microglial expression of the lysosomal-associated antigen CD68, and the clearance of MBP(+) myelin debris appeared similar in TNF and TNF-p55p75 receptor knock out mice compared to C57BL/6 mice. Quantitative PCR and in situ hybridization showed the expression of TNF mRNA to be maximally upregulated 6 h after the lesion, and confirmed that TNF mRNA was still upregulated 5 days after lesion when microglial numbers, CD11b mRNA level, and cellular TNF-p55 and -p75 receptor mRNA level reached maximum. However, in spite of the induction of TNF mRNA, TNF protein level remained at base-line in fascia dentata using immunohistochemistry and ELISA. In conclusion, the results showed a lower than expected lesion-induced increase in TNF protein, and that neither TNF nor its receptors were required for the axonal lesion-induced microglial morphological transformation and proliferation or for the initial clearance of degenerated myelin in the PP-deafferented fascia dentata.

Cytoarchitecture of fibroblast growth factor receptor 2 (FGFR-2) immunoreactivity in astrocytes of neurogenic and non-neurogenic regions of the young adult and aged rat brain.

Fibroblast growth factors (FGFs) are polypeptides that exert diverse biological effects on many cell types and tissues during embryogenesis and adulthood. In the adult brain, FGF-2 is primarily expressed by astrocytes and select groups of neurons. It has been shown that FGF-2 is neuroprotective and can stimulate proliferation of NSCs in neurogenic regions of the adult mammalian brain. Cellular responses to FGFs are mediated through membrane-spanning tyrosine kinase receptors in conjunction with low affinity binding to heparin sulfate proteoglycans. Four FGF receptors (FGFR1-4) have been cloned and characterized to date. In this study, we describe the anatomical distribution of FGFR-2 in young and aged rat brains. We demonstrate that the olfactory bulb, hippocampus, and cerebellum display the most robust FGFR-2 expression and observed age-related decrease in FGFR-2 levels in some but not all brain regions. In addition, we identified astrocytes as the primary source of FGFR-2 expression using immunofluorescence confocal microscopy. The astrocyte populations in the neurogenic areas, the subventricular zone (SVZ) and the subgranular zone (SGZ) of the dentate gyrus, express high levels of FGFR-2 protein, which points to its possible involvement in neurogenesis. We also explored the role of FGFR-2 in response to perforant pathway lesion and observed enhanced FGFR-2 expression by astrocytes surrounding the lesion. Thus, FGF-2 biological effects on astrocytes appear to be mediated through FGFR-2-dependent mechanisms, and this may provide an indirect route by which FGF-2 acts on neuronal populations.

Expression of a familial Alzheimer's disease-linked presenilin-1 variant enhances perforant pathway lesion-induced neuronal loss in the entorhinal cortex.

Alzheimer's disease (AD) is characterized by neuronal loss in the hippocampus and entorhinal cortex that is manifested by progressive memory impairment and cognitive decline. Autosomal-dominant, familial forms of AD (FAD) are caused by mutations in genes encoding amyloid precursor protein, presenilin-1 (PS1), and presenilin 2. Although it is established that expression of mutant PS1 variants leads to increased production of highly fibrillogenic amyloidbeta42 (Abeta42) peptides that deposit in the brains of patients with AD, the mechanism(s) by which Abeta deposition and expression of mutant genes induce lamina- and region-specific vulnerability of neuronal populations is not known. We have examined the hypothesis that expression of transgene-encoded FAD-linked mutant PS1 variants in entorhinal cortex neurons exacerbates the vulnerability of these cells to lesion-induced neuronal loss. To test this notion, we transected the perforant pathway (PP) of transgenic mice harboring either wild-type human PS1 (PS1HWT) or the FAD-linked mutant PS1DeltaE9 variant and examined neuronal survival in layer II of the entorhinal cortex (ECL2). Remarkably, PP transections lead to marked reductions in the numbers of ECL2 neurons in the ECL2 of mice expressing mutant PS1, compared with ECL2 neurons in PP-lesioned PS1HWT mice. Finally, and in contrast to studies in nontransgenic mice and in mice expressing PS1HWT, ECL2 neurons that express mutant PS1 and the calcium binding protein calbindin-D28k in ECL2 are also susceptible to lesion-induced neuronal loss. We conclude that expression of FAD-linked mutant PS1 variants enhances the vulnerability of neurons in the entorhinal cortex to PP lesion-induced cytotoxicity.

GABA(A) receptor gamma subunits in the hippocampus of the rat after perforant pathway lesion.

Immunohistochemical and Western blotting techniques were employed to examine the alterations in immunostaining of the gamma-amino butyric acid (GABA) receptor subunits gamma 1/3 and 2 within the hippocampus of the rat brain at 1, 3, 7, 14, and 30 days after a unilateral perforant pathway lesion. At 1, 3, and 7 days post-lesion, we observed a remarkable decrease in gamma 1/3 neuropil staining in the deafferented zone (i.e., the outer molecular layer of the dentate gyrus ipsilateral to the lesion), although at 3 and 7 days post-lesion, staining intensity was considerably recovered. At 14 days post-lesion, the gamma 1/3 immunostaining was indistinguishable from that of controls and it appeared yet more robust at 30 days post-lesion. We also observed a slight decrease in gamma 2 neuropil staining until 7 days post-lesion, and an increase in gamma 2 staining at 30 days post-lesion. Western blot analysis demonstrated data that was relatively consistent with our immunohistochemical observations, although gamma 3 was hardly detectable. Our study suggests that gamma subunits of the GABA(A) receptor in the dentate gyrus display a plastic response to the deafferentation of the perforant pathway.

Effect of apolipoprotein E deficiency on reactive sprouting in the dentate gyrus of the hippocampus following entorhinal cortex lesion: role of the astroglial response.

This study investigated the effect of apolipoporotein E (apoE) deficiency on hippocampal reactive sprouting responses of the septohippocampal cholinergic (SHC) and commissural/associational fibers (C/A) following an electrolytic lesion of the entorhinal cortex (ECL), using apoE knockout (apoEKO) and age-matched control wild-type mice. Based on recent evidence suggesting that apoE plays a role in the modulation of glial inflammation, we also tested the hypothesis that the pattern of the astroglial response to ECL might be related to the defective reinnervation previously reported in apoEKO mice. Consistent with our hypothesis, we report a differential pattern of astroglial response that concurred with impairments in the sprouting of the SHC and corresponding synaptic replacement in apoEKO mice at 14 and 30 days post-lesion (DPL), a time range covering the onset of axonal/terminal sprouting to synaptogenesis. We also report a limited sprouting of the C/A fiber system in apoEKO relative to control mice at 30 DPL, a period of active dendritic remodeling. The results of the present study confirm and extend previous findings that apoEKO mice display impaired regenerative capacity in response to ECL and argue that in addition to the effect of apoE on lipid trafficking, apoE may also influence the astroglial response to damage, and that both of these effects account for the defective reinnervation observed in apoEKO mice.

Overexpression of myelin-associated glycoprotein after axotomy of the perforant pathway.

Myelin-associated glycoprotein (MAG) contributes to the prevention of axonal regeneration in the adult central nervous system (CNS). However, changes in MAG expression following lesions and the involvement of MAG in the failure of cortical connections to regenerate are still poorly understood. Here, we show that MAG expression is differently regulated in the entorhinal cortex (EC) and the hippocampus in response to axotomy of the perforant pathway. In the EC, MAG mRNA is transiently overexpressed by mature oligodendrocytes after lesion. In the hippocampus, MAG overexpression is accompanied by an increase in the number of MAG-expressing cells. Lastly, the participation of MAG in preventing axonal regeneration was tested in vitro, where neuraminidase treatment of axotomized entorhino-hippocampal cultures potentiates axonal regeneration. These results demonstrate that MAG expression is regulated in response to cortical axotomy, and indicate that it may limit axonal regeneration after CNS injury.

Involvement of interferon-gamma and its receptor in the activation of astrocytes in the mouse hippocampus following entorhinal deafferentation.

The activation of glial cells has been thought to be a universal and important reaction to trauma and pathology in the mammalian central nervous system. The mechanism of glial activation is not completely clear to date, but numerous cytokines have been demonstrated to effectively influence the process in vitro and in vivo. Here we reported the axotomy-induced upregulation of interferon-gamma (IFN-gamma) receptor mRNA in the mouse hippocampus following transections of the entorhinal afferents. Northern blot analysis showed that the transcripts of IFN-gamma receptor were upregulated in a transient manner in the deafferented mouse hippocampus. In situ hybridization confirmed the temporal upregulation of IFN-gamma receptor mRNA specifically in the denervated areas of the mouse hippocampus, which showed that the expression of IFN-gamma receptor mRNA rose slightly at 2 days postlesion, increased remarkably at 3 days postlesion, nearly reached the maximum at 7 days postlesion, and almost returned to control levels at 15 days postlesion. Double labeling further proved that the upregulated IFN-gamma receptor mRNA was confined to reactive astrocytes. At 2 and 3 days postlesion, we also observed the expression of IFN-gamma mRNA by a small number of cells in the denervated areas. We noted that the upregulation of both IFN-gamma and its receptor expression coincided spatiotemporally with astroglial activation, suggesting the potential involvement of IFN-gamma and its receptor in the activation process of astrocytes in the hippocampus following entorhinal deafferentation.

Regulation of Nogo and Nogo receptor during the development of the entorhino-hippocampal pathway and after adult hippocampal lesions.

Axonal regeneration in the adult CNS is limited by the presence of several inhibitory proteins associated with myelin. Nogo-A, a myelin-associated inhibitor, is responsible for axonal outgrowth inhibition in vivo and in vitro. Here we study the onset and maturation of Nogo-A and Nogo receptor in the entorhino-hippocampal formation of developing and adult mice. We also provide evidence that Nogo-A does not inhibit embryonic hippocampal neurons, in contrast to other cell types such as cerebellar granule cells. Our results also show that Nogo and Nogo receptor mRNA are expressed in the adult by both principal and local-circuit hippocampal neurons, and that after lesion, Nogo-A is also transiently expressed by a subset of reactive astrocytes. Furthermore, we analyzed their regulation after kainic acid (KA) treatment and in response to the transection of the entorhino-hippocampal connection. We found that Nogo-A and Nogo receptor are differentially regulated after kainic acid or perforant pathway lesions. Lastly, we show that the regenerative potential of lesioned entorhino-hippocampal organotypic slice co-cultures is increased after blockage of Nogo-A with two IN-1 blocking antibodies. In conclusion, our results show that Nogo and its receptor might play key roles during development of hippocampal connections and that they are implicated in neuronal plasticity in the adult.

Encoding versus retrieval of spatial memory: double dissociation between the dentate gyrus and the perforant path inputs into CA3 in the dorsal hippocampus.

The hippocampus is an essential neural structure for spatial memory. Computational models suggest that the CA3 subregion of the hippocampus plays an essential role in encoding and retrieval of spatial memory. The perforant path (PPCA3) and dentate gyrus (DG)-mediated mossy fibers (MFs) compose major afferent inputs into CA3. A possible functional dissociation between these afferent inputs was attempted using a simple navigation test (i.e., the modified Hebb-Williams maze). Behavioral testing was combined with electrolytic lesions of PPCA3 or neurotoxic lesions of the DG, to eliminate each afferent input into CA3. Lesions in either afferent input into CA3 affected learning of an effective navigational path on the maze. The contributions of the two CA3 afferent inputs, however, were different regarding encoding and retrieval of memory measured based on indices operationally defined for the behavioral paradigm (i.e., encoding, the number of errors reduced within a day; retrieval, the number of errors reduced between days). The DG-lesioned animals exhibited deficits regarding the encoding index, but not the retrieval index, whereas the PPCA3-lesioned rats displayed deficits regarding the retrieval index, but not the encoding index. The results suggest that the two major afferent inputs of CA3 may contribute differentially to encoding and retrieval of spatial memory.

Dynamics of oligodendrocyte responses to anterograde axonal (Wallerian) and terminal degeneration in normal and TNF-transgenic mice.

The inflammatory cytokine tumour necrosis factor (TNF) can both induce oligodendrocyte and myelin pathology and promote proliferation of oligodendrocyte progenitor cells and remyelination. We have compared the response of the oligodendrocyte lineage to anterograde axonal (Wallerian) and terminal degeneration and lesion-induced axonal sprouting in the hippocampal dentate gyrus in TNF-transgenic mice with the response in genetically normal mice. Transectioning of the entorhino-dentate perforant path axonal projection increased hippocampal TNF mRNA expression in both types of mice, but to significantly larger levels in the TNF-transgenics. At 5 days after axonal transection, numbers of oligodendrocytes and myelin basic protein (MBP) mRNA expression in the denervated dentate gyrus in TNF-transgenic mice had increased to the same extent as in nontransgenic littermates. At this time, transgenics showed a tendency towards a greater increase in the number of juxtaposed, potentially proliferating oligodendrocytes. Noteworthy, at day 5 we also observed upregulation of MBP mRNA expression in adjacent hippocampal subregions with lesion-induced axonal sprouting, which were devoid of axonal degeneration, raising the possibility that sprouting axons provide trophic stimuli to the oligodendrocyte lineage. Twenty-eight days after lesioning, oligodendrocyte numbers and MBP mRNA expression were reduced to near normal levels. However, oligodendrocyte densities in the TNF-transgenic mice were significantly lower than in nontransgenics. We conclude that the early response of the oligodendrocyte lineage to axonal lesioning and lesion-induced axonal sprouting appears unaffected by the supranormal TNF levels in the TNF-transgenic mice. TNF may, however, have long-term inhibitory effects on the oligodendrocyte response to axonal lesioning.

GABA(B) receptor activation and limbic network ictogenesis.

Rat brain slices containing interconnected hippocampus and entorhinal cortex (EC) responded to 4-aminopyridine (50 microM) application by generating: (i) CA3-driven interictal discharges that propagated to the EC; and (ii) N-methyl-D-aspartic (NMDA) acid receptor-dependent ictal events originating in EC (cf. J. Neurosci. 17 (1997) 9308 for experiments made in brain slices). Ictal discharges disappeared within 1-2 h, but were re-established by cutting the Schaffer collaterals, which abolished CA3-driven interictal discharge propagation to EC. In intact slices, GABA(B) receptor activation by baclofen (5-40 microM): (i) depressed CA3-driven interictal activity; and (ii) disclosed non-NMDA glutamatergic receptor-dependent ictal discharges originating in CA3 and propagating to EC. These effects were reversed by the GABA(B) receptor antagonist CGP 35348 (0.5 mM). Application of increasing baclofen doses to slices in which hippocampus and EC networks were surgically isolated decreased epileptiform events with an IC50 that was lower in EC (0.6 microM; n = 12) than in CA3 (2.5 microM; n = 12). Hence, under control conditions, EC ictogenesis depends on NMDA receptor function and is controlled by CA3-driven output activity; in contrast, following GABA(B) receptor activation EC excitability is depressed to a greater extent than CA3, which leads to non-NMDA glutamatergic receptor-mediated ictogenesis in CA3. We propose that GABA(B) receptor modulation may represent an important mechanism for setting the site of initiation, the modalities of propagation and the glutamatergic receptor properties of ictogenesis in the limbic system and, perhaps, in mesial temporal lobe epilepsy patients.

Alterations of heterogeneous nuclear RNP A2 and B1 in the hippocampus of the rat after perforant pathway lesion.

We examined alterations in post-transcriptional regulation following deafferentation of the perforant pathway by focusing on heterogeneous nuclear ribonucleoprotein (hnRNP) A2 and B1 in rat hippocampi subjected to perforant pathway lesions. In control brains, immunoreactivity to both was observed in the nuclei of neurons throughout the hippocampus using immunohistochemical techniques. From 1 to 14 days post-lesion, a slight increase in A2 immunoreactivity was observed in neurons within the dentate granular layer as well in the pyramidal cells of the cornus Ammon fields ipsilateral to the lesion. In contrast, we observed a marked decrease in B1 immunoreactivity in the same regions at 1, 3 and 7 days post-lesion. All these alterations, however, were transient. A2 immunoreactivity returned to normal levels by 30 days post-lesion, and B1 immunoreactivity had completely recovered by 14 days post-lesion. The results of immunoblot analysis for A2 and B1 were wholly consistent with immunohistochemical observations. Our study suggests that post-transcriptional regulation in the hippocampal neurons changes after a perforant pathway lesion. Our study further suggests that the functions of hnRNPA2 and B1 are different, as each was differentially involved in the plastic response to deafferentation of the perforant pathway.